Photo-oxidation of methanol using [formula omitted]: Catalyst structure and reaction selectivity

Abstract

Laser Raman spectra, UV-visible spectra, and photo-catalytic activity measurements are described which show that supported MoO3TiO2 catalysts prepared by impregnation possess a layer of surface molybdate species coordinated to the titania support. The saturation coverage of this surface molybdate monolayer corresponds to a surface density of ca. 4 × 1014 Mo atoms per cm2 of TiO2. The properties of this surface monolayer are markedly different from bulk MoO3, as reflected in different vibrational frequencies for the surface molybdate species and for bulk MoO3, and in the higher photo-catalytic activity of the surface molybdate relative to bulk MoO3. The photo-efficiency of the TiO2 catalyst containing the surface molybdate monolayer is about one-fifth of the photo-efficiency of pure TiO2, but the selectivity for suppressing secondary oxidation reactions is significantly greater. The major photo-assisted oxidation product on TiO2 is methyl formate, independent of methanol conversion. In contrast, the molybdate monolayer catalyst has nearly 100% selectivity for dimethoxymethane (the reversible condensation product of CH2O + 2 CH3OH) at low conversions. For both catalysts the selectivity of the primary oxidation step appears to be insensitive to the mode of excitation (i.e., thermal vs photo-assisted). Photo-induced changes in selectivity at a given conversion are due primarily to differences in adsorbate coverage at the different operating temperatures required to give equal conversion for dark vs light reactions.