The Activation of Gold and the Water-Gas Shift Reaction: Insights from Studies with Model Catalysts

The activation of gold in catalytic reactions has been the subject of intensive research that has led to the transformation of one of the least chemically reactive element to a catalyst with excellent activity and selectivity. Scientists have performed numerous systematic experimental and theoretical studies using model systems, which have explained the role of Au in chemical reactions with progressively increasing degrees of structural and chemical complexity. We present an overview of recent studies of model Au(111), CeOx/Au(111), and Au/CeOx/TiO2(110) surfaces that use Au in different structural configurations specifically for the water-gas shift reaction (WGS, CO + H2O -> CO2 + H-2), an important industrial process for the purification of CO. We demonstrate the significance of key structural components of the Au-based supported catalysts such as the metal oxide interface (Au-O-x) toward the WGS catalytic activity, a "structure-activity' relationship. In the WGS reaction, Au(111) or Au nanoparticles have poor catalytic performance due to their inability to activate one of the most important steps of the reaction, the breaking of O-H bonds in the dissociation of water (H2O -> OH + H). The relatively large energetic barrier can be overcome by using O on Au(111) to facilitate the formation of OH at low temperatures, with eventual CO2 and H-2 production upon reaction between CO and the adsorbed OH. However, the inability to replace the reacted 0 prevents a sustainable catalytic process from occurring on Au(111). The addition of a small concentration of CeOx nanopartides on top of the Au(111) surface facilitates this rate-determining step and easily continues the catalytic cycle in the production of H-2. We have discovered that CeOx nanoparticles in contact with Au(111) are rich in Ce3+. They also have a distinct metal oxide interface, which sustains excellent activity for the WGS reaction via the formation of a unique carboxylate Intermediate, making CeOx/Au(111) more active than Cu/ZnO(000 (1) over bar), Cu(100), and Cu(111) which are the typical catalysts for this reaction. Taking this knowledge one step further, bringing these components (oxide and metal nanoparticles) together over a second oxide in Au/CeOx/TiO2 produces a system with unique morphological and electronic properties. The result is a superior catalyst for the WGS reaction, both as a model system (Au/CeOx/TiO2/(110) and as powder material (Au/CeOx/TiO2(anatase)) optimized directly In a series of systematic investigations.