Catalytic Properties of Supported Molybdenum Oxide Catalysts: In Situ Raman and Methanol Oxidation Studies

Abstract

The oxidation of methanol was studied over supported molybdenum oxide catalysts as a function of the specific oxide support (TiO2,ZrO2, Nb2O5, and A1203) and molybdenum oxide loading (surface coverage). The surface molybdenum oxide species were selective for the production of formaldehyde, and the oxide support sites yielded dimethyl ether (alumina and niobia) and methyl formate (zirconia) or were relatively inactive (titania). The turnover frequency (TOF) for the selective oxidation of methanol to formaldehyde varied by a factor of 2-4 with surface molybdenum oxide coverage and a factor of approximately 10 with the specific oxide support at monolayer coverage. The molecular structures of the surface molybdenum oxide species (isolated, tetrahedral at low coverages and polymerized, octahedralketrahedral at high coverages) did not affect the reaction selectivity but did appear to influence the slight increase in TOF with surface coverage. The order of magnitude variation in TOF with the specific oxide support correlated with the reducibility of the support and suggests that the Mo-0-support bond is critical in controlling the TOF. In situ Raman studies during methanol oxidation revealed that the supported molybdenum oxide species were 100% dispersed up to monolayer coverage. The percent reduction of the surface molybdenum oxide species, reflected by the decrease in the Raman intensity of the Mo-0 bond, during methanol oxidation was not a strong function of surface coverage and the specific oxide support. This suggests that the order of magnitude variation in the TOF with the specific oxide support is primarily related to the activity per site of the surface molybdenum oxide species rather than variation in the number of participating sites.