High Water-Gas Shift Activity in TiO2(110) Supported Cu and Au Nanoparticles: Role of the Oxide and Metal Particle Size

The deposition of Cu and Au nanoparticles on TiO2(110) produces very good catalysts for the WGS. Although bulk metallic gold is not active as a WGS catalyst, Au nanoparticles supported on TiO2 (110) have an activity comparable to that of Cu/ZnO(000 (1) over bar). Cu/TiO2(110) is clearly a better catalyst than Cu/ZnO(000 (1) over bar) or Au/TiO2(110). The catalysts that have the highest activity for the WGS have also the lowest apparent activation energy. On Cu(111) and Cu(100), the aparent activation energies are 18.1 and 15.2 kcal/mol, respectively. The apparent activation energy decreases to 12.4 kcal/mol on Cu/ZnO(000 (1) over bar), 10.2 on Au/TiO2 (110), and 8.3 kcal/mol on Cu/TiO2(110). The Cu <-> titania interactions are substantially stronger than the Au <-> titania interactions. This has an effect on the growth mode of the metals on TiO2(110). In images of scanning tunneling miscroscopy, the average particle size in Cu/TiO2(110) is smaller than that in Au/TiO2(110). The Cu particles are dispersed on the terraces and steps of the oxide surface, whereas the Au particles concentrate on the steps. The morphology of Cu/TiO2(110) favors high catalytic activity. The results of density functional calculations indicate that the metal-oxide interface plays an essential role in the catalysis, helping in the dissociation of water and in the formation of an OCOH intermediate, which decomposes to yield CO2 and hydrogen.