The Synthesis of Highly Loaded Cu/Al2O3 and Cu/ZnO/Al2O3 Catalysts by the Two-Step CVD of...

DOI: 10.1002/cvde.200906808

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The Synthesis of Highly Loaded Cu/Al2O3 and Cu/ZnO/Al2O3

Catalysts by the Two-Step CVD of CuIIdiethylamino-2-propoxidein a Fluidized-Bed Reactor**

By Michael Becker, Raoul Naumann d‘Alnoncourt, Kevin Kahler, Jelena Sekulic, Roland A. Fischer, and Martin Muhler*

Highly loaded copper catalysts supported on alumina are synthesized applying the cyclic two-step CVD of the precursor

copper(II)diethylamino-2-propoxide in a fluidized-bed reactor. Copper/zinc oxide/alumina composites are synthesized by

either the CVD of the precursor bis[bis(trimethylsilyl)amido]zinc on Cu/Al2O3, or the CVD of the Cu precursor on

Zn-pretreated alumina, impregnating with diethyl zinc in addition. The composites are extensively characterized by atomic

absorption spectroscopy (AAS), elemental analysis (EA), mass spectrometry (MS), N2 physisorption, N2O reactive frontal

chromatography (RFC), and X-ray diffraction (XRD). The Cu and ZnO nanoparticles originating from the efficient two-step

procedure, consisting of adsorption and subsequent decomposition of the adsorbed species in two separated steps, are highly

dispersed, X-ray amorphous, and, in the case of the Cu-containing catalysts, have high specific Cu surface areas. The catalytic

activities are determined both inmethanol synthesis, to judge the contact between the deposited Cu and ZnOnanoparticles, and

in the steam reforming of methanol (SRM) to probe the stability of the Cu particles. The turn-over frequencies (TOF) in

methanol synthesis of these Cu/ZnO/Al2O3 catalysts are higher than that of a commercial ternary catalyst. The varied sequence

of the CVD of Cu and ZnO on alumina leads to catalysts with similar activities in the case of similar specific Cu areas.

Keywords: copper, fluidized-bed reactor, methanol synthesis, nanocomposites, two-step deposition, zinc oxide

1. Introduction

The production of methanol from synthesis gas (CO/

CO2/H2) is one of the most important processes in the

chemical industry.[1] The ternary copper/zinc oxide/alumina

system is applied as the catalyst in this process, which is also

highly active in the SRM.[2,3] The role of ZnO in the Cu/

ZnO/Al2O3 catalyst is still under debate.[4] Alumina serves

as the structural promoter stabilizing ZnO and thus

producing a high copper surface area.[5] Due to the strong

synergy between Cu and ZnO, a larger contact area results

in a strong increase in the catalytic activity.[6–8]

Industrial catalysts for methanol synthesis are produced

by the coprecipitation of Cu, Zn, and Al salts, mostly

nitrates, which are precipitated by means of basic agents

such as alkali carbonates, hydroxycarbonates, or hydro-

xides.[5] The main disadvantage of coprecipitation is the

[*] Prof. M. Muhler, M. Becker, Dr. R. Naumann d‘Alnoncourt, K. KahlerLaboratory of Industrial Chemistry, Ruhr-University BochumD-44780 Bochum, (Germany)E-mail: [email protected]

Dr. J. Sekulic, Prof. R. A. FischerChair of Inorganic Chemistry II, Ruhr-University BochumD-44780 Bochum, (Germany)

[**] Financial support by the German science foundation (DFG) within theTransfer Project of the Collaborative Research Center (SFB 558)‘‘Metal-Substrate Interactions in Heterogeneous Catalysis’’ is gratefullyacknowledged.

Chem. Vap. Deposition 2010, 16, 85–92 � 2010 WILEY-VCH Verlag Gmb

huge number of experimental parameters, such as purity of

the chemicals, temperature, pH value, concentration, mixing

velocity and aging time, washing cycles, filtration, etc., which

are often difficult to control. Slight changes in the

preparation conditions typically result in significant changes

in the properties of the product leading to a low degree of

reproducibility.

An alternative synthesis method for catalysts, with a

much lower number of involved unit operations, is the CVD

of suitable volatile metal precursors which can be decom-

posed completely without residual poisons such as chlorine

or phosphorous.[9] This technique can be applied under mild

conditions at temperatures below 473K to avoid undesired

reactions or sintering effects. In the case of ternary catalysts,

the role of the sequence of the CVD of the two different

active components can be investigated. For example, Kurtz

et al.[10] developed novel routes for the preparation of

ternary Cu catalysts, including the CVD of diethyl zinc on

Cu/Al2O3 and Becker et al.[11] loaded mesoporous siliceous

matrices with Cu/ZnO by CVD, both obtaining highly active

catalysts. These studies were recently extended by Muller

et al.[12] who loaded MOF-5 with Cu and ZnO nanoparticles

by infiltration with volatile [CpCuL] compounds and ZnEt2.

