PhD Thesis Complete

Florian Huber

Nanocrystalline copper-based mixed oxide catalysts for water-gas shift Doctoral thesis for the degree of philosophiae doctor Trondheim, August 2006 Norwegian University of Science and Technology Faculty of Natural Sciences and Technology Department of Chemical Engineering

The force of habit always tries to close our eyes.

Amazement and questioning is the beginning of

every philosophy, the encounter with the mystery the

greatest experience.

Jean Henri Fabre

Acknowledgements

i

Acknowledgements

First of all, I would like to thank my supervisors Professor Anders Holmen and

Associate Professor Hilde. J. Venvik for their support, encouragement and guidance

throughout my work with this thesis. You have given me a great opportunity to develop

and learn new things. The last four years have been very instructive, from a professional

as well as a personal point of view.

Adjunct Professor John Walmsley, Sintef Materials and Chemistry, is gratefully

acknowledged for the TEM work as well as for interesting and helpful discussions. I

also would like to thank Associate Professor Magnus Rønning for giving me the

opportunity to use X-ray absorption spectroscopy and for interesting and helpful

discussions. Professor De Chen is also gratefully acknowledged for helpful discussions.

Egil Haanæs, Department of Chemical Engineering, and Elin Nilsen, Department of

Material Technology, are gratefully acknowledged for their assistance with the TGA

and the XRD devices, respectively. Zuhair Kamil Sallom, Sintef Materials and

Chemistry, is gratefully acknowledged for his assistance with microwave spray

drying/pyrolysis. The project team at the Swiss-Norwegian Beam Lines at the European

Synchrotron Radiation Facility in Grenoble, in particular Wouter van Beek, are

gratefully acknowledged for their assistance. The Ugelstad laboratory and in particular

Torbjørn Vrålstad are gratefully acknowledged for assistance with the BET device.

Hilde Meland and Cathrine Bræin Nilsen are gratefully acknowledged for preparing

some of the Cu-Zn-based catalysts. Magnus Thomassen, Department of Material

Technology, is gratefully acknowledged for assistance with preliminary cyclic

voltammetry experiments. Edvard Bergene, Asbjørn Lindvåg, Rune Myrstad, Edd A.

Blekkan, Ingrid Aartun, Bozena Silberova and Rune Lødeng are gratefully

acknowledged for interesting discussions and/or technical assistance.

The financial support from the Research Council of Norway, the Department of

Chemical Engineering, and Statoil ASA through the Gas Technology Center NTNU-

SINTEF is gratefully acknowledged.

Acknowledgements

ii

I want to thank my colleagues at NTNU and SINTEF for their support and the

interesting discussions we have had during the last four years.

Finally, I very much want to thank my family and my friends for their patience, support

and encouragement. Thank you for being there for me – laughing with me in good times

and supporting me during hard times.

Every path is only a path, and it is not an infringement

against oneself or others to abandon it if your heart

tells you so.

Carlos Castaneda: The Teachings of Don Juan

Abstract

iii

Abstract

The present work consists of six individual chapters dealing with preparation and

characterization of copper-based mixed oxide catalysts, and their catalytic performance

under water-gas shift conditions.

Paper I investigates the effect of cerium oxide on Cu-Zn-based mixed-oxide catalysts.

The Cu reducibility as well as the TOF of the catalysts, based on N2O chemisorption,

was found not to be significantly improved by the addition of CeO2. Ceria impregnated

on the hydrotalcite-type precipitate prior to calcination was found to improve copper

dispersion and stability of the mixed oxide catalyst.

Paper II comments on passivation of reduced Cu-, Ni-, Fe-, Co-based catalysts. These

catalysts are often subjected to passivation procedures prior to characterization to retain

the reduced state of the active metal. Passivation with N2O or O2 to create a protective

oxide layer results in a certain degree of sub-surface oxidation. The extent of bulk

oxidation depends on the type of oxidant as well as the size of the metal particles, as

shown for copper catalysts. Passivation strategies that involve CO and/or CO2 without

the formation of significant amounts of coke or wax around the metal particles may

comprise both formation of carbonaceous species and an oxide layer. Encapsulation of

reduced metal particles by a protective layer of carbon is found to efficiently preserve

the metallic state, as demonstrated for metallic nickel and iron with carbon nanofibers.

Passivation and encapsulation of reduced catalysts are not ideal strategies for

characterization of reduced systems, but require evaluation by a parallel in situ approach.

Paper III deals with preparation and characterization of Cu-Ce-Zr mixed oxide catalysts.

Cu0.23Ce0.54Zr0.23-mixed oxides were prepared by homogeneous co-precipitation with

urea. The resulting material exhibits high surface area and small nanocrystalline primary

particles with fluorite-type structure. STEM-EDS analysis shows that Cu and Zr are

inhomogeneously distributed throughout the ceria matrix. The EXAFS analysis

indicates the existence of CuO-type clusters inside the ceria-zirconia matrix. This type

of mixed oxide materials should therefore be described as heterogeneous single-phase

Abstract

iv

materials rather than homogeneous solid solutions. The pore structure and surface area

of the mixed oxides are affected by preparation parameters during both precipitation

(stirring) and the following heat treatment (drying and calcination). TPR measurements

show that most of the copper is reducible and not inaccessibly incorporated into the bulk

structure. Reduction-oxidation cycling shows that the reducibility improves from the

first to the second reduction step, probably due to a local phase segregation in the

metastable mixed oxide with gradual copper enrichment at the surface of the Ce-Zr

particles during heat treatment.

Paper IV compares Cu-Zn-Al and Cu-Ce-Zr mixed oxide catalysts under water-gas shift

reaction conditions. The catalysts were prepared by two different methods, co-

precipitation and flame spray pyrolysis. Cu-Ce-Zr catalysts are found not to be

generally superior to Cu-Zn-Al catalysts in terms of activity or short-term stability.

Instead, the difference in activity seems to be related to structural characteristics of the

catalysts as well as the reaction conditions. The apparent activation energy of the Cu-

Ce-Zr MMO catalysts appears to be less affected by increased concentrations of CO2

and H2 than the Cu-Zn-Al MMO catalysts.

Paper V investigates the effect of carbon nanofibers as catalyst dispersant. Carbon

nanofiber (CNF) and Cu-Ce-Zr mixed metal oxide containing nanocomposite catalysts

have been prepared by homogeneous co-precipitation with urea. The CNF-containing

nanocomposite catalysts exhibit similar overall catalytic activity and stability as the

corresponding CNF-free catalyst. 13 wt% of the MMO could be replaced by CNF

without decreasing the overall activity and stability of the catalyst. The specific activity

of the nanocomposites based on the total metal oxide content is similar or higher than

the activity of the CNF-free material, depending on the CNF content. Similar activation

energies are obtained for the CNF-free and CNF-containing materials. CO

chemisorption studies showed no significant CO chemisorption on CNF. CNF may be

regarded as inert dispersant material improving the dispersion of the mixed oxide

particles.

Abstract

v

Paper VI deals with the reducibility of copper in Cu-Ce-Zr and Cu-Zn-Al mixed oxide

catalysts prepared by homogeneous co-precipitation with urea, and the impact of Pt

impregnated on these mixed oxide catalysts. The WGS activity of Cu-Ce-Zr and Cu-Zn-

Al mixed oxide catalysts can be related to the Cu reducibility in these catalysts. Low-

temperature reducibility correlates with low-temperature activity. Pre-reduction is not

absolutely necessary when performing the WGS reaction above the Cu reduction

temperature of the catalyst. An adequate reduction procedure may, however, be applied

in order to optimize the CO conversion. Pt had no significant effect on the Cu-Ce-Zr

mixed oxide catalyst, but altered the properties of the Cu-Zn-Al mixed oxide catalyst. Pt

shifted the Cu reduction to lower temperatures, indicating the existence of an interaction

between Pt and Cu in the bimetallic catalyst. The effect of Pt on the WGS activity and

stability was dependent on the pre-treatment procedure as well as the reaction

conditions.

Table of contents

vi

Table of contents

Acknowledgement i

Abstract iii

Table of contents vi

List of publications and presentations viii

List of symbols and abbreviations xi

1. Introduction 1 1.1. Hydrogen society 1

1.2. Fuel processing for fuel cell applications 1

1.3. Scope of the present work 3

2. Theory and literature 5 2.1. Water-gas shift reaction (WGS) 5

2.2. WGS catalysts for fuel processing 8

2.2.1. Catalyst requirements 8

2.2.2. New catalyst materials 9

2.3. Mixed metal oxide catalysts 11

3. Experimental 13 3.1. Catalyst preparation 13

3.2. Characterization of catalyst materials 13

3.2.1. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) 13

3.2.2. X-ray diffraction (XRD) 14

3.2.3. Nitrogen adsorption-desorption isotherms 14

3.2.4. Thermogravimetric analysis (TGA) 15

3.2.5. Volumetric TPR measurements 15

3.2.6. Transmission electron microscopy (TEM) 15

3.2.7. Transmission X-ray absorption spectroscopy (XAS) 16

3.3. Setup for water gas shift testing 18

Table of contents

vii

4. Results and discussion 23 4.1. Pre-studies 23

4.1.1. Total pore volume and pore size distribution 23

4.1.2. XRD particle size estimates with Win-crysize 25

4.1.3. Reproducibility of the WGS testing – By-pass effects 26

4.2. Summary of results and discussion 30

4.2.1. Relating catalyst structure and composition to the water-gas shift

activity of Cu-Zn-based mixed-oxide catalysts (Paper I) 30

4.2.2. Remarks on the passivation of reduced Cu-, Ni-, Fe-, Co-based

catalysts (Paper II) 34

4.2.3. Preparation and characterization of nanocrystalline, high-surface

area Cu-Ce-Zr mixed oxide catalysts from homogeneous

co-precipitation (Paper III) 38

4.2.4. Comparison of Cu-Ce-Zr and Cu-Zn-Al mixed oxide catalysts for

water-gas shift (Paper IV) 42

4.2.5. Nanocrystalline Cu-Ce-Zr mixed oxide catalysts for water-gas shift:

Carbon nanofibers as dispersing agent for the mixed oxide

particles (Paper V) 43

4.2.6. The effect of platinum in Cu-Ce-Zr and Cu-Zn-Al mixed oxide

catalysts for water-gas shift (Paper VI) 44

5. Concluding remarks 47

6. Suggestions for further work 50

References 51

List of appendices 62

List of publications and presentations

viii


Publications

This thesis is based on the following articles, referred to in the text by their roman

numerals. Reprints of the published articles or manuscripts are given as Appendices I –

VI.

I M. Rønning, F. Huber, H. Meland, H. Venvik, D. Chen, A. Holmen, Relating

catalyst structure and composition to the water-gas shift activity of Cu-Zn-

based mixed-oxide catalysts, Catalysis Today 100 (2005), 249-254.

II F. Huber, Z. Yu, S. Lögdberg, M. Rønning, D. Chen, H. Venvik, A. Holmen,

Remarks on the passivation of reduced Cu-, Ni-, Fe-, Co-based catalysts,

Catalysis Letters, in press.

III F. Huber, H. Venvik, M. Rønning, J. Walmsley, A. Holmen, Preparation and

characterization of nanocrystalline, high-surface area Cu-Ce-Zr mixed oxide

catalysts from homogeneous co-precipitation, Manuscript in preparation.

IV F. Huber, H. Meland, M. Rønning, H. Venvik, A. Holmen, Comparison of Cu-

Ce-Zr and Cu-Zn-Al mixed oxide catalysts for water-gas shift, submitted.

V F. Huber, Z. Yu, J. Walmsley, D. Chen, H. Venvik, A. Holmen,

Nanocrystalline Cu-Ce-Zr mixed oxide catalysts for water-gas shift: Carbon

nanofibers as dispersing agent for the mixed oxide particles, Applied Catalysis

B: Environmental, accepted.

VI F. Huber, J. Walmsley, H. Venvik, A. Holmen, The effect of platinum in Cu-

Ce-Zr and Cu-Zn-Al mixed oxide catalysts for water-gas shift, Manuscript in

preparation.


ix

Presentations

The following presentations were given at national and international meetings based on

results obtained during this work:

H. Venvik, T. Gjervan, I. Aartun, K. A. Johnsen, F. Huber, E. Tangerås, H. Dyrbeck, M.

Fathi, A. Holmen: Propane to hydrogen via stepwise catalytic reactions. Oral

presentation, NKS Catalysis Symposium, Hafjell, November 28 – 29, 2002.

F. Huber, D. Chen, M. Rønning, H. Meland, H. Venvik, A. Holmen: Activity of different

hydrotalcite-based copper catalysts in the water-gas shift reaction. Poster presentation,

International Summer School: Towards a Hydrogen-based Society, Humlebæk,

Denmark, August 9 -15, 2003 and EuropaCat VI, Innsbruck, Austria, August 31 -

September 04, 2003.

M. Rønning, H. Meland, F. Huber, D. Chen, H. Venvik, A. Holmen: Effect of catalyst

composition on the structure and activity of Cu-based water-gas shift catalysts. Poster

presentation, EuropaCat VI, Innsbruck, Austria, August 31 - September 04, 2003.

F. Huber, M. Rønning, H. Meland, D. Chen, H. Venvik, A. Holmen: Structure-activity

relations in water-gas shift catalysts from hydrotalcite precursors. Oral presentation,

NKS Catalysis Symposium, Bergen, November 20 – 21 2003.

M. Rønning, F. Huber, H. Meland, D. Chen, H. Venvik, A. Holmen: Relating catalyst

structure and composition to the water-gas shift activity of Cu-Zn-based mixed-oxide

catalysts. Oral presentation, 11th Nordic Symposium on Catalysis, Oulu, Finland, Mai

23 - 25, 2004.

F. Huber, M. Rønning, H. Meland, D. Chen, H. Venvik, A. Holmen: Thermogravimetric

analysis of copper-based catalysts. Oral presentation. Thermal Analysis Seminar,

Trondheim, June 24, 2004 and Norwegian Hydrogen Seminar, Kvitfjell, November 15

– 16, 2004.


x

F. Huber, H. Venvik, A. Holmen: Cu-based mixed metal oxide catalysts for clean fuel

applications – On preparation, characterization and water-gas shift testing. Oral

presentation, Haldor Topsøe A/S, Lyngby, Denmark, Mai 10, 2006.

F. Huber, H. Meland, Z. Yu, M. Rønning, J. Walmsley, D. Chen, H. Venvik, A.

Holmen: Nanocrystalline Cu-Ce-Zr mixed oxide catalysts for clean fuel applications.

Oral presentation, 12th Nordic Symposium on Catalysis, Trondheim, Norway, Mai 28 -

30, 2006.

F. Huber, Z. Yu, J. Walmsley, M. Rønning, H. Venvik, D. Chen, A. Holmen:

Nanocrystalline Cu-Ce-Zr mixed oxide catalysts for clean fuel applications: Carbon

nanofibers as dispersant for the mixed oxide particles. Oral presentation, ISNEPP 2006,

Hong Kong, China, June 18 - 21, 2006.

List of symbols and abbreviations

xi


Latin symbols

B Line width at half maximum (FWHM) [° or radian]

Be Instrumental line broadening [° or radian]

BET BET surface area [m2/g]

dC Size of the core of passivated metal particles [nm]

dP Particle size estimate [nm]

E Energy [eV]

Ea Activation energy [kJ/mole]

E0 Energy shift to correct for deviation from theoretical edge

value [eV]

I0 Incident X-ray intensity [-]

It Transmitted X-ray intensity [-]

k Wave number of the photoelectron [Å-1]

K Scherrer constant [-]

Keq Equilibrium constant [-]

N Coordination number [-]

R Distance of the central atom to the neighbouring atom (shell) [Å]

stv Surface-to-volume ratio [m2/m3]

T Temperature [K or °C]

xp Fraction of passivated metal atoms [-]

Greek symbols

λ Wavelength [Å]

∆σ2 Debye-Waller factor (disorder in the neighbour distance) [Å2]

Θ Bragg angle [° or radian]

Abbreviations

AFAC Amplitude reduction factor

BET Brunauer Emmet Teller

BJH Barrett Joyner Halenda


xii

CCD Charge-coupled device

CNF Carbon nanofibers

DFT Density functional theory

EDS Energy dispersive X-ray spectroscopy

EG Ethylene glycol

EPR Electron paramagnetic resonance spectroscopy

ESRF European synchrotron radiation facility

EXAFS Extended X-ray absorption fine structure

FT Fourier transform

FWHM Full width half maximum

HCP Homogeneous co-precipitation

HTS High temperature water-gas shift

ICP-AES Inductively coupled plasma atomic emission spectroscopy

LFC Liquid flow controller

LTS Low temperature water-gas shift

MCFC Molten carbonate fuel cell

MFC Mass flow controller

MMO Mixed metal oxides

NLDFT Non-local density functional theory

NPD Nitrate precursor decomposition

OSC Oxygen storage capacity

PAFC Phosphoric acid fuel cell

PCA Principal component analysis

PEMFC Proton exchange membrane fuel cell

or polymer electrolyte membrane fuel cell

PSD Pore size distribution

RMS Root mean square

SOFC Solid oxide fuel cell

SNBL Swiss-Norwegian beam line

SSITKA Steady-state isotopic transient kinetic analysis

STEM Scanning transmission electron microscopy

TCD Thermal conductivity detector


xiii

TEM Transmission electron microscopy

TGA Thermogravimetric analysis

TOF Turnover frequency

TOS Time on stream

TPR Temperature programmed reduction

TPO Temperature programmed oxidation

US Ultrasonic field

VPI Effective core-hole lifetime

WGS Water-gas shift

XANES X-ray absorption near edge structure

XAFS X-ray absorption fine structure

XAS X-ray absorption spectroscopy

XLBA X-ray line broadening analysis

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

Introduction

1

1. Introduction 1.1. Hydrogen society

The predicted depletion of fossil fuels and the increase in energy demand require the

development of alternative low-emission and low-carbon energy systems [1,2].

Hydrogen is expected to play a major role as a future energy carrier, in a society based

on the use of renewable energy to split water into oxygen and hydrogen, and storage and

use of hydrogen as a fuel. Fuel cells are expected to provide electricity and power

vehicles. The only emission from the fuel cell will be pure water [3,4]. It will take more

than a decade before renewable energy sources can compete effectively with fossil fuels

[1], and a transitional approach is therefore necessary to produce hydrogen. Conversion

of natural gas is currently the most economic way to produce hydrogen [4-7].

1.2. Fuel processing for fuel cell applications

Figure 1 illustrates the general concepts of processing gaseous, liquid, and solid carbon-

containing fuels for fuel cell applications. Pure hydrogen, reformate (hydrogen-rich gas

from fuel reforming), and methanol are the fuels available for current fuel cells [8].

Sulphur compounds in carbon-containing fuels poison the catalysts in the fuel

processing and in the fuel cells and must be removed. Synthesis gas, a mixture of CO

and H2, can be generated from reacting the fuel with steam, oxygen or a mixture of both

[9-11]. The product, often referred to as reformate, can be used directly for high-

temperature fuel cells such as SOFC and MCFC. Hydrogen is the preferred fuel for low-

temperature fuel cells such as PEMFC and PAFC, which can be obtained by fuel

reformulation on-site for stationary applications or on-board for automotive applications

[6,12-14].

Using fuel cells to power vehicles, given the cost and size requirements, will require

major breakthroughs in new technologies. The earliest market for fuel cells will be for

stationary power generation. Fuel cells must first be successful for this before they can

be applied to transportation applications. Figure 2 shows that stationary application is

on the critical path for transportation. Also shown is the parallel path to

commercialization for portable power applications, where methanol is likely the fuel of

choice. Molten carbonate and solid oxide fuel cells are also being developed for high-

Introduction

2

power applications (> 250 kW). These require less extensive fuel reformers because this

function occurs partly within the anode compartment [5].

Figure 1. The concepts and steps for fuel processing of gaseous, liquid and solid fuels

for high-temperature and low-temperature fuel cell applications [6].

Figure 2. The road to commercialization for fuel cells [5].

Introduction

3

Distributed power applications can use the current natural gas and commercial propane

infrastructure to generate H2 in homes and at industrial sites. Once this technology is

implemented in the mass market, transportation applications will be more feasible, but

breakthroughs in cost, size, and infrastructure are essential. It is expected that

automotive applications will require on-board storage of H2 distributed from a local

service station. The H2 will be generated from natural gas or from a liquid fuel designed

specifically for fuel cells (low sulphur, high paraffin content, without concern of

optimizing octane or cetane numbers) [9-13]. On-board reformers, using liquid fuels,

are viewed as a transitional solution until new H2 storage technologies are developed [5].

Concerning the fuel processor technology, microreactor, membrane reactor and

monolith reactor concept represent promising approaches for stationary and mobile

applications. For more information on this matter, it is referred to relevant literature

[5,15-27].

A technical challenge for fuel processing catalysts is the development of active, poison-

resistant materials that will result in small catalyst volumes, reduced start-up times,

durability under steady-state and transient conditions at the required temperatures, cost

reductions, and versatility to variations in fuel/feed composition [5,28].

1.3. Scope of the present work

This work has been a part of the Strategic Institute Program “Advanced catalyst/reactor

systems for conversion of hydrocarbons to hydrogen for fuel cells”, supported by the

Research Council of Norway through Grant No. 140022/V30 (RENERGI). The program

was carried out in a joint collaboration between SINTEF Materials and Chemistry and

the Department of Chemical Engineering at the Norwegian University of Science and

Technology (NTNU). The program has focused on synthesis gas production by partial

oxidation or oxidative steam reforming in microstructured and monolithic reactors

[16,17,29,30], and the water-gas shift reaction using conventional and new WGS

catalyst formulations. The WGS reaction has also been studied in a membrane reactor

with a self-supported, hydrogen permeable Pd-Ag membrane [31].

Introduction

4

The aim of the present work was to gain insight into certain aspects related to copper-

based catalysts used for clean fuel applications and small-scale hydrogen production

systems, including preparation, characterization and catalytic performance. The water-

gas shift reaction was used as test reaction. The results of the present study are however

not only relevant to research on the water-gas shift reaction, but in fact also relevant to

other heterogeneous catalytic reactions, since Cu-based catalysts are applied in several

areas of heterogeneous catalysis. Moreover, the results may also be of importance to

catalytic systems where another active metal such as Ni or Co is combined with the

support materials chosen here.

Theory and literature

5

2. Theory and literature 2.1. Water-gas shift reaction (WGS)

The water-gas shift is the reaction of steam with carbon monoxide to produce carbon

dioxide and hydrogen gas. It is a reversible and exothermic chemical reaction:

CO + H2O ↔ CO2 + H2 ∆H0298 = - 41.2 kJ/mol (1)

The WGS is one of the oldest catalytic processes employed in the chemical industry

[32]. The first report on the water-gas shift reaction was published in a British gas

patent from 1888 [33]. Already then it was described how the passage of carbon

monoxide and steam over red-hot refractory material was able to produce carbon

dioxide and hydrogen. Since the early 1940s the WGS reaction has represented an

important step in the industrial production of hydrogen or synthesis gas. The essential

role of the industrial WGS reaction is to enhance the production of hydrogen and

remove CO for refinery hydroprocesses, ammonia synthesis and bulk storage and

redistribution of hydrogen, as well as adjustment of H2/CO ratios in the production of

methanol and synthetic, liquid hydrocarbons through the Fischer-Tropsch synthesis.

Although the WGS reaction is not the primary reaction, it also plays a role in methanol

synthesis, reforming of hydrocarbons, Fischer-Tropsch synthesis, automotive exhaust

catalysis, and selective CO oxidation for removal of trace CO [34].

The WGS is equilibrium controlled, and since the reaction is equimolar, the effect of

pressure on the thermodynamics is minimal considering the range of pressure used for

fuel processing [34]. However, pressure may affect adsorption/desorption of the gas

molecules involved [35] and under certain conditions enhance the rate of reaction [36-

38]. The equilibrium constant Keq as a function of temperature, as estimated from

thermodynamic data [39], is shown in Figure 3. Keq decreases with increasing

temperature due to the exothermic nature of the reaction. Thus, the forward WGS

reaction is thermodynamically favoured by low temperature. Figure 4 shows the

equilibrium CO conversion for two different reactant compositions, a feed containing 5

mole-% CO and 25 mole-% H2O, and a simulated reformate feed containing 5 mole-%

CO, 25 mole-% H2O, 12 mole-% CO2 and 35 mole-% H2. This corresponds to feed


6

compositions used for some of the activity tests performed in this thesis. The

equilibrium conversion in Figure 4 decreases with temperature, and the addition of CO2

and H2 to the reactants shifts the equilibrium towards the reactants.

1

10

100

1000

10000

100 150 200 250 300 350 400 450 500

temperature [ C ]

equi

libriu

m c

onst

ant [

- ]

Figure 3. Equilibrium constant Keq as function of temperature.

From a thermodynamic perspective, the extent of the forward WGS reaction is

maximized at low temperature, high H2O and low H2 concentrations. However, most

catalysts available today are kinetically limited at the low temperatures needed for

nearly complete CO conversion, thus, requiring large reactor volumes. Two or more

consecutive stages of WGS are often used to take advantage of both kinetics and

thermodynamics. By operating an HTS catalyst at higher temperatures the favourable

kinetics can be exploited and the volume of the catalyst can be minimized. By cooling

the syngas between the HTS and LTS stages an active catalyst can take advantage of the

thermodynamic equilibrium at low temperature [34].

The method of producing the syngas affects the WGS equilibrium. Autothermal

reforming produces a syngas with lower H2 concentration due to the dilution of nitrogen

as compared to steam reforming. The lower H2 concentration improves the equilibrium


7

CO conversion whereas the high H2 concentration expected from conventional steam

reforming lowers the WGS equilibrium conversion [34].

0,00

0,20

0,40

0,60

0,80

1,00

100 150 200 250 300 350 400 450 500

temperature [ C ]

CO

con

vers

ion

[ - ]

CO-H2O

CO-H2O-CO2-H2

Figure 4. Equilibrium CO conversion for two different feed conditions: (1) 5 mole-%

CO and 25 mole-% H2O; (2) 5 mole-% CO, 25 mole-% H2O, 12 mole-% CO2 and 35

mole-% H2 (simulated reformate feed), corresponding to feed compositions used for the

activity tests reported in papers IV, V and VI.

Because of its industrial significance, the kinetics and mechanisms of the WGS reaction

have been extensively studied in the past [34,40-47]. Two types of mechanisms have

been proposed for the main catalyst systems, the associative and the regenerative (redox)

[40]. The associative mechanism involves reaction through an intermediate surface

species such as formate, carbonate, or bicarbonate. For instance, water may adsorb and

dissociate to form OH that reacts with CO to form HCOO, which eventually

decomposes to H2 and CO2. In the redox mechanism, successive oxidation and

reduction of the surface occurs. Adsorbed water dissociates into oxygen and hydrogen,

and the oxygen is then titrated by CO [38]. Experimental evidence exists supporting

both reaction pathways. It is possible that either reaction mechanism could proceed and

that the relative rates of the two pathways could depend on the catalyst and the reaction


8

conditions [40]. From a microkinetic analysis on a Cu-based catalyst, it was concluded

that the associative mechanism may be dominant at lower temperatures while the redox

mechanism becomes important at higher temperatures [38,48]. Ovesen et al. [49]

investigated the microkinetics of the WGS over Cu at industrial conditions based upon

the redox mechanism. At low pressure, formate and carbonate type species could be

excluded from the model while at high pressure the synthesis and hydrogenation of

formate type surface species had to be included.

2.2. WGS catalysts for fuel processing

2.2.1. Catalyst requirements

Traditional industrial catalysts used for WGS are iron-based for HTS and copper-based

for LTS. Typical compositions are Fe3O4/Cr2O3 and Cu/ZnO/(Al2O3 or Cr2O3),

respectively [34,40,50-52]. Cu/ZnO catalysts suffer from the drawback of being

pyrophoric (i.e. oxidize spontaneously in air with extensive heat release) and susceptible

to poisons. HTS Fe-Cr catalysts exhibit low activity and suffer from reduced

equilibrium CO conversion at higher temperatures [28].

The requirements for fuel processing are relatively different from the ones for the

industrial use of WGS catalysts [5,19,28,34]. Industrially, the Fe/Cr and Cu/Zn/Al

catalysts are slowly activated by a controlled reduction process. The catalysts must be

purged with inert gas to prevent condensation and reoxidation upon shutdown. If any of

these conditions are not met, the performance of the catalyst is significantly limited.

Simple operation is essential for WGS catalysts for fuel processing applications. Table 1

gives a list over desired requirements of WGS catalysts used for production of hydrogen

for fuel cells. The requirements for mobile applications must compete with the

standards set by internal combustion engines, and the operability, size, weight and cost

targets are therefore rigorous. For stationary applications the catalyst attributes are less

constrained and larger volumes and higher costs are acceptable to compete with the cost

of electricity from the grid [34].


9

Table 1. WGS catalyst requirements for mobile and stationary applications [34].

WGS catalyst attribute Mobile application Stationary application

Volume reduction Critical <0.1 l kW-1 Not as constrained

Weight reduction Critical <0.1 kg kW-1 Not as constrained

Cost Critical <$1 kW-1 Not as critical

Rapid response Critical <15 s Load following

Selectivity Critical Important

No reduction required Critical Important

Oxidation tolerant Critical Important

Condensation tolerant Important Important

Nonpyrophoric Important Eliminate purging

Attrition resistance Critical No constraint

Pressure drop Important Important

Poison tolerant Desired Desired

Conventional Cu/Zn/Al and Fe/Cr WGS catalysts may find a place in fuel processing

due to their proven success with long-term performance. Fe/Cr catalysts are known to

be poison tolerant, selective and stable at high temperature. The low temperature

activity of commercial Cu/Zn/Al catalysts represents a benchmark for new

developments [5,19,34,35,53-55]. Even though conventional Cu-based catalysts must be

treated with special care, engineered solutions are possible that take advantage of the

low temperature activity, low cost and proven performance [13,26,34,56-58].

2.2.2. New catalyst materials

Two trends can be distinguished in the development of new, non-pyrophoric WGS

catalysts for fuel processing [34]: low cost base metal formulations and precious metal

formulations. Alternative base metal formulations include transition metal carbides,

cobalt-vanadium oxides, Co on perovskite, Cu and Ni on ceria, Cu-Al spinel, and other

proprietary base metal formulations. Precious metal formulations are based on platinum


10

group metals or gold supported on (mixed) metal oxides. For further reading on new

developments it is referred to [5,19,28,34,51,59].

The most prominent catalyst formulations are combinations of copper or precious

metals and metal oxides. Promising alternative oxide materials to ZnO (or Al2O3) are

CeO2 [51,52,59], Fe2O3 [60,61], TiO2 [59-62], ZrO2 [24,63,64], La2O3 [65] and V2O3

[64].

A possible strategy for discovering improved WGS catalyst materials based on

metal/metal oxide combinations is outlined in Figure 5.

Figure 5. Possible strategy for improving WGS catalyst materials based on metal/metal

oxide formulations.

A proper combination of metal and metal oxide may be the first task, assuming that the

supporting oxide material affects activity and/or stability of the catalyst [66]. The

combination of Cu and ZnO (or Al2O3) is a classical example. A large number of

alternative formulations have been proposed over the last years, including Cu and CeO2

or ZrO2 [51,52,63], Ru and ZrO2 [24,64] or Fe2O3 [67], Au and CeO2 [68,69], TiO2

[70,71] or Fe2O3 [72], Pt and CeO2 [53,59,73] or TiO2 [59,74], Pd and CeO2 [75], and

more. The performance of the metal/metal oxide formulation can be improved through

optimization of metal loading and metal dispersion on the oxide surface, and further

enhanced by addition of other elements. It has been reported that the catalyst

Improvement of WGS catalyst materials

Metal / Metal Oxide

Alloying Doping Bimetallic systems Mixed oxides Alkali promoter


11

performance can be improved by using mixed metal oxides instead of single metal

oxides [52,56,76-84], as also applied for the classical CuO/ZnO/Al2O3 (or SiO2)

formulation [56,84]. The addition of a second metal resulting in alloy formation or

bimetallic systems can also be applied to improve the performance [35,85-89]. Finally,

the deposition of alkali metals on the metal surface or oxide support is known to

improve the catalyst performance [90-96] and has been successfully employed

commercially.

2.3. Mixed metal oxide catalysts

Mixed metal oxides (MMO) containing copper, cerium and zirconium are applied in

several areas of heterogeneous catalysis. Ce-Zr mixed oxides are extensively used in

three-way catalysts [79,97]. Cu-Ce-Zr-based mixed oxides of various composition are

applied within the field of hydrogen production: Water-gas shift [51,52,85,98], steam

reforming of methanol [99-102] and selective oxidation of CO [98,103-106]. In addition,

they are used as NO reduction catalyst [107], for oxidation of methane [108], wet

oxidation of phenol [109] and acetic acid [110], methanol synthesis [111,112], direct

oxidation of hydrocarbons in solid-oxide fuel cells (SOFC) [113] and storage of reactive

hydrogen for alkadiene hydrogenation [114].

Copper - in its reduced state - is typically regarded as the active catalyst component in

MMO materials, except in SOFCs, where copper is considered to function as electronic

conductor [113]. It is considered the most active metal for WGS on inert support

materials [35,115]. Ceria acts as a reducible oxide support, enhancing the catalytic

activity via catalytic metal-support interaction and/or better dispersion of the active

metal component [100,116]. The main advantage of ceria is its oxygen storage capacity

(OSC). CeO2-containing oxides are able to adsorb and release oxygen under oxidizing

and reducing conditions, respectively, according to the reaction [117]

22 2

2 2

. ( / ). ( / ) x

red H COCeO CeO

ox H O CO − (2)


12

Ceria is also found to stabilize the catalyst against deactivation [100,116] due to a

higher thermal stability of the material and/or better dispersion of the active metal. ZrO2

is also known to improve catalytic activity and stability of MMO supported catalysts

[77,101]. Zirconium added to ceria to form Ce-Zr mixed oxides inhibits the thermal

sintering of CeO2 [79,117-119]. Introduction of Zr into the ceria lattice furthermore

enhances the reducibility of ceria [120-122], resulting in improved catalytic activity of

MMO supports compared to single oxide supports [81,123]. The amount of Zr also

affects surface area and crystallite size of the MMO [123,124] together with the

calcination temperature [125]. Cu inclusion in the fluorite lattice of ceria has been

reported to improve the reducibility of ceria [126,127] and affect particle size and

surface area [112,128], in addition to its function as an active catalyst material.

The catalytic performance of MMO thus depends on the interaction of the single

components. A pre-requisite for an efficient interaction is a homogeneous distribution of

the single components throughout the material without extensive segregation. The

catalytic activity often scales proportionally with the surface area of the active

components [85,129,130] since the reaction takes place at the surface of the catalyst

while the bulk is inaccessible to reactants. Obtaining a homogeneous distribution of the

components in conjunction with a high surface area of the material are therefore

important targets during catalyst preparation.

Co-precipitation of metal (hydrous) oxides in aqueous solution at high pH has been

applied successfully for many different metal oxide catalyst formulations. It has been

stated that co-precipitation results in more active and/or stable catalysts than

impregnation methods [131-135], because of a more homogeneous distribution. For

impregnation-deposition methods the extent of interaction between the different

components depends on the surface area of the support material and the amount of

material impregnated or deposited. The formation of separate phases at higher loadings

is likely - especially when the surface of the support is saturated [85]. At low metal

loadings, however, impregnation-deposition methods may be superior to co-

precipitation, since burying of active components in the bulk is avoided. Thus, the

optimum metal loading may be lower [133], presuming a good metal dispersion.

Experimental

13

3. Experimental 3.1. Catalyst preparation

Different methods have been used for the preparation of the copper-based mixed oxide

catalysts. Most of the preparation methods are described in detail in the corresponding

papers. This chapter contains therefore only additional details on preparations that are

not sufficiently described in the papers.

For the precipitation method used in Paper I, the metal nitrate solution contained 0.1035

mole Cu(NO3)2·3H2O, 0.1382 mole Zn(NO3)2·6H2O and 0.0750 mole Al(NO3)3·9H2O in

approx. 300 ml deionized water. The alkaline solution contained 0.075 mole Na2CO3

and 0.600 mole NaOH in approx. 400 ml deionized water. The nitrate solution was

added to the alkaline solution over a period of 60 – 90 min. Then the pH was adjusted

between 8 and 9. For incipient wetness impregnation with Ce-nitrate, 0.200 g

Ce(NO3)3·6H2O in 0.6 ml water and 0.274 g Ce(NO3)3·6H2O in 1.3 ml water were used

for impregnation before and after calcination, respectively. Additional information is

given in [136].

Catalyst CuZn-CP, used in Paper IV and mentioned in Paper II (estimated Cu particle

size 14 nm), was prepared by co-precipitation [56,137]. The metal nitrate solution

contained 40.3 g Cu(NO3)2·3H2O, 59.8 g Zn(NO3)2·6H2O and 62.5 g Al(NO3)3·9H2O in

1 l of distilled water. The alkaline solution contained 96 g (NH4)CO3 in 1l of distilled

water. The two solutions were mixed under stirring in a volumetric ratio of 1:1. The

light blue precipitate was filtered, washed with distilled water and dried over night at 90

°C. The dry matter was crushed and calcined at 350 °C for 1 h (heating rate: 3 °C/min).

3.2. Characterization of catalyst materials

3.2.1. Inductively coupled plasma atomic emission spectroscopy (ICP-AES)

ICP-AES was used to determine the actual catalyst composition. The analyses were

performed by Molab as. The samples were dissolved in hydrochloric acid before

analysis without any visible residues.

Experimental

14

3.2.2. X-ray diffraction (XRD)

XRD spectra for crystallite size estimation and phase identification were recorded on

Siemens diffractometers D-5000 (monochromatic CuKα-radiation) and D-5005

(dichromatic CuKα+β-radiation). Particle size estimates were calculated by X-ray line

broadening analysis (XLBA, line width at half maximum, B) using the Scherrer

equation [138]. In addition, software-based XLBA was used to determine average

crystal size estimates and crystal size distributions. The analysis was performed in two

steps. Selected experimental XRD peaks were simulated by means of the software

package Profile [139] using the Pearson VII model function. The contribution of CuKβ

to the peak intensities is removed in this step. The program Win-crysize [140], utilizing

the Warren-Averbach method [141], was then used to estimate the crystallite size taking

into account contributions from microstrain (scaled as mean square root of the average

squared relative strain). Contributions from instrumental line broadening were removed

using LaB6 as reference, since this material does not exhibit any broadening due to

crystallite size or strain. The width of the peaks is hence due to instrumental broadening.

3.2.3. Nitrogen adsorption-desorption isotherms

N2 adsorption-desorption isotherms were measured using a Micrometrics TriStar 3000

instrument. The data were collected at -196 °C. The BET surface area was calculated by

the BET equation in the relative pressure interval ranging from 0.01 to 0.30. The pore

volume was estimated by the Barrett-Joyner-Halenda (BJH) method [142] as the

adsorption cumulative volume of pores between 1.7 nm and 300.0 nm width. This

method is based on the assumption of cylindrical pores, and the capillary condensation

in the pores is taken into account by the classical Kelvin equation. The pore size

distributions were calculated by non-local density functional theory (NLDFT, original

DFT model with N2 [143], DFT Plus software package [144]) assuming a slit-like pore

geometry. For comparison, pore size distributions were also calculated from the

Harkins-Jura model (cylindrical geometry), a classical method based on the Kelvin

equation [145].

