Mechanism and surface structural effects in methanol oxidation over molybdates

Abstract

Ferric molybdate is 2–4 times as active as pure MoO3 as a catalyst for methanol oxidation to formaldehyde but shows similar selectivities and kinetics. These catalysts were studied using temperature programmed desorption (TPD), microbalance adsorption, transmission infrared spectroscopy and photoemission spectroscopy. TPD confirms that the mechanism of methanol oxidation, involving loss of a photon from surface methoxy in the rate limiting step, is the same over the two materials. The methanol adsorption kinetics suggest that methanol adsorption is activated and that there is a wider distribution of generally higher activation energies on MoO3 than Fe2(MoO4)3. The wide distribution is the result of the anisotropic, layered structure of MoO3 compared to the pseudo-cubic Fe2(MoO4)3 structure. UPS and TPD measurements on the MoO3 (010) surface support this idea, as no chemisorption of methanol was found on the (010) layer surface, but was found on powder surfaces and ion bombarded MoO3 (010) surfaces. These results lead to the hypothesis that methanol adsorption leading to reaction is facile at coordinatively unsaturated Mo ions. All of the faces of Fe2(MoO4)3 can expose such sites, resulting in the observed higher activity.