Ceria-based supported metal catalysts for the low-temperature water-gas shift reaction

Water-gas shift (WGS) reaction is a crucial step for the industrial production of hydrogen or upgrading the hydrogen generated from fossil or biomass sources by removing the residual CO. However, current industrial catalysts for this process, comprising Cu/ZnO and Fe2O3-Cr2O3, suffer from safety or environmental issues. In the past decades, ceria-based materials have attracted wide attention as WGS catalysts due to their abundant oxygen vacancies and tunable metal-support interaction. Strategies through engineering the shape or crystal facet, size of both metal and ceria, interfacial-structure, etc., to alter the performances of ceria-based catalysts have been extensively studied. Additionally, the developments in the in situ techniques and DFT calculations are favorable for deepening the understanding of the reaction mechanism and structure-function relationship at the molecular level, comprising active sites, reaction path/intermediates, and inducements for deactivation. This article critically reviews the literature on ceria-based catalysts toward the WGS reaction, covering the fundamental insight of the reaction path and development in precisely designing catalysts.

Au and Pt Remain Unoxidized on a CeO2-Based Catalyst during the Water-Gas Shift Reaction

The active forms of Au and Pt in CeO₂-based catalysts for the water-gas shift (WGS) reaction are an issue that remains unclear, although it has been widely studied. On one hand, ionic species might be responsible for weakening the Ce-O bonds, thus increasing the oxygen mobility and WGS activity. On the other hand, the close contact of Au or Pt atoms with CeO₂ oxygen vacancies at the metal-CeO₂ interface might provide the active sites for an efficient reaction. In this work, using in situ X-ray absorption spectroscopy, we demonstrate that both Au and Pt remain unoxidized during the reaction. Remarkable differences involving the dynamics established by both species under WGS atmospheres were recognized. For the prereduced Pt catalyst, the increase of the conversion coincided with a restructuration of the Pt atoms into cuboctahedrical metallic particles without significant variations on the overall particle size. Contrary to the relatively static behavior of Pt⁰, Au⁰ nanoparticles exhibited a sequence of particle splitting and agglomeration while maintaining a zero oxidation state despite not being located in a metallic environment during the process. High WGS activity was obtained when Au atoms were surrounded by oxygen. The fact that Au preserves its unoxidized state indicates that the chemical interaction between Au and oxygen must be necessarily electrostatic and that such an electrostatic interaction is fundamental for a top performance in the WGS process.

Efficient Waste to Energy Conversion Based on Co-CeO2 Catalyzed Water-Gas Shift Reaction

Waste to energy technology is attracting attention to overcome the upcoming environmental and energy issues. One of the key-steps is the water-gas shift (WGS) reaction, which can convert the waste-derived synthesis gas (H2 and CO) to pure hydrogen. Co–CeO2 catalysts were synthesized by the different methods to derive the optimal synthetic method and to investigate the effect of the preparation method on the physicochemical characteristics of Co–CeO2 catalysts in the high-temperature water-gas shift (HTS) reaction. The Co–CeO2 catalyst synthesized by the sol-gel method featured a strong metal to support interaction and the largest number of oxygen vacancies compared to other catalysts, which affects the catalytic activity. As a result, the Co–CeO2 catalyst synthesized by the sol-gel method exhibited the highest WGS activity among the prepared catalysts, even in severe conditions (high CO concentration: ~38% in dry basis and high gas hourly space velocity: 143,000 h−1).

Recent Advances in Design of Gold-Based Catalysts for H2 Clean-Up Reactions

Over the past three decades, supported gold nanoparticles have demonstrated outstanding properties and continue to attract the interest of the scientific community. Several books and comprehensive reviews as well as numerous papers cover a variety of fundamental and applied aspects specific to gold-based catalyst synthesis, characterization by different techniques, relationship among catalyst support features, electronic and structural properties of gold particles, and catalytic activity, reaction mechanism, and theoretical modeling. Among the Au-catalyzed reactions targeting environmental protection and sustainable energy applications, particular attention is paid to pure hydrogen production. The increasing demands for high-purity hydrogen for fuel cell systems caused a renewed interest in the water-gas shift reaction. This well-known industrial process provides an attractive way for hydrogen generation and additional increase of its concentration in the gas mixtures obtained by processes utilizing coal, petroleum, or biomass resources. An effective step for further elimination of CO traces from the reformate stream after water-gas shift unit is the preferential CO oxidation. Developing highly active, stable, and selective catalysts for these two reactions is of primary importance for efficient upgrading of hydrogen purity in fuel cell applications. This review aims to extend the existing knowledge and understanding of the properties of gold-based catalysts for H2 clean-up reactions. In particular, new approaches and strategies for design of high-performing and cost-effective formulations are addressed. Emphasis is placed on efforts to explore appropriate and economically viable supports with complex composition prepared by various synthesis procedures. Relevance of ceria application as a support for new-generation WGS catalysts is pointed out. The role of the nature of support in catalyst behavior and specifically the existence of an active gold-support interface is highlighted. Long-term stability and tolerance toward start-up/shutdown cycling are discussed. Very recent advances in catalyst design are described focusing on structured catalysts and microchannel reactors. The latest mechanistic aspects of the water-gas shift reaction and preferential CO oxidation over gold-based catalysts from density functional theory calculations are noted because of their essential role in discovering novel highly efficient catalysts.