Experimental

15

3.2.4. Thermogravimetric analysis (TGA)

Thermogravimetric analysis was performed with a Thermogravimetric Analyser TGA

Q500 (TA Instruments) and a Thermogravimetric Analyser TGA 7 (Perkin Elmer).

Figure 6 outlines the basic components and the measurement principle of the TGA

devices.

Figure 6. Basic components and measurement principle of the TGA devices.

TGA was used for temperature-programmed oxidation (TPO) and reduction (TPR)

studies, as well as determination of the degree of reoxidation of copper upon passivation

and Cu dispersion measurements by means of selective oxidation via N2O surface

titration [146-148] taking into account the bulk oxidation of copper [149-151].

3.2.5. Volumetric TPR measurements

In paper VI, TPR experiments were carried out with the volumetric device CHEMBET

3000 (Quantachrome Instruments), based on the analysis of the gas stream by means of

a thermal conductivity detector (TCD).

3.2.6. Transmission electron microscopy (TEM)

TEM data were recorded on a JEOL 2010F transmission electron microscope. Small

amounts of catalyst sample were placed in sealed glass containers containing ethanol

Purge gas Reactant gas

Weight chamber

Furnace

Outlet

Thermocouple

Experimental

16

and immersed in an ultrasonic bath to disperse the individual particles. The resulting

suspension was dropped onto a holey carbon film, supported on a titanium mesh grid,

and dried. Conventional TEM images were recorded onto a CCD camera. Scanning

transmission electron microscope (STEM) images were acquired with a nominal probe

size of approx. 0.7nm. Bright field and dark field STEM images were acquired. Energy

dispersive x-ray spectroscopy (EDS) analysis and mapping were performed using an

Oxford Instruments INCA system. Drift compensation was employed to correct for

movement of the sample during the time taken for the acquisition of maps.

3.2.7. Transmission X-ray absorption spectroscopy (XAS)

XAS data were collected at the Swiss-Norwegian Beam Lines (SNBL) at the European

Synchrotron Radiation Facility (ESRF), France. Spectra were obtained at the Cu K-edge

(8.979 keV) using a channel-cut Si(111) monochromator. Higher order harmonics were

rejected by means of a chromium-coated mirror aligned with respect to the beam to give

a cut-off energy of approximately 15 keV. The maximum resolution (∆E/E) of the

Si(111) band pass is 1.4 x 10-4 using a beam of size 0.6 x 7.2 mm. Ion chamber

detectors with their gases at ambient temperature and pressure were used for measuring

the intensities of the incident (I0) and transmitted (It) X-rays.

The amounts of material in the samples were calculated to give an absorber optical

thickness close to 2 absorption lengths. The samples were ground and mixed with boron

nitride to achieve the desired absorber thickness. For X-ray absorption near edge

structure (XANES) measurements, the samples were placed in an in situ reactor-cell

(Figure 7)[152] and reduced in a mixture of 5 vol-% H2 or CO in He (purity: 99.995%;

30 ml/min flow rate at ambient temperature) heating from room temperature to 350 °C

by 6 °C/min. XANES profiles were collected during heating of the samples to follow

the reduction of CuO in H2 or CO.

Experimental

17

Figure 7. Lytle type in situ XAS cell for transmission and fluorescence detection [152].

The recorded XAS spectra were energy-calibrated, pre-edge background subtracted

(linear fit) and normalized using the WinXAS software package [153]. The software

package was also used for XANES analysis. Principal component analysis (PCA) and

linear combination XANES fits were applied for identification of the number and type

of phases in the recorded XANES spectra. The reference spectra of these phases were

used in a least-square fitting procedure to determine the fraction of each phase present

[154]. For Extended X-ray Absorption Fine Structure (EXAFS) analysis, the data were

converted to k-space using WinXAS, and the least-square curve fitting was performed

with the EXCURVE 98 program [155] based on small atom approximation [156] and ab

initio phase shifts calculated by the program. Details on the comparison of the small

atom approximation, a simplified approach resulting in a reduced computation time, and

Water cooling

Gas in

Gas out ~A– +

Liquid

Sample cell

Water cooling

Soller slits

Fluorescent X-rays

Thermocouple

Kapton window

X-rays

Experimental

18

the angle-averaged curved-wave theory have been discussed by [156] and [157], and

references therein.

For more details on X-ray absorption spectroscopy, theory and basic principles,

applications, techniques and data analysis, it is referred to relevant literature [157-168].

3.3. Setup for water-gas shift testing

The setup used for WGS reaction experiments of the catalyst materials is shown in

Figure 8 and Figure 9.

The setup consisted of a dosing system, based on mass flow controller (MFC) and liquid

flow controller (LFC), a water evaporator, two electrical furnaces for the externally

heated tubular fixed-bed reactors, a microchannel heat exchanger and an on-line GC-

analysis system. GC, dosing system and pressure regulation (by a back pressure

controller) were all computer-controlled. A carbonyl trap was connected to the outlet of

the CO gas cylinder to remove possible iron carbonyl formed. Water was dosed by the

LFC from a cylinder pressurized with He at approx. 3 bar overpressure. Helium exhibits

a low solubility in water, lower than Ar and N2. As a consequence, problems with gas

bubbles disturbing the operation of the LFC were less pronounced than experienced

with for example Ar. The water was injected into the vaporizing unit, and the water

vapour was then mixed into the gas stream. The microchannel heat exchanger was

operated with air and used as water condenser after the reactor unit. The dry exit gas

was analyzed with an Agilent G2891A Micro GC equipped with thermal conductivity

detectors. The Micro GC is equipped with four columns; A: Molecular Sieve 5Å (carrier

gas: argon), B: PoraPLOT U (helium), C: Alumina (helium) and D: OV-1 (helium). For

the WGS experiments, column A was used to detect CO, H2, N2 and CH4, column B to

detect CO2. The gas composition in the dry gas was quantified by using a range of

certified calibration gases as external standards.

Experimental

19

Figu

re 8

. Sch

emat

ic d

raw

ing

of th

e ex

perim

enta

l set

up u

sed

for t

estin

g th

e W

GS

perf

orm

ance

of t

he c

atal

yst m

ater

ials

.

N2

Vent

GC

Vent

CH

PC 2

69

SV

LFC

Rea

ctor

II<5

00 o C

Rea

ctor

I <1

000

o C

PC

MFC

O2

MFC

MFC

H2

MFC

HC

Vent

Vent

Vent

CH

PC 1

50

Cal

ib

Vent

MFC

CO

2

CO

Vent

He

Mic

ro h

eat

exch

ange

r

Wat

er ta

nk

Vapo

rizer

Wat

er ta

nk

By-

pass

Air

Car

bony

l tra

p

N2

N2

Vent

GC

GC

Vent

CH

PC 2

69

SV

LFC

SV

LFC

Rea

ctor

II<5

00 o C

Rea

ctor

I <1

000

o C

PC

MFC

O2

MFC

O2

O2

O2

MFC

MFC

MFC

H2

MFC

H2H2

MFC

HC

HC

Vent

Vent

Vent

CH

PC 1

50C

HPC

150

Cal

ibC

alib

Vent

MFC

CO

2C

O2

CO

CO

Vent

He

He

He

Mic

ro h

eat

exch

ange

r

Wat

er ta

nk

Vapo

rizer

Wat

er ta

nk

By-

pass

Air

Car

bony

l tra

p

Experimental

20

Figu

re 9

. Pic

ture

s of t

he e

xper

imen

tal s

etup

use

d fo

r tes

ting

the

WG

S pe

rfor

man

ce o

f the

cat

alys

t mat

eria

ls.

Experimental

21

The CO conversion as well as the carbon balance was calculated from the CO and CO2

concentrations in the dry exit gas. For the reactant mixtures containing CO, H2O and

balance N2 only, the CO conversion was obtained as follows:

2

2

100%( )CO

COXCO CO

= ⋅+

(3)

For the simulated reformate reactant mixture, containing CO/H2O/CO2/H2/N2, the

calculated CO conversion has to take into account the initial composition of the feed gas

(measured via by-pass; subscript ‘0’), as follows:

0

2 2,0

( ) 100%( )CO

CO COXCO CO CO

−= ⋅+ −

(4)

The selectivity was 100 % in all activity measurements. Traces of CH4 were scarcely

detected during the temperature scans as well as the following recording of the short-

term deactivation behaviour. The approach to equilibrium is calculated with the exit gas

concentrations, corresponding to a certain CO conversion, and the equilibrium constant

(Keq):

2 2

2

100%eq

CO HEquilibrium approachK CO H O

⋅= ⋅⋅ ⋅

(5)

Apparent activation energies (Ea) were determined by the integral method (irreversible

reaction, first order in CO and zero order in H2O, plug-flow) or the differential approach

with an Arrhenius-type plot. According to Keiski et al. [169], the water-gas shift

reaction is not a simple first-order reaction in CO, but a first-order rate equation

describes the phenomenon quite well.

Two different reactors were used for the WGS experiments. A tubular stainless steel

reactor with an inner diameter of 9 mm (Figure 10A, Reactor II in Figure 8) was used in

paper I. In order to minimize temperature gradients the catalyst samples (0.15 – 0.25 g)

Experimental

22

were diluted with inert SiC (300 – 600 µm, 4 g). The dilution should not be too high,

otherwise by-pass effects may significantly decrease the reaction extent, especially at

high CO conversion levels [170]. The catalyst-SiC mixture (approx. 2 ml) was placed

on quartz wool and covered with a layer of SiC particles (average diameter 1250 µm,

approx. 8 ml). Quartz-type reactors (Figure 10B, Reactor I in Figure 8), placed in a gold

insulated furnace (Thermcraft Trans Temp), were applied in the papers IV, V and VI.

Two quartz tubes with different inner diameters, 6 and 10 mm, were used as specified in

the papers. The catalyst powder was placed on quartz wool held in place by a quartz

sinter underneath. The free volume of the reactor was reduced by inserting an additional

quartz tube. Heat and transport limitations were considered by applying well-known

empirical evaluation criteria [171].

A)

T

SiC

Catalyst+ SiC

Quartz wool

Innersteal tubewith meshon top

B)

T

CatalystQuartz woolQuartz sinter

Figure 10. Schematic drawing of the tubular fixed bed reactor configurations: A)

Stainless steel reactor, B) quartz-type reactor.

Results and discussion

23

4. Results and discussion 4.1. Pre-studies

4.1.1. Total pore volume and pore size distribution

Table 2 shows the total pore volume for five Cu-Ce-Zr MMO catalysts prepared by

homogeneous co-precipitation (HCP). The pore volume was determined by three

different methods, namely the Harkin-Jura model, the original DFT model (with N2) and

the BJH method, in order to evaluate the impact of the calculation method. The DFT

model and the BJH method give different absolute values, but show similar trends when

arranging the five catalysts according to the size of their pore volume. The results

obtained with the Harkin-Jura model show certain deviations from the other two

methods in terms of the absolute value as well as the size trend.

Table 2. Total pore volume of five Cu-Ce-Zr MMO catalysts determined by three

different methods, namely the Harkin-Jura model, the original DFT model (with N2) and

the BJH method.

Total pore volume (cm3/g)

HCP-1a HCP-1b HCP-1c HCP-2a HCP-2b

Harkin-Jura 0.0598 0.0958 0.0554 0.2233 0.2126

Original DFT 0.0347 0.1710 0.0837 0.2247 0.1630

BJH method 0.1288 0.2678 0.1893 0.3345 0.2201

The pore size distributions (PSD) for the five MMO catalysts were calculated in two

different ways. Figure 11 shows the PSD calculated by original DFT model (with N2),

assuming a slit-like pore geometry. Figure 12 shows the PSD determined with the

Harkins-Jura model, assuming a cylindrical geometry. Although the Harkin-Jura model

and the DFT model gave different absolute values for the total pore volume (Table 2),

the main trends in the pore size distribution are, however, essentially the same for the

samples investigated.


24

0

0,003

0,006

0,009

0,012

1 10 100 1000

pore width [ nm ]

pore

vol

ume

[ cm

³/g S

TP ]

HCP-1aHCP-1bHCP-1cHCP-2aHCP-2b

Figure 11. PSD for five Cu-Ce-Zr MMO catalysts calculated by non-local density

functional theory (NLDFT, Original DFT model with N2 [143], DFT Plus software

package [144]) assuming a slit-like pore geometry.

0

0,002

0,004

0,006

0,008

1 10 100 1000pore width [ nm ]

pore

vol

ume

[ cm

³/g S

TP ]

HCP-1a

HCP-1b

HCP-1c

HCP-2a

HCP-2b

Figure 12. PSD for five Cu-Ce-Zr MMO catalysts determined with the Harkins-Jura

model (cylindrical geometry), a classical method based on the Kelvin equation [145].


25

For further details on the comparison of DFT and classical models based on the Kelvin

equation, it is referred to Rouquerol et al. [172] and Chytil et al. [173], and references

therein.

4.1.2. XRD particle size estimates with Win-crysize

In order to evaluate the particle size estimates obtained from the Win-crysize analysis,

two sensitivity studies were conducted. For a Cu0.23Ce0.54Zr0.23 mixed oxide prepared by

homogeneous co-precipitation (HCP), the Win-crysize procedure was varied to assess

the effect on the particle size estimate. For a Cu0.23Ce0.54Zr0.23 mixed oxide prepared by

nitrate precursor decomposition (NPD), the crysize estimate was compared with particle

size estimates obtained with the Scherrer equation.

Applying the Pearson VII and the Pseudo-Voigt 2 model function, fitting between three

and five peaks ((111), (200), (220), (311), (222)) with Profile and including between

three and four peaks ((111), (200), (220), (311)) in the Win-crysize analysis for the HCP

sample, the average particle size estimate varied between 2.9 and 3.4 nm. The XRD

spectrum of the NPD sample was recorded with both D-5000 and D-5005. Two peaks

were fitted with Profile (Pearson VII) and analyzed with Win-crysize ((111), (220)).

The particle size estimates were 3.7 nm and 3.5 nm for D-5000 and D-5005,

respectively. Analyzing three peaks ((111), (200) and (220)) resulted in 3.8 nm and 3.6

nm, respectively. Estimating the particle size by the Scherrer equation gave 5.0 nm and

4.9 nm for D-5000 and D-5005, respectively. These are average values obtained from

two peaks ((111), (220)), based on the FWHM values, the wavelength for Cu Kα λ =

1.5418 Å and the Scherrer constant K = 0.89 [138]. For the D-5005 spectrum, the

contribution of CuK to the peak intensities was removed prior to determining FWHM.

No background correction was implemented.

TEM and STEM-EDS data obtained for Cu-Ce-Zr MMO catalysts, which were prepared

by co-precipitation (Paper III), suggest that XLBA may somewhat underestimate the

primary crystal size of this type of mixed oxide, possibly with about 2 nm.


26

4.1.3. Reproducibility of the WGS testing – By-pass effects

Initial studies on the WGS reaction over Cu-Ce-Zr MMO catalysts, prepared by the

homogeneous co-precipitation method (Paper III), in the quartz reactor (Reactor I, inner

diameter 10 mm) showed a poor reproducibility of the activity data, indicating that

some important issues were not taken into account under the initial experimental

conditions. A summary of the pre-study on reproducibility as well as some conclusions

are given in this chapter. The WGS reactant mixture contained 5 % CO and 25 % H2O

in nitrogen (see Paper V and VI). Under these conditions, 70 % CO conversion at 300

°C corresponds to an equilibrium approach of about 1 %.

In the initial experiments, the catalyst samples (< 200 µm, 0,15 g) were mixed with SiC

(50 – 150 µm, average diameter 88 µm) in a mass ratio of 1:1 and then poured into the

quartz reactor onto quartz wool held in place by a quartz sinter. The bed height was

approx. 2 mm. The corresponding reproducibility tests in the temperature range 200 –

300 °C showed strong variations (Figure 13), with an estimated standard deviation of 16

– 18 % at all temperatures, based on five runs. The apparent activation energies

determined by the integral approach (irreversible reaction, first order in CO and zero

order in H2O, plug-flow) varied between 25 and 31 kJ/mole.

A certain degree of segregation of the catalyst powder in the catalyst-SiC bed was

observed upon pouring the mechanical catalyst-SiC mixture into the quartz pipe. Some

of the catalyst powder was for example concentrated along the reactor walls or on top of

the bed. Berger et al. [174,175] investigated the influence of inert-diluted catalyst beds

on the conversion in a gas-solid laboratory micro-reactor for an irreversible reaction, i.e.

N2O decomposition over two different catalysts. Vertically and horizontally segregated

beds as well as mixed beds with different degrees of dilution were considered. Their

results showed that catalyst dilution should be applied with caution, since it may

significantly influence the conversion and lead to an erroneous interpretation. If the

catalyst and the diluting particles are not well-mixed, the conversion may be

significantly reduced due to by-pass and axial dispersion. Apparent activation energies

may also be reduced. The effects are stronger at high conversion levels.


27

0

20

40

60

80

100

190 210 230 250 270 290 310

temperature [ C ]

CO

con

vers

ion

[ %

]

rep 1rep 2rep 3rep 4rep 5

Figure 13. Reproducibility of WGS measurements on Cu-Ce-Zr MMO catalysts (< 200

µm) diluted with SiC (50 – 150 µm, average diameter 88 µm) in a quartz reactor (inner

diameter 10 mm).

It is thus assumed that a major source for the poor reproducibility of the activity

measurements shown in Figure 13 can be attributed to by-pass effects in the diluted

catalyst bed. Because of a certain random segregation of the catalyst powder upon

pouring, catalyst powder located for example at the reactor walls may be by-passed,

thus resulting in a reduced conversion level. A significant part of the catalyst powder

exhibited a particle size smaller than 50 µm, while the SiC used exhibited particles

mainly in the range 50 – 150 µm. This fraction of small catalyst particles (< 50 µm is

probably particularly prone to segregation upon pouring. Consequently, when diluting

catalyst samples with inert material, the particle size of both materials should match.

A second study was carried out, where Cu-Ce-Zr catalyst samples were applied without

dilution with inert material. The catalyst powder (0.05 g, particle size < 50 µm, bed

height 0.5 – 1 mm) was placed between two layers of quartz wool. A standard deviation

of 6 – 9 % and apparent activation energies of 28 – 29 kJ/mole (integral method) was

obtained for three repeated experiments. Two of the runs produced similar CO


28

conversion (13.3 % and 12.8 % at 220 °C, and 30.9 % and 30.1 % at 300 °C), while the

third run shows higher conversions (15.0 % at 220 °C and 34.8 at 300 °C).

Approximately 0.02 bar pressure drop was measured over the reactor for the two runs

with similar conversion, and approx. 0.07 bar for the third run with the higher

conversion. This may be an indication of a larger by-pass effect in the two similar

measurements compared to the third with higher conversion.

Three reproductions were also performed on the Cu-Ce-Zr MMO catalysts with

particles in the range 50 – 200 µm. The catalyst powder was placed on quartz wool

(without a covering layer of quartz wool). The amount of catalyst used was 0.10 g (bed

height approx. 1 mm), and no dilution was applied. The apparent activation energies

varied between 32 and 33 kJ/mole in the temperature range 200 – 300 °C. The estimated

standard deviation for these experiments was in the range 2 – 4 %. These values were

later reproduced for Cu-Ce-Zr MMO-CNF nanocomposite catalysts in Paper V and the

MMO catalysts in Paper VI, which were measured under similar reaction conditions.

The average conversions of the experiments with 0.05 g of the (< 50 µm)-fraction show

13 – 19 % lower values than the (50 – 200 µm)-fraction, when scaled up to 0.10 g with

the integral model. An experiment with 0.10 g of the (< 50 µm)-fraction also resulted in

a lower CO conversion than for the (50 – 200 µm)-fraction. This suggests that the (< 50

µm)-fraction suffers more from by-pass effects and should not be used for activity

measurements. The activation energy is not considerably affected by a small degree of

by-passing, especially at low conversion levels [174,175], but the apparent activation

energy will decrease as the extent of by-pass increases [174,175]. In a regime of mass

transport limitations, the apparent activation energy should decrease to about 5 kJ/mole

[176].

The use of Cu-Ce-Zr MMO catalysts with a particle size in the range 50 – 200 µm

without dilution gave the best reproducibility for the WGS reaction experiments in the

quartz reactor with 10 mm inner diameter. To avoid possible limitations from pore

diffusion at high conversion levels under the reaction conditions applied [171], the

upper boundary of the particle range should be decreased. A sieve with mesh size 125

µm was therefore introduced and used in subsequent experiments.


29

Based on the experience gained during this pre-study, I recommend using the following

configurations for catalytic experiments. Even though most of the experiments

described in this study are carried out without catalyst dilution, I recommend applying

catalyst dilution with inert material in order to minimize local temperature gradients

from the heat produced or consumed by the reaction and to assure a certain bed height

thus avoiding an inhomogeneous bed height distribution over the reactor diameter. The

dilution should, however, be not too high in order to minimize random by-pass effects

[170]. Possible axial temperature gradients arising from a larger bed height, depending

on the heating device, have then also to be taken into account. The size of the catalyst

particles should be matched with the size of the inert material in order to minimize

demixing effects. Finally, the catalytic measurements should be carried out at low

conversion levels, smaller than 30 %, since a perfect mixing is not possible and by-pass

effects are more pronounced at higher conversions [174,175]. In this way, the

experiments are also carried out away from the chemical equilibrium, and transport

limitations are avoided.


30

4.2. Summary of results and discussion

This section contains a summary of the results obtained in the Papers I – VI and some

comments on general trends and results in the literature. In some cases, additional

results are presented that are not given in the corresponding paper, because of shortage

of space or because these results were obtained after publication.

4.2.1. Relating catalyst structure and composition to the water-gas shift activity of

Cu-Zn-based mixed-oxide catalysts (Paper I)

Summary:

Four catalyst samples were characterized by means of XRD, in situ XANES and

thermogravimetric analysis in order to investigate the effect of cerium oxide on Cu-Zn-

based mixed-oxide catalysts. The activity of the catalyst samples was tested for the

forward water-gas shift reaction. The TOF of the catalysts, based on N2O chemisorption,

for the water-gas shift reaction was found not to be significantly improved by the

addition of CeO2. Ceria impregnated on the hydrotalcite-type precipitate prior to

calcination was found to improve copper dispersion and stability of the MMO catalyst.

The stabilization of Cu-based catalysts by adding further oxide compounds appears to

be a generally applicable approach. Kristiansen [56] reports that the impregnation of

Cu-Al spinel-based catalysts with ZnO can improve the stability of these catalysts

against sulphur and chlorine poisoning. Jung and Joo [177] report an increase in

stability of Cu-Zn-based catalysts for methanol dehydrogenation upon addition of Al or

Cr. The catalysts were prepared by co-precipitation. Saito et al. [77] report that the

addition of Zr to Cu-Zn-Al MMO catalysts does not significantly improve the activity

of the catalyst prepared by co-precipitation, but make them less affected by pre-

treatments such as calcination and pre-reduction. Cu-Zn-Zr-Al MMO catalysts are thus

considered to be more suitable for practical use. Zhang et al. report that the stability of

Cu-Al MMO catalysts for steam reforming of methanol could be improved by adding

Ce [100] or Zr [101]. The catalysts were prepared by co-precipitation. Wu et al. [178]

report that the activity of Cu-Zn-Al MMO catalysts improved after a certain induction

period by impregnation with an aqueous solution of B2O3. The stability of both, B2O3-

containing and B2O3-free Cu-Zn-Al MMO catalysts could be improved by co-


31

precipitation of Cu-, Zn- and Al-salts with colloidal silica [178,179]. Colloidal silica

was also used for stabilization of a Cu-Zn-Zr-Al MMO catalyst for methanol synthesis

[180]. Chen et al. [181] report that the catalytic activity and stability of Cu/SiO2

catalysts for the high temperature reverse water-gas shift reaction could be improved by

adding iron. Fe was also claimed to prevent the oxidation of Cu via a spillover process

of oxygen. The catalysts were prepared by subsequent impregnation of SiO2 with Cu

and Fe. Iron was also claimed to have similar effects on Cu/Al2O3 and commercial Cu-

Zn-Al MMO catalysts.

Additional aspects and comments:

In the paper, we have reported that the extent of reduction of Cu-aC-350 determined by

TGA-TPR exceeded the maximum copper equivalent determined by ICP-AES

indicating a reduction of components other than copper oxide. A subsequent TGA

analysis (details below) suggested that part of the weight change can be assigned to

decomposition of nitrate precursor remaining after calcination. This illustrates that

reducing atmospheres are more efficient in decomposing nitrate precursors than

oxidizing atmospheres, in line with the decomposition chemistry of nitrates.

The TPR profile of Cu-aC-350 after oxidation treatment in air at 400 °C is shown in

Figure 14 together with the TPR profile of Cu-350. In contrast to the profile shown in

the paper, Cu-aC-350 now exhibits a similar reduction behaviour as Cu-350, since the

heat treatment at 400 °C removed most of the precursor remains, as shown below. The

coverage effect mentioned in the paper, which resulted in a delay of the Cu reduction

during TPR, can thus be related to the decomposition of the precursor remains during

this reduction step.

Table 3 shows the comparison between certain TGA results obtained without and with

the treatment in air at 400 °C after calcination. Comparing the weight loss during

oxidation treatment (subtracted with the weight loss during Ar-drying at 260 °C

described in the paper) with the weight loss corresponding to the difference of the Cu

reduction extent without and with oxidation treatment indicates that most of the

precursor remains removed during the oxidation treatment is also removed during the


32

TPR step reported in the paper. The Cu dispersion has to be corrected by the amount of

precursor remains interpreted as Cu reduction extent in the paper. Thus, the Cu

dispersion is estimated to 8.9 or 7.9 % based on the Cu content determined by TGA-

TPR or ICP-AES, respectively. Consequently, the TOF of Cu-aC-350 has to be

multiplied by a corresponding correction factor resulting in a TOF somewhat below the

TOF values of the other three catalysts. The effect of the precursor remains on the N2O

titration following the TPR can be neglected. For both measurements, before and after

the oxidation treatment, the weight increase assigned to oxygen uptake was estimated to

0.26 · 10-2 mg-O/mg-sample. The Cu particle size estimated by XLBA has to be

corrected for the real thickness of the passivation layer.

-0,10

-0,08

-0,06

-0,04

-0,02

0,00

0 25 50 75 100 125 150

Time [ min ]

Nor

m. d

eriv

. wei

ght

[mg/

(mgC

u*m

in) ]

50

100

150

200

250

300

Tem

pera

ture

[ C

]

Cu-aC-350-ox-red

Cu-350

Temperature

Figure 14. TPR of Cu-aC-350 (7% H2 in Ar, heating rate: 2 °C/min, 260 °C for 2 h,

flow rate: 80 ml/min at ambient temp.) after oxidation treatment in air at approx. 400 °C

(15 min, heating rate: 10 °C/min, flow rate 80 ml/min at ambient temp.). The TPR of

Cu-350 is given for comparison.


33

Table 3. Comparison of TGA results before and after oxidation treatment in air at 400

°C (15 min, heating rate: 10 °C/min, flow rate 80 ml/min at ambient temp.).

Weight loss upon oxidation treatment

(excluding Ar-drying at 260 °C) 6.74 mg/mg-sample

Cu mass frac. [-]

ICP-AES 0.26

TGA-TPR

- without ox. treat.

0.44

Precursor weight loss

(equivalent to Cu mass

frac. of 0.21)

- with ox. treat. 0.23 4.99 mg/mg-sample

Cu dispersion [%] (± 0.4 – 0.8 %)

TGA-N2O

- without ox. treat.

4.8

- with ox. treat. 8.9 based on TGA-TPR Cu content

- with ox. treat. 7.9 based on ICP-AES Cu content

Effect of nitrate precursor on N2O

titration

Without ox. treat. 0.26 · 10-2 mg-O/mg-sample

With ox. treat. 0.26 · 10-2 mg-O/mg-sample

Correction of Cu particle size estimated by Scherrer

equation

Passivation

degree [%] Particle size [nm]

Cu dispersion

from XRD [%]

Without ox. treat. 32 16 5.6

With ox. treat. 59 19 4.6


34

4.2.2. Remarks on the passivation of reduced Cu-, Ni-, Fe-, Co-based catalysts

(Paper II)

Summary:

Catalysts containing metals such as Cu, Ni, Fe, Co are often subjected to passivation

procedures prior to characterization to retain the reduced state of the active metal.

Passivation with N2O or O2 to create a protective oxide layer also results in a certain

degree of sub-surface oxidation. The extent of bulk oxidation depends on the type of

oxidant as well as the size of the metal particles, as shown for copper catalysts. The

final (oxidic) passivation layer requires a certain thickness in order to be stable and

prevent further bulk oxidation in ambient air atmosphere. The studies performed on

reduced metal catalysts based on Cu, Ni, Fe and Co suggest that the result of passivation

procedures should be monitored, in order to quantify the extent of bulk oxidation. Heat

released during exothermic oxidation reactions appear to be a critical parameter, since

the local temperature and hence bulk diffusion and the final extent of bulk oxidation

increases.

Passivation of reduced catalysts is not an ideal strategy for characterization of reduced

systems that are unstable in air. In situ measurements are preferred, but passivated

samples may be used to some extent, e.g. for estimating particle size with XRD, keeping

in mind the limited relevance and accuracy of data derived from this approach. The use

of passivated samples for a detailed X-ray line broadening analysis in correlation with

catalytic activity, distinguishing between particle size and strain effects, is questionable

and requires evaluation by a parallel in situ approach. Strain may also be introduced into

the crystal lattice by the oxidation of the outer metal layers during passivation. Possible

morphological changes depending on the reduction/oxidation potential of the

surrounding atmosphere may also complicate the interpretation of the characterization

results. Finally, the discussion of active reaction sites based on the characterization of

passivated samples is disputable.

Passivation strategies that involve CO and/or CO2 without the formation of significant

amounts of coke or wax around the metal particles may comprise both formation of


35

carbonaceous species and an oxide layer (at the latest when exposed to air), depending

on the conditions used and the metal to be passivated.

Encapsulation of reduced metal particles by a protective layer of carbon is found to

efficiently preserve the metallic state, as demonstrated for metallic nickel and iron with

carbon nanofibers. For certain catalysts, CNF encapsulation could be preferred instead

of passivation by a protective oxide layer. Provided that the carbon layer is impermeable

to oxygen when exposed to air, the reduced metal particles may be preserved in their

metallic, hence active, state. Possible morphological changes as a result of a change in

the reduction/oxidation potential of the surrounding atmosphere should be of less

concern than for passivation with oxygen. However, the particle morphology might be

affected by the CNF growth process.


The Ni-Fe catalyst used in this study was investigated by XAS. The catalyst contains Ni

and Fe in a ratio of Ni:Fe = 8:2 and Al at a molar fraction of 0.25. A least-squares fitting

of the XANES profile at the Ni K-edge with NiO and Ni foil as reference materials gave

a composition of 93 % NiO and 7 % Ni in the calcined sample (Table 4). In the paper it

was claimed that it is unlikely to find 7 % Ni in the calcined sample, and the amount of

Ni found was attributed to uncertainties in the fitting procedures. Figure 15 shows the

XANES profiles of the calcined Ni-Fe catalyst and the reference materials, NiO and Ni

foil. The similarity between the calcined catalyst and NiO is obvious. The deviation in

the high-energy region could be related to data treatment, including energy calibration,

background subtraction and normalization. A small contribution from the Ni profile

function is optimum for the linear combination in order to fit the catalyst profile with

NiO and Ni foil. This example indicates the limits of a simple least-squares fitting

approach with linear combinations of reference spectra. In the end, the reasonability of

the result has to be evaluated by the scientist, and certain error intervals have to be

accepted. We suggest interpreting the data with an error interval of ± 5 % or more.


36

0,00

0,50

1,00

1,50

2,00

8,31 8,33 8,35 8,37 8,39

photon energy [ keV ]

norm

. abs

orpt

ion

[ a.u

. ]Ni-Fe cat

NiO

Ni foil

Figure 15. XAS spectra of calcined Ni-Fe catalyst, NiO and Ni foil recorded at the Ni

K-edge.

The linear combination was also performed with 3 reference materials, Ni foil, NiO and

NiAl2O4 spinel [182]. In the paper, it was argued against the existence of a Ni-Al spinel

phase in the Ni-Fe catalyst. Inclusion of Ni-Al spinel in least-squares fitting implies that

the spinel is partly reducible under the reduction conditions applied and leads to an

unrealistically low passivation degree. The results of both approaches are compared in

Table 4. The average particle size (dP), including core and passivation layer, was

estimated using a simple cubic model:

(6)

Determination of the reduction degree of Ni, Fe or Co by means of magnetic

measurements, as reported by Olafsen et al. for a Ni-Mg-Al mixed oxide [183], is an

interesting alternative method for evaluating the degree of passivation. In a sample with

a total metal loading of approx. 2 wt%, about 11 % of the Ni remained in the reduced

31

(1 )P Cp

d dx

= ⋅−


37

state after oxidation in 2 % O2 in N2 for 1 h at ambient temperature. The non-reducible,

non-ferromagnetic fraction of Ni was estimated to about 13 %. The reduction degree of

the reducible Ni (87 % of Ni in the sample) after passivation can therefore be estimated

to about 13 %. The Ni particle size was estimated to 9 ± 2 nm.

Table 4. Comparison of two or three reference compounds (Ni foil, NiO, NiAl2O4) for

linear combination to reproduce the XANES profile of the calcined and reduced-

passivated Ni-Fe catalyst samples at the Ni K-edge.

Ni metal core [nm] 4.5 (with XRD, after passivation, [182])

Ni metal (%) NiO (%) NiAl2O4 (%)

2 reference compounds

Calcined 7 93

Reduced-passivated 70 30

Passivation degree [%] 30

Particle size [nm] 5.4

3 reference compounds

Calcined 0 62 38

Reduced-passivated 74 6 20

Passivation degree [%] 8

Particle size [nm] 4.7


38

4.2.3. Preparation and characterization of nanocrystalline, high-surface area Cu-

Ce-Zr mixed oxide catalysts from homogeneous co-precipitation (Paper III)

Summary:


urea. The resulting material exhibits high surface area and small nanocrystalline primary

particles. The material consists of a single fluorite-type phase according to XRD, in the

research literature often denoted as solid solution. As a result of the heterogeneous

nature of the co-precipitation process, however, STEM-EDS analysis shows that Cu and

Zr are inhomogeneously distributed throughout the ceria matrix. The EXAFS analysis

indicates the existence of CuO-type clusters inside the ceria-zirconia matrix. This type

of mixed oxide materials should therefore rather be referred to as heterogeneous single-

phase materials. The pore structure and surface area of the mixed oxides are affected by

preparation parameters during both precipitation (stirring) and the following heat

treatment (drying and calcination). TPR measurements show that most of the copper is

reducible and not inaccessibly incorporated into the bulk structure. Reduction-oxidation

cycling shows that the reducibility improves from the first to the second reduction step,

probably due to a local phase segregation in the metastable mixed oxide with gradual

copper enrichment at the surface of the Ce-Zr particles during heat treatment.

It would be interesting to study the dynamic structural change in situ during reduction

and reoxidation, as well as the structure of the mixed oxide in the reduced state (see for

example [184] for in situ XAFS study on dynamic structural changes in Ce-Zr mixed

oxides). Tanaka et al. [185] reported about a so-called ‘intelligent’ Pd-perovskite

catalyst, where Pd shifts between metallic Pd clusters and Pd oxide incorporated in

perovskite depending on the reduction/oxidation potential of the surrounding gas

atmosphere.

The results and discussion given in this paper are not necessarily limited to Cu-Ce-Zr

mixed oxides, but can to some degree apply to other catalyst formulations prepared by

co-precipitation, e.g. other single-phase materials such as Co-Ce-Zr, or multi-phase

materials such as Cu-, Ni- or Fe-based mixed oxides.


39


Figure 16 shows the effect of the metal composition on the surface area of Cu-Ce mixed

oxides, for the surface-to-volume ratio (stv), the mole-based BET (m-BET) and the

specific BET surface area. The BET data are taken from Shen et al. [112] and Lamonier

et al. [128].

0

0,4

0,8

1,2

1,6

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8Cu mole fraction x [-]

surf

ace

area

ratio

A/A

(x=0

or 0

.12)

[-]

BET [Lam]stv [Lam]m-BET [Lam]BET [Shen]stv [Shen]m-BET [Shen]

Figure 16. Effect of material density and composition on the surface area of Cu-Ce

mixed oxides. The BET data are taken from literature [112,128]. The surface area data

are given as surface area ratio normalized with the values of the pure CeO2 (Cu mole

fraction x = 0) [128] or the Cu-Ce mixture with 12 mole-% Cu (x = 0.12) [112]. M-BET

is a mole-based surface area (m2/mole), i.e. BET normalized with the molar mass. The

densities used for the calculation of the surface-to-volume ratio (stv, m2/m3) are based

on a linear combination of tabulated values for CeO2 (7650 kg/m3) and CuO (6310

kg/m3) [186].

A TEM image of a calcined Cu-Ce-Zr mixed oxide sample (sample Bo) is shown in

Figure 17 indicating the crystalline nature of the nanoparticles. Figure 18 shows the

STEM-EDS elemental mapping of sample Bo after calcination. Cu, Ce and Zr are all

distributed over the whole mapping area (roughly 40 x 40 nm2), and the average metal


40

composition is close to the overall composition (Cu:Ce:Zr = 0.23:0.54:0.23). Within the

mapping area, regions of a few nanometers in size with high Cu and Zr concentration

compared to the surroundings are found (Figure 18C, E).

Figure 17. TEM image of a representative Cu-Ce-Zr mixed metal oxide powder

(sample Bo) after calcination at 350 °C (size bar 10 nm).


41

A)

B)

Atomic

%

Metal

frac.

Cu K 10.52 0.27

Zr L 7.73 0.20

Ce L 20.68 0.53

C)

D)

E)

F)

G)

Figure 18. STEM-EDS elemental mapping of a calcined Cu-Ce-Zr mixed oxide powder

(sample Bo). A) Mapping area, marked with white frame. B) Metal ratio in the mapping

area, according to EDS. C) Cu Kα1. D) Ce Lα1. E) Zr Kα1. F) O Kα1. G) EDS spectrum

of the mapping area. The size bar corresponds to 40 nm for all images.


42

4.2.4. Comparison of Cu-Ce-Zr and Cu-Zn-Al mixed oxide catalysts for water-gas

shift (Paper IV)

Summary:

Cu-Zn-Al and Cu-Ce-Zr mixed oxide catalysts were prepared by two different methods,

co-precipitation and flame spray pyrolysis. The performance of the catalysts was

evaluated using the water-gas shift reaction with and without CO2 and H2 added to the

feed. Cu-Ce-Zr catalysts are found not to be generally superior to Cu-Zn-Al catalysts in

terms of activity or short-term stability. Instead, the difference in activity seems to be

related to structural characteristics of the catalysts as well as the reaction conditions. In

general, increasing the surface area of the catalytic material results in increased activity.

The result of the comparison of different catalysts depends strongly on the structural

characteristics of each catalyst, i.e. the catalyst under study is only as good as the

reference catalyst allows it. Thus, the true potential of a catalyst system can only be

evaluated by using a reasonable reference catalyst, and preparation procedures that lead

to comparable structural characteristics.

The apparent activation energy of the Cu-Ce-Zr MMO catalysts appears to be less

affected by increased concentrations of CO2 and H2 than the Cu-Zn-Al MMO catalysts.

Cu-Ce-based catalysts do not display the same increase in apparent activation energy as

Cu-Zn-based catalysts upon increased concentration of hydrogen and CO2 in the feed.