The combination of CVD and fluidized-bed reactors

offers many advantages. The fluidization of the substrate is

achieved by a carrier gas flow streaming vertically from the

bottom to the top of the reactor. This results in vigorous

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mixing of the substrate particles and high heat transfer rates,

thus eliminating temperature and concentration gradients in

the reactor, and maximizing the interface between the

substrate powder and the gas phase.[13,14] By the deposition

of the gaseous precursor a homogenous distribution of

highly dispersed metal particles on the support can be

achieved, resulting in a narrow particle size distribution

and a high degree of reproducibility due to the easily

controllable process conditions. A scale-up of the fluidized-

bed reactor, including a change from the applied batchmode

to continuous conditions, would be possible without any

significant problems. In addition to the deposition of metal

particles, the support material can be coated with another

oxide. Xia et al.[15] prepared ZrO2/SiO2 nanocomposites

using a volatile Zr alkoxide as the precursor. The synthesis

of ZnO/TiO2 nanocomposites on Si(100) and Al2O3

substrates was described by Barreca et al.[16]

Following our preliminary study,[17] we applied the CVD

of organo-metallic copper and zinc compounds. For the

deposition of ZnO two different sources of Zn were used:

bis[bis(trimethylsilyl)amido]zinc (1), which was recently

developed by Maile and Fischer[18] as a suitable Zn

precursor for CVD applications because of its low

evaporation and decomposition temperatures, was chosen.

In addition, liquid impregnation with diethyl zinc, according

to the method reported in the literature,[19] was applied. The

formation of ZnO particles on the substrate surface was

achieved by adsorption of the Zn precursor followed by

exposure to synthetic air (containing 20.5% O2 in N2).

In previous experiments, CuIIdiethylamino-2-propoxide

(2), as well as CuIIdimethylamino-2-propoxide (3), were

used as sources for Cu.[17] For the present experiments (2)

was applied exclusively because of its better thermal

properties with its melting point being 133K lower than

the decomposition temperature, so the precursor can be

vaporized from the liquid state, resulting in steady and

higher evaporation rates compared to the solid precursor

(3). The application of (2) improved the yield and reduced

the duration of the experiment. Both precursors can

be synthesized using commercial, low-cost reagents with

halogen-free ligands to avoid poisoning of the active

catalyst.

Numerous CVD experiments described in literature

were performed combining adsorption and decomposition

of the precursor in one step. The two-step method is a

promising alternative for catalyst preparation, i.e., disso-

ciative adsorption and decomposition are carried out in two

consecutive steps.[15] In the first step, the gaseous precursor

is adsorbed onto the substrate at a temperature sufficiently

below the onset of thermal decomposition under inert

conditions, mostly interacting dissociatively with hydroxyl

groups. Subsequently, the anchored precursor-derived

species is decomposed by reduction with H2 or oxidation

with O2 under mild conditions. This two-step method was

successfully applied to synthesize supported catalysts such as

Pd/SiO2, Pd/C, Fe/C, and ZrO2/SiO2.[15,20–23]

86 www.cvd-journal.de � 2010 WILEY-VCH Verlag GmbH

The aim of this work was the reproducible synthesis of Cu

and ZnO catalysts by CVD in a fluidized-bed reactor.

In order to synthesize nanometer size and uniformly

distributed Cu particles on the ZnO/Al2O3 or Al2O3

supports, the two-step, metal-organic (MO)CVD method

was applied with online monitoring of the adsorption of the

metal-organic precursor (2) by MS. The obtained nano-

composites were characterized by AAS, EA, N2 physisorp-

tion, N2ORFC, and XRD. To achieve a high Cu loading, the

two-step CVD process was repeated up to 7 times. The role

of the sequence of the Cu and Zn CVD was investigated

by depositing Cu on Zn-pretreated Al2O3, or depositing

Zn onto Cu/Al2O3. Finally, the catalytic activities were

determined, both in methanol synthesis to judge the contact

between the deposited Cu and ZnO nanoparticles, and in

the SRM to probe the stability of the Cu particles.

2. Results and Discussion

2.1. Characterization of the Alumina Support

Commercially available Al2O3 (Fluka), which is usually

applied in chromatography, was chosen as the substrate.

When suspended in water, its pH value is 7. The Brunauer-

Emmett-Teller (BET) surface area and the average

diameter of the mesopores were determined by N2

physisorption giving values of 165m2 g�1 and 4 nm,

respectively. The alumina particle size distribution was in

the range 50–150mm. The areal density of OH groups on the

alumina substrate was derived from thermogravimetry

(TG)/MS measurements giving a value of 19.6mmol m�2.

