Ensemble Effects on Methanol Oxidation to Formaldehyde on Ferric Molybdate Catalysts

The properties of Mo-doped iron oxide are compared with those of the single oxides of Fe and Mo, and with stoichiometric ferric molybdate for the selective oxidation of methanol. It is found that Mo oxide segregates to the surface of the iron oxide at low loadings, while at higher loadings, but below the stoichiometric ratio, presents layers of ferric molybdate at the surface. The relationship between bulk loading and surface Mo is explored, and it is concluded that the reactivity is dominated by ensemble effects. Simple modelling indicates that four or more Fe cation ensembles are required to combust methanol to CO2, ensembles of two Mo cations are required for selective oxidation to formaldehyde, whereas it seems that isolated single sites of either Fe or Mo produce CO. Methanol oxidation on the surface of iron oxide (red), giving combustion to CO2, on molybdenum oxide (blue) producing formaldehyde selectively, and on the mixed Mo-Fe surface yielding partial oxidation to CO. image