Photo-oxidation of Methanol Using Mo03/Ti02: Catalyst Structure and Reaction Selectivity

Laser Raman spectra, UV-visible spectra, and photo-catalytic activity measurements are described which show that supported MoO,iTiOr 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 x lOI MO atoms per cm2 of TiO*. The properties of this surface monolayer are markedly different from bulk MOO,, as reflected in different vibrational frequencies for the surface molybdate species and for bulk Moor, and in the higher photo-catalytic activity of the surface molybdate relative to bulk Moor. The photo-efficiency of the TiOr catalyst containing the surface molybdate monolayer is about one-fifth of the photo-efficiency of pure TiO?, but the selectivity for suppressing secondary oxidation reactions is significantly greater. The major photo-assisted oxidation product on TiOr is methyl formate, independent of methanol conversion. In contrast, the molybdate monolayer catalyst has nearly 100% selectivity for dimethoxymethane (the reversible condensation product of CHzO + 2 CH?OH) 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.