Active Sites for Hydrogen Dissociation over TiO_<i>x</i>/Au(111) Surfaces

Abstract

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