www.elsevier.com/locate/apcata

Applied Catalysis A: General 328 (2007) 77–82

Monolithic microfibrous nickel catalyst co-modified with ceria and alumina

for miniature hydrogen production via ammonia decomposition

Ye Liu, Hong Wang, Jianfeng Li, Yong Lu *, Haihong Wu, Qingsong Xue, Li Chen

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China

Received 21 December 2006; received in revised form 17 May 2007; accepted 28 May 2007

Available online 3 June 2007

Abstract

A non-woven microfibrous structure with 15 vol% 8 mm diameter nickel fibers was built using wet-lay papermaking and sintering processes.

Surface of the sinter-locked nickel fibers was then chemically modified with Al2O3 and CeO2, by immersing this novel microfibrous metallic media

in a 65 8C aqueous solution containing each of Al(NO3)3�6H2O and Ce(NO3)3�6H2O or both of them for 2 or 4 h at a constant metal ion

concentration of 0.5 mol/L. Chemical modifications provided a significant increase of the surface nickel atoms per gram catalyst but obviously

suppressed the activity of the metallic nickel sites as indicated by the lowered TOF values. The chemical modification with a mixture solution with

the optimal Al3+/Ce3+ ratio of 9 resulted in a 10-fold increase of the surface nickel atoms per gram catalyst but a 3-fold decrease of the TOF of

ammonia, compared with the neat microfibrous nickel substrate. This chemically modified catalyst was capable of producing roughly 20 W power

hydrogen with >99% ammonia conversion at 650 8C in a bed of 0.9 mL throughout a 100 h continuous test. Activation energies (Ea) for

microfibrous nickel catalysts were all alike in range from 103 to 105 kJ/mol, suggesting that the active site nature was not changed by the chemical

modification treatments.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Microfibrous catalyst; Nickel; Hydrogen production; Ammonia; Fuel cells

1. Introduction

Recently, miniature hydrogen generator to power fuel cells

for portable electronic technologies (PDA’s, notebook compu-

ters, and microelectrochemical systems, etc.) has been the focus

of intense research activities [1–6]. One of the strategies in this

effort is to develop novel microreactor technology to meet the

fundamental criteria needed for a miniature fuel cell power

system [3–7], and meanwhile, to employ simple CO-free

hydrogen production process. The decomposition of ammonia

offers by far the simplest process for COx-free H2 production

[3–8], compared to hydrogen production via steam reforming of

hydrocarbons or oxygenated hydrocarbons [9–13] that requires

a complex combination of multiple processes [14–16] to

achieve required low CO levels [17]. This process allows a

single feed stream, simplicity of start-up and low overall device

weight and volume [3–8], thus making it particularly preferred

* Corresponding author. Fax: +86 21 6223 3424.

E-mail address: ylu@chem.ecnu.edu.cn (Y. Lu).

0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2007.05.034

as an attractive source of hydrogen for micro/portable fuel cell

power supplies.

Many supported metal catalysts, alloys and compounds with

noble metal characters [18] have been developed and tested for

ammonia decomposition. Among them, Ru is the most active

but Ni though lower is not significantly different making it

attractive in terms of cost [4,6,18]. Despite these advances,

development of small hydrogen generators for portable fuel cell

power system still remains challenging, since most of the

reactor designs used so far are the traditional catalyst

particulates packed into microreactors that normally suffer

from poor intraparticle mass/heat transfer, low contacting

efficiency, high pressure drop, mechanical attrition, and

catalyst clumping in a way [1,5]. Although microchannel

technologies combined with catalyst washcoats can be used to

avoid one or more of the frustrated problems mentioned above,

unacceptably low surface area per unit reactor volume is a big

challenge [1,5,19,20].

