Selective oxidation of methanol to formaldehyde has been studied over a series of molybdates, molybdenum oxides, mixed tungsten - molybdenum oxides, and supported molybdenum oxide. Reactor studies, Temperature Programmed Reaction (TPR) and in-situ FTIR have shown that water and methanol adsorb dissociatively and reversibly on the same catalyst sites and that the rate determining step is the breaking of a carbon-hydrogen bond. The reaction is of a redox type and a nearly fully oxidized catalyst is most active and selective. The difference in activity between molybdenum trioxide and ferric molybdate can be explained by a mechanism in which formation of adsorbed methoxys does not occur on the predominant (010) face of MoO3. Oxide-support interactions are responsible for the lower selectivity of silica supported catalysts.
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