Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

Abstract

The molecular structure of silica-supported vanadium oxide (VOx) catalysts over wide range of surface VOx density (0.0002−8 V/nm2) has been investigated in detail under dehydrated conditions by in situ multiwavelength Raman spectroscopy (laser excitations at 244, 325, 442, 532, and 633 nm) and in situ UV−vis diffuse reflectance spectroscopy. Resonance Raman scattering is clearly observed using 244 and 325 nm excitations, whereas normal Raman scattering occurs using excitation at the three visible wavelengths. The observation of strong fundamentals, overtones, and combinational bands due to selective resonance enhancement effect helps clarify assignments of some of the VOx Raman bands (920, 1032, and 1060 cm−1) whose assignments have been controversial. The resonance Raman spectra of dehydrated VOx/SiO2 show a V═O band at a smaller Raman shift than that in visible Raman spectra, an indication of the presence of two different surface VOx species on dehydrated SiO2 even at submonolayer VOx loading. Quantitative estimation shows that the two different monomeric VOx species coexist on silica surface from very low VOx loadings and transform to crystalline V2O5 at VOx loadings above the monolayer. It is postulated that one of the two monomeric VOx species has pyramidal structure and the other is in the partially hydroxylated pyramidal mode. The two VOx species show similar reduction−oxidation behavior and may both participate in redox reactions catalyzed by VOx/SiO2 catalysts. This study demonstrates the advantages of multiwavelength Raman spectroscopy over conventional single-wavelength Raman spectroscopy in structural characterization of supported metal-oxide catalysts.