Hence, it is important to render novel microstructured

catalytic materials for developing miniature H2 generator in

portable fuel cell power system applications. A monolithic

Y. Liu et al. / Applied Catalysis A: General 328 (2007) 77–8278

anodized aluminum microchannel reactor, with an increase in

surface area, has recently been developed for high efficiency

ammonia decomposition [5]. Regardless of high loading of

expensive Ru is required for achieving good performance, the

fatal disadvantage of this reactor rests with the low melting

point (661 8C) of the aluminum body. A new class of

microfibrous carriers consisting of sinter-locked microfibers

has been invented by Tatarchuk and co-workers recently

[21,22]. This microfibrous media provides large void volume,

entirely open structure, large surface-to-volume ratio, high

permeability, high thermal conductivity, and unique form

factors. For applications that entrap small particulates within

the microfibrous network, unique combinations of surface area,

pore size/particle size, thermal conductivity, and void volume

are obtained. In previous work, a novel microfibrous network

consisting of 2–3 vol% metallic fibers (e.g., 8 mm nickel fibers)

have been used to entrap 25–35 vol% 100–200 mm supported

nickel-based catalysts for ammonia decomposition [4],

supported-ZnO sorbent for trace H2S removal from hydrogen

fuel stream [21], and Pt-Co/Al2O3 catalyst for PROX CO from

practical reformate [22]. Microfibrous entrapment of small

ceria-promoted Ni/Al2O3 particulates combined good heat and

mass transfer and provided 4-fold reduction of the overall

weight and volume of the reaction bed at a low pressure drop,

compared with the packed bed with 2 mm catalyst pellets [4].

In the present work, our attempt is to develop a novel

monolithic all-nickel catalyst system for ammonia decomposi-

tion to produce CO-free hydrogen. Three-dimensional sinter-

locked structure of nickel fibers (8 mm diameter) was firstly

built by papermaking/sintering processes. In order to sig-

nificantly improve the catalyst reactivity for ammonia

decomposition, surfaces of the nickel fibers were chemically

modified with trace alumina and/or ceria. By doing so, quite a

number of active sites were directly created on and strongly

adhered to the surface of nickel fibers, which would further

improve heat/mass transfer and avoid the particulate dropout

from the microfibrous structure with entrapment of small

particulates during handling and using stage. Surface Ni atoms

per gram catalyst and TOF values of ammonia were also

determined for better understanding the nature of the chemical

modifications.

2. Experimental

2.1. Fabrication and chemical modification of microfibrous

structure using nickel fibers

The thin-sheet microfibrous nickel substrate was prepared

using wet-lay papermaking process [4,21,22] on a lab-scale

with a 159 mm diameter circular preform using 8 mm diameter

by 2–3 mm length nickel fibers, followed by high-temperature

sintering in hydrogen atmosphere. In this process, 15.0 g metal

fibers were slurried in an aqueous suspension (10 L) with 1.5 g

cellulose fibers (20–40 mm diameter by 100–1000 mm length).

The resulting mixture was then casted into a preform sheet

using a wet-lay process and dried to create a paper. Pre-

oxidation of the paper in air at 500 8C removed the cellulosic

binders and subsequent sintering in hydrogen at 950 8C created

three-dimentional sinter-locked network. This sintered product

was composed of 15 vol% nickel fibers with 85 vol% void

volume. Small pieces of such paper were cut down from that

large sheet product, immersed in an 65 8C aqueous solution

containing each of Al(NO3)3�6H2O and Ce(NO3)3�6H2O or

both of them for 2–4 h at a constant metal ion concentration of

0.5 mol/L, leached of the solution, dried overnight at 100 8C,

and calcined in air at 450 8C for 2 h.