Further studies that ensure the comparability and consider the impact of additional

factors such as temperature range, conversion level, initial deactivation, catalyst

reducibility and Ea estimation method are necessary. Another question that should be

considered is whether the Cu loading and preparation method affect the value of the

apparent activation energy. For example, different Cu species can be identified in Cu-

Ce- or Cu-Zr-based MMO catalyst materials [187,188] depending on Cu loading and

preparation method (i.e. co-precipitation or impregnation on the support). Assuming

different reactivity of the different Cu species, overall activity and apparent activation

energy might vary with the content of the Cu species in the catalyst.


43

Additional data:

Figure 19 shows the TPR profile of CuZn-CP.

-0,05

-0,04

-0,03

-0,02

-0,01

00 25 50 75 100 125 150

time [ min ]

norm

alis

ed d

eriv

ativ

e w

eigh

t [ m

g/(m

gCu

min

) ]

90

140

190

240

290

340

390

tem

pera

ture

[ C

]

rep 1

rep 2

temp

Figure 19. TGA-TPR of catalyst CuZn-CP in 7 % H2/Ar. Heating rate: 2 °C/min, flow

rate: 80 ml/min at ambient conditions, catalyst amount: 56-60 mg. The weight

derivative of two parallels, normalized with the total weight loss during reduction, and

the temperature profile are plotted as a function of time.

4.2.5. Nanocrystalline Cu-Ce-Zr mixed oxide catalysts for water-gas shift: Carbon

nanofibers as dispersing agent for the mixed oxide particles (Paper V)

Summary:

Carbon nanofiber (CNF) and Cu-Ce-Zr mixed metal oxide (MMO) containing

nanocomposite catalysts have been prepared by homogeneous co-precipitation with urea.

The water-gas shift reaction (WGS) has been used as test reaction. The CNF-containing







44

energies are, however, obtained for the CNF-free and CNF-containing materials. We

can not exclude that the CNF material acts as reaction promoter under certain conditions,

but suggest that the impact of CNF addition on the precipitation of the mixed oxide

particles, and hence the catalytic activity relative to the CNF-free MMO, should also be

considered. This is supported by the similar activation energies obtained for the CNF-

free and CNF-containing materials, and CO chemisorption studies resulting in no

significant CO chemisorption on CNF. CNF may be regarded as inert dispersant

material improving the precipitation of the MMO under conditions where the co-

precipitation of the MMO precursors does not result in materials with high surface area.

A possible candidate would be for example the Cu-Zn-Al mixed oxide catalyst

described in Paper VI.

In this study, the CNF-free and the CNF-containing catalysts were prepared under

identical conditions. Under these preparation conditions, the CNF-free catalyst

exhibited a rather high surface area, while for the CNF-containing catalysts, the

precipitation did not result in a complete coverage of the CNF with the mixed oxide.

The CNF rather dispersed the MMO agglomerates. The degree of dispersion of the

MMO by the CNF was therefore limited. Improved preparation conditions, including

lower stirring rate, lower precipitation rate (e.g. via pH-control), good dispersion of the

CNF in the solvent prior to precipitation (e.g. via US treatment and reasonably large

solvent volume), right choice of CNF, high density of oxygen-containing surface groups

on the CNF (e.g. via strong oxidation agent such as a mixture of HNO3 and H2SO4 [189-

191]) and charge matching between support and precipitate should be applied in order

to improve the deposition of the precipitate on the CNF without extensive formation of

agglomerates. In this way, the CNF loading may be increased.

4.2.6. The effect of platinum in Cu-Ce-Zr and Cu-Zn-Al mixed oxide catalysts for

water-gas shift (Paper VI)

Summary:

Cu-Ce-Zr and Cu-Zn-Al mixed oxide catalysts were prepared by homogeneous co-

precipitation with urea. Pt-impregnated Cu-Ce-Zr and Cu-Zn-Al MMO catalysts were


45

prepared by wet impregnation of the precipitates with a Pt precursor salt prior to drying

and calcination.

While the Cu-Ce-Zr mixed oxide catalyst exhibited a high surface area and a good

distribution of all three metals, the Cu-Zn-Al mixed oxide catalyst exhibited a relatively

low surface area and an inhomogeneous distribution of the three metals under the

preparation conditions used. For the Cu-Ce-Zr mixed oxide catalyst, the reduction of Cu

could be achieved at lower temperatures than for the Cu-Zn-Al mixed oxide.

Improvement of the surface area, metal distribution and Cu reducibility in the Cu-Zn-Al

mixed oxide catalyst requires therefore different preparation conditions.

Cu-Ce-Zr mixed oxide catalysts showed WGS activity without pre-reduction under the

reaction conditions used. The not pre-reduced sample was somewhat less active but

more stable than the pre-reduced sample. Pre-reduction is therefore not absolutely

necessary, but an adequate pre-reduction procedure may be applied to optimize the CO

conversion. Pt impregnated on the Cu-Ce-Zr mixed oxide had no significant effect on

Cu reducibility as well as WGS activity and stability.

The WGS activity of the Cu-Zn-Al mixed oxide catalyst correlated with the Cu

reducibility. Below the temperatures required for Cu reduction, the not pre-reduced

sample did not show significant WGS activity. At temperatures where Cu is at least

partly reduced, the catalyst showed significant WGS activity. Above the temperatures

required for partial Cu reduction, the not pre-reduced sample exhibited a somewhat

lower CO conversion than the pre-reduced sample, under similar short-term stability.

Consequently, pre-reduction is not absolutely necessary at these reaction temperatures,

but an adequate pre-reduction procedure may improve the CO conversion.

Pt impregnated on the Cu-Zn-Al mixed oxide catalyst shifted the Cu reduction and

hence also the WGS activity of the unreduced sample to lower temperatures. The not

pre-reduced sample showed similar WGS activity and stability, as compared to the Cu-

Zn-Al mixed oxide catalyst. The pre-reduced sample, however, exhibited a lower CO

conversion and a better stability at 250 °C than the Cu-Zn-Al mixed oxide catalyst


46

under CO/H2O/N2-feed conditions, indicating the existence of an interaction between

Cu and Pt in the bimetallic catalyst. Under CO/H2O/CO2/H2/N2-feed conditions at 300

°C, the impact of Pt diminished and the CO conversion of the Pt-impregnated catalyst

approached the one of the Cu-Zn-Al mixed oxide catalyst with time on stream,

indicating a destruction of the interaction between Cu and Pt.

Concluding remarks

47

5. Concluding remarks Individual conclusions on the topics of the single papers have been given in the

corresponding papers, and will therefore not be repeated in this section. Instead, more

general comments on WGS catalyst requirements for small-scale hydrogen production

systems will be given, based on Table 1 in chapter 2.

The low temperature activity of Cu-based MMO catalysts continues to be the

benchmark for new developments [5,19,34,35,53-55]. Engineered solutions are possible

that take advantage of the low temperature activity, low cost and proven performance

also for small-scale hydrogen production systems [13,26,56-58], especially for

stationary applications, for which the catalyst requirements are less critical than for

mobile applications (Table 1).

A reduction in catalyst weight and volume can be obtained by improving the dispersion

of the catalyst particles, thus increasing the effectiveness factors for pore-diffusional

regimes and avoiding extensive heat and mass transport limitations. This can be

achieved by depositing the active catalyst onto monolith or microreactors instead of

using classical particulate catalyst beds [19,27,192-194]. A further advantage of this

type of reactors is its lower pressure drop compared to conventional fixed bed reactors.

This approach has been successfully applied for production of synthesis gas by partial

oxidation or oxidative steam reforming in microstructured and monolithic reactors

[16,17,29,30] as well as for conversion of CO by water-gas shift and selective oxidation

of CO in a microreactor [24]. As a further advancement of the work on CNF as inert

support and dispersant material (Paper V), the active catalyst may also be supported on

CNF-covered, ceramic monolith reactors [195] or on structured carbon paper made of

CNF [196].

A problem of Cu-based catalysts is the pyrophoricity of reduced copper, i.e.

considerable temperature rise when exposed to air [19]. The extent of heat released

during oxidation in air depends on the amount of reducible Cu in the catalyst. The

reduction degree of Cu in Cu-Ce-Zr-based MMO catalyst is less than 1 (Paper III), in

contrast to conventional Cu-Zn-Al MMO catalysts, in which basically all the Cu is

Concluding remarks

48

reduced during pre-reduction (Paper I and II), including bulk copper which does not

contribute to the catalytic activity. The key factor for avoiding extensive pyrophoricity

is a high copper dispersion, reducing the total Cu content and the fraction of inactive,

but pyrophoric bulk Cu. The benefit of Cu-Ce-Zr mixed oxides is that Cu is in principle

highly dispersed within the mixed oxide crystals, and inactive Cu buried inside the

crystal particles, and hence not accessible to the reactant gas, does not contribute to the

pyrophoricity (Paper III). It should, however, be noted that the MMO crystals undergo

structural changes during heat treatment resulting in an increase in Cu reducibility.

Another class of Cu-containing mixed oxides with reduced pyrophoric behaviour are

spinel structures. The Engelhard company developed a non-pyrophoric base metal

catalyst [19], possibly based on a Cu-Al spinel structure [57,58]. A similar catalyst

formulation, but prepared in a different way and additionally containing ZnO, was also

patented by Norsk Hydro [56]. In principle, spinel-type Cu-Al mixed oxides give rise to

highly dispersed, small metallic Cu particles deposited on the alumina support after

reduction [84]. This also ensures efficient heat transfer from copper to alumina which

has a higher specific heat capacity than the copper component [57]. Different Cu-based

mixed oxides have been investigated, with Cu-Al- and Cu-Mn-based spinels being the

most promising materials [197-199]. Cu-Si-based mixed oxides may also be an

interesting class of materials to look at in this context (see for example [200]).

Condensation and oxidation tolerance as well as use without pre-reduction are important

issues for small-scale applications. Cu-based catalysts can be operated without pre-

reduction, as shown in Paper VI. Cu is then reduced by the reactant gas and at least

partially reoxidized during shutdown. Important for high activity and rapid response

during start-up is that Cu can be reduced already at low temperatures, i.e. WGS activity

and Cu reducibility are highly correlated. Cu-Ce-Zr MMO catalysts have been shown to

exhibit a low temperature reducibility of Cu (Paper VI). Ko et al. compared Cu-Zr

mixed oxide catalysts with Cu-Zn-Al mixed oxide catalysts for WGS and found that the

Cu-Zr mixed oxide catalysts exhibited lower Cu reduction temperatures as well as

higher CO conversions at low temperatures compared to Cu-Zn-Al [201]. This is,

however, not necessarily a unique feature of ceria/zirconia-based systems. If it is

possible to shift the Cu reduction temperature of Cu-Zn-Al mixed oxides to lower

Concluding remarks

49

temperatures, e.g. by decreasing the CuO particle size and hence increasing the surface-

to-volume ratio, then Cu-Zn-Al mixed oxide catalysts may exhibit an increased low-

temperature activity. The Cu reducibility and hence the low-temperature activity can be

improved by depositing noble metals onto the mixed oxide, as shown for Pt on a Cu-Zn-

Al MMO catalyst (Paper VI). Noble metals can also improve the deactivation behaviour,

depending on the preparation conditions (Paper VI). The improved performance must

then, however, be charged up against increased material costs. Appropriate operation

conditions for start-up and shutdown cycles have to be applied in order to improve the

life-time of the catalyst and hence reduce the economic costs [57,202]. Finally, the risk

of hot spots during reduction and reoxidation may be reduced by depositing the Cu-

based catalysts onto monolith or microreactors instead of using classical particulate

catalyst beds.

True individual commitment to a group effort –

that is what makes a team work, a company work,

a society work, a civilization work.

Vince Lombardi

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6. Suggestions for further work Some suggestions for possible continuative studies on the work presented in this thesis

have already been given in the corresponding papers as well as in chapter 5. Further

work should be done on the operations conditions in small-scale systems. The

performance of Cu-based catalysts needs to be evaluated under frequent start-up and

shutdown conditions [57]. This includes further work on the deactivation behaviour.

The deactivation of Cu-Zn-Al MMO catalysts has been addressed in literature [56,203-

207]. Further studies should be performed on deactivation and regeneration of Cu-Ce-Zr

MMO catalysts (e.g. [82,98,208]). Cu-Ce-based mixed oxide catalysts are claimed to

exhibit good stability at high temperatures (up to 600 °C), which would make them

interesting candidates for membrane reactor applications [52]. Another aspect in respect

of deactivation in fuel processing units is the impact of volatile compounds, such as

metal carbonyls, that may be formed in the reforming unit by catalyst leaching [209,210]

and transported to the WGS unit causing deactivation of the Cu-based catalyst [211-

213], as possibly suggested by pre-studies performed in our group [214].

Further studies should be carried out on oxide-supported noble metal catalysts as

alternative to Cu-based catalysts, especially in view of mobile applications

[5,19,28,34,53]. This should also include bimetallic systems continuing the work started

in Paper VI, since this class of catalysts opens new possibilities for tailoring catalyst

activity and stability [215]. An appropriate comparison of Cu-based and noble metal-

based catalysts may then give information about the true potential of these two classes

of catalysts.

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List of appendices

62

List of appendices

I M. Rønning, F. Huber, H. Meland, H. Venvik, D. Chen, A. Holmen, Relating

catalyst structure and composition to the water-gas shift activity of Cu-Zn-

based mixed-oxide catalysts, Catalysis Today 100 (2005), 249-254.

II F. Huber, Z. Yu, S. Lögdberg, M. Rønning, D. Chen, H. Venvik, A. Holmen,

Remarks on the passivation of reduced Cu-, Ni-, Fe-, Co-based catalysts,


III F. Huber, H. Venvik, M. Rønning, J. Walmsley, A. Holmen, Preparation and

characterization of nanocrystalline, high-surface area Cu-Ce-Zr mixed oxide

catalysts from homogeneous co-precipitation, Manuscript in preparation.

IV F. Huber, H. Meland, M. Rønning, H. Venvik, A. Holmen, Comparison of Cu-

Ce-Zr and Cu-Zn-Al mixed oxide catalysts for water-gas shift, submitted.

V F. Huber, Z. Yu, J. Walmsley, D. Chen, H. Venvik, A. Holmen,

Nanocrystalline Cu-Ce-Zr mixed oxide catalysts for water-gas shift: Carbon

nanofibers as dispersing agent for the mixed oxide particles, Applied Catalysis

B: Environmental, accepted.

VI F. Huber, J. Walmsley, H. Venvik, A. Holmen, The effect of platinum in Cu-

Ce-Zr and Cu-Zn-Al mixed oxide catalysts for water-gas shift, Manuscript in

preparation.

Paper I

Relating catalyst structure and composition to the water-gas shift

activity of Cu-Zn-based mixed-oxide catalysts

Catalysis Today 100 (2005), 249-254.

www.elsevier.com/locate/cattod

Catalysis Today 100 (2005) 249–254

Relating catalyst structure and composition to the water–gas

shift activity of Cu–Zn-based mixed-oxide catalysts

Magnus Rønninga,*, Florian Hubera, Hilde Melanda, Hilde Venvikb,De Chena, Anders Holmena

aNorwegian University of Science and Technology (NTNU), Department of Chemical Engineering, N-7491 Trondheim, NorwaybSINTEF Materials and Chemistry, N-7465 Trondheim, Norway

Available online 28 December 2004

Abstract

In order to investigate the effect of cerium oxide on Cu–Zn-based mixed-oxide catalysts four catalyst samples were characterized by means

of XRD, in situ XANES and thermogravimetric analysis. The activity of the catalyst samples was tested for the forward water–gas shift

reaction. Cerium oxide was found to increase the crystallinity of the ZnO phase indicating a segregation of the Cu and ZnO phases. The TOF

of the water–gas shift reaction based on chemisorption data was found to be independent of composition and preparation conditions of the four

catalyst samples. In contrast, the catalyst stability depends on composition and preparation conditions. Cerium oxide impregnated before

calcination of the hydrotalcite-based Cu–Zn precursors leads to a more stable water–gas shift catalyst.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Water–gas shift; Copper; Cerium oxide; XANES; XRD; Thermogravimetric analysis

1. Introduction

The water–gas shift (WGS) reaction (CO + H2O $CO2 + H2) is an important step in the production of H2

from hydrocarbons. Recently, the WGS reaction has received

renewed interest as a key step in fuel processing to reduce

the CO level in hydrogen produced for proton exchange

membrane fuel cell (PEMFC) applications [1]. For low

temperature WGS, Cu is usually preferred as the active com-

ponent because of its proven activity [2,3]. However, there is a

need for catalysts with even higher activity and stability

compared to the traditional CuO–ZnO–Al2O3 system.

It has been shown that oxides with high oxygen storage

capacity such as CeO2 can exhibit high WGS activity in

conjunction with various metal promoters [4–7]. The role of

ceria in such systems is proposed to be via a ceria-mediated

redox process where the oxygen storage capacity of ceria

and the metal are the active elements [4,5,8]. Others have

found evidence of a mechanism involving the reaction

between CO and active OH groups to form surface formates

* Corresponding author. Tel.: +47 73594121; fax: +47 73595047.

E-mail address: [email protected] (M. Rønning).

0920-5861/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cattod.2004.09.059

[7,9–11]. The promoting metal in the form of e.g. Pt, Au or

Cu significantly increases the WGS activity of ceria and

enhances the reducibility of ceria [4,6]. In the presence of

Cu, ceria reduction is proposed to start at low temperatures

(<200 8C) [4]. Ceria is also believed to enhance reducibility

and stabilise the Cu particles towards sintering [12]. For both

cases, ceria as an active component and as a promoter, the

results indicate that ceria is partially reduced in the active

catalyst, most likely by surface reduction. However, the

effect of ceria on the Cu metal particles is not clear.

In this study, we examine the effect of ceria addition on

the Cu metal particles in Cu–ZnO–Al2O3 catalysts derived

from layered hydroxide materials of the hydrotalcite

structure [13]. Mixed-oxide catalysts derived from hydro-

talcite-like materials are shown to possess high activity

and thermal stability together with relatively high metal

dispersion and homogeneous distribution of the components

[14]. WGS activity measurements and deactivation studies

are examined together with structural information from

XRD, XANES and other techniques. However, reaction

mechanisms are not being considered in this report. Kinetic

considerations of the activity measurements will be treated

elsewhere [15].

M. Rønning et al. / Catalysis Today 100 (2005) 249–254250

2. Experimental

A series of Cu- and Zn-based catalysts derived from

hydrotalcite structures were synthesised by co-precipitation

of the precursor salts followed by subsequent aging, filtra-

tion, washing, drying and calcination. The aqueous anionic

solution was made by dissolving Na2CO3 and NaOH.

Nitrates of Cu, Zn and Al were added at 80 8C under

continuous stirring and pH adjustment. The solution was

aged for 15 h before being filtered and washed and dried in

vacuum. The catalysts were impregnated with Ce(NO3)3

either before or after calcination. The sample labels (see

Table 1) are related to the preparation conditions: Cu-350

means a CuO–ZnO–Al2O3 catalyst that was calcined for

6 h at 350 8C. A portion of this sample was impregnated

with cerium nitrate after calcination before undergoing a

second calcination for 6 h at 250 8C, resulting in the sample

Ce-aC-350. Cu-400 is CuO–ZnO–Al2O3 with the same

nominal composition as Cu-350 calcined for 6 h at 400 8C.

Ce-bC-400 is based on Cu-400, but was impregnated with

cerium nitrate before calcination for 6 h at 400 8C. Ce-aC-

350 and Ce-bC-400 have the same nominal composition.

Copper dispersion measurements by means of selective

oxidation via N2O surface titration [16] were performed in a

Thermogravimetric Analyser (Perkin Elmer TGA 7) using

10 vol.% N2O in Argon (temperature: 75 8C, flow rate

80 ml/min at ambient temperature). Temperature-pro-

grammed reduction (TPR) using 7 vol.% H2 or CO in

Argon (heating rate: 2 K/min, flow rate: 80 ml/min at

ambient temperature), as well as the estimation of the weight

loss of the calcined samples assigned to adsorbed water and/

or surface carbonates [6] was carried out in the same

apparatus. The weight loss (at 260 8C for 1 h in Ar) was

determined prior to reduction (at 260 8C for 2 h in 7 vol.%

H2 in Ar) and dispersion measurements. Dispersion

calculations based on oxygen chemisorption from N2O-

decomposition of the reduced catalyst samples were

Table 1

Chemical composition and physical properties for the catalyst samples

Sample ICP-AESa catalyst composition

mass fractionb

BETc (m2/g) H

Cu Zn Al Ce O

Cu-350 0.29 0.39 0.08 0.00 0.24 72.4 5

Ce-aC-350 0.26 0.35 0.08 0.07 0.24 – 3

Cu-400 0.27 0.40 0.09 0.00 0.24 76.2 1

Ce-bC-400 0.25 0.37 0.08 0.06 0.24 – 1

a ICP-AES: inductively coupled plasma-atomic emission spectroscopy with an

performed on calcined samples.b Normalised mass fractions, i.e. only CuO, ZnO, Al2O3 and CeO2 taken intoc Performed on the calcined samples, prior to reduction.d Thermogravimetric measurements performed in Ar on the calcined samples u

adsorbed water and/or surface carbonates [6].e Dispersion based on oxygen chemisorption (from N2O) of the reduced cataf Performed on the reduced and passivated samples. The copper crystallite size

into account the thickness of the passivation layer. The copper dispersion is calcula

contact with the support [18].

corrected for bulk oxidation of copper according to a

method proposed by Sato et al. [17].

XRD spectra for crystallite size estimation and phase

identification were recorded on Siemens diffractometers D-

5000 (monochromatic radiation) and D-5005 (dichromatic

radiation), respectively. Particle size estimates for the

reduced samples were calculated from experimental line

broadening (linewidth at half maximum) of the Cu(1 1 1)

reflection by using the Scherrer equation and LaB6 as a

standard for correction of instrumental line broadening. To

do this, the catalysts were dried (at 260 8C for 1 h in Ar),

reduced (at 260 8C for 2 h in 7 vol.% H2 in Ar) and

passivated (at ambient temperature for 2 h in 0.5 vol.% O2

in Ar) in the TGA before transportation to the X-ray

diffractometer. The crystallite sizes reported in Table 1 are

corrected for the thickness of the passivation layer assuming

a cubic model for the copper particles [18].

Transmission X-ray absorption spectroscopy (XAS) data

were collected at the Swiss–Norwegian Beam Lines (SNBL)

at the European Synchrotron Radiation Facility (ESRF),

France. Spectra were obtained at the Cu K-edge (8.979 keV)

and Zn K-edge (9.659 keV) using a channel-cut Si(1 1 1)

monochromator. Ce K-edge data (40.444 keV) were

recorded using a Si(3 1 1) monochromator. For the

Si(1 1 1) monochromator higher order harmonics were

rejected by means of a chromium-coated mirror aligned

with respect to the beam to give a cut-off energy of

approximately 15 keV. The beam currents ranged from 130

to 200 mA at 6.0 GeV. The maximum resolution (DE/E) of

the Si(1 1 1) bandpass is 1.4 � 10�4 using a beam of size

0.6 mm � 7.2 mm. Ion chamber detectors with their gases at

ambient temperature and pressure were used for measuring

the intensities of the incident (I0) and transmitted (It) X-rays.

The XANES (X-ray absorption near edge structure)

measurements were performed on two of the four catalysts,

Cu-400 and Ce-bC-400. The amounts of material in the

samples were calculated to give an absorber optical

2O/CO2 lossd (%) Cu dispersione (%) XRDf

Cu crystal

size (nm)

Dispersion

(%)

.7 6.5 23 3.8

.2 4.8 16 5.6

.7 5.4 25 3.5

.7 8.2 23 3.8

estimated detection limit of 0.01–0.03 mg/g. The elementary analysis was

account.

p to the reduction temperature (260 8C) resulted in a weight loss assigned to

lyst samples taking into account copper bulk oxidation [17].

is estimated using the Scherrer equation for the Cu(1 1 1) reflection taking

ted from the crystallite size assuming a cubic particle shape with one face in

M. Rønning et al. / Catalysis Today 100 (2005) 249–254 251

Fig. 1. XRD spectra of the (a) calcined and (b) reduced–passivated samples.

The spectra of the calcined samples were recorded with D-5005. The

reduced samples were recorded with D-5000 except the shown spectra

for Ce-aC-350 which was also recorded with D-5005. The symbols on top of

the spectra are related to the different phases: (&) ZnO, (~) CuO, (!) Cu,

(*) CeO2.

thickness close to 2 absorption lengths. The samples were

ground and mixed with the requisite amount of boron nitride

to achieve the desired absorber thickness. The samples were

then loaded into an in situ reactor-cell [19] and reduced in a

mixture of 5 vol.% H2 or CO in He (purity: 99.995%; flow

rate 30 ml/min at ambient temperature) by heating at a rate

of 6 K/min from room temperature to 350 8C. XANES

profiles were collected during heating of the samples to

follow the reduction progression of CuO in H2 or CO. Cu

and Zn metal foils, Cu2O, CuO and ZnO were used as

reference materials.

The software package WINXAS v3.0 [20] was used for

XANES analysis to obtain qualitative and quantitative

information on copper, zinc oxide and cerium oxide bulk

phases under temperature programmed reduction condi-

tions. The XANES data were energy calibrated, pre-edge

background subtracted (linear fit) and normalised. Principal

component analysis (PCA) was applied for identification of

the number and type of phases in the experimental XANES

spectra. The reference spectra of these phases were then

used in a least-square fitting procedure to determine the

fraction of each phase present [21].

The initial water–gas shift activity was tested in an

externally heated tubular fixed-bed reactor with on-line GC

analysis over a wide temperature range (200–300 8C) [15].

In order to minimize temperature gradients the catalyst

samples were diluted with inert SiC (300–600 mm) taking

into account the falsifying effect of dilution on the measured

catalyst conversion [22]. Parameters like catalyst amount,

particle size, feed composition and gas flow rate were chosen

to perform the activity measurements within the kinetic

regime and away from the thermodynamic equilibrium of

the water–gas shift reaction [23]. The catalysts (0.15–

0.25 mg, 200–400 mm) were pre-reduced in situ at 260 8Cfor 3 h in 7 vol.% H2 in nitrogen, and the activity tests were

performed at a total pressure of 3 bar and 1.3 nl/min total

flow (0.7 nl/min N2, 0.3 nl/min CO, 0.3 nl/min H2O). The

deactivation measurements (feed: 1.0 nl/min N2, 0.3 nl/min

CO, 0.3 nl/min H2O) were performed at 300 8C in order to

accelerate the copper sintering process.

3. Results and discussion

The chemical composition and some physical properties of

the catalyst samples are presented in Table 1. Phase identi-

fication was performed by XRD both before and after cal-

cination. The nominal catalyst loadings were confirmed by

inductively coupled plasma (ICP-AES). The copper content

was also confirmed by the TPR-TGA analysis except for Ce-

aC-350. For this catalyst, the extent of reduction exceeded the

maximum copper equivalent indicating a reduction of com-

ponents other than copper oxide. This effect is object for more

studies and will not be discussed here in further detail.

XRD results confirm that the layered hydrotalcite phase

was obtained and that it breaks down during calcination to

produce mixed oxides (see Fig. 1(a)). The diffraction peaks

associated with ZnO are significantly more intense in the

samples containing ceria, for the calcined (Fig. 1(a)) as well

as the reduced–passivated samples (Fig. 1(b)). This can be

due to low crystallinity of ZnO in absence of ceria or simply

arising from smaller ZnO crystallites. The enhanced ZnO

crystallinity in the samples containing CeO2 may indicate

less interaction between ZnO and Cu in these samples. The

shape of the main diffraction peak for CeO2 at around 298indicates either low CeO2 crystallinity or small crystallites.

It has been stated in the literature that ZnO is not an active

reaction site in the water–gas shift reaction [2,3]. The XRD

data suggest that addition of CeO2 to the copper–zinc system

induces segregation of Cu and Zn phases, whereas CeO2

seems to be well dispersed. The Cu crystallite sizes are

calculated from XRD line broadening analysis (XLBA)

using the Scherrer equation and indicate particle sizes

around 20 nm for the Cu(1 1 1) reflection (see Table 1).

The Cu dispersion obtained from oxygen chemisorption

show Cu dispersions of 4–8% (see Table 1). The sample

impregnated with Ce before calcination shows highest

dispersion (8%) whereas the Ce impregnation after

calcination gives lower Cu surface area. These data are in

good agreement with the dispersions calculated from XRD

data (3.5–5.5%) in terms of the order of magnitude of copper

dispersion in the catalyst samples. However, the dispersions

derived from XLBA show a different trend. For turnover

frequency (TOF) calculations, data from oxygen chemisorp-

tion have been used since this method gives a more direct,

and thus presumably more accurate, estimate of the active


surface than the XLBA results from the reduced–passivated

samples. The XLBA method is a bulk technique and derives

the copper surface area indirectly from the crystallite size.

The XLBA-derived dispersions depend on a structural

model used for converting crystallite size into dispersion

and taking into account the passivation layer (here: cubic

model) and might therefore be a less realistic measure of the

copper surface area. The estimates are useful, however, for

validating the range of the chemisorption-based dispersions.

Currently, selective oxygen chemisorption via the

decomposition of N2O on the Cu surface is a widely used

technique for measuring metallic Cu surface area of copper-

based catalysts [16,17]. Nevertheless, uncertainties remain

concerning N2O decomposition on ceria and whether or not

partially reduced cerium oxide corrupts the measured

dispersion [3,18]. It still remains a matter of discussion

whether chemisorption can be applied, or if this standard

procedure needs some corrections.

In situ XANES measurements of the reduction behaviour

of the catalysts at the Cu K-edge show that the bulk copper in

the catalyst is reduced to the metallic state (see Fig. 2(a)).

The reduction proceeds more easily in CO than in H2 (results

not shown here), since CO has a higher reduction potential

than H2 [24,25]. The transition goes directly from CuO to

Cu, and Cu2O is not detected (see Fig. 2(b)) at the given time

resolution of the recorded XANES spectra. This is in

Fig. 2. TPR-XANES of Cu-400 at the Cu K-edge in 5 vol.% H2 in helium.

Heating rate: 6 K/min, flow rate: 30 ml/min at ambient temperature. (a) Cu

K-edge profiles recorded during the reduction procedure. (b) Change in

phase composition as a function of temperature during the reduction

procedure showing the direct transition from CuO to Cu metal. Cu2O

was not detected during the transition process.

agreement with recent studies [26] and suggests that Cu2O is

not a stable component under the applied reduction

conditions. Agreement also exists between the XANES

(Fig. 2(b)) and the TGA results (Fig. 3) concerning the

transition temperature. Slight differences arise from the

limited time resolution in the XANES experiments.

Furthermore, the TGA experiments where carried out using

a slightly higher H2 concentration than the XANES

experiments. According to Fig. 3, the catalyst Ce-aC-350

shows a notably higher reduction temperature than the other

catalysts. A reason for this might be that copper is partially

covered by cerium oxide. This effect will be addressed in

further studies. Ceria does not enhance the reducibility of

copper in any of the investigated catalysts. This is most

likely because the co-operational effect of Cu and ceria in

the WGS reaction is not present when ceria is in a state of

low oxygen storage capacity [5].

The effect of catalysts composition on the metal particles

and the role of Ce will be more closely investigated by the

proceeding EXAFS analysis [15]. However, both XRD and

the EXAFS analysis support the conclusion drawn by

Grunwaldt et al. [24], stating that the Cu metal particles take

a disk-like shape under reducing conditions. This is reflected

in a pronounced anisotropy in particle shape seen when

analysing the particle size in different crystallographic

directions [15].

Zn and Ce K-edge XANES profiles (not presented here)

show that the bulk of ZnO and CeO2 is not reduced under the

applied conditions up to 350 8C, neither in H2 nor in CO

atmosphere. Since XANES is a bulk technique, surface

oxidation of ZnO or CeO2 is not detected but cannot be

excluded.

The initial WGS activity of the four catalysts is shown in

Fig. 4(a). The selectivity of the reaction was 100% towards

CO2 formation. The chemisorption-based turnover frequen-

cies (TOFs) of the different catalysts are similar within the

experimental accuracy, indicating that the WGS reaction

depends on the exposed Cu surface area only [2,3]. Ceria

seems to enhance copper dispersion (N2O-based dispersion,

Table 1) and catalyst stability (see Fig. 4(b)) when

introduced before calcination. This indicates a promoting

effect of ceria on the active copper sites and implies

sufficient contact between both phases. The XRD spectra of

the reduced–passivated catalysts (Fig. 2(b)) with broad ceria

peaks with low intensity leads to the conclusion of ceria

being present in the form of small crystallites, a prerequisite

for a sufficient interface between copper and ceria. Ceria

included after calcination seems to reduce the chemisorp-

tion-based Cu surface area and slightly increase the catalyst

stability. Assuming coverage of Cu by ceria, a reduced

amount of accessible copper surface atoms would result in a

lower catalytic activity of Ce-aC-350 in terms of CO

reaction rate normalised with metallic copper. Since the

chemisorption-based TOF is similar to the other samples

(Fig. 4(a)), the nature of the active site seems to be

unchanged. This also supports the idea of ceria partially

M. Rønning et al. / Catalysis Today 100 (2005) 249–254 253

Fig. 3. Temperature-programmed reduction with 7% H2 in Argon using a thermogravimetric device. Heating rate: 2 K/min, flow rate: 80 ml/min at ambient

temperature. The weight derivative of the four catalyst samples normalised with the total weight loss during the reduction procedure and the temperature profile

are plotted as a function of time.

covering the copper surface. Thus, diffusion limitations as a

result of ceria coverage may explain the higher reduction

temperature of Cu observed in the TPR profile of Ce-aC-350

(Fig. 3).

It is evident that the preparation procedure for introdu-

cing CeO2 is important for the effect of CeO2. As mentioned

above, uncertainties still exist concerning the contribution

Fig. 4. (a) Turnover frequency (TOF) and (b) short-term deactivation behaviour o

N2O) and corrected for loss of H2O/CO2. The deactivation measurements are pe

disappearance of CO vs. time on stream (TOS). The normalised reaction rates are

corresponding catalysts exhibit a quite similar weight loss, this effect is negligible.

out of nitrogen. This has no further effect on the deactivation behaviour of the c

of ceria to the measured dispersion based on oxygen

chemisorption by means of N2O surface titration. Compar-

ing the N2O-based dispersions of Cu-350 and Cu-400 also

reveals a slight effect of the calcination temperature on the

copper dispersion.

Sintering of the copper particles at elevated temperature

close to the Huettig temperature (325 8C for copper), where

f the catalyst samples. TOF is calculated from oxygen chemisorption (from

rformed at 300 8C and are presented as normalised reaction rates for the

not corrected for the H2O/CO2 uptake of the calcined samples, but as the

The step in the deactivation curve for Cu-400 after 20 h stems from running

atalyst.


copper atoms become mobile, is known to be a severe reason

for catalyst deactivation under reaction conditions [27].

Steam enhances the sintering process. Experiments not

shown here, however, indicate that the slight variations

concerning the water vapour pressure between the four

deactivation experiments have no visible influence on the

deactivation behaviour.

4. Conclusions

A comparative study on the effect of cerium oxide on the

water–gas shift activity of Cu–Zn-based mixed-oxide

catalysts has been carried out using mainly XRD, in situ

XANES and thermogravimetric analysis.

According to XRD data, the crystallinity of the ZnO

phase increases when CeO2 is included into the system. This

might indicate a segregation of Cu- and Zn-containing

phases resulting in a reduced interaction between Cu and

ZnO. On the other hand, cerium oxide seems to be well

dispersed. Activity measurements reveal that the chemi-

sorption-based TOF of the investigated catalyst samples is

independent of composition and preparation conditions.

Together with the in situ XANES results, this leads to the

conclusion that metallic copper is the active component. In

contrast, the stability of the catalyst samples under reaction

conditions depends on composition and preparation condi-

tions. Thus, the stability can be increased by adding cerium

oxide. In order to achieve this effect CeO2 should be added

before calcination of the hydrotalcite-based precursor.

Acknowledgements

This work was supported by the Research Council of

Norway. We gratefully acknowledge the project team at the

Swiss–Norwegian Beam Lines (SNBL) at the ESRF for their

assistance. Elin Nilsen (Department of Materials Technol-

ogy, NTNU) and Egil Haanæs (Department of Chemical

Engineering, NTNU) are gratefully acknowledged for their

assistance with XRD and TGA, respectively.

References

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Paper II

Remarks on the passivation of reduced Cu-, Ni-, Fe-, Co-based

catalysts


1

Remarks on the passivation of reduced Cu-, Ni-, Fe-, Co-based catalysts

Florian Huber1, Zhixin Yu1, Sara Lögdberg2, Magnus Rønning1, De Chen1, Hilde

Venvik1,*, Anders Holmen1

1Department of Chemical Engineering, Norwegian University of Science and

Technology (NTNU), N-7491 Trondheim, Norway 2Department of Chemical Engineering and Technology, Royal Institute of Technology

(KTH), SE-100 44 Stockholm, Sweden

* Corresponding author; E-mail: [email protected], Tel: +47-73592831,

Fax: +47-73595047

Catalysts containing metals such as Cu, Ni, Fe, Co in their reduced state are often

subjected to passivation procedures prior to characterization. Passivation with N2O or

O2 to create a protective oxide layer also results in a certain degree of sub-surface

oxidation. The heat released during oxidation is a critical parameter. The extent of bulk

oxidation depends on the type of oxidant as well as on the size of the metal particles, as

shown for copper catalysts. The final, meta-stable passivation layer requires a certain

thickness to sustain exposure to ambient atmosphere. The encapsulation of metal

particles in carbon is an efficient method for preserving the metallic state, as

demonstrated for metallic nickel and iron with carbon nanofibers. The use of passivated

samples for characterization of the active, i.e. reduced, catalyst has limited value.

KEY WORDS: passivation; encapsulation; reduced metals; Cu; Ni; Fe; Co; surface and

bulk oxidation; O2; N2O; CO; CO2; carbon.

1. Introduction

Catalysts comprising the transition metals Cu, Ni, Fe and Co are widely applied in

heterogeneous catalysis. In most cases, the active catalysts contain these transition

metals in their reduced, i.e. metallic, state rather than as an oxide. Under oxidizing

conditions, such as in air, the metallic state of Cu, Ni, Fe and Co is unstable. Depending

2

on the conditions, the metals will be partly or completely reoxidized to their stable

oxides: Cu → Cu2O/CuO [1], Ni → NiO, Fe → Fe3O4/Fe2O3 [2], Co → CoO/Co3O4 [3-

4].

In order to characterize material properties relevant to the catalytic performance of the

catalyst, the metal particles should be studied in the activated state, preferably during

the catalytic reaction, i.e. in situ. Several in situ studies have shown metal catalysts to

undergo dynamic structural changes depending on the reaction atmosphere (oxidizing or

reducing) [5-9]. However, in situ characterization can be time-consuming or require

equipment not readily available. To be able to characterize reduced metal catalysts, a

range of protection methods are applied to retain the reduced state of metal catalysts

during characterization or transfer to the characterization chamber:

1. Sealing in a container under reducing/inert atmosphere after reduction [10]

2. Encapsulation of the reduced metal particles

a. by deposition of polyethylene [11-13] or 1-butene films [14] on the

surface, analogous to the protection of bulk metals against corrosion

by coating with polymer films

b. by growing a layer of carbon (such as carbon nanofibers or paraffinic

wax) around the reduced metal particles [15-17]

3. Surface passivation of reduced metal particles by controlled reoxidation to

create a thin, protective oxide layer. Further oxidation of the bulk is inhibited

by diffusion limitation [18]. A quasi- or meta-stable reduced metal phase is

maintained below the protection layer. Oxidant gases that have been applied

include:

a. O2/air in inert gas [1,18-23]

b. N2O in inert gas [9,24-27]

4. Formation of combined carbonaceous and oxidic layers, applying

a. CO2/O2, CO2/H2O or CO2/H2O/O2, optionally in inert gas [28-31]

b. CO/H2 and O2/air (consecutive) [32]

3

Our group has previously reported the successful application of sealed quartz capillaries

for preserving activated catalysts [10]. In this study, we evaluate and share our

experience with other protection methods, namely O2 and N2O passivation, and relate

our findings to reports on passivation in the literature. In addition, the encapsulation in

carbon nanofibers (CNF) for a range of catalysts used in the ongoing research is

reported and proposed as a possible method.

