Catalysis: Reactivity and Structure

Mechanistic understanding of WGS catalysts from first principles: Au(111) Supported Oxide Nanoparticles

Water-gas shift (WGS, CO + H₂O → H₂ + CO₂) is critical to get clean hydrogen from fuel cells and other industrial applications. A fascinating puzzle has recently emerged: Au/CeO₂ and Au/TiO₂ nanomaterials show very efficient for WGS catalysis. This is remarkable since in bulk form Au, ceria and titania are not known as WGS catalysts. The nature of the active phase(s) in these metal/oxide nanocatalysts is unclear at the present time. Is it AuO_x, metallic Au, or an oxide nanoparticle?  To address these questions, coordinated effects have been made from experiment and theory. Our experimental collaborators have grown and characterized inverse model catalysts: CeO₂ and TiO₂ nanoparticles dispersed on a Au(111) template. The experiments show activities comparable to good WGS catalysts (e.g. Cu(100), Cu(111)). Theoretical calculations based on DFT are also carried out to understand the active sites in the oxide/gold catalysts, by probing reaction scenarios on Au(100) and Au(111) surfaces, a free Ti₂O₄ cluster, and a TiO₂/Au(111) catalyst model structure. In accordance with experiment, our calculations show a very high barrier for the dissociation of water on Au(111) or Au(100) and the formation of very stable formate species on free TiO₂ that prevents the production of H₂ and CO₂. The model TiO₂/Au(111) catalyst overcomes these bottlenecks: the moderate chemical activity of gold is coupled to the more reactive TiO₂ nanoparticle. The dissociation of water takes place on the oxide easily, a reaction that extended surfaces and nanoparticles of Au cannot perform. CO adsorbs on sites of the gold substrate located nearby (bifunctional catalyst). Then all the subsequent steps occur at the oxide-metal interface at a reasonable speed. Our results imply that the high performances of Au/CeO₂ and Au/TiO₂ nanocatalysts in the WGS rely heavily on the direct participation of oxide-metal interface.

Ref. J.A. Rodriguez, S. Ma, P. Liu, J. Hrbek, J. Evans, and M. Perez, Science 318, 1757 (2007).