2.2. Characterization and reactivity test

A FE-SEM instrument (Hitach S-4800, Japan) equiped with

an EDX unit (Oxford, UK) was employed to record the images

of the microfibrous structure, and meanwhile, to analyze the

element compositions in the surface layer of modified nickel

fiber. The specific surface area was determined using BET

method with a commercial unit of Autosorb 3B (Quantra-

chrome, USA), with nitrogen physisorption at �196 8C. The

surface nickel atoms per gram catalyst were measured using

H2S chemisorption method [23] on a unit of TGA/SDTA851e

(Mettler Toledo, Switzerland). The catalyst samples were

reduced in situ with 5 vol% H2/He (30 mL/min) at 500 8Cthoroughly and purged with He (30 mL/min) until to constant

weight. The reduced samples were subsequently exposed to

H2S (by introducing 10 vol% H2S/He at a flow rate of 5 mL/min

from sample gas inlet into the chamber with He carrier gas flow

of 30 mL/min) at the same temperature point for H2S

chemisorption (i.e., Ni + H2S! NiS + H2). The surface Ni

atoms per gram catalyst were then calculated according to the

mass increment (contributed to sulfur).

Catalytic reactivity was measured in an 11.9 mm inner

diameter (i.d.) quartz tube heated by a temperature-controlled

tube furnace. For each experiment, 15 pieces of 12 mm (dia.)

by 0.54 mm (thick) circle chip of the microfibrous catalyst

sample were cut down from a large microfibrous catalyst sheet

and packed carefully into the tube reactor. Prior to reaction, all

catalysts were reduced with hydrogen (30 mL/min) at 500 8Cfor 2 h. Product nitrogen and unconverted ammonia in the

effluent gas was analyzed by an online GC equipped with a

TCD and a 3 m Poropak Q packed column at 100 8C of

auxiliary box, using a hydrogen carrier gas. Ammonia

conversion was calculated by normalized method on the

nitrogen atom basis. Reactant stream of anhydrous ammonia

(99.99%) was controlled with a calibrated mass flow

controller. All experiments were carried out at atmospheric

pressure.

3. Results and discussion

3.1. Microstructure feature

Fig. 1 shows the optical photographs of the microfibrous

structure using 8 mm nickel fibers. Fig. 1a shows the pressed

and dried paper preform before sintering. A unique three-

dimensional open porous structure of nickel microfibers was

trussed up with cellulose fibers as binders. Fig. 1b shows the

Fig. 1. Optical photographs of microfibrous structure prepared using 8 mm Ni

fibers. (a) Preform paper, (b) after pre-oxidation in air at 500 8C for cellulose

removal followed by subsequent sintering in H2 at 950 8C for 45 min.

Y. Liu et al. / Applied Catalysis A: General 328 (2007) 77–82 79

sintered product of microfibrous nickel paper. Clearly, the

sintering process led to a locked three-dimensional network

while the large void volume and entirely open porous structure

remained. The cellulosic binders were completely removed by

pre-oxidation prior to sintering because of no any carbon fibers

with diameter similar to cellulose fibers. The novel micro-

fibrous media provided feature geometry with entirely open

structure, leading to ultrahigh accessibility of surface nickel

atoms because of no any existence of intraparticle diffusion

limitation. Actually, large void volume and the entirely open

structure of the microfibrous network are central to the notion

of increasing the steady-state volumetric reaction rate [24],

while the excellent thermal conductivity of metallic fibers

satisfied the requirement for transient startup. Additionally, this

novel media, with the unique form factors, can be made into

thin sheets (from submillimeters to several millimeters in

thickness) of large area and/or pleated to control pressure drop

and contacting efficiency in a beneficial manner different from

other traditionally employed contacting schemes, like packed

beds, fluid beds, honeycomb, or wovens [25].

3.2. Effect of chemical modification with CeO2 and Al2O3

Despite above mentioned beneficial properties that might

satisfy several of the fundamental criteria needed for useful

operation [3–7], poor reactivity of this paper-like sinter-locked

nickel fibers for ammonia decomposition seems a big problem

[26]. Hence, significant improvement of the reactivity of this

microfibrous nickel catalyst is needed intensively. It is

disclosed that the apparent activation energies of ammonia

decomposition are in range of 180–209 kJ/mol over nickel films

or wires but are significantly decreased to 80–90 kJ/mol when

nickel was supported [27]. Recently, rare earth oxides have

been reported to be able to significantly promote the reactivity

of Ni/Al2O3 catalysts [4,28] while CeO2 showed distinctive

promotive effect [4]. Accordingly, in order to significantly

improve the reactivity the surface of this novel microfibrous

nickel was chemically modified with Al2O3 and CeO2 by

immersing it in an aqueous solution containing each of

Al(NO3)3�6H2O and Ce(NO3)3�6H2O or their both at 65 8C.