The water content was very low (Brockmann activity class

I). The material can be classified as A to B according to the

Geldart classification, and has a suitable fluidization

behavior.[24] For a fluidized bed, typically consisting of

10 g of Al2O3, the pressure drop was 3.7 hPa determined at

20 hPa, and the minimum fluidization velocity was estimated

to be 16.2 sccm. No change of the fluidization behavior with

increasing metal loading was observed.

2.2. Synthesis of Highly Loaded Cu/Al2O3 Catalysts

The methanol synthesis activity of binary Cu/Al2O3 and

ternary Cu/ZnO/Al2O3 catalysts is often linearly related to

the specific Cu surface area.[10] As described in our previous

communication,[17] no free specific Cu surface area was

obtained from Cu/Al2O3 (CA) samples prepared at 448K

by the one-step method consisting of the simultaneous

adsorption and decomposition of precursors (2) or (3) on the

Al2O3 substrate. In order to achieve smaller Cu particles

with larger surface areas, the two-step method was applied.

For a comparison of the CVD methods, sample CA-1 was

prepared by the one-step method. During this experiment

the color of CA-1 changed from white to black. During the

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Fig. 1. Mass spectra obtained during the synthesis of the Cu/Al2O3 samples

CA-1 and CA-2 in the range 124–133m/e. Spectrum a) was obtained during

the one-step CVD synthesis of sample CA-1 in Ar at 448K. CA-2 was

prepared by the two-step CVD method. Spectrum b) was obtained in Ar

at 363K during the 1st step, and spectrum c) during the subsequent exposure

to H2 at 363K (2nd step). The Mþ of 1-diethylamino-2-acetone (129m/e) and

diethylamino-2-propoxide (131m/e) are indicated by the arrows.

deposition of (2) applying the two-step method the color of

the sample CA-2 changed from white to green. In order to

monitor the gaseous reaction products of (2) in the effluent

gas, on-line MS was applied during the CVD preparation of

CA-1 and CA-2. A pumped glass capillary allowed the

analysis of the gas composition above the fluidized bed.

The fragmentation patterns in the range 124–133m/e of the

gaseous products during both experiments are shown in

Figure 1.

During the one-stepmethod of preparing sample CA-1 at

448K the Mþ of the free ligand diethylamino-2-propoxide

(m/e¼ 131) and the by-product 1-diethylamino-2-acetone

(m/e¼ 129) were detected. This by-product was described

by Becker et al.[25] as an oxidation product of the thermal

decomposition of (2) and is shown in Scheme 1. When

applying the two-step method using Ar as the fluidization

gas, only the ligand 1-diethylamino-2-propoxide was

detected by on-line MS (Fig. 1b), suggesting that the OH

Scheme 1. Reactions of (2) with Al2O3 during the one-step and two-step CVD pro

(2) on Al2O3.

Chem. Vap. Deposition 2010, 16, 85–92 � 2010 WILEY-VCH Verlag Gm

groups of the Al2O3 surface served as anchoring sites for

the Cu precursor. The absence of the by-product

1-diethylamino-2-acetone in the effluent gas during the

adsorption step indicates that no thermal decomposition of

(2) occurred at 363K. The occurrence of the free ligand in

the effluent gas during the second step proves that the

adsorbed precursor was decomposed by hydrogenolysis in a

H2 atmosphere. Scheme 1 illustrates the decomposition

reactions of the one-step and two-step CVD processes.

The Cu contents of the as-prepared samples were

determined by AAS. The overall Cu deposition yield was

derived from the initial precursor mass multiplied by its Cu

content and from the total mass and the Cu content of the

sample after deposition. The results of the Cu AAS and the

Cu yields are summarized in Table 1. The amount of

deposited Cu was found to depend on the precursor mass

used, and the number of deposition cycles applied to

increase the Cu loading. To determine the specific Cu

surface area of the reduced samples, RFC was applied using

nitrous oxide (N2O RFC) to oxidize the first monolayer of

Cu atoms as described in detail in the literature.[26]

No detectable Cu surface was found for CA-1. Taking

into account the high Cu content of this sample (10.6wt.-%),

the absence of a detectable Cu surface area points to very

large Cu particles located at the outer surface of the support

particles. In contrast, the CA-2 exposed a specific Cu surface

area of 2.2 m2 g�1, whereas its Cu content was 4.5wt.-%.

Based on the results of the Cu AAS and the N2O RFC, the

ratio of Cu surface atoms to the total number of Cu atoms,

i.e., the degree of dispersion, was derived. Due to the

blocking of small pores, the BET surface area of the samples

was decreased from 165 m2 g�1 to typically 120 m2 g�1 after

the deposition. The results of the N2ORFC experiments, the

BET areas, and the degrees of Cu dispersion are also

summarized in Table 1.