Enhanced Activity and Stability of Gold/Ceria-Titania for the Low-Temperature Water-Gas Shift Reaction

Gold supported on ceria-zirconia is one of the most active low temperature water-gas shift catalysts reported to date but rapid deactivation occurs under reaction conditions. In this study, ceria-titania was evaluated as an alternative catalyst support. Materials of different Ce:Ti compositions were synthesized using a sol-gel methodology and gold was supported onto these using a deposition-precipitation method. They were then investigated as catalysts for the low-temperature water-gas shift reaction. Au/Ce_0.2Ti_0.8O₂ exhibited superior activity and stability to a highly active, previously reported gold catalyst supported on ceria-zirconia. High activity and stability was found to be related to the support comprising a high number of oxygen defect sites and a high specific surface area. These properties were conducive to forming a highly active catalyst with well-dispersed Au species.

Recent Advances in the Gold-Catalysed Low-Temperature Water–Gas Shift Reaction

The low-temperature water–gas shift reaction (LTS: CO + H2O ⇌ CO2 + H2) is a key step in the purification of H2 reformate streams that feed H2 fuel cells. Supported gold catalysts were originally identified as being active for this reaction twenty years ago, and since then, considerable advances have been made in the synthesis and characterisation of these catalysts. In this review, we identify and evaluate the progress towards solving the most important challenge in this research area: the development of robust, highly active catalysts that do not deactivate on-stream under realistic reaction conditions.

Strategies for Carbon and Sulfur Tolerant Solid Oxide Fuel Cell Materials, Incorporating Lessons from Heterogeneous Catalysis

Solid oxide fuel cells (SOFCs) are a rapidly emerging energy technology for a low carbon world, providing high efficiency, potential to use carbonaceous fuels, and compatibility with carbon capture and storage. However, current state-of-the-art materials have low tolerance to sulfur, a common contaminant of many fuels, and are vulnerable to deactivation due to carbon deposition when using carbon-containing compounds. In this review, we first study the theoretical basis behind carbon and sulfur poisoning, before examining the strategies toward carbon and sulfur tolerance used so far in the SOFC literature. We then study the more extensive relevant heterogeneous catalysis literature for strategies and materials which could be incorporated into carbon and sulfur tolerant fuel cells.

Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support

Highly active and stable bimetallic Au-Pd catalysts have been extensively studied for several liquid-phase oxidation reactions in recent years, but there are far fewer reports on the use of these catalysts for low-temperature gas-phase reactions. Here we initially established the presence of a synergistic effect in a range of bimetallic Au-Pd/CeZrO₄ catalysts, by measuring their activity for selective oxidation of benzyl alcohol. The catalysts were then evaluated for low-temperature WGS, CO oxidation, and formic acid decomposition, all of which are believed to be mechanistically related. A strong anti-synergy between Au and Pd was observed for these reactions, whereby the introduction of Pd to a monometallic Au catalyst resulted in a significant decrease in catalytic activity. Furthermore, monometallic Pd was more active than Pd-rich bimetallic catalysts. The nature of the anti-synergy was probed by several ex situ techniques, which all indicated a growth in metal nanoparticle size with Pd addition. However, the most definitive information was provided by in situ CO-DRIFTS, in which CO adsorption associated with interfacial sites was found to vary with the molar ratio of the metals and could be correlated with the catalytic activity of each reaction. As a similar correlation was observed between activity and the presence of Au⁰* (as detected by XPS), it is proposed that peripheral Au⁰* species form part of the active centers in the most active catalysts for the three gas-phase reactions. In contrast, the active sites for the selective oxidation of benzyl alcohol are generally thought to be electronically modified gold atoms at the surface of the nanoparticles.

Highly Active and Stable Pt-Loaded Ce0.75Zr0.25O2 Yolk-Shell Catalyst for Water-Gas Shift Reaction

Multishelled, Pt-loaded Ce0.75Zr0.25O2 yolk-shell microspheres were prepared by a simple spray pyrolysis process for use in the water-gas shift (WGS) reaction. The Pt-loading was optimized, obtaining highly active Pt/Ce0.75Zr0.25O2 yolk-shell nanostructures for the WGS. Of the prepared catalysts, a 2% Pt loading of the Ce0.75Zr0.25O2 yolk-shell microspheres showed the highest CO conversion. The high catalytic activity of the 2% Pt/Ce0.75Zr0.2O2 catalyst was mainly due to its easier reducibility and the maintenance of active catalytic Pt species. The Pt-loaded Ce0.75Zr0.25O2 catalyst microspheres were highly resistant to Pt sintering because of their unique yolk-shell structure. Spray pyrolysis was found to be highly efficient for the production of precious-metal-loaded, multicomponent metal oxide yolk-shell microspheres for catalytic applications.