THE DYNAMIC STATES OF SILICA-SUPPORTED METAL-OXIDE CATALYSTS DURING METHANOL OXIDATION

The in situ Raman spectra of silica-supported metal oxide catalysts (c ontaining surface metal oxide species of V, Nb, Cr, Mo, W and Re) were measured during methanol oxidation. Stable surface methoxy species we re found to form via reaction with the surface Si-OH groups for all th e 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 s urface M-OCH3 were only found for the V2O5/SiO2 catalysts. The surface metal oxide species were all influenced by the methanol oxidation rea ction. The surface rhenium oxide species were removed from the silica surface and the surface molybdenum oxide species were partially agglom erated to crystalline beta-MoO3 particles by the formation of Re-metho xy and Mo-methoxy complexes. The surface niobium oxide and tungsten ox ide species were partially reducing by the net reducing methanol oxida tion environment. In situ Raman spectra for the CrO3/SiO2 catalysts co uld not be obtained due to the formation of reduced chromium oxide spe cies during methanol oxidation which gave rise to sample fluorescence. The in situ Raman observations provided a fundamental basis for under standing the selectivity patterns of the silica-supported metal oxide catalysts during methanol oxidation. However, the mechanism by which t he silica support ligands activate the redox properties of the surface metal oxide species is not completely understood.