The powder XRD pattern of the support and the samples

containing Cu and Al2O3 are shown in Figure 2. The XRD

pattern of Al2O3 shows that the substrate mainly consists of

g-Al2O3, but fractions of d- and h-Al2O3 can also be

identified. CA-1, prepared by one-step CVD, exhibits sharp

reflections at 2Q¼ 43.158, 50.358, and 74.078 assigned to the

cesses. The intermediate product (2)0/Al2O3 is derived from the adsorption of

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Table 1. CVD method, Cu content (vCu), Cu surface area (SCu), BET surface area, degree of dispersion (dCu), and Cu yield (YCu) of Cu/Al2O3 samples.

Sample CVD method vCu [wt.-%] [a] SCu [m2 g�1] [b] BET area [m2 g�1] dCu [%] YCu [%]

CA-1 One-step 10.6 <1 115.0 – 67

CA-2 Two-step 4.5 2.2 136.0 7.6 45

CA-3 Two-step 4.2 1.6 129.1 5.9 44

CA-4 Two-step repeated 7� 10.7 7.7 119.5 11.2 91

[a] determined by AAS; [b] determined by N2O RFC.

reflections of the (111), (200), and (220) crystal planes of

metallic Cu. The Cu particles have a mean diameter of 15–

21 nm, estimated from the line width of the XRD reflections

using Scherrer’s equation (K¼ 0.9). The diffraction pattern

of CA-2, prepared by the two-step method, contains no

reflections of Cu or CuxOparticles, indicating that the size of

Cu nanoparticles prepared by the two-step CVD method is

below the XRD detection limit of about 3 nm. Based on

these results the two-step CVD was chosen for all following

experiments to synthesize Cu/Al2O3 and Cu/ZnO/Al2O3

catalysts.

For technical reasons the synthesis of samples CA-1 and

CA-2 had to be carried out under atmospheric pressure to

enable the on-line monitoring by the quadrupole (Q)MS. In

order to increase the evaporation rate of (2), sample CA-3

was prepared under reduced pressure. The Cu content and

the Cu yield of this experiment were of the same magnitude

as those of CA-2. Surprisingly, the Cu dispersion of CA-3

was somewhat lower.

For preparing CA-4, seven successive two-step CVD

cycles consisting of the deposition of 1 g of (2) in Ar, and

subsequent reduction in H2, both at 363K, were carried out

on the same sample. The aim of this series was to monitor

the step-wise increase of the Cu content and to achieve a

high metal loading and a high specific Cu area. For each

experiment 1.0 g of (2) was used, and the amount of Al2O3

Fig. 2. Powder XRD images of the as-prepared Cu/Al2O3 (CA) samples;

a) CA-1, b) CA-2, c) CA-4, d) Al2O3 substrate. The reflections of polycrystal-

line copper are indicated at the abscissa (JCPDS No. 4-826).


used in the first CVD experiment was 10.0 g. The green color

of the sample darkened with increasing number of Cu

deposition cycles. After each deposition a sample amount of

200mg was analyzed by AAS and N2O RFC to determine

the Cu content and the specific Cu surface area. The results

are shown in Figure 3. The Cu content increased almost

linearly with the number of experiments, indicating a high

degree of reproducibility for this method. The Cu yield in

this series was in the range 84–92%. Due to the low Cu

content and the detection limit of the N2O RFC technique,

no specific Cu surface area was detectable for the first three

samples. Starting with the fourth experiment the copper

surface area increased, but not linearly. The degree of Cu

dispersion in the last three samples was calculated to be 11–

12%. The XRD pattern of sample CA-4 displayed no

reflections of Cu or CuxO particles (Fig. 2). These results

demonstrate that the application of successive CVD cycles

leads to a high Cu content and avoids the formation of large

Cu particles.

2.3. Synthesis of Cu/ZnO/Al2O3 Catalysts

All Cu/ZnO/Al2O3 samples were synthesized using

Al2O3 as the support and (2) as the source of Cu, whereas

two different sources for Znwere applied. Samples prepared

Fig. 3. The seven successive preparation steps of sample CA-4; the Cu content

and the specific Cu area were derived from Cu AAS and N2O RFC,

respectively, as a function of the number of deposition cycles.


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Table 2. Synthesis of Cu/ZnO/Al2O3 catalysts by liquid impregnation and

CVD. Et2Zn: Diethyl zinc; (1): Bis[bis(trimethylsilyl)amido]zinc; (2):

CuIIdiethylamino-2-propoxide.

Sample Synthesis method

CZA-1-Et2Zn 1. Al2O3þEt2Zn !ZA-Et2Zn

2. ZA-Et2Znþ (2) !CZA-1-Et2Zn

CZA-2-Et2Zn 1. Al2O3þEt2Zn !ZA-Et2Zn

2. ZA-Et2Znþ (2) !CZA-2-Et2Zn

ZCA-3-Et2Zn 1. Al2O3þ (2) !CA-3

2. CA-3þEt2Zn !ZCA-3-Et2Zn

ZCA-4-(1) 1. Al2O3þ (2) !CA-4

2. CA-4þ (1) !ZCA-4-(1)

CZA-5-(1) 1. Al2O3þ (1) !ZA-(1)

2. ZA-(1)þ (2) !CZA-5-(1)

by the liquid impregnation with diethyl zinc were labeled

with -Et2Zn, samples prepared by the CVD of precursor

(1) were labeled with -(1). In addition, the sequence of

deposition of Cu and Zn was varied. The sample label CZA

implies that first Zn and then Cu was deposited, ZCA

specifies that first Cu and then Zn was deposited on Al2O3.

