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.

The influence of a hydrophobic carrier, reactant and product during H2O adsorption on Pd surface for the oxidative esterification of methacrolein to methyl methacrylate

Taking the one-step oxidative esterification of methacrolein (MAL) to methyl methacrylate (MMA) as a model reaction and because H2O that was generated easily formed a film of water on the catalyst surface, which restricted the diffusion of the reactants to the active sites, the effects of the hydrophobic carrier styrene-divinylbenzene (SDB) copolymer, the reactant CH3OH and the product MMA during the adsorption of H2O on a Pd surface were investigated. For a Pd/SDB catalyst, the interactions between the active component and the carrier were first calculated using Pd4 clusters. The results implied that Pd4 clusters were chemisorbed on the SDB carrier. By comparing the adsorption energy of H2O molecules on Pd4 clusters with or without SDB, it was found that the adsorption energy of the former was reduced by about 50%, indicating that the hydrophobic carrier SDB reduced the adsorption of H2O on Pd4 clusters. This was also confirmed by the results for the partial density of states, differences in charge density and comparative Mulliken charge analysis. The influences of the reactant CH3OH and the product MMA on the adsorption of H2O were investigated using the Pd(111) surface. The results of co-adsorption simulations showed that some of the electrons on CH3OH molecules were transferred to H2O molecules that strengthened the electronic interaction between H2O molecules and the Pd surface and led to a change in the adsorption of isolated H2O molecules from physisorption to chemisorption. However, the product MMA when chemisorbed on the Pd surface had little effect on the adsorption of H2O molecules on the Pd(111) surface.

Imaging and chemically probing catalytic processes using field emission techniques: a study of NO hydrogenation on Pd and Pd–Au catalysts

Nitric oxide hydrogenation is investigated on palladium and gold–palladium alloy crystallites, i.e. the extremity of sharp tip samples aimed at modelling a single catalytic grain.

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.