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 FeVO 4 mixed metal oxide catalyst. The bulk V 2 O 5, α-Fe 2 O 3, and supported 4% V 2 O 5 /α-Fe 2 O 3 metal oxide systems were also investigated to better understand the surface composition and surface chemistry of the bulk FeVO 4 catalyst. Raman spectroscopy confirmed that the bulk FeVO 4 was phase pure and has no contribution from excess V 2 O 5 and α-Fe 2 O 3 phases. IR spectroscopy confirmed that the model supported 4% V 2 O 5 /α-Fe 2 O 3 catalyst consists of an amorphous surface VO x monolayer on the α-Fe 2 O 3 support. The surface chemistry of the metal oxides was chemically probed with temperature programmed CH 3 OH-IR spectroscopy and revealed that both intact surface CH 3 OH* and CH 3 O* species are present on the catalysts. On acidic α-Fe 2 O 3, the surface CH 3 OH* and CH 3 O* intermediates yield CH 3 OH and dimethyl ether (DME), respectively. For the redox V 2 O 5, FeVO 4 and supported 4% V 2 O 5 /α-Fe 2 O 3 catalysts: however, both surface intermediates primarily give rise to HCHO. These results confirm that the surface of bulk FeVO 4 and supported 4% V 2 O 5 /α-Fe 2 O 3 catalysts are similar and that surface VO x species are the catalytic active sites for methanol oxidation to formaldehyde over bulk FeVO 4 catalysts. This conclusion is supported by HR-TEM images that reveal an amorphous VO x enriched layer of ∼1 nm at the outer surface of the bulk FeVO 4 catalysts. Methanol oxidation over bulk FeVO 4 was found to proceed via a Mars−van Krevelen mechanism, where the reduced surface VO x 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 FeVO 4 mixed oxide reside in the outermost surface layer and not in the bulk lattice structure.