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 AuOx, 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).
Last Modified: October 25, 2013