Mo+TiO₂(110) Mixed Oxide Layer: Structure and Reactivity

We present a STM/XPS/TPD/LEED study of the structural and electronic properties of Mo+Ti mixed oxide layers on TiO₂(110), and of their interaction with water, methanol and ethanol. Several different preparation procedures were tested and layers with different degrees of Mo/Ti mixing were prepared. Ordered mixed oxide surface phases with distinct LEED patterns could not be found; for all investigated Mo concentrations a TiO₂(110) like pattern was observed. Mo tends to agglomerate on the surface where it is found predominantly as Mo⁶⁺ at low coverages and as Mo⁴⁺ at high coverages. Mo⁴⁺ was also identified in the bulk of the mixed oxide layer. The Mo3d binding energies categorize the Mo⁴⁺ species as being dimeric. A third Mo3d doublet is attributed to a Mo species (Mo ⁿ⁺) with an oxidation state between those reported for Mo in MoO₂ and metallic Mo. Two types of Mo-induced features could be identified in the STM images for low Mo concentrations (in the range of 1 %). At higher Mo concentrations (~50 %) the surface is characterized by stripes with limited lengths in [001] direction. The concentration of bridging oxygen vacancies, which are common defects on TiO₂(110), is reduced significantly even at low Mo concentrations. Methanol and ethanol TPD spectra reflect this effect by a decrease of the intensity of the features related to these surface defects. At elevated MoO _x coverages, the yield of reaction products in methanol and ethanol TPD spectra are somewhat smaller than those found for clean TiO₂(110) and the reactions occur at lower temperature.