Rules for Selective Oxidation Exemplified by Methanol Selective Oxidation on Iron Molybdate Catalysts

A number of simple rules are proposed which dictate for high selectivity to formaldehyde during methanol oxidation on iron molybdate catalysts. The reaction is of the Mars–van Krevelen type, that is, it is surface lattice oxygen that is the active species. However, we also show that the material is quite a flexible one, in that the bulk oxygen anions become mobile enough at moderate temperatures to exchange rapidly with the surface vacancies that are created during the reaction. Further, and essential to high selectivity, is the fact that Mo is preferentially segregated to the surface layer of the catalyst, with no Fe present there. Finally, it can also be shown that the important oxidation state for the cations at the surface is the highest one (6+ for Mo), with lower oxidation states being detrimental to selectivity. It is likely that these basic rules are also important for a range of other catalytic processes/catalysts.