The sample preparation is summarized in Table 2.

The results of the Cu and Zn AAS, specific Cu surface

areas, degrees of Cu dispersion, and the Cu yields of the

as-prepared samples are summarized in Table 3. The sample

ZA-Et2Zn was obtained by liquid impregnation of Al2O3

using diethyl zinc and served as the support for CZA-1-

Et2Zn and CZA-2-Et2Zn. The BET surface area of this

sample was 119 m2 g�1. In preparing CZA-2-Et2Zn, three

successive CVD cycles were applied on the same support.

High degrees of Cu dispersion of about 11.4% and 13.4%

were found for the samples CZA-1-Et2Zn and CZA-2-

Et2Zn, respectively. Liquid impregnation was also applied

to the calcined sample CA-3 containing only Cu and Al2O3,

to prepare sample ZCA-3-Et2Zn. A surprisingly high ZnO

content of 11wt.-%was found. In order to synthesize sample

ZCA-4-(1), ZnO was added by the CVD of (1) on the

sample CA-4, which had been prepared by seven CVD

cycles. The deposition resulted in a decrease of the specific

Cu surface area from 7.7 to 7.0 m2 g�1. In preparing sample

CZA-5-(1), first the CVD of (1) and then the CVD of (2)

were applied on the Al2O3 support. The highest degree

of Cu dispersion of all samples was achieved by this

preparation procedure.

Table 3. Composition, Cu and Zn content (vCu, vZn), Cu surface area (SCu), BET su

samples. Type of Zn source: Et2Zn¼ prepared by impregnation with Et2Zn; (1)¼pre

were made.

Sample Composition vCu [wt.-%] vZn [wt.-%]

ZA-Et2Zn ZnO/Al2O3 – 6.8

CZA-1-Et2Zn Cu/ZnO/Al2O3 3.4 5.9

CZA-2-Et2Zn Cu/ZnO/Al2O3 7.9 5.0

ZCA-3-Et2Zn Cu/ZnO/Al2O3 4.2 11.0

ZCA-4-(1) Cu/ZnO/Al2O3 10.1 3.2

CZA-5-(1) Cu/ZnO/Al2O3 2.9 4.5


Depending on the used amount of (2), the content of Cu

was in the range 2.9–10.1wt.-%, and the BET surface area

decreased to about 115 m2 g�1. As expected, all samples

containing Cu exposed accessible specific Cu surface areas.

The amounts of nitrogen, hydrogen, and carbon originating

from residual precursor ligands were determined by EA.

The nitrogen content never exceeded 0.8wt.-%, and the

hydrogen content was in the range 1.0–1.3wt.-%. The

carbon content varied from 2.8 to 6.0wt.-% depending on

the number of deposition cycles. The amounts of N,H, andC

were significantly reduced by calcination in air. Figure 4

shows the XRD patterns of the samples ZA-Et2Zn, CZA-2-

Et2Zn, and ZCA-4-(1). All XRD patterns display the

reflections of Al2O3, but only very broad reflections of ZnO

were found. Due to the application of the two-step CVD, no

reflections of Cu or CuxO were detectable in the samples

CZA-2-Et2Zn and ZCA-4-(1). The results of the XRD and

Zn AAS measurements demonstrate that liquid impregna-

tion with Et2Zn, as well as the CVD of (1), are suitable for

the deposition of X-ray amorphous ZnO.

2.4. Modification of a Conventional TernaryCu/ZnO/Al2O3 Catalyst

In addition to the synthesized catalysts, an industrially

applied ternary catalyst was modified by the deposition of

ZnO. A sample of the ternary catalyst was reduced in

streaming H2 and then modified by the CVD of (1). The Zn

content of the modified catalyst (conventional catalyst-(1))

increased from 14.6 to 20.9wt.-%, but the deposition resulted

in a strong decrease of the specific copper surface area.

Additionally, the BET areas were determined before and

after the CVD experiment. The original catalyst had a BET

area of 93.6 m2 g�1, which decreased to 67.3 m2 g�1 due to the

modification (Table 4). The decrease of the Cu surface area

and the lower BET area indicate the presence of ZnO on the

Cu particles and the blocking of small pores, respectively.