2. Experimental

2.1. Catalyst samples

The copper catalysts are mixed metal oxides derived from hydrotalcite precursors. Cu-

350 and Cu-400 contain CuO, ZnO and Al2O3 after the final calcination at 350 and 400

°C, respectively, and their hydrotalcite precursors were prepared by coprecipitation. Ce-

bC-400 was made from Cu-400 by incipient wetness impregnation of the dried sample

with a cerium nitrate solution. Details on preparation, sample notation and catalyst

properties can be found elsewhere [33].

The Ni-Fe catalyst is designed for carbon nanofiber (CNF) production and contains both

metals in a molar ratio Ni:Fe = 8:2. The catalyst, composed of NiO, Fe oxides (Fe2O3

and Fe3O4) and Al2O3 after calcination, was prepared by coprecipitation to produce a

hydrotalcite precursor. The CNF were synthesized by catalytic chemical vapour

deposition from C2H4/CO/H2 (30/10/10 ml/min) at 600 °C for 1 h. Further details on

catalyst preparation and properties have been previously reported [15].

The Fischer-Tropsch (FT) catalyst containing 12 wt% cobalt was prepared by incipient

wetness impregnation of silica (PQ corp. CS-2133) with an aqueous solution of

Co(NO3)2·6H2O. The impregnated powder was dried in air at 120 °C for 3 h and

calcined at 300 °C for 16 h, increasing the temperature from 120 °C to 300 °C at a rate

of 1 °C /min. Further details on this type of catalyst have been published elsewhere

[3,34].

4

2.2. Characterization

Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) was used to

determine the actual amount of copper in the catalyst samples after calcination. The

samples were dissolved in hydrochloric acid prior to this analysis without any visible

residues.

Copper dispersion and passivation measurements by means of N2O decomposition

[22,27,35,36] according to the exothermic reaction

2 Cu + N2O → Cu2O + N2 (1)

were performed in a Thermogravimetric Analyser (Perkin Elmer TGA 7) using approx.

50 mg sample and 10 vol-% N2O in argon (reaction temperature: 75 ºC, flow rate: 80

ml/min at ambient pressure/temperature, 1 h under N2O atmosphere until the baseline

was stabilized). Temperature-programmed reduction (TPR) using 7 vol-% H2 in argon

(260 ºC, 2 h, heating rate 2 K/min, flow rate 80 ml/min at ambient pressure/temperature)

as well as the estimation of weight loss assigned to adsorbed water and/or surface

carbonates of the calcined samples [37], were carried out in the same apparatus. TPR

was also carried out with 7 vol-% CO in argon over Ce-bC-400, with the reduced

catalyst being cooled down in CO/Ar prior to N2O titration. The weight loss (at 260 ºC

after 1 h in Ar) was determined prior to reduction (at 260 ºC for 2 h) and dispersion

measurements. Dispersion calculations based on oxygen chemisorption from N2O

decomposition with a simplified reaction stoichiometry of Cu:O = 2 corresponding to

equation (1) were corrected for bulk oxidation of copper according to the method

proposed by Sato et al. [38].

The passivation of the copper catalysts with O2 was also performed in the TGA. In a

typical experiment, the catalyst sample was dried (260 ºC, 1 h, Ar), reduced (260 ºC, 2

h, 7 vol-% H2 in Ar) and passivated (ambient temperature, 2 h, 1 vol-% oxygen in Ar,

100 ml/min). The exothermic reoxidation can be written as follows [1,20]:

2 Cu + ½ O2 → Cu2O (2)

5

Hence, a simplified reaction stoichiometry of Cu:O = 2 is assumed. Pernicone et al. [20]

have discussed the reaction stoichiometry and the possibility of non-integer

stoichiometry. For the erroneous impact of the support (especially reducible metal

oxides like ceria or zirconia, and partly ZnO) on the quantification of dispersion and

passivation extent, we refer to Bartley et al. [27]. Concerning the stability of Cu2O

phases, Palkar et al. [40] report that cubic Cu2O is more stable than monoclinic CuO at

small crystallite sizes (< 25 nm). This is related to the increasing ionic character of

solids with decreasing particle size and is also dependent on calcination conditions.

XRD spectra for crystallite size estimation and phase identification of the (O2-

passivated) copper catalysts were recorded on Siemens diffractometers D-5000

(monochromatic CuKα-radiation) and D-5005 (dichromatic CuKα+β-radiation),

respectively. Particle size estimates (dP) for the reduced-(O2)passivated copper samples

were calculated by X-ray line broadening analysis (XLBA, linewidth at half maximum,

B) of the Cu(111) reflection at the Bragg angle Θ = 21.67° using the Scherrer equation

and LaB6 as a standard for correction of instrumental line broadening (Be)

2 2 cos( )

P

e

KdB B

⋅ λ=− ⋅ Θ

(3)

with λ = 1.5418 Å, the wavelength for Cu Kα, and K = 0.89, the Scherrer constant. The

value of the constant factor K depends on the definition of B, being set equal to 1.00

when using the integral breadth and 0.89 when using the full width at half maximum

[39]. The crystallite size of the reduced-passivated samples reported in Table 1 takes

into account the thickness of the passivation layer assuming a cubic shape for the copper

particles.

The passivation of the Co/SiO2 catalyst was characterized by XRD using the Siemens

D-5005 diffractometer. The sample (1 g) was reduced in flowing hydrogen at

atmospheric pressure and 350 °C for 16 h (heating rate: 5 °C/min up to 70 °C and then 1

°C/min up to 350 °C). According to Storsæter et al. [34] and references therein, most of

the reducible cobalt oxide is reduced after this procedure. Subsequently, the sample was

6

cooled down to ambient temperature, and the quartz reactor was flushed with N2 5.0 for

2 h. The consecutive passivation was carried out in either of two ways: In one

experiment, a gas mixture of 0.5 vol-% of O2 in nitrogen was used (first, 1 h at 50

ml/min and then 1 h at 140 ml/min). In a second experiment, 5 vol-% of N2O in

nitrogen was applied (2 h at 140 ml/min). The XRD analysis was started within 2 h after

passivation.

Transmission X-ray absorption spectroscopy (XAS) data were recorded for calcined,

reduced-O2-passivated and CNF-encapsulated Ni-Fe samples at the Swiss-Norwegian

Beamline (SNBL) at the European Synchrotron Radiation Facility (ESRF) in France.

Spectra were obtained at the Fe K-edge (7.112 keV) and Ni K-edge (8.333 keV) using a

channel-cut Si(111) monochromator. Higher order harmonics were rejected by means of

a chromium-coated mirror aligned with respect to the beam to give a cut-off energy of

approximately 15 keV. The software package WINXAS v3.1 [41] was used for XANES

analysis (X-ray absorption near edge structure) to obtain qualitative and quantitative

information on Ni/Fe oxide and metallic phases. The XAS data were calibrated, pre-

edge background subtracted (linear fit) and normalized. XAS spectra were collected

both for the calcined material, in the reduced-passivated state and after use in CNF

production. The passivation was conducted with about 1 vol-% of O2 in nitrogen. Ni and

Fe metal foils, NiO, α-Fe2O3 and Fe3O4 were used as reference materials. The phase

composition of the catalyst samples under study was determined by linear combination

of the reference XANES profiles applying a least-square fitting procedure in WINXAS.

3. Results

3.1. Copper catalysts

A representative experimental curve from TGA measurements of the copper-containing

samples is shown in Figure 1. Initially, water and/or carbonate species adsorbed on the

surface [37] are removed by increasing the temperature to the final reduction

temperature under Ar atmosphere. In this way, the weight change resulting from

desorption of adsorbed species does not interfere with the weight change caused by the

subsequent reduction. CuO is reduced to metallic copper and oxygen is released as

water molecules during TPR, decreasing the sample weight thereby. After reduction and

7

flushing with Ar, N2O decomposition was carried out to reoxidize the copper atoms in

or close to the surface. By plotting oxygen uptake versus time and scaling the abscissa

with the square root of time, surface and bulk oxidation contributions can be separated.

Since bulk oxidation is diffusion limited, it will appear as a linear segment in such a

diagram. Further details about this approach can be found elsewhere [38,42].

Weight losses caused both by adsorbates and copper reduction for the three copper

samples are given in Table 1. The copper content as determined by TGA is in good

agreement with the ICP-AES results. The copper dispersion of the samples varies

between 5 and 8 % assuming the Cu/N2O = 2 stoichiometry according to equation (1).

The oxygen uptake during N2O decomposition is correspondingly 7 – 11 %. This

indicates that the passivation layer is relatively thin and that the reoxidation of copper

does not reach far into the bulk. In contrast, Cu passivation with O2 results in a

considerably thicker passivation layer, extending to about 40 % of the Cu atoms.

The crystallite size of the Cu core embedded in the oxide layer is estimated from XRD

spectra recorded for reduced-(O2)passivated samples (Figure 2a). XRD spectra of

calcined samples before reduction/passivation are included for comparison (Figure 2b).

For all three catalyst samples, the size of the Cu core (dC) is estimated to about 20 nm

(Table 1). Using a cubic model the average size of the copper particles (dP), including

core and passivation layer, can be estimated by the following equation:

31

(1 )P Cp

d dx

= ⋅−

(4)

with xp being the fraction of the passivated Cu atoms. According to this estimation, the

thickness of the oxide layer lies in the range of a few nanometers (Table 1), which is in

agreement with literature ([20] and references therein). The structural model applied

does not, however, take into account possible structural changes during passivation.

Several studies have shown that both surface structure as well as particle shape may

change (dynamically) upon changes in the gas phase conditions [5-9]. Despite the

significant degree of reoxidation, the oxide layer does not appear in the XRD spectra

8

(Figure 2a). The passivation layer is only a few nanometers thick and will therefore

appear amorphous to XRD [5,43]. Thus, from XRD alone, the passivation layer appears

insignificant. Kvande et al. [44] identified a crystalline Cu2O phase in addition to

metallic copper by XRD after passivation of a reduced Cu/CNF catalyst in O2/He,

indicating a significant degree of bulk oxidation.

Pernicone et al. [20] report the passivation of a copper-containing catalyst with 60 % of

the Cu atoms being reoxidized. This copper catalyst contained smaller copper particles

(Cu core: 10.2 nm, with passivation layer: 13.8 nm) than the samples in Table 1. If we

assume that the passivation layer must reach a certain thickness in order to protect the

bulk against further oxidation, the fraction of reoxidized metal atoms should increase

with decreasing particle diameter (equation (4)). This trend was also observed within

our passivation studies. Another Cu-Zn-Al mixed oxide catalyst, prepared by

coprecipitation under different conditions but with a similar composition as the copper

catalysts described in chapter 2.1, showed a passivation degree of about 58 % (with

oxygen). The size of the copper core was estimated to 10.6 nm, and the particle size,

including the passivation layer, to 14.2 nm, in good agreement with the results obtained

by Pernicone et al.

Figure 3 shows the normalized weight increase of Ce-bC-400 during N2O titration as a

function of the square root of time, based on the total weight loss upon reduction. The

pre-reduction was carried out with H2/Ar and CO/Ar. In both cases, the reduced catalyst

was cooled down in the reduction gas prior to N2O titration. The sample pretreated with

CO/Ar exhibits a slower oxidation kinetics and a lower total oxygen uptake than the

sample pretreated with H2/Ar. The dispersions determined from the dispersion-square

root of time plot correspond to approx. 8.2 % and 6.2 % for pre-reduction with H2 and

CO, respectively. We assume that carbonaceous species on the surface of the reduced

Cu particles slow down and confine the re-oxidation of metallic Cu in the case of pre-

reduction with CO.

9

3.2. Nickel-iron catalyst

The Ni-Fe catalyst was investigated by XAS, in particular in the XANES region. By

recording data at both the Ni K-edge and the Fe K-edge, the oxidation state of both

metals can be determined. Figure 4 and 5 show the XAS spectra of Ni and Fe,

respectively, for the calcined and reduced-passivated samples and for the sample used in

the synthesis of CNFs. The spectra of NiO, a Ni foil, α-Fe2O3, Fe3O4 and a Fe foil are

used as references.

Figure 6 shows a least squares fit of a linear combination of the XANES spectra of

model compounds, reconstructing the profile of a catalyst sample. NiO and Ni foil are

combined in order to model the XANES profile of the reduced-passivated Ni-Fe catalyst

at the Ni K-edge. According to this fit, 70 % of the Ni atoms in the sample are in the

metallic state after O2-passivation while 30 % are reoxidized to NiO. Table 2 contains

the phase composition for the three Ni-Fe catalyst samples recorded at both the Ni and

the Fe K-edge. The Ni sample profiles are modelled with two reference materials, since

Ni(0) and Ni(2+) are the most common oxidation states of nickel. The Fe sample

profiles are reconstructed with three reference materials, since Fe(0) under low partial

pressure of oxygen first oxidizes to Fe3O4 (= FeO·Fe2O3) and then further to Fe2O3 [2].

The XANES analysis of the Ni K-egde of the sample used for CNF production shows

that essentially all Ni is preserved as Ni(0). Reduced Ni particles encapsulated in carbon

nanofibers (see also [15]) thus preserve their metallic state also upon exposure to air.

The encapsulation in carbon is better than passivation by O2, where about 30 % of the

Ni atoms are reoxidized to NiO, assuming that both H2 reduction and reduction under

CNF production conditions result in complete conversion of NiO to metallic Ni prior to

oxygen exposure. About 80 % of the iron atoms in the CNF-encapsulated sample retain

the metallic state, as compared to about 60 % with oxygen passivation. The remaining

20 % or 40 % are present as a mixture of Fe3O4 and Fe2O3. The 20 % non-metallic Fe

atoms in the CNF-encapsulated sample have either been reoxidized under air exposure

or they represent a fraction of iron atoms not reducible under the given pre-reduction

and CNF-production conditions (see also [45]). Correspondingly, the oxide fraction in

10

the O2-passivated sample may also contain the non-reducible part, in addition to the

oxide formed under passivation conditions.

3.3. Cobalt catalyst

Figure 7 shows XRD spectra of the calcined Co/SiO2 catalyst, as well as after reduction

and passivation with oxygen. The XRD profile recorded after passivation with N2O is

identical to the one after passivation with oxygen, and is hence not included in the

figure. The nine main peaks in the spectrum for the calcined sample can be assigned to

Co3O4. In addition, there are two broad peaks between 10 and 30° stemming from the

silica support. For the passivated sample, the characteristic Co3O4 peaks have vanished

and the apparent peaks can be assigned to CoO and metallic cobalt. The broad peak at

36 – 37° is assigned mainly to CoO, but it can not be excluded that Co3O4 contributes to

the right shoulder of this peak. The broad shoulder between 46 – 54° can be assigned to

different metallic cobalt phases.

4. Discussion

From dispersion measurements of copper catalysts [20,22,27,38,42], it is well-known

that the reoxidation of the reduced metal with O2 or N2O is not limited to the surface,

but also reaches sub-surface layers. As a consequence, these measurements are either

conducted at low temperatures (for O2 [20]) or the contribution of the bulk oxidation is

taken into account applying a diffusion model (for N2O [38,42]).

4.1. Thermodynamics and kinetics

Standard reaction enthalpies for the oxidation of Cu, Ni, Fe and Co by O2 and N2O are

shown in Table 3. They are estimated from the standard enthalpies of formation of the

compounds participating in the reactions, based on bulk data. The oxidation processes

investigated in this study are limited to a few nanometers into the bulk of the metals.

Thus, pure bulk data may not be accurate for quantifying the energies involved in these

reactions, but can be used for qualitative comparisons.

The values for the O2 oxidation are calculated for 1 mole O2 and not for 1 mole O,

which one might prefer when comparing with the values for N2O oxidation. We prefer

11

the O2-based enthalpies because each time an O2 molecule reacts with the metal, two

oxygen atoms are involved simultaneously in the oxidation reaction releasing the double

amount of energy of one oxygen atom. Thus, we prefer to compare N2O and O2 based

on the amount of energy these two molecules contain in total.

The following trends can be deduced from the thermodynamic data in Table 3:

1. The oxidation reactions are all exothermic. This is reflected in the well-

known pyrophoric behaviour of reduced transition metal catalysts in

contact with air.

2. The oxidation reaction with N2O is less exothermic than with O2, i.e.

during the consumption of one O2 molecule more energy is released as

per N2O molecule.

3. For N2O, the energy release during oxidation of the different metals

increases in the following order: Cu < (Ni, Co) < Fe. The same order is

obtained for oxidation with O2, if Fe2O3 is the main phase formed. The

stability of the metallic phase decreases in the same order.

4. Copper may be considered the thermodynamically most stable among the

four metals. Ni and Co behave relatively similar, while for Fe the energy

released depends strongly on the type of oxide formed. These

thermodynamic properties are to some extent responsible for Ni, Co and

Fe being more difficult to reduce than Cu, as reflected in the typical

reduction temperatures for these metals.

Apart from thermodynamics, there are also kinetic effects to be considered. According

to Pernicone et al. [20], temperature is a main parameter influencing the extent of bulk

oxidation using O2. Because the passivation process is a non-catalytic gas-solid type

reaction [46], the passivation rate and thus the extent of bulk oxidation depends on the

diffusion rate in the solid material with the diffusion coefficient being a function of

temperature. The kinetic rate constant of the oxidation reaction itself is also enhanced

by increasing temperature, although we assume that the diffusion process is the rate-

limiting step during bulk oxidation with O2. In principle, this is also the case for

passivation with N2O [27,38,42].

12

Via the interrelation between temperature and passivation kinetics, the thermodynamics

affects the kinetics of the process. Because of the exothermic nature of the oxidation

reactions, energy is released and the local temperature increases. The energy release

may thus enhance the diffusion rate and increase the extent of bulk oxidation.

According to this, the extent of bulk oxidation should be higher for passivation with O2

than with N2O, not taking into account the effect of different oxidation kinetics for O2

and N2O. Bartley et al. [27] identify local temperature increases as a significant cause

for bulk oxidation of Cu in dispersion measurements with O2 and N2O. They stress the

need for low oxidant concentrations, large sample amounts and characterization setups

with efficient heat dissipation, resulting in a more controlled release of the heat of the

oxidation reactions.

4.2. Copper catalysts

The higher extent of bulk oxidation with O2 as compared to N2O (Table 1) we assume

does not only stem from the effect discussed above, especially since the passivation

with N2O is conducted at higher temperatures and higher gas concentration than the

passivation with O2. Other factors could be the somewhat higher sample amount used

for the O2 passivation (because of requirements for the subsequent XRD analysis) and

the different reaction rate constants for N2O and O2, with N2O enabling a more

controlled passivation. This might also be reflected in that Cu dispersion measurements

conducted with O2 are performed at very low temperatures while the N2O method can

be performed at ambient temperatures and even up to 90 °C.

The question remains whether the thin oxide layer produced with N2O is sufficient to

protect the metallic copper core against further bulk oxidation in air. The observed

dependence of the passivation degree on the size of the copper particles (Chapter 3.1,

and similar discussion about higher reactivity of small metal particles in [27] and

references therein) implies that a certain, stable passivation layer thickness is required.

Furthermore, the similar XRD profiles for the Co catalyst sample passivated in O2 and

N2O (Figure 7) point in the same direction. Bartley et al. [27] report that they could not

prevent some continued reoxidation of copper after applying a N2O passivation

procedure that resulted in a thin oxide layer around the metallic copper core. If N2O

13

initially creates a passivation layer thinner than O2 does, yet not stable enough to

survive in ambient air atmosphere, further bulk oxidation leads to a thicker, meta-stable

passivation layer of similar thickness as created by O2 passivation. Hence, even though

N2O does not lead to a stable passivation layer, it can be used in a first passivation step

to grow an oxide layer that will slow down further reoxidation with O2 and prevent local

temperature increase, thereby keeping the final thickness of the passivation layer at a

minimum.

Moreover, in analogy with electrochemical processes [47], it is possible that Cu(2+)

might be formed in ambient atmosphere at the surface of the passivation layer by

adsorbing oxygen and/or H2O (from air moisture) leading to formation of Cu(OH)2

and/or CuO at the surface [2].

4.3. Nickel-iron catalyst

The Ni(2+) detected by XAS in the Ni-Fe-Al sample is identified as NiO. This

oxidation state could also be assigned to a Ni-Al spinel structure known to form at high

calcination temperatures [48]. However, Ni incorporated in such a structure is less

reducible, and would not be reduced under the conditions applied here [15]. The spinel

structure is detectable in XRD if formed to a significant extent [48], but was not found

for the Ni-Fe-Al sample calcined at 480 °C [15]. Ni K-edge data of the CNF-

encapsulated sample also show that almost 100 % of Ni remains in the metallic state

after CNF production at 600 °C. Consequently, the sample probably does not contain

XRD amorphous Ni-Al-spinel, since basically all Ni can be reduced. The reduced-

passivated sample is treated up to the same temperature. It is thus likely that the

detected Ni(2+) stems from NiO formed during passivation.

The reliability of the phase composition data obtained by the least-squares fitting

procedure depends on a reasonable choice of reference materials, i.e. fitting of the

experimental curve might still be possible even when using unreasonable model

compounds. The quality of the recorded spectra is also vital to the fitting procedure.

According to the XANES fit, the calcined sample contains around 7 % Ni metal. It

appears unlikely that Ni particles oxidized in air at 480 °C contains significant amounts

14

of metallic nickel. The XANES curves of the calcined sample and the NiO reference

(not shown) display similar profiles. The difference between these curves is a certain

deviation in the high energy part of the recorded spectra, believed to emerge from

measurement (e.g. thickness effects) and/or data treatment rather than being related to

the material itself. As a consequence, the 7 % of Ni metal obtained for the calcined

sample should be treated as an error, indicating the accuracy limit of the linear

combination approach for this system. The values presented in Table 2 should thus be

viewed as average values with an accuracy not better than ± 5 % composition

percentage. For comparison, the standard deviations given by Overbury et al. [49]

represent a qualitative assessment of the accuracy limit of a linear combination of

XANES data.

Yu et al. [15] performed an EXAFS analysis for the Ni-Fe sample investigated here. In

their structural model, the extent of reoxidation during passivation was estimated to

about 26 % for Ni and 35 % for Fe, when comparing the coordination numbers (N) of

the oxygen shells of the calcined and reduced-passivated samples (Iron: first Fe-O shell

N = 1.1 and second Fe-O shell N = 3.5 for the calcined sample, first Fe-O shell N = 0.3

and second Fe-O shell N = 1.3 for the passivated sample; Nickel: first Ni-O shell N =

5.4 for the calcined sample, first Ni-O shell N = 1.4 for the passivated sample). These

values are in the same range as the values in Table 2.

The thermodynamic prediction that Ni is more stable than Fe (when forming Fe2O3) can

be supported by the XAS data. The extent of bulk oxidation upon O2-passivation of Cu

in the TGA (Table 1) is, however, slightly higher than the one obtained for Ni with

XAS. The stability of the metal phase might depend on particle size and co-additives in

the catalytic material, as can be seen from the comparison of the three copper catalysts,

thus making interrelationships more complex than comprised in a mere discussion of

thermodynamic trends. As previously mentioned, a practical aspect with regard to hot

spots is for example the choice of equipment used during passivation [27]. The Cu

catalysts were passivated in a TGA sample pan with small dimensions, i.e. each sample

was concentrated in a small volume implying a certain risk for local temperature

increase. In contrast, the Ni-Fe catalyst was passivated in a conventional calcination

15

reactor with large dimensions, the sample being distributed over a larger area compared

with the TGA pan and a lower risk for hot spots.

4.4. Cobalt catalyst

XRD was used to qualitatively identify the main phases present in the samples after

passivation (Figure 7). The oxidation of cobalt is similar to the oxidation of nickel from

an energetic point of view (Table 3). Not taking into account kinetics, it should

therefore be possible to passivate cobalt in the same manner as nickel. The XRD profile

of the O2-passivated sample in Figure 7 confirms the presence of metallic cobalt

encapsulated in a CoO shell. The reduction of Co3O4 to metallic cobalt proceeds via

CoO [3,4]. Under mild conditions (low temperature and low oxygen concentration) the

reoxidation may only proceed to the intermediate CoO (in analogy with the behaviour of

copper), since CoO is stable under certain conditions [2]. Yet, the existence of a Co3O4

phase (a mixture of Co(2+)O and Co(3+)2O3 [2]) in the passivated samples can not be

ruled out. The CoO detected by XRD may originate either from the passivation process

or from CoO not reduced during the reduction step. Cobalt oxide supported on silica is

easily reduced from Co3O4 to CoO independent of particle size, morphology and

support properties, but the reduction of CoO to Co is more difficult and depends on

particle size and support properties [4]. According to Storsæter et al. [34], about two-

thirds of the silica-supported cobalt is reducible (determined by oxygen titration) upon

reduction in hydrogen at 350 °C for 16 h. About one-third remains unreduced even upon

a subsequent TPR up to 900 °C, and might be assigned to cobalt silicate or cobalt oxide

encapsulated in silica [4,21]. An in situ XAS analysis suggested that the degree of

reduction is around 80 % which is somewhat higher than determined by oxygen titration

[3].

4.5. Encapsulation in carbon

Encapsulation of reduced metals in carbon appears to be an efficient way to protect

metallic phases against reoxidation in air. The encapsulation in CNF is, however, not a

generally applicable technique, since it applies only to catalysts active for carbon

formation. Jacobs et al. [17] used the solidified, paraffinic Fischer-Tropsch wax product

as an encapsulation matrix to preserve the reduced state of cobalt in a used Fischer-

16

Tropsch catalyst. A more generally applicable encapsulation procedure mentioned in

literature is the deposition of polyethylene [11-13] or 1-butene [14] films at the catalyst

surface, in analogy with the protection of bulk metals against electrochemical corrosion

by coating with polymer films.

4.6. Methods combining carbon and oxide formation

The treatment in a mixture of CO and H2 to form a layer of carbonaceous species on the

catalyst surface was reported to be superior to passivation with oxygen for reduced

cobalt particles [32]. However, a measured temperature increase upon air exposure

(lower than for passivation with oxygen [32]) can be interpreted as an indication for the

formation of an oxide layer. The conditioning of reduced catalysts with CO2 at elevated

temperature (e.g. 200 °C) has been applied to the passivation of Ni-Cr/Al2O3 steam-

reforming-methanation catalysts [50]. Furthermore, the XPS and XRD data of Cu-based

[51] and Cu-Co-based [52] catalysts that had been exposed to CO/CO2-containing gas

mixtures during the synthesis of alcohols are relevant also to passivation. Metallic

copper and Cu2O were identified by XRD for a Cu-ZnO-Al2O3 catalyst after use in

methanol synthesis [51]. However, we suggest to assign the corresponding XPS spectra

to Cu(1+) rather than to Cu(0) or a mixture of both species, in agreement with literature

[47,53] and the well-known instability of Cu(0) in oxidizing atmosphere (i.e. air). For

the Cu-Co-based catalysts used in the synthesis of higher alcohols, metallic Co as well

as oxidized Co species were identified by XPS [52].

Passivation strategies that involve CO and/or CO2 without the formation of significant

amounts of coke or wax around the metal particles may comprise both formation of

carbonaceous species and an oxide layer (at the latest when exposed to air), depending

on the conditions used and the metal to be passivated. Deactivation studies carried out

on ceria-supported precious metal catalysts indicate that CO and CO2 can adsorb at

catalyst surfaces during reactor shutdown forming stable surface carbonates [54].

Vissokov [29] claims that the rate of oxidation could be lowered, hence the passivation

improved, by the ability of certain metals to form surface complexes (e.g. metal

carbonyl), but oxidation could not be prevented completely under the conditions used.

These findings are in line with the results shown in Figure 3. Pretreatment with CO

17

instead of H2 resulted in a slower oxidation kinetics and a lower oxygen uptake during

N2O titration, which might be related to the formation of carbonaceous surface species

limiting the access of N2O. Such methods may therefore be considered intermediate

between encapsulation with carbon and formation of a protective oxide layer, with the

carbonaceous surface layer suppressing the extent of bulk oxidation.

5. Conclusions

Studies of reduced metal catalysts based on Cu, Ni, Fe and Co show that the result of

passivation procedures should be monitored, in order to quantify the extent of bulk

oxidation. Heat released during exothermic oxidation reactions appear to be a critical

parameter, since the local temperature and hence bulk diffusion and the final extent of

bulk oxidation may be increased. Furthermore, the (oxidic) passivation layer requires a

certain thickness in order to be stable and prevent further bulk oxidation in ambient air

atmosphere.

Passivation of reduced catalysts is not an ideal strategy for characterization of reduced

systems that are unstable in air. In situ measurements are prefered, but passivated

samples can be used to some extent, e.g. for estimating particle size with XRD [20,33],

keeping in mind the limited relevance and accuracy of data derived from this approach.

The use of passivated samples for a detailed X-ray line broadening analysis, in

correlation with catalytic activity, distinguishing between particle size and strain effects

is questionable and requires evaluation by a parallel in situ approach. Through the

passivation procedure, strain may be introduced into the crystal lattice by the oxidation

of the outer metal layers. Possible morphological changes depending on the

reduction/oxidation potential of the surrounding atmosphere may also complicate the

interpretation of the characterization results. Finally, the discussion of active reaction

sites based on the characterization of passivated samples is disputable.

Encapsulation of reduced metal particles by a protective layer of carbon is found to

efficiently protect Ni particles. For certain catalysts, CNF encapsulation could be

preferred instead of passivation by a protective oxide layer. Provided that the carbon

layer is impermeable to oxygen when exposed to air, the reduced metal particles may be

18

preserved in their metallic, hence active, state. Possible morphological changes as a

result of a change in the reduction/oxidation potential of the surrounding atmosphere

should be of less concern than for passivation with oxygen. However, the particle

morphology might be affected by the CNF growth process [55].

With this paper, we want to call more attention to the passivation procedure, its effect

and limited usability as sample treatment prior to characterization.

Acknowledgements

This work was supported by the Research Council of Norway through Grant No.

140022/V30 (RENERGI). Statoil ASA through the Gas Technology Center NTNU-

SINTEF is also acknowledged for their support. We gratefully acknowledge the project

team at the Swiss-Norwegian Beam Lines (SNBL) at the ESRF for their assistance. Elin

Nilsen (Department of Materials Technology, NTNU) and Egil Haanæs (Department of

Chemical Engineering, NTNU) are gratefully acknowledged for their assistance with the

XRD devices and the TGA, respectively. Hilde Meland and Cathrine Bræin Nilsen

(Department of Chemical Engineering, NTNU) are gratefully acknowledged for

preparing the copper catalysts.

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22

Figure captions

Figure 1. A typical TGA experimental curve, obtained over Cu-350, including removal

of water/carbonate species adsorbed on the surface, TPR of Cu and N2O decomposition

for determination of Cu dispersion and passivation.

Figure 2. XRD spectra of (a) reduced-(O2)passivated (D-5000 diffractometer and (b)

calcined (D-5005 diffractometer) Cu catalysts. The symbols refer to different crystalline

phases: █ = ZnO, ▲ = CuO, ▼ = Cu, ○ = CeO2

Figure 3. Weight increase during N2O titration (10 vol-% N2O/Ar, 75 ºC, flow rate 80

ml/min) normalized with the total weight loss upon reduction on Ce-bC-400 as a

function of square root of time. Pre-reduction was performed either with 7 vol-% H2 or

with CO in argon (260 ºC, 2 h, heating rate 2 K/min, flow rate 80 ml/min). The reduced

sample was cooled in the reduction gas prior to N2O titration.

Figure 4. XAS spectra of the Ni-Fe mixed oxide catalyst recorded at the Ni K-edge.

XANES profiles are shown for the calcined, reduced-(O2)passivated and used samples

together with the reference compounds NiO and Ni-foil.

Figure 5. XAS spectra of the Ni-Fe mixed oxide catalyst recorded at the Fe K-edge.

XANES profiles are shown for the calcined, reduced-(O2)passivated and the used

samples together with the reference compounds α-Fe2O3, Fe3O4 and Fe-foil.

Figure 6. Least-squares fit (dashed line) of the XANES spectra (solid line)of the

reduced-(O2)passivated Ni-containing sample at the Ni K-edge. The fit is produced by

linear combination of the XANES spectra of the model components NiO and Ni-foil

using a Levenberg-Margquardt least squares algorithm in the WINXAS software.

Figure 7. XRD spectra of Co/SiO2 for calcined and reduced-(O2)passivated samples.

The symbols refer to different crystalline phases: ▼ = Co3O4, ● = CoO, = Co. The

profile of the reduced-(N2O)passivated sample is identical to the spectra of the O2-

passivated sample and is therefore not shown in the figure.

23

Figure 1

41

42

43

44

45

46

47

48

49

50

0 100 200 300 400 500 600 700 800 900time [min]

wei

ght l

oss

[ m

g ]

0

50

100

150

200

250

300

tem

pera

ture

[ C

]

Cu-350programme temperature

water,carbonates

N2O decomposition

Cureduction

41

42

43

44

45

46

47

48

49

50

0 100 200 300 400 500 600 700 800 900time [min]

wei

ght l

oss

[ m

g ]

0

50

100

150

200

250

300

tem

pera

ture

[ C

]

Cu-350programme temperature

water,carbonates

water,carbonates

N2O decompositionN2O decomposition

Cureduction

Cureduction

temperature program

24

Figure 2

0

1000

2000

3000

4000

5000

6000

25 30 40 50 60 70

Cu-350

Cu-400

Ce-bC-400

(b)

sign

al [a

.u.]

2 Θ [ ° ]

0

1000

2000

3000

4000

30 40 50 60 70 80 90

Cu-350

Cu-400

Ce-bC-400

(a)

sign

al [a

.u.]

2 Θ [ ° ]

25

Figure 3

0

0,02

0,04

0,06

0,08

0 2 4 6 8

t^(1/2) [ min^(1/2) ]

norm

aliz

ed w

eigh

t inc

reas

e [ -

]

Ce-bC-400-H2

Ce-bC-400-CO

26

Figure 4

0.5

1.0

1.5

8.328.34

8.368.38

2.25

4.5

Y

X

Z


NiO Calcined

Red-pass Used

Ni foil

norm

. abs

orpt

ion

[ a.u

.]

27

Figure 5

0.5

1.0

1.5

7.1

7.15

7.2 norm

. abs

orpt

ion

[ a.u

.]

photon energy [ keV ] Fe foil

Used Red-pass

Calcined α-Fe2O3

Fe3O4

28

Figure 6

0.5

1.0

8.32 8.34 8.36 8.38

norm

. abs

orpt

ion

[a.u

.]

photon energy [keV]

29

Figure 7

sign

al [a

.u.]

2 Θ [ ° ]

calcined reduced-(O2)passivated

30

Table 1. Chemical composition and physical properties of the copper catalyst samples.

Cu crystallite size

(with O2 passivation)

Sample H2O/CO2

weight lossa

Total Cu mass fraction

in the samples

[ - ]

Cu

dispersiond

Cu passivated

with N2Oe

Cu passivated

with O2e Cu core

with XRDf

Core + pass.layer

(XRD + TGA)g

[ wt.% ] ICP-AESb TGA-TPRc [ % ] [ % ] [ % ] [ nm ] [ nm ]

Cu-350 5.7 0.29 0.28 6.5 10 40 19 23

Cu-400 1.7 0.27 0.26 5.4 7 45 20 25

Ce-bC-400 1.7 0.25 0.25 8.2 11 40 20 24

a Thermogravimetric measurements performed in Ar on the calcined samples up to the reduction temperature (260 C) resulted in a weight loss assigned to

adsorbed water and/or surface carbonates. b Normalised mass fractions, i.e. only CuO, ZnO, Al2O3 and CeO2 taken into account, with an estimated detection limit of 0.01 – 0.03 mg/g.

The elementary analysis was performed on as-prepared calcined samples, thus containing water/carbonates adsorbed on the surface. c For better comparison with the ICP-AES results the amount of copper determined by TGA-TPR is normalized with the sample mass before the drying procedure. d Copper dispersion measured by means of selective oxidation via N2O surface titration of the reduced catalyst samples taking into account copper bulk oxidation

[38,42], with percentage values being based on mole fraction. e Normalized with amount of Cu determined by TGA-TPR prior to passivation, with percentage values being based on mole fraction. f Performed on the reduced and passivated samples. The copper crystallite size is estimated using the Scherrer equation for the Cu (111) reflection. g The thickness of the passivation layer is taken into account assuming a cubic particle shape.

31

Table 2. Phase composition of the Ni-Fe catalyst sample (passivated with O2),

determined by a linear combination of XANES data of the reference materials at the Ni

and the Fe K-edge applying a least-squares fitting algorithm in the software package

WINXAS.

Ni K-edge:

Sample Ni metal [ % ] NiO [ % ]

Calcined 7 93

Reduced-passivated 70 30

Used 98 2

Fe K-edge:

Sample Fe metal [ % ] α-Fe2O3 [ % ] Fe3O4 [ % ]

Calcined 0 65 35

Reduced-passivated 57 39 4

Used 82 10 8

32

Table 3. Thermodynamics of the metal oxidation by O2 and N2O, based on the

standard enthalpies of formation for the bulk metals.

a From standard enthalpies of formation: N2O: 82 kJ/mole, Cu2O: -168 kJ/mole, NiO: -236 kJ/mole,

Fe2O3: -815 kJ/mole, Fe3O4: -1103 kJ/mole, CoO: -234 kJ/mole, Co3O4: -885 kJ/mole. Per definition,

the standard enthalpy of O2 is zero. The enthalpy of the metals is set to zero as well, although small

enthalpies of formation (absolute values < 7 kJ/mol) are tabulated for some metallic phases of Ni, Fe

and Co ([56] and references therein).

Standard reaction enthalpy [ kJ/mole ]a

N2O (per mole N2O) O2 (per mole O2)

Cu → Cu2O -250 -337

Ni → NiO -318 -472

Fe → Fe2O3 -354 -543

Fe → Fe3O4 -358 -408

Co → CoO -316 -468

Co → Co3O4 -303 -442

Paper III

Preparation and characterization of nanocrystalline, high-surface area

Cu-Ce-Zr mixed oxide catalysts from homogeneous

co-precipitation

Manuscript in preparation.

1

Preparation and characterization of nanocrystalline, high-surface area Cu-Ce-Zr

mixed oxide catalysts from homogeneous co-precipitation

Florian Huber1, Hilde Venvik1,*, Magnus Rønning1, John Walmsley2, Anders Holmen1


Technology (NTNU), N-7491 Trondheim, Norway 2SINTEF Materials and Chemistry, N-7465 Trondheim, Norway


Fax: +47-73595047


urea. The resulting materials exhibit high surface area and nanocrystalline primary

particles. The material consists of a single fluorite-type phase according to XRD and

TEM. STEM-EDS analysis shows that Cu and Zr are inhomogeneously distributed

throughout the ceria matrix. EXAFS analysis indicates the existence of CuO-type

clusters inside the ceria-zirconia matrix. This type of mixed oxide materials should

therefore be described as heterogeneous single-phase materials rather than

homogeneous solid solutions. The pore structure and surface area of the mixed oxides

are affected by preparation parameters during both precipitation (stirring) and the

following heat treatment (drying and calcination). TPR measurements show that most of

the copper is reducible and not inaccessibly incorporated into the bulk structure.

