EFFECTS OF SUPPORT COMPOSITION ON THE CATALYTIC ACTIVITY OF ISOLATED VANADATE SPECIES FOR METHANOL OXIDATION TO FORMALDEHYDE

摘要

Isolated vanadate species are active as catalysts for the oxidation of methanol to formaldehyde. While the structure of such sites is found to be virtually the same on different supports, the turnover frequency of such sites can differ by orders of magnitude depending on support composition. The origins of the strong effects of support composition have been explored both experimentally and theoretically for isolated vanadate species supported on silica and titania. We find that independent of the support the precursor to formaldehyde are V-OCH3 species formed by the reaction of methanol with V- O-S (S = Si or Ti) bonds. The rate limiting step in both cases is the transfer of an H atom from the methyl group of V-OCH3 to the vanadyl group associated with the active site. The principal difference between VO_x/SiO2 and VO_x/TiO2 is in the activation energy for the rate limiting step, which is higher for VO_x/SiO2 than for VO_x/TiO2. It has been hypothesized that this difference is caused by a change in the intrinsic properties of the supported vanadate species resulting from differences in the electronic properties of the support. However, this idea is not supported by theoretical analysis. What is found instead is that the lower activation barrier measured for isolated vanadate species supported on titania is due to the occurrence of defects in the support adjacent to the active center. The kinetics of methanol oxidation on VO_x/TiO2 developed on the basis of the proposed mechanism are in excellent agreement with those observed experimentally. In an extension of these ideas, we have investigated the effects of placing VO4 units at various positions on the surface of a defected Ti (001) surface.