The selective oxidation of methanol on iron molybdate catalysts

The role of Mo in the selective oxidation reaction is considered in some detail, focusing on the selective oxidation of methanol to formaldehyde. The reaction mechanism and kinetics will be described. It is notable that Mo tends to segregate to the surface of iron molybdate catalysts, proven by scanning transmission electron microscopy and XPS, and so it dominates the surface, even at very low loadings. This is manifest in reaction data too: for instance, the selectivity to formaldehyde for a catalyst with only 20% Mo present is 50% at 50% conversion, whereas for pure iron oxide it is close to zero at all conversions. The active site for the reaction is Mo(VI), which cycles through Mo(IV) during the reaction. Mo(IV) itself is shown to be unselective for the reaction. Lattice oxygen in the material can readily re-oxidise the surface at temperatures above 300 degrees C. Some of the mechanistic behaviour is analogous to the role of Mo in enzymatic processes, such as xanthine oxidation, and the two areas of catalysis by molybdenum are compared and contrasted. The role of different types of oxygen, such as 'lattice' oxygen, 'surface' oxygen, bridging and terminal oxygen species will be defined and clarified, and the modified Mars-van Krevelen description of the heterogeneous reaction will be given.