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 TiO 2 (110) produces very good catalysts for the WGS. Although bulk metallic gold is not active as a WGS catalyst, Au nanoparticles supported on TiO 2 (110) have an activity comparable to that of Cu/ZnO(000ı̅). Cu/TiO 2 (110) is clearly a better catalyst than Cu/ZnO(0001̅) or Au/TiO 2 (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(0001̅), 10.2 on Au/TiO 2 (110), and 8.3 kcal/mol on Cu/TiO 2 (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 TiO 2 (110). In images of scanning tunneling miscroscopy, the average particle size in Cu/TiO 2 (110) is smaller than that in Au/TiO 2 (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/TiO 2 (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 CO 2 and hydrogen.