2.5. Catalytic Activity

The samples were tested in methanol synthesis and in the

SRM to assess the catalytic activity. All investigated samples

rface area, degree of dispersion (dCu), and Cu yield (YCu) of ZnO-containing

pared by CVD of (1). No investigations of the ZnO/Al2O3 intermediate ZA-(1)

SCu [m2 g�1] BET area [m2 g�1] dCu [%] YCu [%]

– 119.2 – –

2.5 110.6 11.4 32

6.8 105.0 13.4 63

1.3 92.0 5.8 –

7.0 113.3 10.8 –

2.7 118.0 14.5 44


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Table 5. Rates of methanol production (mmolMeOH g�1cat h

�1), TOF, Cu

SSA.

Sample Rate of production [a] TOF Cu SSA [b]

[mmolMeOH g�1 h�1] [h�1] [m2 g�1]

CZA-1-Et2Zn 84 10.3 4.1

CZA-2-Et2Zn 162 10.4 6.9

ZCA-3-Et2Zn 51 23.5 1.3

CA-4 0 0 1.4

ZCA-4-(1) 141 9.5 7.0

conventional catalyst 436 8.9 20.5

conventional catalyst-(1) 249 14.6 6.9

[a] Methanol synthesis was performed under ambient pressure at 493K for20 h. Feed gas: 14% He, 72% H2, 4% CO2, and 10% CO; [b] SSA wasdetermined after the methanol synthesis.

Fig. 4. XRD patterns of the as-prepared samples: a) ZCA-4-(1), b) CZA-2-

Et2Zn, c) ZA-Et2Zn, d) the Al2O3 substrate. The reflections of polycrystalline

ZnO are indicated at the abscissa (JCPDS No. 89-0510).

were calcined in synthetic air at 613K and reduced first in

diluted and then in pure H2. The methanol synthesis

activities for samples CZA-1-Et2Zn, CZA-2-Et2Zn, ZCA-3-

Et2Zn, and ZCA-4-(1), as well as the original and the

modified conventional ternary catalyst (conventional cata-

lyst-(1)) are summarized in Table 5. The catalytic activities

are expressed as rate of production in mmol h�1 g�1cat. Also

the TOF in h�1, defined as the number of methanol

molecules produced per surface Cu atom per hour, were

derived. The specific surface area (SSA) of Cu was

determined after methanol synthesis by N2O RFC. As

expected, sample CA-4, containing only Cu and alumina,

showed no activity in methanol synthesis. All Cu/ZnO/

Al2O3 samples exhibit a stable catalytic behavior with no

significant changes of the methanol content in the effluent

gas during testing. The catalytic activity of the conventional

ternary catalyst was much higher than the CVD samples,

obviously due to the higher content of Cu and ZnO.

Interestingly, TOFs of all CVD-prepared samples were

higher than the TOF of the industrial catalyst, indicating

higher intrinsic catalytic activity.

The Cu content of the calcined sample ZCA-4-(1) is

10.1wt.-% with a Cu surface area of 7.0 m2 g�1, whereas the

sample CZA-2-Et2Zn has 7.9wt.-% Cu and a Cu surface

area of 6.9 m2 g�1. It is interesting to note that both catalysts

had similar high activities indicating that the sequence of the

deposition of Cu and ZnO does not have a major influence,

Table 4. Cu and Zn content (vCu, vZn), Cu surface area (SCu), degree of dispersio

Sample vCu [wt.-%] [a] vZn [wt.-%]

Conventional catalyst 38.1 14.6

Conventional catalyst-(1) 37.9 20.9

[a] Cu contents determined by TPR in H2.


either on the Cu surface area or on the catalytic activity.

Further studies employing high-resolution transmission

electron microscopy (HRTEM) are in progress to elucidate

the microstructure of these two catalysts.

The SRM is also a suitable reaction to characterize the

catalytic performance of Cu/Al2O3 and Cu/ZnO/Al2O3

samples in the presence of water vapor. The reaction

network is described in detail in the literature.[2] The results

of the SRM expressed as overall rates of production of H2

and CO2 at 493K and 523K are summarized in Table 6.

The Cu and ZnO contents of the samples tested in the

SRM are listed in Tables 1 and 3. The samples CZA-2-

Et2Zn, CA-4, and CZA-4-(1) were active, but deactivated

slowly during the first few hours under reaction conditions.

After 3 h at 523K, the catalytic performance reached steady

state. This typical deactivation behavior can be attributed to

sintering of the Cu particles under SRM conditions.[2]

Sample CA-1 showed no activity due to the too small Cu

surface area. In contrast, sample CA-4 was highly active

containing an almost equal amount of Cu, but exposing a

much higher Cu surface area. This observation confirms that

a small Cu particle size and a high accessible Cu surface area

are necessary for good catalytic performance. The higher

rates of production of sample ZCA-4-(1) compared to

sample CA-4 prove that the presence of ZnO (3.2wt.-%) is

also beneficial for the SRM; however, it is not mandatory as

demonstrated by the high activity of the ZnO-free catalyst

CA-4. The rates of H2 production of sample CZA-2-Et2Zn

are similar to sample ZCA-4-(1) with 10.7wt.-%, indicating

again that the sequence of deposition of Cu and ZnO does

not play a major role.

n (dCu).

