Different binding sites for methanol dehydrogenation and deoxygenation on stoichiometric and defective TiO_2(110) surfaces

摘要

Methanol adsorption on stoichiometric and defective TiO_2(1 1 0) surfaces was studied by means of TPRS and XPS. Anion vacancy defects were created by electron bombardment in order to minimize the structural damage inflicted to the surface. Methanol adsorbed dissociatively on the TiO_2(1 1 0) surfaces at room temperature, forming methoxide and hydroxide groups. On the stoichiometric surface most methoxide groups recombined at low temperature (320 K) to form methanol, while a small fraction (～15%) converted to formaldehyde and methanol at high temperature (630 K). On the electron-irradiated (defective) surfaces, a significant fraction of methoxy groups (28-36%) reacted to form methane above 450 K. The activation energy for methane desorption decreased with increasing number of anion vacancy defects. Methane formation from methoxy is favored in the presence of defects because the oxygen released is returned to the surface, healing the anion vacancy defects. Two different binding sites for the methoxy species (Ti~(4+) cations and anion vacancy defects) are proposed to be responsible for the different reaction products. Oxygen post-dosed after methanol onto the defective surfaces results in the formation of methanol and formaldehyde at high temperature, as on the stoichiometric surface.