Reduction-oxidation cycling shows that the reducibility improves from the first to the

second reduction cycle, probably due to a local phase segregation in the metastable

mixed oxide with gradual local copper enrichment during heat treatment.

KEY WORDS: homogeneous alkalinization; urea hydrolysis; solid solution; Cu-Ce-Zr

mixed metal oxide; reducibility; drying; surface-to-volume ratio.

2

1. Introduction

1.1. Cu-Ce-Zr mixed metal oxides (MMO)

Mixed metal oxides (MMO) containing copper, cerium and zirconium are applied in

several areas of heterogeneous catalysis. Ce-Zr mixed oxides are extensively used in

three-way catalysts [1,2]. Cu-Ce-Zr-based mixed oxides are applied within the field of

hydrogen production: Water-gas shift [3-6], steam reforming of methanol [7-10] and

selective oxidation of CO [6,11-17]. In addition, they are used as NO reduction catalysts

[18], for oxidation of methane [19,20], wet oxidation of phenol [21] and acetic acid [22],

methanol synthesis [23,24], direct oxidation of hydrocarbons in solid-oxide fuel cells

(SOFC) [25-27], and storage of reactive hydrogen for alkadiene hydrogenation [28].

Copper - in its reduced state - is typically regarded as the active catalyst component in

MMO materials (except for SOFCs, where copper the main function is electronic

conductivity [25]). Ceria acts as a reducible oxide support, enhancing the catalytic

activity via metal-support interaction and/or improved dispersion of the active metal

component [8,29]. An important property of ceria is the oxygen storage capacity (OSC),

i.e. the ability to adsorb and release oxygen under oxidizing and reducing conditions,

respectively, according to the reaction [30]:

22 2

2 2

. ( / ). ( / ) x

red H COCeO CeO

ox H O CO − (1)

In addition, ceria is found to stabilize the catalyst against deactivation [8,29] due to a

higher thermal stability and/or better dispersion of the active metal. ZrO2 is also known

to improve the activity and stability of MMO-based catalysts [9,31]. Zirconium added to

ceria to form Ce-Zr mixed oxides inhibits the thermal sintering of CeO2 [2,30,32,33].

Incorporation of Zr into the ceria lattice enhances the reducibility of ceria [34-36],

which may improve the catalytic activity of MMO catalysts relative to single oxides

[37,38]. The amount of Zr-dopant also affects surface area and crystallite size of the

MMO [37,39], in conjunction with the calcination temperature [40]. In addition to its

presumed function as an active catalytic species, Cu as a dopant in the fluorite lattice is

3

found to improve the reducibility of ceria [41,42], and affect particle size and surface

area [24,43].

The activity and stability of MMO catalysts therefore depends on the interaction

between the single components, for which a homogeneous distribution of the

components throughout the material without pronounced segregation is a pre-requisite.

The catalytic activity often scales proportionally with the surface area of the active

components [4,44] since the reaction takes place at the surface of the catalyst. A

homogeneous distribution of the components and a high surface area of the material

have to be assessed simultaneously during catalyst preparation.

Co-precipitation of metal (hydrous) oxides in aqueous solution at high pH has

successfully been applied for preparing different metal oxide catalyst formulations. It

has been stated that co-precipitation results in more active and stable catalysts than

impregnation methods [17,20,45-47], because of a more homogeneous distribution of

the elements. For impregnation-deposition methods, the interaction between the

different components depends on the surface area of the support material and the

amount of material impregnated or deposited. The existence of separate phases at higher

loadings is likely [4].

1.2. Homogeneous co-precipitation with urea (HCP)

Homogeneous alkalinization via urea hydrolysis is an efficient co-precipitation

procedure for preparation of MMO with high surface area and well-defined particle size

and shape [3,19,48-51]. Urea decomposes at elevated temperatures in a two-step

reaction releasing ammonium and carbonate ions into the metal salt solution

accompanied by a simultaneous increase in pH, which leads to the precipitation of basic

carbonates [48,50,52]:

22 2 2 4 3( ) 2 2CO NH H O NH CO+ −+ → + (2)

The decomposition rate strongly depends on the temperature [52], the rate constant

increasing by a factor of about 200 as the temperature increases from 60 to 100 °C

4

[50,52]. The kinetics of the metal ion hydrolysis, and hence the nucleation rate, can be

tuned through controlled release of hydroxide ions to obtain well-defined particle

shapes with a narrow particle size distribution [48-50]. Constant-pH co-precipitation

procedures with other hydroxide ion sources, such as NaOH, are claimed to facilitate

stronger agglomeration of primary particles into irregularly shaped clusters, resulting in

a broader particle size distribution [50,51].

In terms of synthesizing MMO solid solutions, co-precipitation in general has to be

regarded as a heterogeneous process. Because of different hydrolytic properties of the

metal ions in aqueous solution, simultaneous nucleation is rarely the case. A phase

containing only one cation usually nucleates to serve as site for the heterogeneous

nucleation of a second solid. Further growth proceeds incorporating both cations at

different rates [49]. As a result, the internal local composition of such composites

usually varies from the center to the periphery of each particle [48]. The detection of a

homogeneous solid solution inside the MMO material depends on the characterization

technique used. The detection of single-phase MMO by conventional XRD techniques

does not exclude the presence of nanodomains of single-component-rich phases [2],

since XRD averages properties over a macroscopic scale as well as being insensitive to

amorphous phases and ordered structures below about 2 nm [53,54].

1.3. Effect of synthesis parameters

Synthesis parameters that may affect the resulting catalyst material properties such as

surface area, particle size and morphology are [21,48,50]: Type of precursor salt,

organic additives (from simple organic solvents to surfactants), total metal

concentration, metal ratio, urea concentration, procedure of urea addition (at ambient or

high temperature), pH, procedure of mixing, synthesis temperature, aging time and post-

treatment (drying and calcination conditions).

Nitrate precursor salts can easily be decomposed (an exothermic reaction), are

inexpensive and have high solubility in aqueous media. Sulphates and chlorides are

usually excluded since their residue in the catalyst material might accelerate catalyst

5

deactivation [57,58]. The type of anion present in the solution can, however, have an

impact on the precipitated particles [48].

The urea concentration (and initial ratio of urea to total metal in the solution), synthesis

temperature and initial metal ratio in the solution can affect the pH evolution rate, and

hence the nucleation and growth. Small particles with a narrow particle size distribution

has be obtained for high urea-total metal ratio and high reaction temperature [48,50].

Total metal concentration and acidic/basic additives affect pH and hence actual metal

contents and phases present in the final product [48,49]. A high total metal

concentration in the solution may result in enhanced particle agglomeration that reduces

the surface area [48]. The aging time can affect crystallinity, particle size [50] and phase

changes [49,59] and influence the final metal content [48,49]. All the parameters

mentioned in this paragraph affect the nucleation and growth of mixed metal (hydrous)

oxides/carbonates at different stages in aqueous solution, and may be highly correlated.

The effect of calcination temperature is well established for ceria-based systems. As for

many systems, the particle size generally increases with a corresponding decrease in the

surface area with increasing calcination temperature. As a consequence, the oxygen

storage capacity of ceria and the catalytic activity of the material decrease

[4,21,33,40,43,60-63].

There appears to be an optimum range for the metal loading of ceria-based mixed

oxides, irrespective of preparation method [20]. The best redox properties and highest

OSC for Ce-Zr solid solutions have been obtained within the range 0.2 ≤ x ≤ 0.4, where

x is the atomic fraction of Ce atoms replaced by Zr relative to pure CeO2 [1]. A

maximum activity for water-gas shift [38] and for CO2 reforming of methane [64] on

Ce-Zr supported Pt catalysts was found with x = 0.5. A maximum methanol

decomposition activity was reached at x = 0.3 for a Pd-Ce-Zr system [37]. For the same

components, a maximum at x = 0.2 was found for CO and C3H8 oxidation [40].

A roughly linear relationship between the rate constant and Cu content up to 20 at% was

found for Cu-Ce mixed oxide catalysts for wet oxidation of phenol [21]. During

6

selective CO oxidation over Cu-Ce catalysts, the highest reaction rate was obtained for a

Cu-content of 14 at% as compared to 7 and 21 at% [15]. This catalyst was also the one

exhibiting the highest surface area. For the same reaction, catalyst system and

preparation method, albeit higher surface areas, Kim et al. [6] obtained the highest

activity at 20 at% Cu as compared to 10 and 50 at%, but state that the difference

between the three catalyst formulations is not large. Tang et al. [17] reported the CO

conversion at low temperatures to be higher for 24 at% Cu than for 7 and 13 at% Cu in

co-precipitated catalysts. In methanol steam reforming over Cu-Ce catalysts, the highest

methanol conversion was observed for about 50 at% Cu [46]. The maxium TOF was

obtained for about 10 at%. Shen et al. [24] found the initial maximum space-time yield

in the methanol synthesis for 47 at% Cu, but after about 25 h on stream the maximum

shifted to the Cu-Ce catalyst with 23 at% Cu. The difference between the 12, 23 and 47

at% catalysts decreased with time on stream. For oxidation of CO and CH4 over

Cu/ZrO2, the catalyst containing 20 at% Cu in ZrO2 was found to be most active [65].

Kundakovic et al. [19] achieved higher methane oxidation reaction rates for 15 at% than

for 5 at% Cu in a Cu-Ce-La catalyst containing 4.5 at% La. For the water-gas shift

reaction, the highest CO conversion was measured for 15 – 20 at% Cu in Cu-Ce-La

catalysts containing 10 at% La, but the difference is not large between catalysts with 5

at% to 40 at% under the reaction conditions used [3]. In a more recent study of the same

system [5], a 10 at% Cu catalyst was found to perform slightly better than 5 and 15 at%

catalysts, and all were better than a 40 at% catalyst. All these catalysts contained 8 at%

La in ceria, and the reaction conditions were similar in the two studies. Investigating the

impact of the third metal component, they observed increased activity in the order 30

at% La > 24 at% Zr > 8 at% La, and conclude that this effect is of chemical origin and

not scalable with the surface area [5]. An improvement in the long-term stability and

suppressed CO formation during methanol steam reforming over Cu-Ce-Zr catalysts by

increasing the amount of Cu from 4 to 12 at% has also been reported [66]. No

significant enhancement was observed above 12 at%, and this could be related to the

detection of a separate CuO phase for samples with higher loading. The molar

composition (Cu:Ce:Zr) of the 4 at% and 12 at% catalyst were 4.4:51.0:44.6 and

12.1:44.3:43.6, respectively. Usachev et al. [14] achieved the best selectivity for

selective oxidation of CO in excess hydrogen on Cu-Ce-based catalysts for Cu:Ce:Zr =

7

0.23:0.54:0.23. Interestingly, an optimum composition for Co-Ce-Zr catalysts for

hydrogen production by ethanol steam reforming was found at Co:Ce:Zr =

0.225:0.500:0.275 [67].

Qualitative conclusions that can be drawn from the brief literature review above with

regard the Cu-Ce-Zr composition are:

(1) A Ce:Zr ratio between 3:1 and 2:1 appears to be a reasonable choice.

(2) The optimum amount of copper incorporated into the ceria structure lies in the

range 10 - 20 at %, probably limited by the amount of Cu that can be dispersed

in ceria (or zirconia) without forming separate CuO phases.

(3) In 3-component-mixtures, good performance is achieved with 40 - 50 % of the

Ce-atoms replaced by Cu and Zr in equimolar amounts, i.e. an atomic ratio

Cu:Ce:Zr ≈ 1:2:1.

Some of the studies suggest that the catalytic properties of these MMO are insensitive to

variations in the Cu content within a range of 10 – 25 at%. In a study dealing with the

preparation method, an experimental optimization of the composition may thus not be

first priority, and reasonable values taken from literature should be adequate. An

optimization of the metal composition has to be established for the specific application

under relevant reaction conditions.

When using urea for precipitation of copper salts, the ammonia complexes (NH4+/NH3)

released during urea decomposition (equation (2)) interact with copper ions to form the

deep blue copper-ammonia complex cations, [Cu(NH3)4]2+ or [Cu(NH3)6]2+, depending

on the ammonia concentration. These complexes retain the copper ions in solution and

decrease the amount of Cu in the final mixed metal precipitate. In a closed system

(reflux conditions), an equilibrium will be established between precipitated copper

hydroxide-carbonate phases and complex copper ions in solution. In an open system,

this equilibrium can be shifted in favour of copper precipitation by releasing ammonia

into the gas phase at elevated temperatures.

8

1.4. Description of the surface area of MMO

When characterizing the surface area of a certain material, the interesting parameter is

the surface-to-volume ratio (stv, m2/m3), i.e. the fraction of the material exposed to the

surrounding gas. The BET-deduced surface area (m2/g) is useful for comparison of

solids of equal composition. For materials with varying composition, however, the

effect of the density has to be taken into account [68]. If the structure remains more or

less unaffected by varying composition, the molar mass can simply be used to eliminate

the mass effect, resulting in a mole-based BET value (m2/mole). The BET results

obtained by Kapoor et al. [37], for mesostructured Ce-Zr mixed oxides, and Hirano et al.

[39], for non-ordered microporous Ce-Zr mixed oxides, are useful for exemplifying the

effect of the metal composition on the surface area (Figure 1). The stv (m2/m3) curves

show the true impact of varying metal composition on the surface area of the materials.

The densities used to calculate the stv from the BET data are based on a linear

combination of tabulated values. The use of measured densities in such evaluations

would further increase the precision. Comparing the surface-composition dependence

for the specific BET (m2/g) and the mole-based BET (m2/mole) indicates the impact of

the change in molar mass (with varying metal composition) on the BET surface-

composition dependence. Comparing the surface-composition dependence for mole-

based BET and stv shows the impact of the change in lattice spacing (with varying

metal composition) on the mole-based BET surface-composition dependence. In this

simplified re-evaluation, the effect of a change in lattice spacing and hence the volume

of the crystal unit cell by varying the metal composition affects the BET surface area

only to a minor degree. A considerable contribution to the change in specific BET

surface area with variation of the metal composition arises from the different molar

mass of cerium and zirconium.

The effect of copper loading on the surface area of Cu-Ce mixed oxides can be

evalutated in a similar way, for example by using data from references [24] and [43].

Here, however, the surface area decreases with increasing Cu loading after an initial

strong increase at very low Cu level. These curves (not shown) contain local minima

and maxima, and the overall trend depends on the resolution of the experimental points.

The same might be true for the Ce-Zr system. The BET data obtained for a series of

9

coprecipitated Cu-Zr mixed oxides can be used in a similar way to evaluate these binary

MMO [69].

1.5. Aim of the study

The aim of the present study is to determine relevant material properties of Cu-Ce-Zr

mixed oxides prepared by homogeneous co-precipitation. This includes the actual

catalyst composition relative to the nominal composition, as well as particle

morphology, surface area, pore structure, metal distribution or degree of homogeneity,

local atomic structure, and finally redox behaviour.

The initial concentration of the metal salts in aqueous solution (= nominal MMO

composition) is kept constant. The nominal molar metal ratio used is: Cu:Ce:Zr =

0.23:0.54:0.23. This ratio is based on the literature, as described above. The parameters

studied, in terms of their effect on surface area/particle size and pore structure, are:

Synthesis setup with (a) two types of stirring/heat transfer configurations and (b) reflux

conditions vs. open system, drying conditions and heating rate during calcination.

An elementary analysis of the actual catalyst composition is carried out with Inductively

Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). Morphology and particle

shape are examined with Transmission Electron Microscopy (TEM). The particle size of

the MMO is estimated with X-ray Diffraction (XRD) and TEM. The homogeneity of the

MMO is investigated using XRD (phase identification) and Scanning-TEM (STEM,

mapping of metal composition on a nm-scale). X-ray Absorption Spectroscopy (XAS)

has been applied for eludication of the oxidation state of bulk copper and its local

environment (first coordination shell). The redox behaviour is characterized by means

of Temperature Programmed Reduction and Oxidation (TPR-TPO) cycles. The pore

structure of the MMO is deduced from nitrogen adsorption-desorption isotherms. The

specific surface area is estimated by the Brunauer-Emmet-Teller (BET) method.

10

2. Experimental

2.1. Synthesis

Copper(II)-nitrate-trihydrate (> 99 %), cerium(III)-nitrate-hexahydrate (> 99.5 %) and

zirconyl(IV)-nitrate-hydrate (> 99.5 %) were purchased from Acros Organics. Urea (>

99.5 %) was purchased from Merck. Ethylene glycol (EG) (> 99.5 %) was purchased

from Fluka. Ethanol was purchased from Arcus. All chemicals were used as-received.

Deionized water was used for all catalyst preparations.

The two different setups used for catalyst preparation are shown in Figure 2. The first

setup consists of a hot plate equipped with a magnetic stirrer(Figure 2A). The precursor

solution is stirred in a 600-ml glass beaker with a magnetic stirring bar rotating at

approx. 1200 rpm. The temperature of the mixture is monitored with a thermometer

immersed in the solution. This setup is operated in an open mode, i.e. water evaporating

during operation is compensated by continuous refilling. The glass beaker is partly

covered to limit the evaporation. The second setup consists of a 500-ml 5-neck glass

flask immersed in a stirred oil bath (Figure 2B). The temperature is monitored with an

immersed thermometer, and the aqueous mixture is stirred with a blade agitator at

approx. 750 rpm. In the closed mode, a reflux condenser is connected to the flask, and

the unused flask necks are plugged. In the open mode, the three unused flask necks are

left open to allow the evaporation as well as the continuous refilling of water.

A total amount of 0.1047 mole of Cu-, Ce- and Zr-salt (nominal composition: Cu:Ce:Zr

= 0.23:0.54:0.23) were dissolved in 450 ml of water at ambient temperature. 0.9037

mole of urea were then added to the solution. After all solid components were dissolved

to give a pH of approx. 1.8 at 20 °C, the light blue, transparent solution was placed on

the hot plate/in the hot oil bath and heated up to 95 °C (unless otherwise noted) under

stirring. It took about 30 min. to reach the final temperature for both setups. The mixture

was kept on the hot plate/in the oil bath for a period of 8 hours, including precipitation

and aging. The precipitation started after approx. 30 min in both setups. The colour of

the suspension then changed from light-blue to green over the next 45 mins. and then

remained unchanged for the rest of the time. In the open mode, water at ambient

temperature was added continuously to the suspension to compensate the evaporation.

11

Ethanol was added instead of water in a single experiment, and in total 1,5 l of ethanol

was added during synthesis.

The suspension was removed from the hot plate/oil bath after 8 hours, and cooled to

room temperature using water. The solid precipitate was filtered off and washed twice

with 200 ml of deionized water (40 – 50 °C) under stirring (20 – 30 min.). Finally, the

precipitate was dried (about 11 h at 100 °C, unless otherwise noted) and calcined in a

muffle furnace for 2 h at 350 °C, either by placing the sample directly into the hot

furnace (normal method) or by increasing the calcination temperature at 2 °C/min up to

350 °C.

In two preparations, an ethylene glycol-water mixture (18 mole% or 40 V% EG) was

used instead of pure water, thereby increasing the boiling point of the mixture by about

5 °C. The synthesis temperatures were 95 °C and 100 °C. The resulting samples were

used for studying the effect of the drying conditions. The pore size distribution of the

sample synthezised at 100 °C was different from the typical distribution of the other

samples. For more information on this matter, we refer to Adachi-Pagano et al. [50].

The sample notation takes into account deviations from the typical synthesis conditions

given above and is classified as follows: HP = hot plate, B = oil bath, c = closed system,

o = open system, EG = ethylene glycol–water mixture, Eth = ethanol refilling, s98 =

synthesis temperature raised to 98 °C, d150 = drying temperature 150 °C, dry = dried,

not calcined, 2C = calcination rate 2 °C/min, N = drying in nitrogen, redox = after

reduction and reoxidation.

2.2. Characterization


determine the actual amount of copper, cerium and zirconium after calcination of the

precipitated materials. The samples were dissolved in hydrochloric acid before analysis

without any visible residues.

12

Nitrogen adsorption-desorption isotherms were measured using a Micrometrics

TriStar 3000 instrument. The data were collected at -196 °C. The BET surface area

was calculated by the BET equation in the relative pressure interval ranging from 0.01

to 0.30. The pore volume was estimated by the Barrett-Joyner-Halenda (BJH) method

[70] as the adsorption cumulative volume of pores between 1.7 nm and 300.0 nm width.

This method is based on the assumption of cylindrical pores, and the capillary

condensation in the pores is taken into account by the classical Kelvin equation. The

pore size distributions were calculated by non-local density functional theory (NLDFT,

Original DFT model with N2 [71], DFT Plus software package [72]) assuming a slit-like

pore geometry. For comparison, pore size distributions were also determined with the

Harkins-Jura model (cylindrical geometry), a classical method based on the Kelvin

equation [73]. Although the models gave different absolute values for the total pore

volume, the trends were essentially the same for the samples investigated. For more

details on the comparison of DFT and classical models based on the Kelvin equation,

we refer to Rouquerol et al. [74] and Chytil et al. [75], and the references cited therein.

XRD data were recorded on a Siemens diffractometer D-5005 (dichromatic CuKα+β-

radiation). Average crystal size estimates and crystal size distributions for the MMO

powders were obtained from a software-based X-ray line broadening analysis (XLBA).

The analysis was performed in two steps. Selected experimental XRD fluorite peaks

((111), (200) and (220)) were simulated by means of the software package Profile [76]

using the Pearson VII model function. The contribution of CuKβ to the peak intensities

is removed in this step. The program Win-crysize [77], utilizing the Warren-Averbach

method [78], was then used to estimate the crystallite size taking into account

contributions from microstrain (scaled as mean square root of the average squared

relative strain). Contributions from instrumental line broadening were removed using

LaB6 as reference (selected peaks: (110) at 30.38°, (111) at 37.44° and (210) at 48.96°).

Thermogravimetric Analysis (TGA) was performed with a Thermogravimetric

Analyser (Perkin Elmer TGA 7). The device was used to check the metal oxide content

in the nitrate precursors, the weight loss of the calcined samples assigned to

water/carbonates adsorbed on the oxide powders, and small residues of precursor

13

species not removed after calcination at 350 °C (approx. 1 wt% at temperatures up to

500 °C). TGA was also used for temperature-programmed reduction and reoxidation

(redox). The redox procedure was conducted as follows using crushed oxide powders:

(1) pre-oxidation in air up to 500 °C (heating rate: 10 °C/min, dwell: 10 min) in order to

remove water/carbonates and precursor residues; (2) cooling down to ambient

temperature in air, flushing and stabilization of the baseline, (3) reduction in 7 vol-% H2

in nitrogen up to 300 °C (5 °C/min); (3) cooling down in H2/N2, flushing and

stabilization; (4) reoxidation in air up to 350 °C (5 °C/min, 1 h); (5) cooling down in air,

flushing and stabilization; (6) reduction in 7 vol-% H2 in nitrogen up to 300 °C (5

°C/min). The gas flow rate was 80 Nml/min in all steps.

Transmission XAS data were collected at the Swiss-Norwegian Beam Lines (SNBL) at

the European Synchrotron Radiation Facility (ESRF), France. Spectra were obtained at

the Cu K-edge (8.979 keV) using a channel-cut Si(111) monochromator. Higher order

harmonics were rejected by means of a chromium-coated mirror aligned with respect to

the beam to give a cut-off energy of approximately 15 keV. The maximum resolution

(∆E/E) of the Si(111) bandpass is 1.4 x 10-4 using a beam of size 0.6 x 7.2 mm. Ion

chamber detectors with their gases at ambient temperature and pressure were used for

measuring the intensities of the incident (I0) and transmitted (It) X-rays. The catalyst

powder sample was diluted with boronitride. A copper foil, and Cu2O and CuO powders

diluted with boron nitride were used as reference materials. The recorded XAS spectra

were energy calibrated, pre-edge background subtracted (linear fit) and normalised

using the WinXAS software package [79]. For Extended X-ray Absorption Fine

Structure (EXAFS) analysis the data were converted to k-space using WinXAS, and the

least-square curve fitting was performed with the EXCURVE 98 program [80] based on

small atom approximation and ab initio phase shifts calculated by the program.


amounts of the catalyst samples were put into sealed glass containers containing ethanol

and placed in an ultrasonic bath for a couple of minutes to disperse the individual

particles. The resulting suspension was dropped onto a holey carbon film, supported on

a titanium mesh grid, and dried. Conventional TEM images were recorded onto a CCD

14

camera. Samples were also examined in scanning transmission electron microscope

(STEM) mode, with a nominal probe size of approx. 0.7nm. Bright field and dark field

STEM images were acquired. Energy dispersive x-ray spectroscopy (EDS) analysis and

mapping were performed using an Oxford Instruments INCA system. Drift

compensation was employed to correct for movement of the sample during the time

taken for the acquisition of maps.


3.1. General features of the synthesized Cu-Ce-Zr mixed oxides

The colour change from light-blue via dark blue/green to green observed during the first

45 min of precipitation is an indication of the heterogeneous nature of the co-

precipitation of different metal ions. The pH at which the precipitation of the single

metals begins depends on temperature, concentrations and the anions present (see e.g.

the titration experiment performed by Lamonier et al. [43] for Cu-Ce mixtures). With

urea as the source for hydroxide ions, the formation of carbonate precipitates, which

depends on the solubility of the corresponding metal carbonates in aqueous solution as

well as the interaction of Cu cations with NH3, has to be taken into account. This holds

especially for rare-earth metals whose carbonates are insoluble in water [81]. In addition,

the metal ions may affect each other in respect of their precipitation behaviour. A

detailed experimental study, similar to the one performed by Soler-Illia et al. [49] on

Cu-Zn basic carbonates, would be necessary to elucidate the reaction path of the

nucleation and growth process in the temperature-dependent phase diagram of the

Cu/Ce/Zr-nitrate-urea system.

The total (not optimized) yield of precipitation was approximately 80 wt% based on the

nominal oxide composition, including losses due to the post-synthesis treatment.

3.1.1. TEM

TEM images of a calcined Cu-Ce-Zr mixed oxide sample are shown in Figure 3. Co-

precipitation of copper, cerium and zirconium ions results in the formation of

nanocrystalline Cu-Ce-Zr mixed oxide particles. Primary particles formed through

nucleation and growth have agglomerated to larger clusters (Figure 3A), perhaps

15

already during the aging of the precipitate in solution. According to Matijevic [48], the

degree of particle agglomeration in solution increases with initial metal concentration.

By using lower concentrations of the nitrate precursors, the degree of agglomeration

could be reduced and the total surface area of the powder possibly improved. However,

particle agglomeration will also occur during drying and calcination. Zhang et al. [82]

suggested the addition of an anionic surfactant before drying in order to reduce the

degree of agglomeration during drying and calcination.

Figure 3B displays the crystalline nature of the nanoparticles. A rough particle size

estimate lies in the range 3-5 nm. The lattice spacing is visible, and nanocrystalline

particles are randomly oriented. The d-spacing mainly observed in the individual

particles is approximately 3.1 Å, corresponds to the (111) spacing of the fluorite

structure for the CeO2 and Ce-Zr mixed oxides. The lattice constant is thus estimated to

5.4 Å, in accordance with literature data for Ce-Zr mixed oxides [2,83]. The image of

Figure 3B is obtained after reduction and reoxidation of the calcined sample in Figure

3A. Comparable images (not shown here) were obtained for the calcined sample,

displaying similar features. The fluorite-type structure is therefore preserved upon redox

treatment.

3.1.2. XRD

Representative XRD spectra of the Cu-Ce-Zr mixed oxides after different steps in the

synthesis are included in Figure 4. Figure 4a is recorded after drying at 100 °C. The

characteristic peaks of the fluorite-type structure show that this structure is present

already after drying. This is in agreement with Hirano et al. [39], who observed the

fluorite structure in as-precipitated Ce-Zr (hydrous) oxide samples after drying at 60 °C.

Lamonier et al. [43] made a corresponding observation for Cu-Ce mixed oxides dried at

90 °C. No significant change in the fluorite structure can be observed upon calcination

(Figure 4c) and redox treatment (Figure 4d), in accordance with the TEM images.

Figure 4b will be discussed in section Particle size estimation from XRD data (Table 1,

Bo-samples) shows that the particles present after drying of the precipitate are preserved

in size after calcination and redox treatment, with a slight tendency towards sintering.

Figure 5 shows the crystal size distribution and the microstrain as a function of crystal

16

size for sample Bo as obtained by XLBA. The average crystal size for this sample is

approx. 3 nm (Figure 5a, Table 1). The primary crystal XLBA size estimate reaches up

to 9 nm, with 90 % of the particles being smaller than about 6 nm (Figure 5b). The

XLBA and TEM result are thus in good agreement. 10 % of the particles with size ≤ 1

nm may, however, be questionable. This is further discussed in chapter 3.1.4.

3.1.3. XAS

The XANES profile of a typical calcined Cu-Ce-Zr mixed oxide (sample Bo-d120) at

the Cu K-egde is compared with the profile of reference materials: Cu foil, CuO and

Cu2O powder in Figure 6. The XANES fingerprint of Cu in the MMO resembles that of

CuO, indicating a major fraction of the Cu atoms to be in the (2+)-oxidation state.

A standard EXAFS analysis (i.e. k ≥ 3 [54,84]) was performed on the XAS spectra of

the same sample. The structural model is limited to the first oxygen shell around the

copper atoms. The fitting was carried out in different ways to check whether the

structural parameters obtained are sensitive to the fitting procedure: The k-intervals

used for fitting were [3;8], [3;9] and [3;12]. For the range k = [3;9], the analysis

includes both fitting with k1- and k3-weighting in order to account for the high

correlation between the coordination number N and the Debye-Waller factor ∆σ2 [84].

In Table 2, the structural parameters obtained by fitting the structural model for the first

oxygen shell to the experimental data are shown (R = distance of the oxygen shell to the

copper central atom, E0 = energy shift to correct for deviations from the theoretical edge

value). The initial values for the fitting were taken from the crystallographic data for

CuO (E0 = 0, ∆σ2 = 0.01). The variance takes into account the uncertainty given by

EXCURVE as well as the variation of different fitting approaches. Figure 7A shows the

experimental and theoretical EXAFS spectra in k-space with k = [3;9] and k3-weighting.

The Fourier transform profile indicates that the first oxygen shell is the dominant shell

in the experimental EXAFS spectra (Figure 7B). This is consistent with spectra reported

by Shen et al. [24] for Cu-Ce mixed oxides. It is thus reasonable to limit the fitting to

the first shell. No significant improvement was obtained by inclusion of further shells.

17

Table 2 also includes the structural parameters obtained for the CuO reference powder.

Here, the coordination number was not included in the fitting procedure (initial value: 4),

but was simply refined at the end of the fitting. The structural parameters for the first

oxygen shell around the Cu atoms in the mixed oxide are similar to the ones in CuO, in

accordance with the XANES comparison. It can therefore be concluded, that the major

part of the Cu atoms in the MMO exhibit the typical chemical and structural

environment of Cu in copper-(2+) oxide. Ramaswamy et al. [85] and Lamonier et al.

[43] identified different Cu species in electron paramagnetic resonance (EPR) studies on

sol-gel prepared, nanocrystalline Cu-Zr oxides and co-precipitated Cu-Ce oxides,

respectively. They distinguished framework-substituted Cu ions and extra-lattice

species, such as interstitial copper ions or copper dimers, dispersed ions bound to the

surface and CuO-type clusters or small particles. The ratio between these species was

reported to depend on the amount of Cu in the samples and the temperature pre-

treatment. Crystalline CuO, which is not visible to EPR but is to XRD [43], was

reported to form at high Cu loadings. Consistent with our XAS results, Lamonier et al.

[43] conclude from their EPR studies on coprecipitated Cu-Ce mixed oxides calcined at

400 °C, that a major part of copper exists as CuO-like clusters well dispersed in the

solid matix.

The reduced coordination number (N1) of oxygen atoms compared to CuO (Table 2)

may relate to the small CuO cluster size, with a significant number of low-coordinated

Cu atoms at the particle surface [53-56]. Neglecting other possible effects [54], a rough

estimate for the size of spherical CuO clusters would be 8 Å (with error deviation: 5-15

Å) [55], i.e. in the range of the STEM resolution. However, defects in the oxide

structure, such as oxygen vacancies, can not be excluded. Standard EXAFS analysis

tends to underestimate coordination numbers, especially for small particles [54].

Applying the correction function developed by Clausen & Nørskov [54] in a modified

form (N-bulk = 4 instead of 12) to our system, the coordination number does not

increase significantly. This is only a rough indication, since the correction was

developed for metallic copper with fcc structure and not for copper oxide. A multi-data-

set EXAFS analysis [56] including asymmetric pair distribution functions for nano-

sized particle clusters [54], could further minimize correlation effects.

18

3.1.4. STEM-EDS

As a result of the nucleation and growth process, the synthesized material should exhibit

gradient in composition at the nanoscale. Figure 8 shows the STEM-EDS elemental

mapping of sample Bo-redox. Cu, Ce and Zr are distributed over the whole mapping

area (approx. 30 x 50 nm2), and the average metal composition is close to the ICP-AES

measured composition (Table 3). Within the mapping area, regions of a few nanometers

high in Cu or Zr concentration relative to the surroundings could be observed (Figure

8C, E). This is in agreement with the XAS results (chapter 3.1.3) that suggested the

existence of CuO-type clusters. As mentioned above, initially formed nuclei probably

serve as sites during co-precipitation for the heterogeneous nucleation of a second phase,

which grows incorporating different cations at different rates [49]. Thus, the particles

should incorporate the three metals in an inhomogeneous way, although governed by

their hydrolytic behaviour. Ce and Zr do not necessarily precipitate simultaneously or at

the same rate under the conditions used here (in contrast to the findings in reference

[39]) and/or the formation of single element clusters might be more favourable than a

homogeneous distribution of all components.

A similar elemental mapping was conducted for sample Bo, i.e. the same material,

calcined but before any redox treatment. Inhomogeneous metal distributions at the local

scale were found similar to those shown in Figure 8. Therefore, the redox treatment

itself does not significantly change the metal distribution. Qi et al. [5] found by XPS

analyses of Cu-Ce-La mixed oxides that the amount of surface Cu increased after water-

gas shift reaction as compared to the freshly calcined material. Lamonier et al. [43]

observed enrichment of Cu at the surface of Cu-Ce mixed oxide particles with

increasing temperature. For a discussion on the phase segregation in Ce-Zr mixed

oxides under high temperature treatment we refer to Di Monte & Kašpar [2] and

references cited therein.

It is known that the lattice parameters (a/b/c) of the crystalline fluorite phase change

upon doping of ceria with other metals [2,7,43,46,86]. The corresponding 2Θ-shift in

the XRD peaks depends on type and amount of dopant. A powder sample can be

considered as a collection of crystallites with different d-spacings with a broadening of

19

the XRD peaks as a result of the bandwidth of the d-value within the powder. An

inhomogeneous composition therefore introduces uncertainty into the determination of

particle size from XLBA analysis, since the peak broadening in locally inhomogeneous

phases no longer only stems from particle size, strain or the instrument. Based on the

TEM images in Figure 3, it may be assumed that the error introduced is not significant,

but the XLBA crystallite size distribution appears to produce somewhat smaller

diameters than observed in TEM, down to 1 nm. This phenomenon should not be

ignored in quantitative analyses of materials inhomogeneous composition by XRD, and

the use of complementary characterization techniques, such as TEM, is advisable.

3.1.5. TGA-TPR

The calcination temperature was chosen low enough to avoid sintering of the mixed

oxide particles, and high enough to ensure the removal of most of the synthesis residue

as gaseous decomposition products (N2/NOx, CO2 and H2O) from the basic hydrous

oxides. The freshly calcined samples were examined by TGA. Temperature-

programmed oxidation (TPO, data not shown) confirmed that most synthesis residues

were removed upon calcination. A weight loss observed at 100 – 150 °C can be

assigned to adsorbed water and carbonate species [29]. A second weight loss of about 1

wt% close to 500 °C may correspond to the amount of synthesis chemicals remaining

after calcination at 350 °C. Prior to further redox experiments in the TGA set-up, the

mixed oxide samples were therefore calcined at 500 °C to eliminate disturbances to the

reduction quantification.

Figure 9 shows the TPR profiles of the calcined Cu-Ce-Zr mixed oxide obtained during

redox cycling in the TGA. The maximum of the reduction peak lies at approx. 170 °C

with the peak of the second TPR slightly shifted to higher temperatures. This could be

caused by densification during the redox treatment also in agreement with changes in

BET and pore volume of the corresponding samples Bo and Bo-redox in Table 1.

Reoxidation of the reduced catalyst after the first TPR step results in a temperature

increase of 50 - 100 °C for 1 g of sample in flowing air at ambient temperature, due to

the pyrophoric properties of reduced copper. The temperature range for the reduction

peaks observed coincide with reduction temperatures obtained for Cu-Ce mixed oxides

20

prepared by conventional co-precipitation [6,12,46]. The main reduction peaks are

found at similar temperatures as for classical Cu-Zn-Al mixed oxide catalysts [29].

While Cu-Ce-Zr mixed oxides have the fluorite structure as the only XRD/TEM-visible

crystalline phase, the Cu-Zn-Al systems usually exhibit different oxide phases; Cu/CuO,

ZnO, according to XRD as well as (sometimes amorphous) Al2O3. Ce-Zr mixed oxides

are known to be reducible, but the degree of reduction is low for T < 200 °C according

to Overbury et al. [87]. A major contribution to the low temperature reduction stems

from the formation of OH-groups at the surface [35]. This can be neglected in TGA

because of the marginal weight of hydrogen atoms. The formation of oxygen vacancies

by H2O release would be more TGA sensitive, but plays a significant role above 500 °C

only [35]. Addition of reducible metals may improve the low temperature reducibility of

Ce-Zr mixed oxides, but this is also mainly related to the formation of OH-groups [35].

Cu has a moderate effect on the reducibility of ceria [41] that is dependent on the Cu

content [42]. In agreement with Overbury et al. [87] and Norman & Perrichon [35], who

studied the impact of noble metals, Wrobel et al. [42] found the Cu-promoted reduction

of Ce4+ to Ce3+ in Cu-Ce mixed oxides with Cu/Ce < 0.5 to occur mainly above 200 °C.

No quantitative discrimination between the formation of OH-groups and oxygen

vacancies was made in this study.

With reference to the literature summarized above, we assign the observed reduction of

the Cu-Ce-Zr mixed oxide at T < 200 °C mainly to the reduction of copper oxide. The

small, constant weight loss at T > 200 °C may originate from the reduction of Ce and

possibly to a fraction of less reducible Cu species. The existence of a broad TPR peak

with a pronounced shoulder may be related to a stepwise reduction of Cu(2+) via Cu(1+)

[42], as well as to the inhomogeneous distribution of copper in the mixed oxide sample,

as observed by STEM (Figure 8). The reducibility of single Cu atoms incorporated in

the MMO bulk could be lower than that of surface Cu or Cu particles due to

accessibility and/or stability of the oxide state. This interpretation of varying

reducibility of different copper species incorporated in the Ce-Zr mixed oxide is in

accordance with results obtained by Ramaswamy et al. [85] for nanocrystalline Cu-Zr

oxides. They found the extra-lattice species to be reduced more easily than the

framework-substituted ions. Consistently, Wrobel et al. [42] reported Cu clusters in Cu-

21

Ce oxides to be more reducible than isolated Cu(2+) ions. However, based on the

assumed heterogeneous mechanism of the co-precipitation, also sterical hindrances, i.e.

encapsulation of in principle reducible copper species by the Ce-Zr matrix, probably

causes variations in the reduction behaviour of copper.

