Active species and working mechanism of silica supported MoO3 and V2O5 catalysts in the selective oxidation of light alkanes

The catalytic performance of a series of silica supported (2-7 wt%) MoO3 and (2-20 wt%) V2O5 catalysts has been comparatively evaluated in both the partial oxidation of methane to formaldehyde (MPO) and the oxidative dehydrogenation of propane to propylene (POD) in the range 500-800 degrees C and 500-525 degrees, respectively. V2O5 acts as a promoter of the reactivity of the SiO2 for both MPO and POD, while MoO3 plays a generally negative effect on the catalytic functionality of the SiO2 surface acting however as a promoter in the MPO at T>650 degrees C. A direct relationship between the density of reduced sites of low-medium loaded silica based oxide catalysts and reaction rate in both MPO and POD strongly suggests the occurrence of a concerted reaction mechanism involving the activation of gas-phase Oz on the reduced sites of the catalyst surface. The nature of the active sites has been defined on the basis of a complete characterization of the surface and redox features of MoO3/SiO2 and V2O5/SiO2 catalysts.