Table 1 summarizes the characteristics and the ammonia

decomposition performance of each of the seven microfibrous

all-nickel catalysts studied. Catalyst A represented the base

case: neat microfibrous nickel substrate. Catalysts B to G were

modified ones with CeO2 and/or Al2O3. All microfibrous

catalysts provided a void volume fraction of 85% since the

same microfibrous substrate consisting of 15 vol% nickel fibers

was used in all cases. The microfibrous all-nickel catalysts were

all observed using a SEM/EDX instrument. SEM images

indicated that the chemical modification treatments made the

surface of nickel microfibers more rough while creating pores

on the microfibers (see Fig. 2 for selected catalysts A and C).

N2-BET analyses indicated that the modification treatments

made the specific surface area increased to around 5–6 m2/g,

2–3 times higher than that of catalyst A (�1.5 m2/g). Element

analyses with EDX detection showed that Al and Ce contents in

the surface layer of �2 mm (EDX detection depth) varied in

parallel with their concentration in the solutions but the Al/Ce

ratios in the surface layer were inconsistent with those in the

solutions (see Table 1).

As shown in Table 1, catalyst A demonstrated very low

ammonia conversion: 39.0% at 600 8C and 65.7% at 650 8C,

respectively. Interestingly, chemically modified catalysts B to

G all displayed significant increase in ammonia conversion

compared with the base catalyst A. The co-modifications

together with both ceria and alumina resulted in higher

ammonia conversion than the modifications with either single

alumina (catalyst B) or single ceria (catalyst F). Note that the

increase of ammonia conversion was strongly dependent on the

Al3+/Ce3+ ratio in the solutions used in the chemical

modification processes. In comparison with catalyst F

(modified using single ceria), ammonia conversion increased

slowly with increasing the Al3+/Ce3+ ratio in the solution to 1/9

(catalyst E) and then to 1/1 (catalyst D), but presented a sharp

increase with further increasing the Al3+/Ce3+ ratio up to 9/1

(catalyst C). Catalyst B (modified using single alumina) gave an

ammonia conversion comparable with that of the catalyst F.

Moreover, modification with longer immersing time period

Table 1

Characteristics and NH3 decomposition reactivity of the microfibrous all-nickel catalysts chemically modified with ceria and/or alumina

Microfibrous nickel catalyst A (base) B C D E F G

Feature of microfibrous matrix

Nickel fiber composition (vol%) 15 15 15 15 15 15 15

Void volume fraction (vol%) 85 85 85 85 85 85 85

Apparent density (g/mL) 1.56 1.56 1.56 1.56 1.56 1.56 1.56

Chemical modification treatment

Bathing temperature/time (8C/h) 65/2 65/2 65/2 65/2 65/2 65/2 65/4

Al3+/Ce3+ ion concentration of

the solutiona (mol/L)

0/0 0.5/0 0.45/0.05 0.25/0.25 0.05/0.45 0/0.5 0.45/0.05

Atomic composition in fiber surface layerb (at.%)

Ni 100 71.43 95.49 88.97 87.34 99.86 –

Ce 0 0 0.04 0.27 1.04 0.14 –

Al 0 28.57 4.49 10.76 11.62 0 –

Surface Ni atoms per gram catalystc 2.1 � 1019 2.0 � 1020 2.4 � 1020 1.5 � 1020 1.8 � 1020 9.0 � 1019 2.2 � 1020

Reactivity of microfibrous bed with 0.9 mL catalysts @ a 150 mL/min NH3 feed gas rate and 600 8C (and 650 8C)

NH3 conversiond (mol%) 39.0 (65.7) 62.6 (88.7) 81.0 (99.4) 68.9 (92.8) 66.2 (90.9) 64.6 (89.6) 81.8 (99.4)