SCu [m2 g�1] BET area [m2 g�1] dCu [%]

20.5 93.6 8.3

6.9 67.3 2.8


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Table 6. Rates of production of the SRM (mmolproduct g�1

cat h�1). The SRM

was performed under ambient pressure for 8 h at 293K, followed by 3 h at

523K, and 4 h at 293K. The feed gas contained 6.4% water and 6.4%

methanol in argon. The listed data were obtained after a time on stream

of 11 h (523K) and 15 h (493K).

Sample Rate of production

H2 at 493K H2 at 523K CO2 at 493K CO2 at 523K

CA-1 0 0 0 0

CZA-2-Et2Zn 150 410 40 117

CA-4 151 381 30 78

ZCA-4-(1) 178 467 35 99

3. Conclusions

Highly dispersed Cu and ZnO particles were successfully

deposited on alumina in a fluidized-bed reactor using the

compounds bis[bis(trimethylsilyl)amido]zinc and copper

(II)diethylamino-2-propoxide as CVD precursors. Only

with samples prepared under mild conditions using the

two-step method, were small Cu particles and large Cu

surface areas achieved, whereas the deposition of the Cu

precursor at high temperatures in one step led to the

formation of large Cu particles with diameters above 15 nm.

By repeating the two-step Cu CVD, the Cu content was

found to increase linearly, and a specific Cu surface area of

7.7 m2 g�1 was obtained after 7 cycles. The Al2O3-supported

samples containing both Cu and ZnO were catalytically

active in methanol synthesis and exhibited high TOFs. The

varied sequence of the CVD of Cu and ZnO on alumina led

to catalysts with similar activities in the case of similar

specific Cu areas. The additional deposition of ZnO on a

commercial ternary catalyst resulted in a strong decrease of

the Cu surface area, but in an increase of the methanol TOF.

All samples prepared by the two-step method were also

active in the SRM.

4. Experimental

Synthesis of Copper(II)diethylamino-2-propoxide: In the first step,copper methoxide and lithium methoxide were dissolved in dried methanol.In the second step, diethylamino-2-propanol was added to a stirredsuspension of Cu(OMe)2. The resulting violet solution was stirred for 2 h.The product was obtained after removal of the solvent and slow sublimationunder vacuum conditions at 348K.

Synthesis of Bis-[bis(trimethylsilyl)amido]zinc: Sodium bis(trimethylsi-lyl)amide and zinc(II) chloride were suspended in dried diethyl ether. Themixture was refluxed for 1 h and then filtered. The solvent of the filtrate wasremoved under vacuum and the product was purified by distillation undervacuum conditions at 328K.

The CVD System: The CVD experiments were carried out in a home-built, vertical, fluidized-bed reactor made of Duran glass described in detail inthe literature [17]. The reactor had a diameter of 20mm and a height of420mm. A ceramic filter element with a diameter of 5mm served as a gasdistributor. Additionally, 1 g of glass beads with mean diameter of 1mm wereplaced at the bottom of the reactor to improve the gas distribution and toavoid the formation of compression layers within the bed. The gas outletsection was conical in shape.

The on-line gas analysis of the exhaust line of the reactor was carried outwith a QMS (Thermostar, Pfeiffer Vacuum) with a range of 300 amu. As the


gas inlet system of theMS is built for analyzing gas streams under atmosphericpressure, no MS data were available for experiments preformed underdynamic vacuum conditions.

For a typical CVD experiment, the glass saturator was filled with 1 g

precursor under inert atmosphere. The saturator was connected to the reactorand the bypass of the reservoir was purged with Ar to remove O2 andmoisture. After purging, 10 g of the alumina substrate was dried in-situ undervacuum conditions at 373K for 1 h to remove any physisorbed water, whilethe precursor chamber was kept at room temperature. The substrate wasfluidized by an Ar flow of 30 sccm. The minimum fluidization velocity (umf)was determined to be 5.9T 10�3 m s�1. The ratio between the appliedfluidization velocity (u) and umf was chosen to be u/umf¼ 4.3.

The precursor was stored in a precursor reservoir and heated to 348K.

The Ar carrier gas (99.999%) was passed over the liquid precursor. Thevaporization was carried out either under reduced pressure (20 hPa) toincrease the rate of evaporation, or under atmospheric pressure, in case ofusing the QMS to monitor the exhaust gas. As indicated by the very lowsublimation temperature of 348K, precursor (2) is very volatile, rendering ithighly suitable for CVD under both atmospheric and reduced pressures. TheCVD experiment was started by switching the precursor reservoir on-line.The stream of argon was passed through the reservoir and loaded withvaporized precursor before entering the reactor. The vapor deposition wasperformed at 448K when using precursor (1) or when the one-step method isapplied with precursor (2). When applying the two-step method the reactortemperature was 363K during the deposition of (2). The experiment wasfinished when the precursor was completely consumed. The reservoir wasthen switched off-line. Usually, the deposition was finished after 12 h.

