Activity and stability of low-content gold–cerium oxide catalysts for the water–gas shift reaction

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Abstract

We report here on the high activity and stability of low-content gold–cerium oxide catalysts for the water–gas shift reaction (WGS). These catalysts are reversible in cyclic reduction–oxidation treatment up to 400 °C, are non-pyrophoric, and are thus potential candidates for application to hydrogen generation for fuel cell power production. Low-content (0.2–0.9 at.%) gold–ceria samples were prepared by single-pot synthesis by the urea gelation/coprecipitation method; and by sodium cyanide leaching of high-content (2–8 at.%) gold–ceria materials prepared by various techniques. The low-content gold–ceria catalysts are free of metallic gold nanoparticles. Gold is present in oxidized form, as verified by a variety of analytical techniques. However, these materials display the same WGS activity as the high-content gold ones, and remain free of gold nanoparticles after use in a reaction gas stream composed of 11% CO–26% H2O–26% H2–7% CO2–balance He up to 300 °C. We show that the determining factor for the retention of active gold in ceria is the surface properties of the latter. Measurements of lattice constant expansion indicate gold ion substitution in the ceria lattice. The turnover frequency of WGS under the assumption of fully dispersed gold is the same for a variety of low-content gold–ceria preparations. The stability of gold–ceria in various gas compositions and temperatures was good. The most serious stability issue is formation of cerium hydroxycarbonate in shutdown operation.

Introduction

The water–gas shift (WGS) reaction is an integral part of fuel processing for the production of hydrogen. When hydrogen is used for fuel cell power generation, the WGS catalysts should be both active and stable in cyclic operation and in exposure to air and condensed water, and of course, economical. The state-of-the-art low-temperature WGS catalyst in chemical plants is Cu–ZnO [1]. However, this type catalyst is very sensitive to temperature excursions, pyrophoric if exposed to air, and requires very careful pre-activation. It has been assessed unsuitable for application to PEM fuel cells, especially for use in transportation. WGS catalysts based on nanocrystalline cerium oxide (ceria) have been investigated in recent years as alternatives to Cu–ZnO for fuel cell applications [2], [3], [4], [5], [6]. They are non-pyrophoric and can be used without activation [5]. It is well known that platinum metals (PM) supported on cerium oxide in the three-way automotive catalyst enhance the low-temperature WGS reaction and are much more active than PM/alumina [2], [7]. Previous work in our laboratory has shown that Cu- and other transition metal-containing nanocrystalline cerias are active and stable catalysts in low- and high-temperature redox reactions [5], [6], [8], [9], [10], [11], [12], [13]. Cu–ceria was found active for WGS over a wide temperature window [5]. The reducibility and catalytic activity of CeO2 are significantly enhanced by the presence of a small amount of a transition metal, which does not have to be a platinum group metal. Platinum was the earliest case demonstrated of a metal additive having a considerable effect on ceria reducibility [14]. Among the metal–ceria systems examined in the literature, Au–ceria is a particularly active and stable catalyst for low-temperature CO oxidation [10], [11], [15], methane oxidation [10], [11], and the WGS reaction [6], [12], [13], [16].

The literature of fine gold particles supported on reducible oxides has focused on the gold particle size and properties [17], [18], [19]. However, it is possible that the oxide support is much more important than given credit to date [6], [12], [13], [16]. Little is known about the interaction of Au with ceria that might be responsible for the observed high activity. We recently reported that nonmetallic gold and platinum species on ceria are associated with the active sites for the water–gas shift reaction [6]. In the present work, we further examine the gold–ceria interaction and evaluate the catalyst under realistic operating conditions.

Section snippets

Catalyst preparation and characterization

Lanthana- or gadolinia-doped ceria and undoped ceria materials were prepared by the urea gelation/coprecipitation (UGC) method [5]. Gold–ceria samples were prepared by deposition precipitation (DP), coprecipitation (CP) and the above UGC method [5], as described in detail elsewhere [12].

Leaching of gold from calcined gold–ceria samples took place in an aqueous solution of 2% NaCN under O2 gas sparging at room temperature. Sodium hydroxide was added to keep the pH at ∼12. Cyanide leaching is a

Fresh catalyst characterization

Table 1 shows the physical properties of doped and undoped ceria prepared by UGC; and the properties of gold-containing cerias. Doping of ceria with La2O3 was used in our previous work to stabilize the crystal growth of ceria [9], [12], [13]. Here we show that doping ceria with gadolinia, Gd2O3, is equally effective. The ionic radii of both La3+ (0.116 nm) and Gd3+ (0.105 nm) are bigger than Ce4+ (0.097 nm) [7]. Their substitution in the ceria lattice increases the lattice constant α, as is

Summary/conclusions

In this work, we examined the activity and stability of Au–ceria catalysts under realistic water–gas shift reaction conditions. Metallic gold nanoparticles as well as ionic gold were found in gold–ceria catalysts. Leaching with NaCN solutions, removed all the gold species that were weakly bound on ceria. This included all the metal Au nanoparticles. The leached samples contained only a small fraction of the original amount of gold in the form of cations, apparently bound in the ceria lattice as

Acknowledgement

The financial support of this work by the NSF/EPA, Grant No. CTS-9985305 and by the NSF Nanotechnology Interdisciplinary Research Team (NIRT) Grant No. 0304515, is gratefully acknowledged.

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    Present address: Cabot Corporation, Albuquerque, NM, USA.

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