The degree of reduction was quantified by integrating over the low temperature

reduction peaks and assuming a stoichiometry of Cu:O = 1, not taking into account

(possible) partial reduction to Cu(1+) [42]. The obtained copper content in Bo is given

in Table 3 (Bo-TPR-1 and BoTPR-2). Systematic errors may result from the choice of

baseline and the simplified reduction stoichiometry. The deviation between the fraction

of Cu obtained from TPR-1 (0.16) to that determined by ICP-AES (0.23(5)) indicates

that Cu is not completely reduced upon the first TPR. The reduction degree increases

upon reoxidation and a second TPR, but even then a small amount of Cu seems to

remain in the oxidised state. A similar increase in the extent of Cu reduction from the

first to the second TPR was observed for a sample prepared according to the same

procedure and, in addition, aged in ethanol under reflux conditions, with a somewhat

higher overall reduction degree (TPR-1: 0.18, TPR-2: 0.22). The improvement in

accessibility and reducibility of the Cu could be related to changes in the nanostructure

of the primary particles upon redox cycling. The segregation of copper atoms to the

surface of the mixed oxide particles would be in agreement with XPS results reported

by Qi et al. [5] for a used Cu-Ce-La catalyst sample and Lamonier et al. [43] for a high

temperature treated Cu-Ce sample. Cu segregation is not necessarily in conflict with our

STEM-EDS results, since a slow structural change is not as easily quantified as with

XPS.

The enhanced reducibility upon redox cycling was not observed in a Cu-Ce-Zr mixed

oxide prepared by a nitrate decomposition method that resulted in two XRD-visible

phases (CuO and fluorite, data not shown). Cu-Zn-Al mixed oxides with separate phases

(CuO, ZnO, and presumably XRD-amorphous Al2O3) also usually reach complete

reduction after a single TPR [88]. It may therefore be concluded that the main part of

copper in Cu-Ce-Zr mixed oxides prepared by co-precipitation has similar reduction

behaviour as in Zn-Al supported systems, whereas a certain part of the Cu is

22

incorporated in the fluorite structure (and/or interacting with Ce atoms [42]) and hence

more difficult to reduce. The key factor for such reduction properties appears to be the

existence of the fluorite structure, with the improved reducibility interpreted as a sign of

segregation within the metastable MMO phase.

3.1.6. N2 adsorption-desorption isotherm

Figure 10 shows a N2 adsorption-desorption isotherm representative of the Cu-Ce-Zr

mixed oxides prepared by co-precipitation. The isotherm exhibits a type-IV character

[89] with a hysteresis loop commonly interpreted as a consequence of capillary

condensation in the mesopores present in the material [90]. The inset in Figure 10

displays the pore size distribution of sample Bo obtained by NLDFT analysis of the

adsorption isotherm indicating which pore sizes that contribute to the overall pore

volume. The pore sizes in the MMO material mainly lie in the mesoporous range (2 –

50 nm).

3.2. Effect of ammonia

Table 3 shows the composition of some of the prepared Cu-Ce-Zr mixed metal oxides

determined by ICP-AES. Bo is representative of an open system (setup with oil bath)

preparation with evaporation and refilling of water. The measured metal composition of

Bo is corresponds well with the nominal composition based on the amount of precursor

salts used. Bo-d120 and Bo were prepared similarly and represent an indication of the

reproducibility of the preparation in terms of elemental composition. Bc was prepared

under reflux conditions in the same apparatus. Ammonia formed after urea

decomposition was not evaporated but remained in the solution under these conditions.

As a consequence, ammonia reacts with copper to form complexes that keep a

significant amount of copper in solution, while the ratio between Ce and Zr is not

significantly affected. An open system is therefore preferred for an optimal utilization of

the copper precursor, as well as a straightforward control of the catalyst composition via

the initial precursor concentrations.

23

3.3. Effect of synthesis temperature

Spectrum (b) in Figure 4 belongs to sample Bo-s98, similar to Bo except the

temperature during precipitation and aging being gradually increased to approximately

98 °C. The colour of the precipitate gradually changed from green to dark green/grey.

Copper hydroxide is known to decompose to black CuO when boiling in aqueous

solution, may also do so at lower temperature at a lower rate [91,92]. This explains the

darkening of the solution when approaching the boiling point of water. The additional

XRD peaks could not be ascribed to one, specific compound. Instead, the peaks can be

assigned to a mixture of basic copper hydroxide and hydroxynitrate phases, including

tenorite (CuO), malachite (CuCO3·Cu(OH)2), azurite (2CuCO3·Cu(OH)2) and

gerhardtite (Cu2(OH)3NO3) ([92]). These additional phases disappeared after calcination

of the dried sample (spectrum not shown), to retain the typical peaks of the fluorite-type

structure (such as spectrum (c)).

Sample Bo-s98 has suffered loss of surface area and pore volume compared to Bo

(Table 1). This indicates a larger extent of agglomeration of the primary crystallites,

since these are of similar size. One may suspect that the formation of intermediate Cu

hydroxide species results in a higher degree of Cu segregation in the final MMO, even if

a separate CuO phase could not be detected in XRD after calcination at 350 °C.

Lamonier et al. [43] investigated Cu-Ce mixed oxides with different Cu loading and

related the existence of intermediate copper hydroxynitrates at higher Cu loadings to the

appearance of a CuO phase in the calcined MMO.

Sample BoEth (Table 1) was precipitated in the standard way (as Bo), except the

gradual addition of ethanol instead of water about 1 h after the onset of the precipitation,.

The synthesis temperature was simultaneously reduced to the boiling point of the water-

ethanol mixture (approx. 83 °C at the end of the synthesis). The aging of the precipitate

thus proceeded in boiling solution, and the colour of the suspension changed from green

via grey to red-brown in the course of precipitation. This colour change was also

observed for precipitation in the water-ethylene glycol solution at 100 °C (BoEG-s100).

During washing with ethanol and filtration in contact with air, the colour of the

precipitate changed from red-brown to (light) green. The XRD spectra of the dried as

24

well as calcined samples contained only the typical fluorite peaks and no additional

phases related to Cu species. BoEG-s100 has lower surface area and smaller pore

volume than Bo, as well as a larger primary crystallite size.

The lower Cu content of BoEth (Table 3) is probably caused by the less efficient

removal of ammonia at reduced temperature, or a change in the decomposition rate of

urea with a change in temperature and solvent. McFadyen & Matijevic [93] also

observed a red-brown precipitate during precipitation of colloidal copper hydrous oxide

sols from copper nitrate solutions at 75 °C and high pH. The red-brown material, with

larger particle size than a blue-coloured mixture of copper hydroxide and hydroxynitrate,

was assigned to CuO. Alternatively, the colour change to red-brown could be related to

reduction of Cu2+ to Cu1+, accompanied by oxidation of ethanol, in analogy with

chemical reduction of transition metals by alcohols or polyols. The colour change

during washing may thus be interpreted as the reverse redox process.

Synthesis temperatures too close to the boiling point of the solvent apparently have an

undesirable effect on surface area and pore volume, and possibly also the homogeneity

of the Cu-Ce-Zr mixed oxides. Increasing the temperature during aging enhances the

agglomeration of the primary crystallites and to give lower pore volume and accessible

surface area. Too high temperature during the early stage additionally results in larger

crystallites. An optimal MMO preparation requires optimization of synthesis

temperature and aging time, parameters that are connected to the pH of the solution via

urea decomposition. An elucidation of the reaction path of the precipitation in the

temperature-dependent phase diagram of the Cu/Ce/Zr-nitrate-urea system may explain

the appearance of additional copper phases in the dried precipitate under some

conditions.

3.4. Effect of set-up

According to the ICP-AES data in Table 3, the use of different preparation set-ups had

no significant effect on the resulting catalyst composition. HPo was prepared in the

simple set-up with the hot plate, (Figure 2A), and exhibits within experimental

25

uncertainty the same metal composition as Bo that was prepared in the oil bath (Figure

2B).

In terms of structural characteristics (Table 1), Bo has a higher surface area and pore

volume than HPo, while the size of the primary crystallites is comparable. Both

catalysts have pores in the mesoporous range (2 – 50 nm, Figure 10, pore size distr. of

HPo not shown), but pores around 20 – 30 nm are more abundant in Bo than in HPo.

The higher mesoporosity of Bo is in agreement with the trend obtained by the BJH

method (Table 1). The synthesis set-up therefore affects the agglomeration of the

primary particles during aging rather than the formation of the primary particles. Since

the time scale for temperature increase, onset of precipitation and change of colour were

similar in both set-ups, heat management and hence urea decomposition should be

comparable. This may explain why the primary particle formation is unaffected by the

choice of set-up.

The agglomeration of the primary particles is presumably affected by the flow field of

the synthesis reactor. Different types of stirrers are used in the two set-ups (Figure 2),

that probably create different flow patterns. The flow pattern controls the fluid shear

rate acting on the primary crystallites in the solution and hence affects the rate of

collisions between the particles. An increase in the average fluid shear rate may

decrease the aggregation rate due to reduced contact time between two colliding

particles [94]. Hocevar et al. [21] observed an increase in the catalytic activity of Cu-Ce

mixed oxides with increasing stirring velocity during co-precipitation. The BET surface

area of these oxides was not reported, but beside the effect of enhanced metal

distribution, the surface area of the final oxide powder may also be affected. Kunz et al.

[95] showed that ultrasonic vibration (US) can be used to enhance mixing during

precipitation to give small particles and high surface area. Thus, an increase in the

disruptive hydrodynamic forces acting on the particles in the flow field of the synthesis

reactor, by increased stirring, application of ultrasound or use of a setup with more

efficient mixing (increased average fluid shear rate), should slow down the

agglomeration of primary particles and lead to a more open pore structure.

26

3.5. Effect of drying and calcination conditions

Several samples were calcined under identical conditions but dried under different

conditions. The comparison of sample Bo (drying in static air at 100 °C) to Bo-d150

(drying in static air at 150°C), as well as BoEG-s100 (drying in static air at 100°C) to

BoEG-s100-d250 (drying in flowing air at 250°C, heating rate: 5 °C/min) in Table 1

indicates that the drying temperature affects the surface area and pore structure of the

subsequently calcined MMO. Increasing the drying temperature to 150°C or 250 °C

decreases the BET surface area approximately 29 % relative to Bo and BoEG-s100,

respectively. Figure 11 shows the pore size distributions of BoEG-s100 and BoEG-

s100-d250. The higher drying temperature has resulted in a collapse of pores larger than

10 nm. Pei et al. [96] observed a substantial reduction in pore volume and surface area

when increasing the drying temperature from 90 °C to 250 °C for mesoporous silica

prepared by spray drying. A similar, ultrasonic-treated mixed oxide exhibited a decrease

of the surface area of about 13 % upon drying at 250 °C (flowing air, heating rate: 5

°C/min) relative to 100 °C (static air, 113 m2/g). The treatment in an ultrasonic bath for

2 h hence resulted in a more dense material with lowered pore volume and surface area

already before drying. For more details on ultrasonic treatment we refer to [95].

The pore structure formed by agglomeration during aging in solution is presumably

fragile with weak bonds between the primary particles (probably OH- and a few O-

bridges). Too high drying temperature may then cause collapse of the structure because

of thermal stress and onset of precursor decomposition. The extent of structural collapse

depends on the type of material and the preparation conditions prior to drying. Materials

with high initial surface area and large pore volume may be more brittle in this context

than samples with a dense structure. Drying the precipitates at lower temperature (e.g.

90 – 100 °C) may facilitate consolidation of the pore structure by strengthening of the

bonds between primary particles through a condensation-type reaction that forms

oxygen bridges [97], while thermal stress and precursor decomposition are kept at a

minimum. The densification during following heat treatment is less pronounced.

The gas atmosphere during drying may also surface area and pore volume of the

calcined mixed oxide. For the mixed oxide subjected to ultrasonic treatment before

27

drying, drying in flowing nitrogen at 250 °C resulted in 12 % higher surface area than

drying in flowing air under equivalent conditions. Samples prepared with ethylene

glycol and water at 95 °C were dried both in static air at 100°C (BoEG) and in flowing

nitrogen at 250 °C (BoEG-d250N; analog to BoEG-s100-d250, heating rate: 5 °C/min in

flowing air). The surface areas were similar despite the different drying temperatures, in

contrast to BoEG-s100 vs BoEG-s100-d250. The pore volume of BoEG-d250N is 13 %

lower than BoEG (0.3031 cm3/g), as compared to 34 % lower for sample BoEG-s100-

d250 in flowing air relative to in static air BoEG-s100 (Table 1). The same trend was

also found for a sol-gel prepared Cu-Ce-Zr mixed oxide catalyst dried at 150 °C. Drying

in flowing nitrogen resulted in 29 % higher surface area than flowing air. Xu et al. [98]

observed a surface area increase of about 23 % for nitrogen-dried ZrO2 powder pre-

calcined at 270 °C compared to the air-dried sample.

The effect of the gas atmosphere should be related to precursor decomposition. For the

mixed oxides prepared by co-precipitation, the main decomposition peak in air lies

around 200 °C. Using nitrogen instead of air or oxygen may retard the precursor

decomposition under elevated temperatures and allow consolidation of the pore

structures. At temperatures below the decomposition temperature of remaining

precursor compounds, the impact of the gas atmosphere is negligible. This was

confirmed by Cu-Ce-Zr precipitate samples dried at 100 °C in static air, flowing air and

flowing nitrogen. Approximately the same surface area was measured within

experimental uncertainty for all three drying conditions: 123 m2/g, 119 m2/g and 118

m2/g, respectively.

Bo and HPo were placed directly in a muffle furnace at 350 °C to calcine, whereas Bo-

2C and HPo-2C were heated at 2 °C/min up to 350 °C the muffle furnace. Table 1

shows that the heating rate during calcination has a minor effect on pore volume and

surface area. Drying at 100 °C for about 11 h seems to sufficiently consolidate the

structure to tolerate the different heating rates without significant impact. An increase in

the heating rate and hence in thermal stress and rate of precursor decomposition can

possibly induce a slight densification of the materials [97]. This would depend on the

28

initial pore structure, however, since large surface area materials with open pore

structure appear more susceptible to structural collapse.

3.6. Comparison with conventional co-precipitation

Conventional co-precipitation is normally carried out with NaOH or Na2CO3 as base.

Precursor solution and alkaline solution are prepared separately and then mixed in the

precipitation, either by successively adding the alkaline solution to the precursor

solution or the other way around. Some selected examples from the literature are:

(1) Hocevar et al. [21]: 10 at% Cu in ceria, base: Na2CO3, pH < 5.5, ambient temp.,

calcination at 500 °C for 1 h, surface area: 22.5 m2/g or 0.17 m2/cm3 (with the

densities given in Table 1).

(2) Liu et al. [7,46]: 10 at% Cu in ceria, base: NaOH, pH = 10, ambient temp. (then

1h at 90 °C), calcined at 450 °C for 3 h, surface area: 96 m2/g or 0.72 m2/cm3.

(3) Kim et al.[6]: 20 at% Cu in ceria, base: NaOH, pH = 10, 70 °C, calcined at 500

°C for 5 h, surface area: 91 m2/g or 0.67 m2/cm3.

(4) Shen et al. [24]: 12 at% Cu in ceria, base: Na2CO3, added to precursor, 70 °C,

calcined at 350 °C for 12 h, surface area: 191 m2/g or 1.43 m2/cm3.

(5) Lamonier et al. [43]: 17 at% Cu in ceria, base: NaOH, precursor solution into

NaOH solution, start pH = 14, 60 °C, calcined at 400 °C for 4 h, surface area: 80

m2/g or 0.59 m2/cm3.

The surface area strongly depends on the method used and not only the calcination

conditions. According to the study of Shishido et al. [51] on Cu-Zn oxide catalysts,

homogeneous co-precipitation yields catalyst materials superior to the ones obtained by

conventional co-precipitation.Conventional co-precipitation can under certain

conditions result in surface areas comparable to those obtained by homogeneous co-

precipitation (see example (4)). Homogeneous co-precipitation with urea as base

precursor allows premixing of all components in the initial solution, and no dosing

system is required. Salt and base precursors are mixed on a molecular level in the

solution, whereas efficient mixing of salt and alkaline solution during dosing is

necessary to minimize concentration gradients for conventional techniques [95]. The

29

use of urea (or ammonia for the conventional technique) as alkaline source facilitates

easy removal of the decomposition products, while metal cations from ordinary bases,

such as NaOH or Na2CO3, might be incorporated into the precipitate and hence affect

the properties of the final material.

4. Conclusions

Homogeneous co-precipitation with urea as base precursor results in a Cu-Ce-Zr mixed

oxide of composition close to the nominal (0.23:0.54:0.23), as given by the amount of

precursor salts in solution. The material exhibits high surface area (> 100 m2/g) and

nanocrystalline primary particles (3-5 nm) composed of a single fluorite-type phase

according to XRD and TEM. This type of single-phase material is often denoted as a

solid solution, but STEM-EDS elemental mapping shows that Cu and Zr are

inhomogeneously distributed throughout the ceria matrix as a result of the

heterogeneous nature of the co-precipitation process. The EXAFS analysis indicates the

existence of CuO-type clusters within the ceria-zirconia matrix. We therefore propose

that this type of mixed metal oxide materials is viewed and denoted as heterogeneous

single-phase materials rather than as homogeneous solid solutions. XRD-based

characterization methods, such as XLBA or conventional Rietveld profile fitting, are

found to be insufficient when dealing with this type of heterogeneous single-phase

materials, and complementary characterization techniques such as TEM are needed.

The pore structure and surface areas of the mixed oxide catalysts are affected by

preparation parameters related to the precipitation stage and the subsequent heat

treatment, i.e. drying and calcination. The surface area is governed by the degree of

agglomeration of the primary crystallites.

Temperature programmed reduction (TPR) experiments find most of the Cu atoms to be

reducible and not inaccessibly incorporated into the bulk, which is important since

reduced copper is the active catalyst component in most cases. The reducibility of the

mixed oxide improves from the first to the second reduction, between which the catalyst

was re-oxidized. We attribute this to improved accessibility of the reducible components

(mostly copper) upon heat treatment, as a result of gradual, local phase segregation in

30

the metastable mixed oxide, leading to copper enrichment at the surface. Both XRD and

TEM confirm that the crystal structure of the mixed oxide was preserved upon reduction

and reoxidation. No phase separation could be detected using these two techniques,

which have limitations with respect to local variations in compositions and structure.

The results and discussion given in this paper are not necessarily limited to Cu-Ce-Zr

mixed oxides, but can to some degree apply to other catalyst formulations prepared by

co-precipitation, e.g. other single-phase materials such as Co-Ce-Zr, or multi-phase

materials such as Cu-, Ni- or Fe-based mixed oxides.

Acknowledgements


140022/V30 (RENERGI) and 158516/S10 (NANOMAT). Statoil ASA through the Gas

Technology Center NTNU-SINTEF is also acknowledged for their support. We

gratefully acknowledge the project team at the Swiss-Norwegian Beam Lines (SNBL) at

the ESRF for their assistance with XAS. Elin Nilsen (Department of Materials

Technology, NTNU) is gratefully acknowledged for her assistance with the XRD device.

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Figure captions

Figure 1. Effect of material density and composition on BET data on Ce-Zr mixed

oxides. Reported in the literature [37,39]. The surface area data of Kapoor et al. [37]

(left axis) and Hirano et al. [39] (right axis) are given as surface area ratio normalized

with the values of the pure CeO2 samples (Ce mole fraction x =1). M-BET denotes the

BET surface area normalized with the molar mass (m2/mole). The surface-to-volume

ratio (stv, m2/m3) is calculated using densities based on a linear combination of

tabulated values [99] for CeO2 (7650 kg/m3) and ZrO2 (monoclinic for pure ZrO2: 5680

kg/m3, tetragonal for Ce-Zr mixed oxides: 6100 kg/m3, according to the Ce-Zr phase

diagram [1,2,30]).

Figure 2. The two different set-ups used for catalyst preparation. A: Hot plate equipped

with a magnetic stirrer, B: 500-ml 5-neck glass flask stirred with a blade agitator and

immersed in a stirred oil bath.

Figure 3. TEM images of a representative Cu-Ce-Zr mixed metal oxide powder (sample

Bo). A) As calcined sample (scale bar 50 nm). B) After pre-oxidation (heating rate: 10

°C/min, dwell: 10 min at 500 °C), reduction (up to 300 °C, 5 °C/min) and reoxidation (5

°C/min, 1 h at 350 °C). Scale bar: 5 nm. Insertion: Lattice fourier transform.

Figure 4. XRD spectra of Cu-Ce-Zr mixed metal oxide powders at different stages in

the preparation: A) Bo-dry; dried at 100 °C for about 11 h, not calcined. B) Bo-s98;

aging temperature approx. 98 °C. C) Bo; sample of (A) after calcination at 350 °C for 2

h. D) Bo-redox; sample of (C) after reduction (up to 300 °C, 5 °C/min) and reoxidation

(5 °C/min, 1 h at 350 °C).

Figure 5. Crystal size distribution and microstrain (RMS = root mean square) for the

Cu-Ce-Zr mixed metal oxide (sample Bo) as estimated from XRD data by means of a

software-based X-ray line broadening analysis (XLBA). A) Relative frequency. B)

Cumulative frequency. C) RMS strain.

37

Figure 6. Normalized Cu K-edge XANES spectra of a typical calcined Cu-Ce-Zr mixed

metal oxide powder (sample Bo-d120) and reference materials (Cu foil, CuO and Cu2O

powder). The spectra of Cu2O and Cu foil are shifted upwards along the ordinate axis

for visualisation.

Figure 7. A) Cu K-edge EXAFS spectra and B) corresponding Fourier transforms of a

Cu-Ce-Zr mixed oxide catalyst (sample Bo-d120, k3 weighting, fitting interval: ∆k = 3-

9 Å-1), containing both the experimental and the fitted theoretical curve based on small

atom approximation. The structural model takes into account the first oxygen shell

around Cu.

Figure 8. STEM-EDS elemental mapping of a calcined Cu-Ce-Zr mixed oxide powder

(sample Bo-redox) after reduction (up to 300 °C, 5 °C/min) and reoxidation (5 °C/min,

1 h at 350 °C). A) Mapping area, marked with white frame. B) EDS metal ratio of the

mapping area. C) Cu Kα1. D) Ce Lα1. E) Zr Kα1. F) O Kα1. G) EDS spectrum of the

mapping area. Scale bar: 30 nm in all images.

Figure 9. TPR profiles of a Cu-Ce-Zr mixed metal oxide (sample Bo) obtained during

the first (TPR1) and second (TPR2) in the TGA. Between TPR1 and TPR2 the catalyst

was reoxidised in air (5 °C/min, 1 h at 350 °C).

Figure 10. N2 adsorption-desorption isotherm of a Cu-Ce-Zr mixed metal oxide (sample

Bo) and the corresponding pore size distribution estimated from the adsorption isotherm

by NLDFT.

Figure 11. Effect of the drying temperature on the pore size distribution estimated from

the adsorption isotherm by NLDFT. BoEG-s100 was dried at 100 °C whereas BoEG-

s100-d250 was dried at 250 °C in air.

38

Figure 1

1

1,2

1,4

1,6

1,8

2

0 0,2 0,4 0,6 0,8 1 1,2Ce mole fraction x [ - ]

surf

ace

area

ratio

A/A

(x =

1)

[ - ]

1

3

5

7

9BET [Kap]stv [Kap]m-BET [Kap]BET [Hir]stv [Hir]m-BET [Hir]

39

Figure 2

T

T

M

A) B)

40

Figure 3

A)

B)

41

Figure 4

5 20 30 40 50 60 70 80

2 theta

sign

al [a

. u.]

(a)

(b)

(c)

(d)

42

Figure 5

A)

0

5

10

15

20

25

0 2 4 6 8 10

crystal size estimate [ nm ]

rela

tive

freq

uenc

y [ %

]

B)

0

20

40

60

80

100

0 2 4 6 8 10crystal size estimate [ nm ]

cum

ulat

ive

freq

uenc

y [ %

]

C)

0

0,4

0,8

1,2

1,6

2

0 2 4 6 8 10crystal size estimate [ nm ]

RM

S st

rain

[ 1e

-2, -

]

43

Figure 6

0,00

0,50

1,00

1,50

2,00

8,96 8,98 9,00 9,02 9,04 9,06


norm

. abs

orpt

ion

[ - ]

Bo-d120CuOCu2OCu-foil

44

Figure 7

A)

B)

FT k

3 ·χ

(k)

k3 ·χ

(k)

FT k

3 ·χ

(k)

k3 ·χ

(k)

45

Figure 8

A)

B)

Atomic

[ % ]

Molar frac.

[ - ]

Cu K 10.14 0.27

Zr L 9.91 0.26

Ce L 17.88 0.47

C)

D)

E)

F)

G)

46

Figure 9

-0,4

-0,3

-0,2

-0,1

0

60 100 140 180 220 260

temperature [ °C ]

deriv

ate

wei

ght [

mg/

min

]

TPR1

TPR2

47

Figure 10

20

60

100

140

180

220

0 0,2 0,4 0,6 0,8 1

relative pressure [ - ]

nitr

ogen

ads

orbe

d [ c

m³/g

STP

]

0,000

0,002

0,004

0,006

0,008

0,010

1 10 100pore width [ nm ]

pore

vol

ume

[ cm

³/g S

TP ]

20

60

100

140

180

220

0 0,2 0,4 0,6 0,8 1

relative pressure [ - ]

nitr

ogen

ads

orbe

d [ c

m³/g

STP

]

0,000

0,002

0,004

0,006

0,008

0,010

1 10 100pore width [ nm ]

pore

vol

ume

[ cm

³/g S

TP ]

48

Figure 11

0

0,002

0,004

0,006

0,008

0,01

0,012

1 10 100

pore width [ nm ]

pore

vol

ume

[ cm

³/g S

TP ]

BoEG-s100

BoEG-s100-d250

49

Table 1. Structural parameters of Cu-Ce-Zr mixed metal oxides. For simplification, the

densities used for the calculation of the surface-to-volume ratio (stv) and the pore

volume in cm3/cm3 (based on the sample volume) are estimated by linear combination

of the tabulated densities of CeO2 (7650 kg/m3), ZrO2 (5680 kg/m3, tetragonal phase)

and CuO (6310 kg/m3).

BETa stv pore volume XRD crystal size

[ m2/g ] [ m2/mm3 ] [ cm3/g ] [ cm3/cm3 ] [ nm ]

Bc 121.13 0.86 0.1347 0.96

HPo 89.24 0.62 0.1288 0.90 3.1

HPo-2C 89.26 0.62 0.1352 0.95

Bo-d120b 145.20 1.01 0.2720 1.90 3.8

Bo-2C 163.79 1.15 0.2632 1.84 3.4

Bo 154.42 1.08 0.2678 1.87 3.3

Bo-dry 3.1

Bo-redox 141.69 0.99 0.2464 1.72 3.6

Bo-d150 109.34 0.2344

BoEth 110.49 0.78 0.1943 1.37 5.1

BoEth-dry 4.2

BoEG-s100 170.65 1.19 0.3345 2.34 3.1

BoEG-s100-dry 2.8

BoEG-s100-d250 120.76 0.85 0.2201 1.54 3.5

Bo-s98 115.29 0.1893 3.6

a Estimated error of BET smaller than 5 % of the total value. b Sample Bo-d120 suffered a temporary temperature hot spot in the initial period of the precipitation, and

the nominal drying temperature was higher than for Bo (approximately 120 °C in a different type of

drying furnace); similar XRD profile as Bo, with fluorite structure only, as well as similar N2 isotherm

and pore size distribution.

50

Table 2. Structural parameters of the first oxygen coordination shell around the Cu

central atom in nanocrystalline Cu-Ce-Zr mixed metal oxide (sample Bo-d120, fitting

interval: ∆k = 3-9 Å-1) and the reference compound CuO (k3 weighting, fitting interval:

∆k = 3-12 Å-1) obtained from a standard EXAFS analysis at the Cu K-edge. The

EXCURVE parameter AFAC reflects the amplitude reduction factor. The AFAC value

used for fitting the oxide sample was obtained by fitting the reference material CuO

with k3 weighting of the EXAFS function, but deviated only slightly from the

theoretical value 1. The parameters given for Bo-d120 result from fitting with both k1

and k3 weighting.

AFAC 0.94

E0 R1 N1 ∆σ2

[ eV ] [ Å ] [ - ] [ Å2 ]

Bo-d120 -10.5 ± 1.0 1.92 ± 0.01 2.5 ± 0.5 0.003 ± 0.002

CuO -15.2 ± 0.8 1.96 ± 0.01 3.9 ± 0.2 0.008 ± 0.001

CuOa 1.96 4.0

CeO2a 2.34 8.0

a Crystallographic data obtained from literature for CuO [101] and CeO2 [102].

51

Table 3. Composition of Cu-Ce-Zr mixed metal oxides with ICP-AES. The deviation

between the parallel runs was within 5 %. The estimated detection limit is 2000 mg/kg

for Ce, 300 mg/kg for Cu and 200 mg/kg for Zr (based on sample weight). In addition,

the copper amount determined by TPR-TGA (integration over the TPR peaks) is also

included.

Catalyst composition - molar fraction of metals

[ - ]

Cu Ce Zr

Nominal comp. 0.23 0.54 0.23

Bc 0.10 0.63 0.27

HPo 0.22 0.55 0.23

Bo-d120 0.23 0.54 0.23

Bo 0.23(5) 0.54 0.22(6)

Bo-TPR 1 0.16a

Bo-TPR 2 0.20

BoEth 0.19 0.58 0.23

BoEG-s100 0.22 0.55 0.23

a The first TPR was reproduced three times for the freshly calcined catalyst. The deviation between the

reduction degrees determined for these three runs was less than 1 % (with 0.16 corresponding to 100 %).

Paper IV

Comparison of Cu-Ce-Zr and Cu-Zn-Al mixed oxide catalysts for

water-gas shift

Manuscript submitted.

1

Comparison of Cu-Ce-Zr and Cu-Zn-Al mixed oxide catalysts for water-gas shift

Florian Huber, Hilde Meland, Magnus Rønning, Hilde Venvik*, Anders Holmen

Department of Chemical Engineering, Norwegian University of Science and Technology

(NTNU), N-7491 Trondheim, Norway


Fax: +47-73595047

Cu-Zn-Al and Cu-Ce-Zr mixed oxide catalysts were prepared by two different methods,

co-precipitation and flame spray pyrolysis. The performance of the catalysts was

evaluated using the water-gas shift reaction with and without CO2 and H2 added to the

feed. Cu-Ce-Zr catalysts are found not to be superior to Cu-Zn-Al catalysts in terms of

initial activity and short-term stability. Their apparent activation energy appears to be

less affected by increased concentrations of CO2 and H2.

KEY WORDS: Cu-Zn-Al; Cu-Ce-Zr; mixed metal oxides; water-gas shift.

1. Introduction

The water-gas shift (WGS) reaction is one of the oldest catalytic processes employed in

the chemical industry. There is renewed interest in this reaction because of its relevance

for producing hydrogen for use in fuel-cell systems as well as its key role in automotive

exhaust processes, since the hydrogen produced is an efficient reductant for NOx

removal [1]. Improved WGS catalysts with high activity at relatively low temperatures

and better stability than commercial Cu-Zn-Al catalysts are needed. Ceria is at present

one of the most investigated metal oxides, especially in connection with its oxygen

storage capacity (OSC) applied in three-way catalyst systems [2]. CeO2 is reported to

improve the stability of classical Cu-Zn-Al formulations [3], as is ZrO2 [4]. In addition,

the application of Ce-Zr mixed oxides has become attractive [5], since CeO2 doped with

Zr shows improved OSC and better stability [2]. In the present study, Cu-Zn-Al and Cu-

Ce-Zr mixed metal oxide (MMO) catalysts were prepared both by co-precipitation and

2

flame spray pyrolysis, and investigated under WGS conditions. The aim of the study

was to compare classical Cu-Zn-Al formulations to novel Cu-Ce-Zr MMO catalysts

from different preparation procedures at various reaction conditions.

2. Experimental

Four MMO catalysts were prepared. CuZn-CP was prepared by co-precipitation from

nitrate salts with (NH4)2CO3 in aqueous solution at ambient temperature according to a

patent of Norsk Hydro, dried at 90 °C and calcined at 350 °C (3 °C/min, 1h) [6]. CuCe-

UCP was prepared by homogeneous co-precipitation from nitrate precursors with urea

in a ethylene glycol-water mixture at 95 °C, dried at 100 °C and calcined at 250 °C (2

°C/min, 30 min) [7]. CuZn-FSP and CuCe-FSP were prepared by flame spray pyrolysis

(FSP) of organo-metallic salts dissolved in toluene [8]. The MMO catalysts were

characterised by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-

AES), X-ray diffraction (XRD) and N2 adsorption-desorption (BET). For more details

on preparation and characterization of the catalysts we refer to [7] and [8] for the co-

precipitated and flame spray pyrolysed catalysts, respectively. The Cu dispersion of Cu-

Zn-Al catalysts has previously been determined by N2O titration (e.g. 8 % for CuZn-CP)

[3,9]. However, N2O titration was not applied for Cu-Ce-based catalysts, since

contributions from ceria may obscure the results [10].

The WGS activity was tested at atmospheric pressure in an externally heated tubular

fixed-bed reactor setup with on-line GC analysis, with the catalysts pre-reduced in

H2/N2. More details on the reduction and WGS testing of CuZn-CP/CuCe-UCP and

CuZn-FSP/CuCe-FSP are given in [7] and [8], respectively. Three different feed

compositions were applied; CuZn-CP and CuCe-UCP were investigated under a simple

WGS reactant mixture (25/125/350 Nml/min CO/H2O/N2, catalyst amount: 0.10 g) and

a simulated reformate product mixture (25/125/60/175/115 Nml/min

CO/H2O/CO2/H2/N2, 0.20 g). CuZn-FSP and CuCe-FSP were studied under a simple

WGS reactant mixture (50/100/50 Nml/min CO/H2O/N2, 0.10 g). The selectivity was

100 % in all measurements. Activation energies (Ea) were determined by the integral

method (irreversible reaction, first order in CO and zero order in H2O, plug-flow) for

3

the simple WGS reactant mixture applied to CuZn-CP and CuCe-UCP, and by the

differential approach with an Arrhenius plot for the two other WGS conditions.


The chemical composition and structural characteristics of the catalysts are given in

Table 1. Figure 1 shows the WGS activity of the flame-sprayed (A) and co-precipitated

(B) Cu-Zn-Al and Cu-Ce-Zr MMO catalysts as a function of reaction temperature at

different WGS feed conditions. Based on the catalyst mass, the Cu-Ce-Zr catalysts show

either lower, similar or higher WGS activity compared to classical Cu-Zn-Al

formulations. Cu-Ce-Zr catalysts appear not to be generally superior to classical Cu-Zn-

Al formulations. Instead, the difference in activity seems to be related to structural

characteristics of the catalysts as well as the reaction conditions. Koryabkina et al. [10]

suggest that neither ceria nor ZnO promotes the WGS activity of copper. In an earlier

study, a promoting effect of ceria on the WGS turnover frequency of copper could not

be confirmed [3]. The activity curves for Cu-Ce-Zr in Figure 1A and 1B in the low CO

conversion range (< 20 %) can be normalized by the ratio of the Cu content in the

corresponding Cu-Zn-Al and Cu-Ce-Zr samples. The Cu-Ce-Zr catalysts then show a

similar or higher relative activity as compared to Cu-Zn-Al, indicating a good utilization

of the active material. Beside a somewhat lower Cu content in the Cu-Ce-Zr catalysts

studied here, the system has a general disadvantage in the higher molar masses of Ce

and Zr relative to Zn and Al using mass-based criteria.

The apparent activation energies (Ea) for the MMO catalysts under the chosen WGS

conditions are shown in Table 1. Figure 1C shows the Arrhenius-type plot used to

estimate Ea for CuZn-FSP/CuCe-FSP under simple WGS reactant conditions and CuZn-

CP/CuCe-UCP under simulated reformate feed conditions. The estimated activation

energies vary between 35 and 48 kJ/mole for the simple WGS reactant mixtures. Ea

increases from 44 to 83 kJ/mole with the addition of CO2 and H2 into the WGS feed for

CuZn-CP. This can be explained by a change in the rate-determining step [11]. Ea

remains unaffected by the addition of CO2 and H2 for CuCe-UCP, similar to Cu-Ce-Zr

MMO/carbon nanofiber composite catalysts [7]. Mann et al. [12] obtained activation

energies of 48 and 86 kJ/mole for forward and reverse WGS, respectively, over a Cu-

4

Zn-Al catalyst. A corresponding trend for Cu-Zn-Al MMO catalysts is found from other

reports [10,13-15]. Thus, Cu-Zn-Al MMO catalysts appear to exhibit a higher apparent

activation energy under reaction conditions where reverse WGS has to be considered,

e.g. simulated reformate feed. Consistent with our results, activation energies of 56 and

32 kJ/mole have been obtained for Cu-Ce and Cu-Ce-Al MMO catalysts, respectively,

under reformate feed conditions [10]. Ea increased from 54 to 71 kJ/mole for a Cu-Ce-

La MMO catalyst with the addition of CO2 and H2, while for a different metal ratio and

lower calcination temperature, 60 kJ/mole was obtained under a different simulated

reformate feed composition [16], as well as 19 and 30 kJ/mole under simple WGS

conditions [17]. It therefore appears that Cu-Ce-based catalysts do not display the same

increase in apparent activation energy as Cu-Zn-based catalysts upon increased

concentration of hydrogen and CO2 in the feed. Further studies that ensure the

comparability and consider the impact of additional factors such as temperature range,

conversion level, initial deactivation and Ea estimation method are necessary.

Figure 2 shows the normalized short-term deactivation behaviour of the Cu-Zn-Al and

Cu-Ce-Zr catalysts under the different WGS feed conditions and constant reaction

temperatures (300, 310 or 350 °C). All catalysts exhibit a similar deactivation behaviour

between 300 and 350 °C, independent of the feed conditions. The Cu-Zn-Al and Cu-Ce-

Zr catalysts do not differ much with respect to short-term deactivation. CuZn-FSP

shows a somewhat higher deactivation than CuCe-FSP, but has a higher surface-to-

volume ratio (Table 1). Hence, stronger deactivation of the flame-sprayed catalyst may

be related to a higher and more unstable surface, prone to sintering. CuZn-CP exhibits a

slightly lower deactivation than CuCe-UCP at 300 °C under simple WGS feed

conditions. Beside the different chemical composition of the two catalyst formulations,

a higher surface-to-volume ratio could be connected to the higher deactivation.

In summary, the performance of the catalysts depends on reaction conditions and

catalyst properties. Cu-Ce-Zr catalysts appear not to be generally superior to Cu-Zn-Al

catalysts in terms of activity or short-term stability. Their apparent activation energies

appear, however, to be less affected by the increased concentrations of CO2 and H2.

5

Acknowledgements

This work was supported by the Research Council of Norway and Statoil ASA through

the Gas Technology Center NTNU-SINTEF. Cathrine Bræin Nilsen (Depart. of Chem.