In a separate step, the adsorbed precursor was decomposed feeding a

stream of 30 sccm of pure H2 into the reactor at 513K when using (1), and at363K when using (2). After 1 h the flowing gas was switched to Ar in order toflush H2, and the reactor was cooled to room temperature. The vacuum linewas closed, and the gas flow was switched to synthetic air. The total pressurewas increased and the as-prepared catalyst was slowly oxidized under mildconditions to avoid sintering of the nanoparticles. During this step, the flow ofgas was slowly increased to keep the reactor bed in a fluidized state. Whenatmospheric pressure was reached, the gas flow was stopped and the samplewas removed from the reactor. The calcination of the sample was carried outin a flow of 100 sccm synthetic air at 623K for 3 h in a laboratory kiln. Thecalcination is a necessary step to remove precursor residues, because Cucatalysts applied in methanol synthesis are very sensitive to poisons.Furthermore, it ensures the comparability with the conventional catalyst,which is calcined typically at 600K to transform the Cu- and Zn-containinghydroxy carbonates originating from the precipitation into a CuO/ZnOmixture.

An alternative preparation route for the deposition of ZnO on the

substrate was the liquid impregnation with ZnEt2. A sample of alumina wassuspended in pure liquid ZnEt2 for 1 h. The sample was exposed to dynamicvacuum to remove the excess of liquid ZnEt2 at 300K for 2 h. This methodwas developed by Schroder et al. [19] in order to deposit ZnO on mesoporoussiliceousMCM-48 matrices and applied to alumina in an analogous manner inthe present work. Subsequently, the sample was oxidized under mildconditions with synthetic air. This procedure was applied to sample ZCA-3-Et2Zn after the deposition of Cu, and to samples CZA-1-Et2Zn and CZA-2-Et2Zn before the deposition of Cu.

The catalytic activity in methanol synthesis was measured in a

standardized test under ambient pressure at 493K in catalytic test equipment.These measurements were based on the procedure described in the literature[10, 27]. The activity tests were conducted in a glass-lined U-tube reactortypically using 100mg of samples containing Cu/ZnO/Al2O3. The sampleswere treated in diluted hydrogen (2% H2 in He) for 16 h at 448K.Subsequently, the temperature was increased to 513K, and the sample wasreduced in pure H2 at that temperature for 30min to ensure completereduction.

The Cu surface area was determined using N2O RFC. After reduction in

H2, the catalyst was exposed to flowing N2O (1% N2O in He, 298K), and theCu surface area was derived from the amount of N2 produced (density ofsurface Cu atoms: 1.47� 1019 m�2). The inlet feed gas composition formethanol synthesis was 14% He, 72% H2, 4% CO2, and 10% CO. Gases ofhigh purity (>99.9995%) were used. A modified space velocity of 500N mL(min gcat)

�1 was chosen. The synthesis was performed under ambient pressureat 493K. The quantitative gas analysis of the outlet flow was achieved with acalibrated QMS (GAM 422, Balzers). All samples were tested for at least20 h. After methanol synthesis an additional N2O RFC experiment wasperformed in order to determine the Cu surface area.

The SRM was carried out in a glass-lined U-tube reactor at various

temperature steps from 493K to 523K under ambient pressure. Typically,100mg of a sample was reduced in diluted H2 (2% H2 in He) for 3 h at 293K.


Full Paper

An equimolar mixture of water and methanol, both HPLC grade, werepumped by a syringe pump into a vaporizer. After vaporization, a gasmixture of 6.4% water and 6.4% methanol was established using Ar(99.999%) as the carrier gas. A space velocity of 3000N mL (min gcat)

�1 waschosen. The quantitative gas analysis of the outlet flow was achieved with acalibrated QMS (Thermostar, Pfeiffer Vacuum). All samples were tested for15 h. The temperature program consisted of 8 h at 293K, followed by 3 h at523K, and 4 h again at the starting temperature.

For characterization AAS (6 Varia, Analytik Jena), EA (Vario EL,Elementar), N2O RFC described in detail in the literature [26], on-line MS(Thermostar, Pfeiffer Vacuum), temperature-programmed reduction (TPR)(Hydros 100, Rosemount), XRD (theta-theta powder diffractometer,PANalytical), and N2 physisorption at 77K using a gas adsorption apparatus(Autosorb-1 MP, Quantachrome) were used. The pressure drop over thefluidized bed was determined using a pressure gauge (Baratron 222BA-01000DDS, MKS Instruments Inc.)

Received: May 7, 2009

Revised: August 24, 2009

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The Synthesis of Highly Loaded Cu/Al2O3 and Cu/ZnO/Al2O3 Catalysts by the Two-Step CVD of...

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