Active Sites for Hydrogen Dissociation over TiO x /Au(111) Surfaces

We carried out H2–D2 exchange reactions over TiO x /Au(111) (x = 0–2) model surfaces to clarify the active sites for hydrogen dissociation over the Au/TiO2 catalyst. Ti oxides with controlled oxidation states were prepared on the Au(111) surface by regulation of O2 exposure and temperature. The HD formation rate depended on the concentration of TiO2 over Au(111), indicating that hydrogen dissociation sites were created between the stoichiometric TiO2 and Au. Atomic force microscopy observations indicated that the number of monolayer TiO2 islands with diameters of about 20 nm increased with increasing Ti coverage below ΘTi = 0.65 and that as ΘTi was increased further, a TiO2 multilayer began to form over Au(111). The turnover frequencies for HD formation over the TiO2/Au(111) surface, which were calculated from the length of the perimeter interface between the TiO2 islands and the Au substrate, were consistent with the frequencies reported for Au/TiO2(110) surfaces. Furthermore, the activation energies for HD formation over TiO2/Au(111) agreed well with those for Au/TiO2(110). We thus speculated that the active sites for hydrogen dissociation over Au/TiO2 were the Auδ+–Oδ-–Ti sites, which were formed at the perimeter interface between the stoichiometric TiO2 and Au.