H2 production rate (mL/min) 87.8 (147.8) 140.9 (199.6) 182.3 (223.7) 155.0 (208.8) 149.0 (204.5) 145.4 (201.6) 184.1 (223.7)

TOFNH3 at 600 8C (and 650 8C) (s�1) 1.2 (2.1) 0.21 (0.30) 0.30 (0.37) 0.30 (0.41) 0.18 (0.25) 0.51 (0.69) 0.33 (0.40)

Activation energy (kJ/mol) 115 109 105 109 103 113 –

a Total metal ion concentration was kept at a constant value of 0.5 mol/L.b Analyzed by EDX unit with 2 mm detection depth.c Determined by H2S chemisorptions at 500 8C.d Each reaction condition was run for 2 h, during which the experimental data were collected.

Y. Liu et al. / Applied Catalysis A: General 328 (2007) 77–8280

would not lead to further improvement on the reactivity for

ammonia decomposition, as indicated by the quite close

ammonia conversions on both catalysts C and G (immersing for

6 h would make microfibrous structure loose). A catalytic bed

packed with 0.9 mL of catalyst C could yield �220 mL/min

hydrogen (equivalent to �20 W fuel cell power output) with

>99% conversion of ammonia at 650 8C.

For better understanding the reactivity of the catalysts, it is

essential to provide the turnover frequency (TOF) in addition to

the conversion values since the TOF values are normalized with

respect to the surface Ni metal sites. The surface Ni atoms per

gram microfibrous all-nickel catalysts and the calculated

TOFNH3 values are also listed in Table 1. As shown in Table 1,

surface Ni atoms per gram modified catalysts were in a range of

0.9–2.4 � 1020, 3- to 10-fold higher than that of neat

microfibrous nickel (catalyst A). The chemical modification

treatments might make the continuous metallic surface of the

nickel microfibers rearranged into isolated nickel particles

thereby leading to a burst of the number of surface Ni atoms

compared to the base catalyst A. TOFNH3 values of the modified

catalysts ranged from 0.18 to 0.51 s�1 at 600 8C and from 0.30

to 0.69 s�1 at 650 8C, lower than that of the catalyst A (1.2 s�1

at 600 8C and 2.1 s�1 at 650 8C, respectively). Diffusion

limitation that might exist in the porous surface layer created on

the nickel fibers by chemical treatments reduced the utilization

of active sites and therefore decreased the TOFNH3, because the

following activation energy calculation evidenced that the

treatments did not change the nature of active site for

microfibrous all-nickel catalysts. For instance, the TOFNH3

of catalyst C at 600 8C was 3-fold lower but surface Ni atoms

per gram the catalyst C was 10-fold higher than that of the

catalyst A. As a result, the catalyst C still presented much

higher ammonia conversion at 600 8C: 81.0% of the catalyst C

versus 39.0% of the catalyst A. It was disclosed by previous

UPS/XPS results that ammonia decomposition is a pronounced

structure-sensitive reaction over Ni [29,30]; the reduction of the

average size of the supported metallic Ni particles from 4 to

2 nm led to a 10-fold promotion of TOF [29,30]. It seems not

consistent with our results, probably because the catalyst

systems, reaction conditions and the investigation methods are

quite different.

To gain in sight into the chemical nature effect of alumina

and ceria, activation energies for all catalyst samples were

calculated using the differential reactor data. First, reaction

rates were measured, according to the law of XNH3/r = Wcat./

FNH3, where XNH3 is the ammonia conversion (mol%), r

reaction rate (mol/(g s)), Wcat. catalyst weight (g), FNH3 flow

rate of ammonia (mol/s). The reaction conditions were chosen

to yield NH3 conversions <10% to satisfy the needs of Mears’

criterion [31] and Thiele moduli [32] for a differential reactor.

