The dynamic states of silica-supported metal oxide catalysts during methanol oxidation

The in situ Raman spectra of silica-supported metal oxide catalysts (containing surface metal oxide species of V, Nb, Cr, Mo, W and Re) were measured during methanol oxidation. Stable surface methoxy species were found to form via reaction with the surface Si---OH groups for all the catalysts investigated. The surface Si---OH groups were also titrated by the surface metal oxide species and the surface concentration of Si---OH groups decreases with increasing metal oxide loading. The stable surface M---OCH₃ were only found for the V₂O₅/SiO₂ catalysts. The surface metal oxide species were all influenced by the methanol oxidation reaction. The surface rhenium oxide species were removed from the silica surface and the surface molybdenum oxide species were partially agglomerated to crystalline β-MoO₃ particles by the formation of Re-methoxy and Mo-methoxy complexes. The surface niobium oxide and tungsten oxide species were partially reducing by the net reducing methanol oxidation environment. In situ Raman spectra for the CrO₃/SiO₂ catalysts could not be obtained due to the formation of reduced chromium oxide species during methanol oxidation which gave rise to sample fluorescence. The in situ Raman observations provided a fundamental basis for understanding the selectivity patterns of the silica-supported metal oxide catalysts during methanol oxidation. However, the mechanism by which the silica support ligands activate the redox properties of the surface metal oxide species is not completely understood.