Eng., NTNU) and Bjørnar Arstad (Sintef Materials and Chemistry) are gratefully

acknowledged for preparing one of the copper catalysts and performing the WGS

measurements on the FSP samples, respectively. Tue Johannessen et al. are gratefully

acknowledged for the flame-spray synthesis.

References

[1] D. Andreeva, V. Idakiev, T. Tabakova, L. Ilieva, P. Falaras, A. Bourlinos, A.

Travlos, Catal. Today 72 (2002) 51.

[2] A. Trovarelli, Catalysis by Ceria and Related Materials, Catalytic Science Series,

Vol. 2 (Imperial College Press, London, 2002).


Today 100 (2005) 249.


11.

[5] R. Di Monte, J. Kašpar, J. Mater. Chem. 15 (2005) 633.

[6] L.A. Kristiansen, US Patent US4308176, 1981.

[7] F. Huber, Z. Yu, J. Walmsley, D. Chen, H. Venvik, A. Holmen, Appl. Catal. B,

in press.

[8] H. Meland, T. Johannessen, B. Arstad, H.J. Venvik, M. Rønning, A. Holmen,

Stud. Surf. Sci. Catal., accepted.

[9] F. Huber, Z. Yu, S. Lögdberg, M. Rønning, D. Chen, H. Venvik, A. Holmen,

Catal. Lett., in press.


Catal. 217 (2003) 233-239.

[11] M.S. Spencer, Catal. Lett. 32 (1995) 9-13.

[12] R.F. Mann, J.C. Amphlett, B. Peppley, C.P. Thurgood, Int. J. Chem. React. Eng.

2 (2004) A5.

[13] R.L. Keiski, O. Desponds, Y.F. Chang, G.A. Somorjai, Appl. Catal. A 101 (1993)

317-338.

6

[14] Y. Choi, H.G. Stenger, J. Power Sources 124 (2003) 432-439.

[15] C.V. Ovesen, B.S. Clausen, B.S. Hammershøi, G. Steffensen, T. Askgaard, I.

Chorkendorff, J.K. Nørskov, P.B. Rasmussen, P. Stoltze, P. Taylor, J. Catal. 158

(1996) 170-180.



7

Figure Captions

Figure 1. A) WGS activity of CuZn-FSP and CuCe-FSP as function of reaction

temperature (50/100/50 Nml/min CO/H2O/N2, catalyst amount: 0.10 g). CuCe-FSP-

norm corresponds to CuCe-FSP normalized with the ratio of the Cu content in CuZn-

FSP (28 wt%) and CuCe-FSP (10 wt%). B) WGS activity of CuZn-CP and CuCe-UCP

as function of reaction temperature for two different feeds: SWR (25/125/350 Nml/min

CO/H2O/N2, 0.10 g) and SRP (25/125/60/175/115 Nml/min CO/H2O/CO2/H2/N2, 0.20

g). CuCe-UCP-SWR/SRP-norm corresponds to CuCe-UCP-SWR/SRP normalized with

the ratio of the Cu content in CuZn-CP (24 wt%) and CuCe-UCP (10 wt%). C)

Arrhenius-type plot for CuZn-CP/CuCe-UCP (25/125/60/175/115 Nml/min

CO/H2O/CO2/H2/N2, 225 – 275 °C) and CuZn-FSP/CuZn-FSP (50/100/50 Nml/min

CO/H2O/N2, 180 – 310 °C) with all CO conversions lower than 13 %.

Figure 2. Normalized short-term deactivation behaviour, based on the initial CO

conversion. A) CuZn-FSP/CuCe-FSP at approx. 310 °C (50/100/50 Nml/min

CO/H2O/N2). The initial CO conversion is obtained from the y-interception of the linear

fit at TOS = 0 h. B) CuZn-CP/CuCe-UCP at 300 °C (SWR: 25/125/350 Nml/min

CO/H2O/N2) and 350 °C (SRP: 25/125/60/175/115 Nml/min CO/H2O/CO2/H2/N2). The

initial CO conversion is calculated as the average of the first three measurements

obtained at intervals of 3 minutes.

8

Figure 1

A)

0

3

6

9

12

15

180 200 220 240 260 280 300 320

temperature [ C ]

CO

con

vers

ion

[ % ]

CuZn-FSP

CuCe-FSP

CuCe-FSP_norm

B)

0

20

40

60

80

100

160 210 260 310 360temperature [ C ]

CO

con

vers

ion

[ % ]

CuCe-UCP_CO+H2O CuZn-CP_CO+H2OCuCe-UCP_reformate CuZn-CP_reformateCuCe-UCP_CO+H2O_norm CuCe-UCP_reformate_norm

9

C)

-1

0

1

2

3

0,0016 0,0018 0,002 0,0022 0,0024

1/T [ 1/K ]

ln(C

O c

onve

rsio

n) [

- ]

CuCe-UCP

CuZn-CP

CuZn-FSP

CuCe-FSP

10

Figure 2

A)

0

0,2

0,4

0,6

0,8

1

1,2

0 2 4 6 8 10 12

TOS [ h ]

norm

aliz

ed C

O c

onve

rsio

n [ -

]

CuZn-FSP

CuCe-FSP

B)

0

0,2

0,4

0,6

0,8

1

1,2

0 4 8 12 16 20

TOS [ h ]

norm

aliz

ed C

O c

onve

rsio

n [ -

]

CuCe-UCP_reformate

CuZn-CP_reformate

CuCe-UCP_CO+H2O

CuZn-CP_CO+H2O

Table 1. Chemical composition, structural characteristics and apparent WGS activation energies (Ea) of the catalysts.

catalyst composition

molar metal fraction a BET b csurface

volume XRD d Ea

Cu Ce or Zn Zr or Al crystal size CO+H2O reformate

[-] [-] [-] [m2/g] [m2/mm3] [nm] [kJ/mole]

CuZn-FSP 0.32(7) 0.43(6) 0.23(7) 160 0.86 n.d. 37

CuCe-FSP 0.23 0.54 0.23 93 0.65 n.d. 48

CuZn-CP 0.27 0.40 0.33 80 0.41 14 44 f 83

CuCe-UCP 0.23 0.54 0.23 164 1.15 2 32 f 33

a Nominal composition for CuZnAl-FS and CuCeZr-FS, composition of CuZnAl-CP determined by ICP-AES, nominal composition of CuCeZr-UCP verified by

ICP-AES. b Performed on the calcined samples, after drying at 150 °C under vaccum for 1.5 h. c Surface-to-volume ratio estimated by linear combination of tabulated densities for CeO2 (7650 kg/m3), ZrO2 (5680 kg/m3, tetragonal phase), CuO (6310 kg/m3),

ZnO (5660 kg/m3) and γ-Al2O3 (3650 kg/m3). d Crystal size of CuZn-CP estimated with Scherrer equation for the Cu (111) reflection taking into account the thickness of the passivation layer [3,9]; for CuCe-

UCP estimated by software-based X-ray line broadening analysis [7]. f Ea for the temperature range 165 – 210 °C.

Paper V

Nanocrystalline Cu-Ce-Zr mixed oxide catalysts for water-gas shift:

Carbon nanofibers as dispersing agent for the mixed oxide particles

Applied Catalysis B: Environmental, accepted.

1

Nanocrystalline Cu-Ce-Zr mixed oxide catalysts for water-gas shift:

Carbon nanofibers as dispersing agent for the mixed oxide particles

Florian Hubera, Zhixin Yua,1, John C. Walmsleyb, De Chena, Hilde J. Venvika,*, Anders

Holmena

aDepartment of Chemical Engineering, Norwegian University of Science and

Technology (NTNU), N-7491 Trondheim, Norway bSINTEF Materials and Chemistry, N-7465 Trondheim, Norway


Fax: +47-73595047 1 Present address: SINTEF Materials and Chemistry, N-7465 Trondheim, Norway

Nanocomposite catalysts containing carbon nanofiber (CNF) and Cu-Ce-Zr mixed metal

oxide (MMO) have been prepared by homogeneous co-precipitation with urea. The

water-gas shift reaction (WGS) has been used as test reaction. The CNF-containing






energies are, however, obtained for the CNF-free and CNF-containing materials. We

can not exclude that the CNF material acts as reaction promoter under certain conditions,

but suggest that the impact of CNF addition on the precipitation of the mixed oxide

particles, and hence the catalytic activity relative to the CNF-free MMO, should also be

considered. CNF may be regarded as inert dispersing agent material improving the

precipitation of the MMO under conditions where the co-precipitation of the MMO

precursors does not result in materials with high surface area.

KEY WORDS: Carbon nanofibers; dispersing agent; Cu; Ce; Zr; mixed metal oxides;

nanocomposites; homogeneous co-precipitation; water-gas shift.

2

1. Introduction

Mixed metal oxide (MMO) catalysts containing Cu, Ce and Zr are widely used in

environmental heterogeneous catalysis. Examples include reactions relevant in

hydrogen production for fuel cell applications (methanol steam reforming [1-4], water-

gas shift [5-7], selective CO oxidation[8-13]), methanol synthesis [14,15] and selective

catalytic NO reduction for automotive emission control [16]. Copper in its reduced state

is typically held as the active catalyst component in these materials. Ceria and zirconia

as support materials enhance catalytic activity and stability via metal-support

interactions and/or improved dispersion of the active metal component [2,3,17-19]. A

large and stable catalyst surface area, as well as a homogeneous metal distribution, are

prerequisites for high catalytic activity.

Carbon nanofibers (CNF) have been attracting increasing interest as catalyst support

material over the recent years [20]. The material exhibits high mechanical strength, high

electrical conductivity and medium to high specific surface area. Moreover, the metal

dispersed on CNF can be readily recovered from a spent catalyst by burning off the

nanofibers. In MMO formulations, CNF can be incorporated as dispersing agent for the

metal oxide particles. By decreasing the degree of agglomeration of the metal oxide

particles, the exposed metal surface area and thereby the number of active surface sites

can be increased. CNF is in some cases proposed to serve as adsorbent and activator for

the reactant molecules [21,22].

Homogeneous alkalinization via urea hydrolysis is a useful technique for preparation of

MMO with high surface area, homogeneous metal distribution and well-defined particle

size and shape [5,7,23-26]. In principle, urea decomposes at elevated temperatures in a

two-step reaction releasing ammonium and carbonate ions into the metal salt solution

accompanied by a simultaneous increase in the pH, which leads to the precipitation of

metal basic carbonates [23,25,27]:

22 2 2 4 3( ) 2 2CO NH H O NH CO+ −+ → + (1)

The decomposition rate strongly depends on the reaction temperature [25,27].

3

In terms of synthesizing MMO ‘solid solutions’, one has to realize that co-precipitation

in general has to be regarded as a heterogeneous process. A phase containing only one

cation first nucleates to serve as locus for the heterogeneous nucleation of a second solid.

Further growth proceeds incorporating the different cations at different rates [24]. As a

result, the internal chemical composition of such composites usually varies from the

center to the periphery [23].

In the present study, a series of CNF-containing Cu-Ce-Zr mixed metal oxide catalysts

were prepared by homogeneous co-precipitation. The water-gas shift (WGS) reaction

was used as test reaction. The catalytic performance of the CNF-containing samples was

compared with that of the corresponding CNF-free Cu-Ce-Zr mixed oxide sample, thus

examining the effect of the carbon nanofibers in the CNF-metal oxide nanocomposite

catalyst.

2. Experimental

2.1. Chemicals

Copper(II)-nitrate-trihydrate (> 99 %), cerium(III)-nitrate-hexahydrate (> 99.5 %) and

zirconyl(IV)-nitrate-hydrate (> 99.5 %) were purchased from Acros Organics. Urea (>

99.5 %) was purchased from Merck. Ethylene glycol (EG) (> 99.5 %) was purchased

from Fluka. All chemicals were used as-received. All catalyst preparations were

conducted with deionized water.

2.2. Catalyst preparation

CNF with platelet structure were synthesized by catalytic chemical vapor deposition as

previously described [28], from CO/H2 (40/10 ml/min) decomposition on Fe3O4

nanoparticles at 600 °C. Such a procedure results in CNF with the graphite sheets

oriented perpendicular to the fiber axis, as shown in Figure 1, and average diameter of

approx. 116 nm. The XRD diffraction pattern exhibits sharp, graphitic (002) reflections,

indicating a high degree of structural order. The value of d-spacing examined from the

(002) line is 3.45 Å, close to that of graphite. The as-grown CNF were boiled in

concentrated nitric acid for 3 hours, followed by washing with water and drying

overnight in a muffle furnace at 120 °C. After such oxidation treatment, the remaining

4

amount of catalyst (Fe2O3) is decreased from 3.1% to 0.6%, and surface oxygen groups

have been introduced at the CNF surface to increase the hydrophilicity.

In the MMO/CNF synthesis, a total amount of 0.105 mole of Cu-, Ce- and Zr-salt

(nominal composition: Cu:Ce:Zr = 0.23:0.54:0.23) was dissolved in 300 ml of a mixture

of 40 vol% ethylene glycol (EG) in water at ca. 30 °C under stirring. The CNF (11.1,

22.2 and 44.2 g CNF/mole total metal) was suspended in 150 ml 40 vol% EG-water and

treated in an ultrasonic bath before adding to the salt solution. Finally, 0.904 mole of

urea was added to the mixture, resulting in a pH of approx. 2 of the mixture. The

mixture was poured in a 500-ml 5-neck glass flask, placed in a hot oil bath and rapidly

heated up to 95 °C under vigorous stirring with a blade agitator at approx. 750 rpm. The

temperature of the solution was monitored with an immerged thermometer.

The setup was operated in an open mode in order to avoid loss of Cu through

complexation with NH3 in the solution [29]. Water was allowed to evaporate through

the unused flask necks, and the loss was compensated by continuous refilling with water

(at ambient temperature, not pre-heated). The addition of water was carried out in the

same way for all catalyst preparations, but variations may influence the formation of the

precipitate and thereby the final material properties. The investigation of this parameter

was not part of this study.

The mixture was kept in the oil bath for a total period of 8 hours, including heating (ca.

15 – 20 min), precipitation and aging. The suspension was then removed and rapidly

cooled down to room temperature using cold water. The solid precipitate was filtered

off and washed three times with 200 ml of deionized water (40 – 50 °C). The precipitate

was dried for about 12 h at 100 °C and calcined for 30 min at 250 °C (heating up: 2

°C/min; cooling down: ca. 2 °C/min) in a muffle furnace.

The CNF-free sample was prepared in the same way as the three CNF-containing

samples, except that the nitrate precursors were dissolved in 450 ml 40 vol% EG-water.

Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) has been used

to determine the actual amount of copper, cerium and zirconium on CNF-free MMO

5

after calcination. The actual composition of the MMO catalysts was in good agreement

with the nominal composition: Cu:Ce:Zr ≈ 0.23:0.54:0.23 [29].

The catalysts are denoted CNF00, CNF07, CNF13 and CNF24 with the digits

specifying the CNF content (in wt%) in the CNF-metal oxide nanocomposites. 11.1,

22.2 and 44.2 g CNF/mole total metal corresponds to 7.2, 13.4 and 23.6 wt% CNF in

the nanocomposite, respectively. Pure platelet CNF after oxidation treatment is denoted

as P-CNF.

2.3. Characterization techniques

Nitrogen adsorption-desorption isotherms were measured using a Micromeritics TriStar

3000 instrument. The data were collected at -196 °C. The BET surface area was

calculated in the relative pressure interval ranging from 0.01 to 0.30. The pore volume

was estimated by the Barrett-Joyner-Halenda (BJH) method [30] as the adsorption

cumulative volume of pores between 1.7 and 300.0 nm width. This method is based on

the assumption of cylindrical pores, and the capillary condensation in the pores is taken

into account by the classical Kelvin equation.

XRD data were recorded on a Siemens diffractometer D-5005 (dichromatic CuKα+β-

radiation). Average crystal size estimates and crystal size distributions for the mixed

oxide powders were obtained from a software-based X-ray line broadening analysis

(XLBA). Selected experimental XRD peaks ((111), (200) and (220)) were simulated by

means of the software package Profile [31], using the Pearson VII model function,

removing the contribution of CuKβ to the peak intensities. The program Win-crysize

[32], utilizing the Warren-Averbach method [33], was then used to estimate the

crystallite size taking into account contributions from microstrain (scaled as mean

square root of the average squared relative strain). Contributions from instrumental line

broadening were removed using LaB6 as a reference (selected peaks: (110) at 30.38°,

(111) at 37.44° and (210) at 48.96°).

A Thermogravimetric Analyser (TA Instruments TGA Q500) in a temperature-

programmed oxidation mode (TPO) was applied to check the metal oxide content in the

6

nitrate precursors and the weight loss of the dried and calcined samples. A heating rate

of 2 °C/min and a gas flow composed of 90 ml/min air and 10 ml/min nitrogen (for

flushing the weight chamber) was applied over approx. 15 mg of sample for each TPO

uptake.


samples were put into sealed glass containers containing ethanol, and placed in an

ultrasonic bath to disperse the catalyst. A drop of the resulting suspension was placed

onto a holey carbon film, supported on a titanium mesh grid and dried. Conventional

TEM images were recorded onto a CCD camera. Samples were also examined in

scanning transmission electron microscope (STEM) mode, with a nominal probe size of

approx. 0.7 nm, acquiring bright field and dark field STEM images. Energy dispersive

x-ray spectroscopy (EDS) analysis and mapping were performed using an Oxford

Instruments INCA system, employing drift compensation to correct for sample

movement during the acquisition of maps.

2.4. Catalyst testing

The water-gas shift (WGS) activity was tested at atmospheric pressure in an externally

heated tubular fixed-bed reactor made of quartz with inner diameter of 10 mm. The

catalyst particles (50 – 125 µm) were loaded onto quartz wool. The quartz reactor was

heated inside an electric furnace. A carbonyl trap was connected to the outlet of the CO

gas cylinder to remove possible iron carbonyls. The gas flows were controlled by mass

flow controllers. Water was dosed with a liquid flow controller from a He-pressurized

cylinder. The water was injected into a vaporizing unit, and the steam was then mixed

into the gas stream. A microchannel heat exchanger (Forschungszentrum Karlsruhe)

operated with air was used for condensation of liquid products prior to analysis of dry

product gas with an on-line Agilent G2891A Micro GC equipped with thermal

conductivity detection (TCD). The gas composition in the dry gas was quantified by

using a range of certified calibration gases as external standards.

The catalyst samples were pre-reduced in 10 % H2 in nitrogen at 250 °C for 1 h (total

flow: 300 Nml/min; heating rate: 5 °C/min), and cooled in the H2/N2 atmosphere to the

7

starting temperature of the activity measurements. The water-gas shift reaction was

studied under two different feed conditions. A simple WGS reactant mixture (25

Nml/min CO, 125 Nml/min H2O and 350 Nml/min N2) was fed over 0.1 g (approx. 0.1

ml) of catalyst. A simulated reformate product mixture (25 Nml/min CO, 125 Nml/min

H2O, 60 Nml/min CO2, 175 Nml/min H2 and 115 Nml/min N2) was applied in

experiments with 0.2 g catalyst (approx. 0.2 ml). The initial stabilization of the feed was

carried out in the by-pass line. The temperature was increased in steps of 15 – 25 °C

during the activity measurements, and stabilized for 15 min. (approx. 30 min for the

starting temperature) before three GC analyses were taken at three minute intervals. The

short-term deactivation behaviour of the catalysts was recorded at the final temperature

for about 15 h.

The CO conversion (XCO) for the feed containing only CO and H2O (and balance N2)

can be calculated from the CO and CO2 concentrations in the dry exit gas measured at

each temperature:

2

2

100%( )CO

COXCO CO

= ⋅+

(2)

For the simulated reformate feed, the calculated CO conversion must take into account

the initial composition of the feed gas:

0

2 2,0

( ) 100%( )CO

CO COXCO CO CO

−= ⋅+ −

(3)

The selectivity was 1 for all activity measurements. Trace CH4 was detected during the

temperature scan as well as the following short-term deactivation experiment.

Apparent activation energies (Ea) were estimated from the temperature scans. For the

CO and H2O containing feed, the activation energy was determined by the integral

method assuming an irreversible reaction of first order in CO and zero order in H2O as

well as plug-flow conditions. According to Keiski et al. [34], the water-gas shift

8

reaction is not a simple first-order reaction in the CO activity, however, the first-order

rate equation describes the phenomenon quite well. The model was fitted to the

experimental data in the temperature range 165 – 210 °C applying the least-square

method. The CO conversion was below 30 % and hence the approach to WGS

equilibrium composition less than 1 % legitimating the use of an irreversible reaction

model. The approach to equilibrium is calculated with the exit gas concentrations and

the equilibrium constant:

2 2

2

100%eq

CO HEquilibrium approachK CO H O

⋅= ⋅⋅ ⋅

(4)

For the simulated reformer feed, the activation energies were estimated from an

Arrhenius plot in the temperature range 225 – 275 °C by the differential approach, with

the CO conversion always lower than 15 %. Possible heat and transport limitations were

considered by applying well-known empirical evaluation criteria [35].

The standard deviation of the CO conversion was estimated to be 4 %. The carbon

balance for CO and CO2 was within approx. 1 %, except for the three highest

temperature levels in the experiments with the simulated reformate feed. Here, the

negative deviation from the initial carbon content gradually increased to 3 %, as a result

of a systematic error connected to the GC calibration of CO2.

3. Results

3.1. Catalyst characterization

Figure 2 displays TPO profiles of the dried catalyst materials CNF00 and CNF13 as

well as calcined CNF13. The CNF-free MMO material shows a major decomposition

peak around 200 °C corresponding to about 10 % weight loss. This peak disappeared for

the corresponding calcined sample (data not shown) verifying that most of the precursor

remainding in the dried sample was removed by the calcination treatment. In addition, a

small peak corresponding to approx. 1 % weight loss is observed around 400 °C. The

CNF13 sample exhibits two major peaks after drying, at approx. 200 °C and 400 °C.

The peak at 200 °C, corresponding to approx. 9 % weight loss, can be assigned to

9

remaining precursor material in the dried sample. The peak at 400 °C corresponds to

approx. 13.5 % weight loss, and can be assigned to the oxidation of the carbon

nanofibers to CO and CO2. The pure CNF material oxidizes at around 500 °C (data not

shown). The metal oxides thus catalyze the oxidation of the nanofibers, decreasing the

oxidation temperature thereby. Only the peak at 400 °C remains after calcination of

CNF13, corresponding to approx. 13 % weight loss, i.e. the calcination treatment

removes remaining precursor material but does not oxidize the carbon nanofibers.

Temperature-programmed reduction (TPR) measurements performed on a CNF-free

Cu-Ce-Zr mixed oxide sample showed that Cu is reduced at around 170 °C, and that the

degree of Cu reduction increases from the first to the second reduction [29].

Figure 3 shows XRD spectra of the nanocomposites and the CNF-free material after

drying, as well as P-CNF for reference. The powder samples exhibit the fluorite-type

structure with the typical (111), (200), (220) and (311) reflections after drying at 100 °C.

With increasing CNF content, the graphitic (002) reflection becomes more significant in

the nanocomposites. The fluorite crystal size estimated from the XRD spectra (Table 1)

is similar for all samples and hence not significantly affected by the CNF content. The

XRD spectra and hence the estimated crystal size does not change significantly upon

calcination.

Figure 4 shows TEM images of CNF13 after calcination at 250 °C. The low-resolution

image (Figure 4A) displays agglomerates built up of the primary CuCeZr MMO

crystallites. The agglomerates are dispersed to a certain degree by the CNF, depending

on the CNF content. The high-resolution image (Figure 4B) displays the crystalline

structure of the CuCeZr MMO primary particles. The lattice spacing demonstrates the

random orientation of the nanocrystalline particles. These TEM data confirm the

existence of primary crystallites in the range of a few nanometers, for which the

software-based XLBA appears to slightly underestimate the size (Table 1). A reasonable

estimate for the primary crystallite size would be 3 – 5 nm.

10

3.2. Catalytic activity for water-gas shift

Figure 5 shows the conversion of CO over the catalysts for the simple WGS reactant

mixture as a function of temperature. CNF13 exhibits the highest WGS activity among

the three nanocomposite catalysts, and its performance is similar to the activity of

CNF00. In Table 2, the CO conversion of the three nanocomposites is normalized with

the weight content of the MMO and divided by the CO conversion of the CNF-free

catalyst. CNF07 and CNF24 show a relative WGS activity close to 1, i.e. the metal

oxide components in these two nanocomposites exhibit a catalytic performance similar

to the metal oxide in CNF00. CNF13 shows a relative WGS activity of 1.2, as reflected

in an overall WGS activity similar to CNF00 despite a lower MMO content. The

estimated activation energies of the four catalyst materials all lie in the range 31 – 34

kJ/mole (Table 2). The addition of the platelet CNF does not significantly affect the

WGS activation energy under the given temperatures and reaction conditions. Figure 6

shows the normalized short-term deactivation behaviour of all four catalysts at 300 °C

for the simple WGS reactant mixture. The catalysts show similar deactivation up to 15 h,

with CNF24 exhibiting a slightly higher deactivation rate than the other catalysts. The

addition of the platelet CNF is not found to improve the short-term stability of the metal

oxide catalyst under the given reaction conditions.

Figure 7 shows the CO conversion over CNF00, CNF13 and CNF24 for the reformate

reactant mixture as a function of temperature. CNF13 exhibits a WGS activity higher

than CNF24 and similar to CNF00, as for the CO/H2O/N2 mixture. CNF24 shows a

lower CO conversion than CNF00 and CNF13 at the highest temperatures (> 280 °C).

The MMO weight-normalized CO conversion of CNF13 and CNF24 divided by the CO

conversion of the CNF-free catalyst is given in Table 3. Under the reformate feed

conditions, the metal oxide components in both CNF13 and CNF24 exhibit a relative

WGS activity higher than 1. The activation energies of the three catalysts as determined

from the Arrhenius plot in Figure 8 are in the range 33 – 39 kJ/mole (Table 3). The

addition of the platelet CNF does not significantly affect the WGS activation energy for

the given temperatures and reaction conditions, as for to the CO/H2O/N2 mixture.

Furthermore, the activation energies are rather similar for the pure WGS and the

simulated reformate reactant mixture, i.e. the addition of CO2 and H2 does not affect the

11

activation energies considerably, bearing in mind that the values for pure WGS and the

reformate reactant mixture are determined in different ways.

Figure 9 shows the normalized short-term deactivation behaviour of CNF00, CNF13

and CNF24 at 350 °C for the reformate product mixture. The catalysts display similar

deactivation up to 16 h. In general, the addition of the platelet CNF does not improve

the short-term stability of the metal oxide catalyst under the reaction conditions applied.

The short-term deactivation behaviour is similar under pure WGS and the simulated

reformate product mixture. The deactivation test temperatures are, however, somewhat

different, and the approach to equilibrium conditions is 1 % for the pure WGS mixture

as compared to 50 % for the reformate reactant mixture.

4. Discussion

Dong et al. [21] found the optimum amount of Herringbone-type CNF as dispersing

agent in Cu-Zn-Al mixed oxide catalysts for methanol synthesis to lie around 11 g

CNF/mole total metal (approx. 13 wt% CNF). The same group also found the optimum

amount for Co-Cu catalysts for synthesis of higher alcohols to lie around 10 g

CNF/mole total metal (approx. 11 wt%) [22]. To some extent, the optimum amount of

CNF dispersing agent appears not to depend strongly on the MMO catalyst and the type

of reaction chosen. In our study, the catalyst prepared with 11 g platelet CNF/mole total

metal (approx. 7 wt%, CNF07) did not improve the catalyst properties significantly.

Instead the optimum amount appears to lie around the double amount, 22 g CNF/mole

(13 wt%, CNF13), with reservations concerning the limited resolution of the data points

for the CNF loading. Despite the decreasing amount of MMO in CNF07 relative to

CNF13, the catalyst performance remains at the same level. A further decrease in MMO

content and increase in CNF loading to 44 g/mole (24 wt%, CNF24) decreases the

catalyst activity and indicates that there is a limit to how much of the active catalyst

component can be replaced by inert material. The Cu-Ce-Zr mixed oxide materials

prepared and studied here are capable of utilizing the double amount of CNF per mole

total metal compared to the two studies mentioned above, whereas based on the weight

content of the CNF in the MMO catalyst our results are similar. The optimum amount of

CNF dispersing agent for MMO catalysts may thus depend on the type of CNF used, the

12

conditions applied for preparing the nanocomposites, as well as the basis for the

comparison.

All CNF-containing catalysts exhibit a superior overall activity compared to the CNF-

free samples in the previously mentioned studies [21,22]. Dong et al. [21] as well as

Zhang et al. [22] suggest the CNF to be more than a mere dispersing agent material. As

adsorbent and activator of reactant molecules (e.g. H2-adsorption/spillover and electron

transfer), they may facilitate higher stationary-state concentrations of the active species

on the catalyst surface, thereby improving activity and affecting the selectivity. This

effect would be less distinctive in our study, but can not be generally excluded (see for

example CNF-supported noble metal catalysts for hydrogenation of cinnamaldehyde

[36,37]). In our study, the CNF-containing samples exhibit a higher activity than the

CNF-free catalyst (Table 2 and Table 3) only when based on the metal content.

The influence of the preparation conditions when comparing CNF-containing and CNF-

free catalysts is, however, also important. The CNF may influence the precipitation,

even if appearing inert under reaction conditions. For CNF-free MMO catalysts

prepared by co-precipitation, the resulting surface area depends on the mixing efficiency

during the co-precipitation process [29,38,39]. Efficient stirring results in small particles

with high surface area and hence high catalytic activity. Small particles can be

introduced as seeds into the precipitation reactor to offer nucleation centers in the

growth process [40]. For carbon nanofibers with carboxylic acid surface groups, metal

ion adsorption may occur prior to nucleation of the metal hydroxides. Small adsorbed

metal clusters may then serve as nucleation sites for the formation of mixed metal

hydroxide crystallites. Further nucleation and crystal growth may then occur on these

primary nuclei [41,42]. Shape, size distribution and agglomeration degree of the final

mixed oxide crystals as well as the degree of attachment to the carbon support may

depend on size and shape of the CNF [40].

The deposition-precipitation on CNF may lead to smaller particles, dispersed by the

seed material, and hence higher surface area of the active materials. The addition of

CNF may thus compensate inefficient mixing during precipitation, whereas the impact

13

of CNF on nucleation and crystal growth may diminish under efficient mixing

conditions. The efficiency of the mixing in [21] and [22] is not clear. However, the

lower reduction temperatures of the CNF-containing materials as compared to the CNF-

free MMO may indicate the active particles to be of different size or exhibit a different

degree of agglomeration. The preparation of MMO by homogeneous co-precipitation

with urea as base precursor has been shown to result in high surface area materials

under the preparation conditions chosen [29], and CNF addition may thus not greatly

improve the particle dispersion during precipitation. The fluorite crystal size in our

catalysts is not significantly affected by the CNF addition, within experimental

uncertainty (Table 1). The similar overall activity and the slightly increased specific

activity of the nanocomposites compared to the CNF-free MMO in the present study

can be interpreted accordingly. The small difference in apparent activation energy

(Table 2 and Table 3) also indicates that the active particles are not fundamentally

different, and the addition of CNF does not appear to have an impact on the rate-

determining step. Frøseth et al. [43] observed no significant CO chemisorption to occur

on CNF identical to the ones used in our study, using steady-state isotopic transient

kinetic analysis (SSITKA). The study was performed at 100 °C and 1.85 bar with

CO/inert (1.5/33.5 Nml/min) and CO/H2/inert (1.5/15/33.5 Nml/min) gas mixtures.

Chemisorption experiments (Micrometrics ASAP 2010) at 40 °C also showed no

significant H2 chemisorption on the pure CNF material.

The addition of CNF did not improve the short-term stability of the nanocomposite

catalysts compared to the CNF-free MMO in our study. Consequently, the deactivation

of the active phase is not affected by the CNF-MMO interface. According to the

combined XRD and TEM data, the nanocomposites consist of agglomerates of mixed

oxide crystallites that are dispersed by the CNF. One reason for deactivation could be a

decrease in the catalytically active surface area of the MMO by sintering of the

secondary mixed oxide agglomerates. Another cause could be phase separation within

the mixed oxide structure, as described earlier [29]. Both effects are not directly

affected by the CNF, hence CNF should not have an influence on catalyst deactivation

caused by these two processes.

14

The impact of CNF on the catalyst activity and stability could be related to size-

matching [44]. The mixed oxide crystallites and the CNF exhibit a rather large size

difference in our study (Table 1). The addition of CNF might improve the catalyst

activity and stability in cases where CNF with smaller diameters are combined with

precipitation procedures resulting in larger mixed oxide crystals than under the given

conditions, possibly at the expense of a lower initial, catalytically active surface area. In

summary, effects of CNF size, structure and surface properties as well as all parameters

concerning preparation of the active phase have to be considered before concluding on

the optimum amount of CNF in nanocomposites, and especially if conclusions on the

activity of the CNF itself are to be drawn.

5. Conclusions

Under the preparation and reaction conditions chosen in this study, the CNF-containing

nanocomposite catalysts exhibit overall activity and stability similar to the CNF-free

catalyst. The main advantage of the nanocomposites relates to conserving raw materials

for MMO catalysts, since about 13 wt% of the MMO could be replaced by CNF without

decreasing the overall activity of the catalyst. CNF are also a recycling-friendly additive

in MMO materials, since they can be easily removed by oxidation at 400 °C.

The specific activity of the nanocomposites, based on the total metal oxide content, is

similar to or higher than the activity of the CNF-free material, with 13 wt% CNF found

as optimum in our study. We can not exclude the possibility that the CNF material acts

as a reaction promoter under certain conditions, but suggest that the impact of CNF

addition on the precipitation of the mixed oxide particles, and hence the catalytic

activity relative to the CNF-free MMO, is important. This is supported by the similar

activation energies obtained for the CNF-free and CNF-containing materials in our

study and SSITKA results on CO chemisorption on CNF. CNF may therefore be

considered as an inert dispersing agent material improving the precipitation of the

MMO under conditions where the co-precipitation of the MMO precursors does not

result in materials with high surface area. The size distribution and surface properties of

the CNF will also influence the precipitation.

15

Acknowledgements



Technology Center NTNU-SINTEF is also acknowledged for their support. Elin Nilsen

(Department of Materials Technology, NTNU) and Jon Arvid Lie (Department of

Chemical Engineering, NTNU) are gratefully acknowledged for their assistance with

the XRD and the TGA devices, respectively.

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18

Figure Captions

Figure 1. TEM images of as-grown carbon nanofibers (CNF) produced at 600 °C from

decomposition of CO/H2 (40/10 ml/min) over Fe3O4 nanoparticles. A) Scale-bar 10 nm.

B) Scale-bar 500 nm. The high-resolution image (A) confirms the platelet structure of

the CNF.

Figure 2. Temperature-programmed oxidation profiles of the catalyst samples CNF00

(dried sample) and CNF13 (both dried and calcined samples).

Figure 3. XRD patterns of CNF00, CNF07, CNF13 and CNF24 after drying and of P-

CNF after oxidation treatment.

Figure 4. TEM images of CNF13 after calcination at 250 °C. A) Scale-bar 50 nm. B)

Scale-bar 5 nm.

Figure 5. WGS activity of the CNF-containing and CNF-free Cu-Ce-Zr mixed oxide

catalysts as function of reaction temperature under the CO/H2O/N2 (25/125/350

Nml/min) reactant mixture.

Figure 6. Short-term deactivation of the CNF-containing and CNF-free Cu-Ce-Zr

mixed oxide catalysts at 300 °C under the CO/H2O/N2 (25/125/350 Nml/min) reactant

mixture. All curves are normalized with the initial CO conversion, calculated as the

average of the first three analyses obtained at of 3 min. intervals (CNF00: 63.77 %,

CNF07: 56.92 %, CNF13: 60.87 %, CNF24: 51.75 %).

Figure 7. WGS activity of the CNF-containing and CNF-free Cu-Ce-Zr mixed oxide

catalysts as function of reaction temperature under the CO/H2O/CO2/H2/N2

(25/125/60/175/115 Nml/min.) reactant mixture.

19

Figure 8. Arrhenius plots for the CO/H2O/CO2/H2/N2 (25/125/60/175/115 Nml/min.)

reactant mixture in the temperature range 225 – 275 °C with all CO conversions lower

than 15 %.

Figure 9. Short-term deactivation of the CNF-containing and CNF-free Cu-Ce-Zr

mixed oxide catalysts at 350 °C under the CO/H2O/CO2/H2/N2 (25/125/60/175/115

Nml/min.) reactant mixture. All curves are normalized with the initial CO conversion,

calculated as the average of the first three analyses obtained at of 3 min. intervals

(CNF00: 49.20 %, CNF13: 47.85 %, CNF24: 41.81 %).

20

Figure 1

A)

B)

21

Figure 2

22

Figure 3

20 30 40 50 60

CNF00

CNF07

CNF13

CNF24

P-CNF

(111) (200) (220) (311)

(002)

23

Figure 4

A)

B)

24

Figure 5

0

10

20

30

40

160 180 200 220 240 260

Temperature [ °C ]

CO

con

vers

ion

[ % ]

CNF00

CNF07

CNF13

CNF24

25

Figure 6

0

0,2

0,4

0,6

0,8

1

1,2

0 3 6 9 12 15

TOS [ h ]

Nor

mal

ized

CO

con

vers

ion

[ - ]

CNF00

CNF07

CNF13

CNF24

26

Figure 7

0

10

20

30

40

50

160 200 240 280 320 360

Temperature [ °C ]

CO

con

vers

ion

[ % ]

CNF00

CNF13

CNF24

27

Figure 8

1

1,5

2

2,5

3

0,0018 0,00185 0,0019 0,00195 0,002 0,00205

1/T [ 1/K ]

ln(C

O c

onv.

) [ -

]

CNF00

CNF13

CNF24

28

Figure 9

0

0,2

0,4

0,6

0,8

1

1,2

0 2 4 6 8 10 12 14 16

TOS [ h ]

Nor

mal

ized

CO

con

vers

ion

[ - ]

CNF00

CNF13

CNF24

29

Table 1. Structural characteristics of the dried and calcined CNF and Cu-Ce-Zr MMO

nanocomposites.

BET Pore volume XRD crystal sizea

[ m2/g ] [ cm3/g ] [ nm ]

P-CNF 172 0.326

CNF00 164 0.317 2.4 (3.4)

CNF07 n.d. n.d. 2.8

CNF13 164 0.307 2.8

CNF24b 139 0.262 3.1 (3.1)

a Determined on the dried samples. The values in parenthesis correspond to calcined

samples: CNF00 at 350 °C, placed directly into the hot furnace; CNF24 at 2 °C/min

up to 250 °C. b This sample was prepared with a different batch of platelet CNF, but with similar

structural properties after oxidation treatment: BET 155 m2/g, pore volume 0.297

cm3/g.

30

Table 2. Comparison of WGS activity between the nanocomposites and the CNF-free

catalyst for the CO/H2O/N2 (25/125/350 Nml/min) reactant mixture. The CO conversion

of the nanocomposites is normalized with the MMO weight content and divided by the

CO conversion of the CNF-free catalyst. The activation energies (Ea) determined by the

integral method in the temperature range 165 – 210 °C are also given.

CNF00 Rel. CO conv. normalized with metal oxide content

Temperature CO conv. CNF07/CNF00 CNF13/CNF00 CNF24/CNF00

[ °C ] [ % ] [ - ] [ - ] [ - ]

165 10.46 0.95 1.22 1.01

180 14.25 0.95 1.22 1.07

Ea [ kJ/mole ] 32 31 31 34

(165 – 210 °C)

31

Table 3. Comparison of WGS activity between the nanocomposites and the CNF-free

catalyst for the CO/H2O/CO2/H2/N2 (25/125/60/175/115 Nml/min.) reactant mixture.

