Catalysis Science of Methanol Oxidation over Iron Vanadate Catalysts: Nature of the Catalytic Active Sites

Bulk mixed metal oxides are widely used in industry for various oxidation reactions, but there is still debate in the heterogeneous catalysis literature about the nature of their catalytic active sites. In the present study, the two-component iron vanadate mixed metal oxide system is employed to investigate the outermost surface composition and surface chemistry of the bulk FeVO4 mixed metal oxide catalyst. The bulk V2O5, alpha-Fe2O3, and supported 4% V2O5/alpha-Fe2O3 metal oxide systems were also investigated to better understand the surface composition and surface chemistry of the bulk FeVO4 catalyst. Raman spectroscopy confirmed that the bulk FeVO4 was phase pure and has no contribution from excess V2O5 and alpha-Fe2O3 phases. IR spectroscopy confirmed that the model supported 4% V2O5/alpha-Fe2O3 catalyst consists of an amorphous surface VOx monolayer on the alpha-Fe2O3 support. The surface chemistry of the metal oxides was chemically probed with temperature programmed CH3OH-IR spectroscopy and revealed that both intact surface CH3OH* and CH3O* species are present on the catalysts. On acidic alpha-Fe2O3, the surface CH3OH* and CH3O* intermediates yield CH3OH and dimethyl ether (DME), respectively. For the redox V2O5, FeVO4 and supported 4% V2O5/alpha-Fe2O3 catalysts: however, both surface intermediates primarily give rise to HCHO. These results confirm that the surface of bulk FeVO4 and supported 4% V2O5/alpha-Fe2O3 catalysts are similar and that surface VOx species are the catalytic active sites for methanol oxidation to formaldehyde over bulk FeVO4 catalysts. This conclusion is supported by HR-TEM images that reveal an amorphous VOx enriched layer of similar to 1 nm at the outer surface of the bulk FeVO4 catalysts. Methanol oxidation over bulk FeVO4 was found to proceed via a Mars-van Krevelen mechanism, where the reduced surface VOx species are reoxidized by bulk lattice oxygen rather than gas phase molecular O-2. This study demonstrates that the catalytic active sites for oxidation reactions over the bulk FeVO4 mixed oxide reside in the outermost surface layer and not in the bulk lattice structure.