Molybdenum Oxide on Fe2O3 Core−Shell Catalysts: Probing the Nature of the Structural Motifs Responsible for Methanol Oxidation Catalysis

A series of MoOx-modified Fe2O3 catalysts have been prepared in an attempt to make core−shell oxidic materials of the type MoOx/Fe2O3. It is conclusively shown that for three monolayers of Mo dosed, the Mo stays in the surface region, even after annealing to high temperature. It is only when the material is annealed above 400 °C that it reacts with the iron oxide. We show by a combination of methods, and especially by XAFS, that at temperatures above 400 °C, most of the Mo converts to Fe2(MoO4)3, with Mo in a tetrahedral structure, whereas below that temperature, nanocrystalline MoO3 is present in the sample; however, the active catalysts have an octahedral MoOx layer at the surface even after calcination to 600 °C. This surface layer appears to be present at all temperatures between 300 and 600 °C, and it is the nanoparticles of MoO3 that are present at the lower temperature that react to form ferric molybdate, which underlies this surface layer. It is the MoOx layer on the Fe2(MoO4)3 underlayer that makes the surface active and selective for formaldehyde synthesis, whereas the iron oxide surface itself is a combustor. The material is both activated and improved in selectivity due to the dominance of the methoxy species on the Mo-doped material, as opposed to the much more stable formate, which is the main intermediate on Fe2O3.