The activation energies could then be obtained according to the

law of ln (r2/r1) = (Ea � (T2 � T1))/(R � T2 � T1), where Ea is

the activation energy, R gas constant (8.314 J/K), r2 and r1

reaction rates at temperatures of T1 and T2 (K). As noted in

Table 1, catalyst samples offered close values of activation

energy from 103 to 115 kJ/mol (see Table 1), implying that

chemical modification treatments here did not change the active

site nature but just made active site number increased.

In addition, the maximum TOFNH3 achieved over all-nickel

microfibrous catalysts was 1.2 s�1 at 600 8C. However, the

reported TOFNH3 is 4.8 s�1 at 600 8C over Ni/Al2O3 entrapped

into the microfibrous carrier [4]. For revealing this difference,

both Ni/Al2O3 and Ni/CeO2/Al2O3 (100–200 mm) were taken

out from the composites same as used in reported work [4] and

Fig. 2. SEM images of morphology of (a) catalyst A: neat microfibrous nickel,

(b) catalyst C: after chemical modification treatment using an aqueous solution

with Al3+/Ce3+ ratio of 9 at 65 8C for 2 h.

Y. Liu et al. / Applied Catalysis A: General 328 (2007) 77–82 81

were subjected to the same activation energy measurements.

The activation energies were calculated to be 83 kJ/mol on Ni/

Al2O3 (in good agreement with the literature number (80–

90 kJ/mol) [27]) and 72 kJ/mol on Ni/CeO2/Al2O3, which were

30–40 kJ/mol lower than those of the all-nickel microfibrous

catalysts. This might be the real reason for the above difference

in TOFNH3 values between all-nickel microfibrous catalysts and

supported nickel catalysts.

3.3. Stability

A longer-term test of the catalyst C (bed volume of 0.9 mL)

was carried out at 650 8C with a GHSV of 10 000 h�1.

Ammonia conversion remained at >99% in the entire test of

100 h. The microfibous nickel catalyst kept its monolithic shape

as perfect as the fresh one after the 100 h test while sinter-

locked network microstructure remained robust.

4. Conclusion

A thin-sheet microfibrous structure consisting of 15 vol%

8 mm diameter nickel fibers was built through regular wet-lay

paper-making followed by subsequent sintering process, and its

reactivity for ammonia decomposition was significantly

improved through the chemical modifications with alumina

and/or ceria. This new approach opens an opportunity on the

development of high efficiency miniature hydrogen generator

for the use with portable fuel cell power system by taking

advantage of enhanced heat/mass transfer, high permeability

and monolithic structure with convenient shape design. The

chemical modification by immersing in a mixture solution with

Al3+/Ce3+ (nitrite salts as precursors) ratio of 9 at 65 8C for 2 h

led to 2-fold promotion of the reactivity of the microfibrous

nickel catalyst for ammonia decomposition compared with the

neat microfibrous nickel. Roughly 20 W power output

hydrogen (�220 mL/min) could be achieved with an ammonia

conversion of >99% in a bed volume of 0.9 mL at 650 8Cthroughout a 100 h test.

While the microfibrous all-nickel catalyst systems show

great promise, improvements are necessary to render the

reactor useful, because currently a bed volume of 0.9 mL is

required to achieve 99.4% ammonia conversion for 150 sccm

ammonia flow rate at 650 8C but it was only 0.5 mL to achieve

>99.99% conversion at comparable conditions when employ-

ing microfibrous entrapped ceria-promoted Ni/Al2O3 catalyst

composite [4]. We believe that such improvements are possible,

for example, by using much smaller diameter microfibers (but

higher pressure drop), anodizing to increase surface area,

plating Ru layer through electrochemical deposition, and/or

using other promoters such as alkali (earth) metals. The works

along with these lines are in progress.

Acknowledgement

Y.L. gratefully thanks the Program for New Century Excellent

Talents in University (NCET-06), Shuguang Project (06SG28),

and Qimingxing Project (05QMX1418). This work is supported

by grants to Y. Lu from National Natural Science Foundation of

China (20590366, 20570360), and the Science & Technology

Commission of Shanghai Municipality (05DJ14002).

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