The CO conversion of the nanocomposites is normalized with the MMO weight content

and divided by the CO conversion of the CNF-free catalyst. The activation energies (Ea)

determined by the differential method from an Arrhenius plot in the temperature range

225 – 275 °C are also given.

CNF00 Rel. CO conv. normalized with metal

oxide content

Temperature CO conv. CNF13/CNF00 CNF24/CNF00

[ °C ] [ % ] [ - ] [ - ]

225 6.23 1.19 1.10

250 8.97 1.30 1.27

275 9.66 1.31 1.25

Ea [ kJ/mole ] 33 38 39

(225 – 275 °C)

Paper VI

The effect of platinum in Cu-Ce-Zr and Cu-Zn-Al mixed oxide

catalysts for water-gas shift

Manuscript in preparation.

1

The effect of platinum in Cu-Ce-Zr and Cu-Zn-Al mixed oxide catalysts for water-

gas shift

Florian Huber1, John Walmsley2, Hilde Venvik1,*, Anders Holmen1


Technology (NTNU), N-7491 Trondheim, Norway 2SINTEF Materials and Chemistry, N-7465 Trondheim, Norway


Fax: +47-73595047


precipitation with urea. Pt was wet-impregnated on mixed oxide catalysts. The WGS

activity of Cu-Ce-Zr and Cu-Zn-Al mixed oxide catalysts can be related to the Cu

reducibility in these catalysts. Low-temperature reducibility correlates with low-

temperature activity. Pre-reduction is not absolutely necessary when performing the

WGS reaction above the Cu reduction temperature of the catalyst. An adequate

reduction procedure may, however, be applied in order to optimize the CO conversion.

Pt had no significant effect on the Cu-Ce-Zr mixed oxide catalyst, but altered the

properties of the Cu-Zn-Al mixed oxide catalyst. Pt shifted the Cu reduction to lower

temperatures, indicating the existence of an interaction between Pt and Cu in the

bimetallic catalyst. The effect of Pt on the WGS activity and stability was dependent on

the pre-treatment procedure as well as the reaction conditions.

KEY WORDS: homogeneous alkalinization; urea; Cu-Ce-Zr; Cu-Zn-Al; mixed oxide;

bimetallic; platinum; pre-reduction; Cu reducibility; water-gas shift.

1. Introduction

The water-gas shift (WGS) reaction (CO + H2O ↔ CO2 + H2) is one of the oldest

catalytic processes employed in the chemical industry [1]. It is an important step in the

industrial production of hydrogen or synthesis gas. The role of WGS is to enhance the

2

production of hydrogen and remove CO before ammonia synthesis, refinery

hydroprocesses and bulk storage and redistribution of hydrogen, or to adjust the H2/CO

ratio in the production of methanol and the Fischer-Tropsch synthesis. The WGS

reaction has received renewed interest as a key step in fuel processing to optimise

hydrogen production and reduce the CO level for proton exchange membrane fuel cell

(PEMFC) applications [2-4].

Copper-based catalysts are considered as the most active for WGS [1,6]. Cu-Zn-Al

mixed oxides are classical catalyst formulations for WGS [2,7,8], while Cu-Ce or Cu-

Ce-Zr mixed oxides represent more recently studied systems [9-11]. The Cu-Ce-based

catalysts are claimed to be stable at high temperatures and applicable without activation

in a reducing gas such as hydrogen [9,10].

Supported bimetallic catalysts containing Cu and a noble metal, such as Pd or Pt, have

also been studied for the WGS reaction [12-14], in addition to CO oxidation [15-18],

methanol steam reforming [19], NOx reduction [15,20] and nitrile hydrogenation [21].

Bimetallic formulations can be used to tailor the reactivity of catalysts, governed by

electronic, ensemble and/or geometric effects [22,23].

In the present study, Cu-Zn-Al and Cu-Ce-Zr mixed metal oxide catalysts were

prepared by co-precipitation with urea as base precursor. A part of the precipitated

material was impregnated with Pt. The Pt-impregnated Cu-Zn-Al and Cu-Ce-Zr mixed

oxides were studied under WGS conditions and compared to the unimpregnated Cu-Zn-

Al and Cu-Ce-Zr systems, to investigate the effect of Pt on the catalyst performance.

2. Experimental

2.1. Synthesis

Cu(II)-nitrate-trihydrate (≥ 99 %), Ce(III)-nitrate-hexahydrate (≥ 99.5 %) and Zr(IV)-

nitrate-hydrate (≥ 99.5 %) purchased from Acros Organics, Zn(II)-nitrate-hexahydrate

(≥ 99 %) purchased from Fluka and Al(III)-nitrate-nonahydrate (≥ 98.5 %) purchased

from Riedel-de Haën were used as metal precursors. Tetraamineplatinum(II)-nitrate

(99.995 %) was purchased from Aldrich Chem. Co. Urea (≥ 99.5 %) was purchased

3

from Merck. Ethanol was purchased from Arcus. All chemicals were used as-received.

Deionized water was used for all catalyst preparations.

In a typical synthesis of Cu-Ce-Zr MMO, a total amount of 0.21 mole of Cu-, Ce- and

Zr-salt (nominal composition: Cu:Ce:Zr = 0.23:0.54:0.23) was dissolved in 1000 ml of

water at ca. 30 °C under stirring. 0.904 mole of urea was added to the mixture, resulting

in a pH of approx. 1.7 of the mixture. The mixture was poured into a 1000-ml 5-neck

glass flask, placed in a hot oil bath and rapidly heated up to 95 °C under vigorous

stirring with a blade agitator at approx. 750 rpm, while monitoring the temperature with

a thermometer immersed into the solution.

The setup was operated in an open mode, i.e. water was allowed to evaporate through

the unused flask necks, and the loss was compensated by continuous refilling with water

(at ambient temperature, not pre-heated). The addition of water was conducted in the

same way in all preparations, but variations in this manual procedure may influence the

formation of the precipitate and thereby the final material properties. The investigation

of this parameter was not part of the study.

The mixture was kept in the oil bath for a total period of 8 hours, including heating (ca.

20 min), precipitation and aging. The resulting suspension was then removed and

rapidly cooled down to room temperature using cold water. The solid precipitate was

filtered off and washed with deionized water and ethanol. The precipitate was then

divided into two equal parts. One part was dried for about 12 h at 100 °C and calcined

for 1 h at 350 °C (heating at 2 °C/min; cooling at approx. 2 °C/min) in a muffle furnace.

The other part was redispersed in 50 ml water at ambient temperature. 0.8 mmole of Pt-

salt was dissolved in 20 ml water and added dropwise to the redispersed precipitate

under stirring. The suspension was stirred for 1 h at ambient temperature. The

temperature was then increased to 70 °C, evaporating most of the water within approx.

1.5 h. The precipitate was then dried for about 14 h at 100 °C and calcined for 1 h at 350

°C in a muffle furnace (heating at 2 °C/min; cooling at approx. 2 °C/min).

4

The Cu-Zn-Al MMO catalysts were prepared in the same way as the Cu-Ce-Zr MMO,

with a total amount of 0.21 mole of Cu-, Zn- and Al- salt (nominal composition:

Cu:Zn:Al = 0.23:0.54:0.23). The initial pH of the salt-urea mixture was approx. 3.4. 0.8

mmole of Pt-salt was added to one half of the precipitate.

The four copper-containing catalysts are denoted as CeZr, CeZr-Pt, ZnAl and ZnAl-Pt.

2.2. Characterization techniques


determine the actual metal content in the samples after calcination of the precipitated

materials. The samples were dissolved in acid before analysis without any visible

residues.

Nitrogen adsorption-desorption isotherms were obtained using a Micrometrics

TriStar 3000 instrument. The data were collected at -196 °C. The BET surface area

was calculated by the BET equation in the relative pressure interval ranging from 0.01

to 0.30. The pore volume was estimated by the Barrett-Joyner-Halenda (BJH) method

[24] as the adsorption cumulative volume of pores between 1.7 nm and 300.0 nm width.

This method is based on the assumption of cylindrical pores, and the capillary

condensation in the pores is taken into account by the classical Kelvin equation. The

pore size distributions were calculated by non-local density functional theory (NLDFT,

original DFT model with N2 [25], DFT Plus software package [26]), assuming a slit-like

pore geometry.

X-ray diffraction spectra (XRD) were recorded on a Siemens diffractometer D-5005

using dichromatic CuKα+β-radiation.

Transmission electron microscopy (TEM) images were recorded on a JEOL 2010F

transmission electron microscope. Small amounts of catalyst sample were placed in

sealed glass containers containing ethanol and immersed in an ultrasonic bath for a

couple of minutes to disperse the individual particles. The resulting suspension was

dropped onto a holey carbon film, supported on a titanium mesh grid, and dried.

5

Conventional TEM images were recorded onto a CCD camera. Samples were also

examined in scanning transmission electron microscope (STEM) mode, with a nominal

probe size of approx. 0.7nm. Bright and dark field STEM images were acquired. Energy

dispersive x-ray spectroscopy (EDS) analysis and mapping were performed using an

Oxford Instruments INCA system. Drift compensation was employed to correct for

movement of the sample during the time taken for the acquisition of maps.

Temperature-programmed reduction (TPR) experiments were performed with

CHEMBET 3000 from Quantachrome Instruments, based on the volumetric principle

with the gas flow being analyzed by a thermal conductivity detector (TCD). The catalyst

samples, 0.065-0.066 g and 0.133-0.137 g for Cu-Zn-Al- and Cu-Ce-Zr-based catalysts,

respectively, were loaded into a quartz reactor and held in place by quartz wool. The

samples were heated in 5 vol-% O2/Ar at 5 °C/min to 260 °C, and kept there for 10 min.

After cooling down to ambient temperature and flushing with Ar, the samples were then

heated in 7 vol-% H2/Ar at 2 °C/min.

2.3. Reaction experiments

The water-gas shift activity and stability were measured at atmospheric pressure in an

externally heated tubular fixed-bed reactor made of quartz. Quartz tubes with inner

diameters of 10 mm and 6 mm were used for the Cu-Zn-Al- and Cu-Ce-Zr-based

catalysts, respectively. The particle size fraction used for the experiments was 125 – 200

µm and 50 – 125 µm for the Cu-Zn-Al- and Cu-Ce-Zr-based catalysts, respectively.

Quartz wool was applied below and above the catalyst layer. A carbonyl trap was

connected to the outlet of the CO gas cylinder to remove possible iron carbonyls. The

gas flows were controlled by mass flow controllers. Water was dosed from a He-

pressurized cylinder using a liquid flow controller. The water was injected into a

vaporizing unit, and the steam was then mixed into the gas stream. A microchannel heat

exchanger (Forschungszentrum Karlsruhe) operated with air was used for condensation

of liquid products prior to analysis of the dry product gas with an on-line Agilent

G2891A Micro GC equipped with thermal conductivity detection (TCD). The gas

composition in the dry gas was quantified by using a range of certified calibration gases

as external standards.

6

The catalysts were studied under reaction conditions with and without pre-reduction.

Pre-reduction was conducted in 10 vol-% H2 in nitrogen at 250 °C for 1 h (total flow:

300 Nml/min; heating rate: 5 °C/min). The catalyst sample was then cooled in the H2/N2

atmosphere to the starting temperature of the activity measurements. When no pre-

reduction was carried out, the catalyst sample was heated to 250 °C (heating rate: 5

°C/min) in 300 Nml/min air and kept there for 45 min. The catalyst sample was then

flushed with 300 Nml/min N2 5.0 for 15 min and cooled down to the starting

temperature of the activity measurements.

The water-gas shift reaction was studied under two different reactant mixtures. All four

catalysts were subjected to a simple WGS reactant mixture containing 25 Nml/min CO,

125 Nml/min H2O and 350 Nml/min N2. The amount of catalyst used for these

experiments was 0.051 g. ZnAl and ZnAl-Pt were also studied under a simulated

reformate product mixture containing 25 Nml/min CO, 125 Nml/min H2O, 60 Nml/min

CO2, 175 Nml/min H2 and 115 Nml/min N2, using 0.110 g of catalyst. An initial

stabilization of the feed was carried out in a by-pass line. The temperature was

increased in steps of 15 – 25 °C during the activity measurements, and stabilized for 15

– 30 min. before four GC analyses were taken at three minute intervals. The short-term

deactivation behaviour of the catalysts was recorded at the final temperature.

The conversion of CO (XCO) for the feed containing CO, H2O and balance N2 only can

be calculated from the CO and CO2 concentrations in the dry exit gas:

2

2

100%( )CO

COXCO CO

= ⋅+

(1)

For the simulated reformate feed that also contains H2 and CO2, the calculated CO

conversion must take into account the initial composition of the feed gas:

0

2 2,0

( ) 100%( )CO

CO COXCO CO CO

−= ⋅+ −

(2)

7

The conversion of CO as a basis for comparison of the catalysts with and without Pt

impregnated can be justified, since equal amounts of catalyst were used and the mass

difference introduced by Pt is relatively small. Furthermore, the turnover frequency is

not easily defined or determined in such systems.

The selectivity was 100% in all experiments. Trace CH4 was detected in some of the

temperature scans as well as the following short-term deactivation experiment. The

standard deviation of the CO conversion was estimated to 4 %. The carbon balance for

CO and CO2 was always within ±1 %.


3.1. Catalyst characterization

Table 1 shows chemical composition and structural parameters of the four catalysts. The

actual composition of the Cu-Zn-Al-based catalysts (Cu:Zn:Al = 0.25:0.53:0.22 for

ZnAl) is close to the nominal composition (Cu:Zn:Al = 0.23:0.54:0.23), while for the

Cu-Ce-Zr-based catalysts the actual composition (Cu:Ce:Zr = 0.19:0.62:0.19 for CeZr)

deviates somewhat from the nominal one. The Pt-content is approx. 1 wt% in CeZr-Pt

and 2 wt% in ZnAl-Pt, and hence the total molar metal fraction approx. 0.8 % and 1.0 %,

respectively.

The Cu-Ce-Zr- and Cu-Zn-Al-based catalysts have been prepared under identical

conditions, but the Cu-Ce-Zr-based systems exhibit significantly higher surface areas

and pore volumes than the Cu-Zn-Al-based systems (Table 1). This indicates that the

two systems exhibit different precipitation behaviour and/or behaviour under heat

treatment (drying/calcination). Adjusted preparation conditions have to be applied if

optimization of the surface area of the Cu-Zn-Al mixed oxide system is desired.

The N2 adsorption-desorption isotherms and the corresponding pore size distributions

for the four catalysts are shown in Figure 1. The majority of the pores in all four

catalysts lies in the mesoporous range (2-50 nm). The Cu-Ce-Zr-based systems show a

broad unimodal pore size distribution with a maximum at around 25 nm, while the Cu-

8

Zn-Al-based systems show a bimodal distribution with a pronounced maximum at

around 8 nm and a local maximum at around 28 nm.

The catalyst bed density was estimated to approximately 0.3 g/cm3 and 0.9 g/cm3 for

ZnAl and CeZr, respectively. CeZr thus had a bed density three times higher than ZnAl.

The impregnation step for deposition of Pt on the precipitates before drying resulted in a

decrease of the pore volume, and in case of CeZr-Pt also the surface area (Table 1). The

change in pore volume is more pronounced in the Cu-Ce-Zr- than in the Cu-Zn-Al-

based system, probably due to the larger surface area and pore volume, and hence more

fragile pore structure of the former. As can be seen in Figure 1B, the impregnation of

the Cu-Ce-Zr precipitate with Pt resulted in a break-down of pores larger than 25 nm,

reflected in a significant pore volume decrease.

Figure 2 shows the XRD spectra of the four catalysts after calcination. The Cu-Ce-Zr-

based systems exhibit the fluorite-type structure of ceria and zirconia. No additional

CuO phase is visible. The spectrum of CeZr-Pt does not show any sign of crystalline Pt

or Pt-oxide. The Cu-Zn-Al-based systems show reflections assignable to CuO, ZnO and

Al2O3, and possibly mixed oxide structures of these elements. The spectrum of ZnAl-Pt

shows weak reflections that may be assigned to crystalline Pt and/or PtO2, as compared

to the spectrum of ZnAl.

Cu-Ce-Zr mixed oxide catalysts have been investigated with TEM and STEM-EDS in

an earlier study [27], and will therefore not be discussed in detail in this work. In short,

the Cu-Ce-Zr mixed oxides exhibit a good distribution of all three metals, but without

forming a true solid solution. The distribution of Pt over the Cu-Ce-Zr mixed oxide in

CeZr-Pt was investigated with STEM-EDS, finding Pt distributed over the regions

studied with local variations in concentration (not shown). Figure 3 shows TEM images

of ZnAl-Pt. The bright, dense particles in Figure 3A and B are rich in Zn, with a

Cu:Zn:Al ratio of 1:8-9:1-2. The grey, porous areas are closer to the overall composition

as determined by ICP-AES, with a typical Cu:Zn:Al ratio of 1: 2-4:2-3. The overall

Cu:Zn:Al ratio in Figure 3A is 1:3.9:1.7. The particles in Figure 3C mainly contain Cu

9

and Zn, with Al present only in small quantities. Cu and Zn are not homogeneously

distributed in this region, since both Cu- and Zn-rich areas are found. The overall

Cu:Zn:Al ratio in Figure 3C is 1:0.7:0.02. Different preparation conditions and metal

ratios have to be applied if an optimization of the metal distribution of the Cu-Zn-Al

mixed oxide catalyst is desired..

The bimodal pore size distribution of the Cu-Zn-Al-based catalysts, shown in Figure 1B,

can be explained with the TEM images in Figure 3. The pores with a size around 8 nm

are probably located in the grey, porous areas, while the pores with a size around 28 nm

are probably built up by the bright, dense particles. The distribution of Pt over the Cu-

Zn-Al mixed oxide in ZnAl-Pt was investigated with STEM-EDS. The analysis showed

that Pt was distributed over the regions studied with local variations in concentration.

The TPR profile of the four catalysts is shown in Figure 4. The Cu-Ce-Zr-based

catalysts are reduced at lower temperatures than the Cu-Zn-Al-based catalysts, with the

maximum signal around 160 °C. In addition, CeZr and CeZr-Pt exhibit a small signal

already below 100 °C, a feature not present in the Cu-Zn-Al-based catalysts. The

addition of 1 wt-% Pt does not have a large effect on the reducibility of the Cu-Ce-Zr

mixed oxide catalyst. It slightly enhances the reduction at lower temperatures. ZnAl

exhibits a broad reduction peak between 180 and 280 °C. ZnAl-Pt exhibits a broad

reduction peak of different shape than ZnAl between 135 and 235 °C. The addition of 2

wt-% Pt to the Cu-Zn-Al mixed oxide clearly shifts the Cu reduction to lower

temperatures, possibly due to hydrogen spillover [28,29] from Pt to CuO, thus

indicating a significant interaction between Pt and Cu in the catalyst. Huang & Sachtler

[21] reported that the addition of Ru, Rh, Pd or Pt to NaY-supported Cu catalysts

enhanced the reduction of Cu2+.

3.2. Catalyst activity and stability

3.2.1. Cu-Ce-Zr-based mixed oxide catalysts

Table 2 summarizes the conversion of CO obtained for CeZr and CeZr-Pt under the

simple WGS reactant mixture between 165 and 195 °C. Both catalysts demonstrated

WGS activity without pre-reduction, and the activity did not increase significantly upon

10

pre-reduction. Hence, Cu-Ce-Zr mixed oxide catalysts do not require pre-reduction, but

the CO conversion can be optimized through an adequate pre-reduction procedure.

CeZr-Pt exhibited a somewhat lower CO conversion than CeZr. This may be related to

the lower surface area (Table 1). A slight negative impact of Pt on the reactivity can,

however, not be excluded, but Pt did not significantly alter the WGS activity of the Cu-

Ce-Zr mixed oxide catalyst under the given conditions. Hungría et al. [15] reported that

the catalytic performance of a Cu-Ce-Zr mixed oxide supported on alumina and

impregnated with 1 wt% Pd was governed by the Cu-(Ce,Zr)Ox character of its active

sites, when used for CO oxidation and NO reduction. In a similar way, the catalytic

activity of CeZr-Pt may be dominated by active Cu sites that are not significantly

altered by Pt under the given conditions, as also reflected in the reducibility (Figure 4).

This slight shift in reducibility to lower temperatures may, however, be correlated with

a lower impact of the pre-reduction of CeZr-Pt as compared to CeZr. Pt appears to make

the WGS activity of Cu-Ce-Zr mixed oxide catalyst more insensitive to the pre-

treatment.

Both catalysts, CeZr and CeZr-Pt, were more stable without than with pre-reduction, as

can be seen in Figure 5. The short-term stability was studied at 250 °C under the simple

WGS reactant mixture. Under these conditions, the not pre-reduced catalysts display a

somewhat lower CO conversion, 80 % and 95 % of the pre-reduced catalysts for CeZr

and CeZr-Pt, respectively.

3.2.2. Cu-Zn-Al-based mixed oxide catalysts

Figure 6 shows the CO conversion of ZnAl and ZnAl-Pt as function of reaction

temperature under the CO/H2O/N2 reactant mixture. In contrast to the Cu-Ce-Zr mixed

oxide catalysts, the Cu-Zn-Al mixed oxide catalyst ZnAl exhibited a different behaviour

with and without pre-reduction. Without pre-reduction, ZnAl did not show WGS

activity at 165 °C and then ignited between 180 and 195 °C, reaching more or less the

activity of the pre-reduced sample. The higher CO conversion of the not pre-reduced

catalyst in the range 195 – 220 °C as compared to the pre-reduced can be ascribed to a

contribution from Cu reduction by CO. This conclusion was reached through

comparison of the CO and H2 concentrations detected in the dry exit gas.

11

The reactivity of the not pre-reduced catalyst can be further explained with the TPR

profile of ZnAl in Figure 4, neglecting the impact of the type of reduction gas used. The

reduction of Cu in ZnAl started at about 180 °C and correlates thus with the onset of the

CO conversion, indicating the importance of reduced Cu for the WGS activity of the

catalyst. The TPR profile of CeZr exhibited a maximum reduction peak at around 160

°C and correspondingly showed a significant WGS activity already at 165 °C (Table 2).

A low Cu reduction temperature is therefore a prerequisite for WGS activity at low

temperature. Ko et al. [30] reported lower Cu reduction temperatures and higher

conversions in the WGS reaction for Cu-Zr relative to Cu-Zn-Al mixed oxide catalysts

at low temperatures. Tang et al. [31] stated that the redox ability of Cu-Ce mixed oxide

catalysts at low temperatures plays an essential role for the catalytic activity in CO

oxidation, and that the redox properties are determined by the dispersion of the copper

species and the degree of interaction between these species and ceria. Shen & Song [32]

reported that Cu-Zn-Al mixed oxide catalysts with lower Cu reduction temperature

showed higher activity for production of hydrogen from methanol steam reforming.

Above 210 °C, the CO conversion of not pre-reduced ZnAl was slightly lower than of

the pre-reduced ZnAl. Above the onset temperature for Cu reduction, pre-reduction is

not absolutely necessary, but as for Cu-Ce-Zr mixed oxides, an appropriate reduction

procedure may enhance catalyst activity and stability [33-42] and ensure avoidance of

hot spots (especially in fixed bed reactors) [43].

The addition of Pt to the Cu-Zn-Al mixed oxide in ZnAl-Pt resulted in an increased

reactivity of the not pre-reduced sample below 195 °C as compared to not pre-reduced

ZnAl (Figure 6). This can also be explained by the TPR profiles of both catalysts

(Figure 4) and neglecting the impact of the type of reduction gas used. Pt shifted the Cu

reduction to lower temperatures with a certain amount of Cu being reduced at 165 °C.

Above 195 °C, the not pre-reduced ZnAl-Pt showed a similar CO conversion as

compared to not pre-reduced ZnAl.

ZnAl-Pt without pre-reduction exhibits a similar initial WGS activity as the Cu-Zn-Al

mixed oxide catalyst ZnAl (Figure 6). The pre-reduced ZnAl-Pt sample, however,

showed a significantly lower activity than both ZnAl and not pre-reduced ZnAl-Pt.

12

Utaka et al. [14] also observed a decreased activity of pre-reduced, Pt-impregnated Cu-

Zn-Al mixed oxide catalysts for oxygen-assisted WGS as compared to pre-reduced Cu-

Zn-Al. Huang & Sachtler [21] reported that the addition of Ru, Rh, Pd or Pt to NaY-

supported Cu catalysts lowered the activity for acetonitrile hydrogenation.

Figure 7 shows the short-term stability of ZnAl and ZnAl-Pt at 250 °C. ZnAl and ZnAl-

Pt without pre-reduction exhibited a similar normalized deactivation behaviour (Figure

7B). The pre-reduced ZnAl-Pt sample, however, showed a completely different trend.

After a slight increase in the first 10 h on stream, the CO conversion remained constant

for the following 15 h on stream. The initial increase in CO conversion may be related

to structural changes and the formation of a slightly more active and stable phase under

the given reaction conditions.

The type of pre-treatment procedure appears to have a significant effect on initial

activity and stability of supported, bimetallic Pt-Cu catalysts. Epron et al. [44] also

report that the pre-treatment significantly affects the interaction between Pt and Cu in an

Al2O3-supported, bimetallic Pt-Cu system, and the catalytic activity for nitrate reduction

in water. The origin for the decreased activity and enhanced stability of the pre-reduced

ZnAl-Pt, as compared to ZnAl may be related to surface alloy effects [22,45-50]. Zhou

et al. [20] reported a higher activity and stability of Pt-on-Cu shell-core nanoparticles in

NO reduction compared to Cu-on-Pt shell-core particles and Pt-Cu alloy particles.

Persson et al. [51] reported a lower initial activity and increased stability of an Al2O3-

supported, bimetallic Pd-Pt catalyst for catalytic combustion of CH4, as compared to

Pd/Al2O3. The decreased initial activity and enhanced stability of this catalyst was

related to a partial formation of a Pd-Pt alloy. Besenbacher et al. [52] reported on a

supported Au-Ni surface alloy catalyst for steam reforming being less active and more

stable than a supported Ni catalyst, and ascribed it to a change in the surface reactivity

of Ni by Au.

The WGS activity and stability of pre-reduced ZnAl and ZnAl-Pt were also studied

under the simulated reformate product mixture, as shown in Figure 8. Pre-reduced

ZnAl-Pt exhibits lower CO conversion than pre-reduced ZnAl also under these reaction

13

conditions (Figure 8A). The stability as function of time on stream of both catalysts at

300 °C is indicated in Figure 8B. ZnAl shows the typical deactivation behaviour of Cu-

Zn-Al mixed oxide catalysts. ZnAl-Pt shows an increase in the CO conversion during

the first 6 h on stream, as observed under the simple WGS reactant mixture at 250 °C.

The CO conversion then decreases and approaches the conversion of ZnAl. A possible

explanation would be that the structure responsible for the high stability at 250 °C under

the simple WGS reactant conditions also existed initially at 300 °C under the simulated

reformate feed conditions but is not stable over a longer period under these reaction

conditions. Hungría et al. [15] reported that the Pd-Cu alloy formed in an Al2O3-

supported Pd-Cu/(Ce,Zr)Ox catalyst for NO reduction was gradually degraded under

reaction conditions at temperatures above 230 °C. Epron et al. [44] reported on

segregation of Pt and Cu and hence diminishing interaction at 400 °C, under reducing as

well as oxidizing conditions in an Al2O3-supported, bimetallic Pt-Cu catalyst.

3.3. Important aspects on bimetallic Pt-Cu

The change in reactivity of bimetallic surfaces is generally attributed to electronic,

ensemble and/or geometric effects [22,23,53].

Pt has a lattice constant that is approx. 9 % larger than the one of Cu. A pseudomorphic

Pt overlayer on a Cu substrate is therefore compressed, since the individual Pt atoms

have less space and thus interact/overlap more with surrounding neighbour states. This

results in a broader d-band and a downshift of the d-band center, away from the Fermi

level. Consequently, the binding energy/strength of adsorbates such as CO on the

surface decreases [45,53]. A decrease in the CO adsorption energy relative to pure Cu

may then result in a decreased WGS turnover rate of the bimetallic catalyst [54]. The

reactivity of the bimetallic surface may depend on the Pt coverage. Regarding the

supported Au-Ni surface alloy catalyst for steam reforming, it was shown that the

dissociation probability of CH4 decreases with increasing Au coverage on Ni [52,53]. In

analogy with Pd on Cu [22], real pseudomorphic Pt overlayers may not exist on Cu

because of the strong compression of the Pt layer by the Cu substrate lattice. Pt probably

forms a surface alloy with Cu [23,49]. Consequently, the above mentioned trends

14

derived from pseudomorphic overlayer systems are more of a qualitative rather than a

quantitative nature [22,53].

The thermodynamic behaviour of bimetallic systems is an important issue when

discussing surface reactivity. Two important quantities are the segregation energy and

the mixing energy [53]. Pt on Cu has a small negative surface segregation energy [50,53]

and a positive mixing energy [49,53], thus forming alloys in the surface layers of the Cu

lattice without significant Pt migration into the Cu bulk when considering energetics

without taking into account the temperature-dependent impact of entropy [53]. The

temperature may, however, also play an important role. With increasing temperature,

the entropy may eventually become dominant resulting in Pt migration into the bulk [53]

or destruction of the interactions between Cu and Pt [44]. The effect of the surrounding

gas may also have an important effect on the surface composition of the bimetallic

system [53]. The dependence of the catalyst activity on the pre-treatment conditions,

observed in this study and by Epron et al. [44], may be an indication for that.

While the decreased activity of the pre-reduced ZnAl-Pt, as compared to ZnAl, may be

explained by a d-band shift, the reason for the improved stability is not clear. Further

studies on the deactivation behaviour of Cu-Zn-Al mixed oxide catalysts as well as the

impact of Pt are necessary. The improved stability of a supported, bimetallic Au-Ni

steam reforming catalyst, as compared to a supported Ni catalyst, was explained with a

decrease in the coke formation rate [52]. For the bimetallic Pt-Cu catalyst, the improved

stability may be related to the suppression of gradual deterioration of active sites by

secondary chemical processes [35], in analogy with Au-Ni. It is known that the structure

of Cu particles/surface depends on the reaction environment [37,41]. A possible

question is therefore, whether reduced, active copper sites undergo gradual degradation

processes, possibly including sintering, Cu-Zn alloy formation and formation of surface

spinel species [33,38], partial reoxidation or degradation by H2O or CO2 [43,55-58], and

whether Pt could suppress these processes. The mechanism behind regeneration of

deactivated Cu-based catalysts by oxidation and subsequent reduction [35] is also of

significance in this respect.

15

4. Conclusions


precipitation with urea. While the Cu-Ce-Zr mixed oxide catalyst exhibited a high

surface area and a good distribution of all three metals, the Cu-Zn-Al mixed oxide

catalyst exhibited a relatively low surface area and an inhomogeneous distribution of the

three metals under the preparation conditions used. For the Cu-Ce-Zr mixed oxide

catalyst, the reduction of Cu could be achieved at lower temperatures than for the Cu-

Zn-Al mixed oxide. Improvement of the surface area, metal distribution and Cu

reducibility in the Cu-Zn-Al mixed oxide catalyst requires therefore different

preparation conditions.

Cu-Ce-Zr mixed oxide catalysts showed WGS activity without pre-reduction under the

reaction conditions used. The not pre-reduced sample was somewhat less active but

more stable than the pre-reduced sample. Pre-reduction is therefore not absolutely

necessary, but an adequate pre-reduction procedure may be applied to optimize the CO

conversion. Pt impregnated on the Cu-Ce-Zr mixed oxide had no significant effect on

Cu reducibility as well as WGS activity and stability.

The WGS activity of the Cu-Zn-Al mixed oxide catalyst correlated with the Cu

reducibility. Below the temperatures required for Cu reduction, the not pre-reduced

sample did not show significant WGS activity. At temperatures where Cu is at least

partly reduced, the catalyst showed significant WGS activity. Above the temperatures

required for partial Cu reduction, the not pre-reduced sample exhibited a somewhat

lower CO conversion than the pre-reduced sample, under similar short-term stability.

Consequently, pre-reduction is not absolutely necessary at these reaction temperatures,

but an adequate pre-reduction procedure may improve the CO conversion.

Pt impregnated on the Cu-Zn-Al mixed oxide catalyst shifted the Cu reduction and

hence also the WGS activity of the unreduced sample to lower temperatures. The not

pre-reduced sample showed similar WGS activity and stability, as compared to the Cu-

Zn-Al mixed oxide catalyst. The pre-reduced sample, however, exhibited a lower CO

conversion and a better stability at 250 °C than the Cu-Zn-Al mixed oxide catalyst

16

under CO/H2O/N2-feed conditions, indicating the existence of an interaction between

Cu and Pt in the bimetallic catalyst. Under CO/H2O/CO2/H2/N2-feed conditions at 300

°C, the impact of Pt diminished and the CO conversion of the Pt-impregnated catalyst

approached the one of the Cu-Zn-Al mixed oxide catalyst with time on stream,

indicating a destruction of the interaction between Cu and Pt.

Improvement of the performance of the bimetallic Pt-Cu catalyst requires optimization

of the Pt-loading as well as further studies on appropriate temperatures and ambient gas

atmospheres in order to facilitate the interaction between Pt and Cu.

Acknowledgements



Technology Center NTNU-SINTEF is also acknowledged for their support.

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Figure captions

Figure 1. N2 adsorption-desorption isotherms (A) and corresponding pore size

distributions (B) of the four catalysts after calcination.

Figure 2. XRD spectra of the four catalysts after calcination.

Figure 3. TEM images of ZnAl-Pt after calcination.

Figure 4. TPR profiles of the four catalysts. The samples, 0.065-0.066 g and 0.133-

0.137 g for ZnO- and CeO2-based catalysts, respectively, were heated in 5 vol-% O2/Ar

to 260 °C at a heating rate of 5 °C/min, kept there for 10 min, cooled down to ambient

temperature and flushed with Ar, and then heated in 7 vol-% H2/Ar at a heating rate of 2

°C/min.

Figure 5. Normalized short-term deactivation of CeZr and CeZr-Pt at 250 °C under the

CO/H2O/N2 (25/125/350 Nml/min) reactant mixture.

Figure 6. Initial CO conversion of ZnAl and ZnAl-Pt as function of reaction

temperature under the CO/H2O/N2 (25/125/350 Nml/min) reactant mixture. The

catalysts (0.051 g) were studied with and without pre-reduction.

Figure 7. A) Short-term deactivation of ZnAl and ZnAl-Pt at 250 °C under the

CO/H2O/N2 (25/125/350 Nml/min) reactant mixture, with and without pre-reduction. B)

Normalized short-term deactivation of ZnAl and ZnAl-Pt. All curves are normalized

with the initial CO conversion, calculated as the average of the first three analyses

obtained at 3 min. intervals.

Figure 8. A) CO conversion of ZnAl and ZnAl-Pt as function of reaction temperature

under the CO/H2O/CO2/H2/N2 (25/125/60/175/115 Nml/min.) reactant mixture. The

catalysts (0.110 g) were pre-reduced at 250 °C for 1 h in 10 vol-% H2/N2. B) Short-term

deactivation of pre-reduced ZnAl and ZnAl-Pt at 300 °C under the CO/H2O/CO2/H2/N2

(25/125/60/175/115 Nml/min.) reactant mixture.

21

Figure 1

A)

0

100

200

300

0 0,2 0,4 0,6 0,8 1Relative pressure [-]

Nitr

ogen

ads

orbe

d [c

m3 /g

STP

] CeZrCeZr-PtZnAlZnAl-Pt

B)

0

0,004

0,008

0,012

0,016

0,02

1 10 100 1000

Pore width [nm]

Pore

vol

ume

[cm

3 /g S

TP]

CeZrCeZr-PtZnAlZnAl-Pt

22

Figure 2

0

10000

15 20 30 40 50 60 70 80

2 [ ° ]

Inte

nsity

[a. u

.]

CeZr

CeZr-Pt

ZnAl

ZnAl-Pt

Pt

PtO2

0

10000

15 20 30 40 50 60 70 80

2 [ ° ]

Inte

nsity

[a. u

.]

CeZr

CeZr-Pt

ZnAl

ZnAl-Pt

Pt

PtO2

23

Figure 3

A)

B)

100 nm100 nm100 nm

200 nm200 nm200 nm

24

C)

50 nm50 nm50 nm

25

Figure 4

-20

20

60

100

140

180

30 70 110 150 190 230 270 310

Temperature [ °C ]

Sign

al [

a. u

. ]

CeZrCeZr-PtZnAlZnAl-Pt

26

Figure 5

0,5

0,7

0,9

1,1

0 4 8 12 16

TOS [ h ]

Nor

mal

ized

CO

con

vers

ion

[ - ]

CeZr redCeZr not redCeZr-Pt redCeZr-Pt not red

27

Figure 6

0

4

8

12

16

20

24

150 170 190 210 230 250 270

temperature [ C ]

CO

con

vers

ion

[ %

]

ZnAl red

ZnAl not red

ZnAl-Pt red

ZnAl-Pt not red

28

Figure 7

A)

0

4

8

12

16

20

0 5 10 15 20 25

TOS [ h ]

CO

con

vers

ion

[ %

]

ZnAl red

ZnAl not red

ZnAl-Pt red

ZnAl-Pt not red

B)

0

0,4

0,8

1,2

0 5 10 15 20 25

TOS [ h ]

Nor

mal

ized

CO

con

vers

ion

[ - ]

ZnAl redZnAl not redZnAl-Pt redZnAl-Pt not red

29

Figure 8

A)

0

5

10

15

20

25

170 190 210 230 250 270 290 310

temperature [ C ]

CO

con

vers

ion

[ %

]

ZnAl red

ZnAl-Pt red

B)

0

6

12

18

24

0 8 16 24 32 40

TOS [ h ]

CO

con

vers

ion

[ %

]

ZnAl red

ZnAl-Pt red

30

Table 1. Composition and structural parameters of the mixed oxide catalysts.

Catalyst composition – metal content

Cu Ce/Zn Zr/Al Pt BET

asurfacevolume

pore volume a

[% (g/g-cat.)] [m2/g] [m2/mm3] [cm3/g] [cm3/cm3]

CeZr 9.4 69.4 14.0 161 1.12 0.40 2.79

CeZr-Pt 9.2 67.2 13.5 0.7 147 0.30

ZnAl 19.4 42.0 7.3 74 0.40 0.20 1.08

ZnAl-Pt 18.8 40.3 7.1 2.2 79 0.18

a Surface-to-volume ratio and pore volume in [cm3/cm3] estimated by linear

combination of tabulated densities for CeO2 (7650 kg/m3), ZrO2 (5680 kg/m3,

tetragonal phase), CuO (6310 kg/m3), ZnO (5660 kg/m3) and γ-Al2O3 (3650 kg/m3).

31

Table 2. Initial CO conversion of CeZr and CeZr-Pt under the CO/H2O/N2 (25/125/350

Nml/min) reactant mixture. The catalysts (0.051 g) were studied with and without pre-

reduction.

CO conversion [ % ]

Temperature CeZr CeZr-Pt

[ °C ] pre-reduced not pre-reduced pre-reduced not pre-reduced

165 6.47 6.39 5.14 5.76

180 9.27 8.39 7.94 8.36

195 12.53 10.55 11.10 11.10

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