Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

Abstract

The molecular structure of silica-supported vanadium oxide (VO_<i>x</i>) catalysts over wide range of surface VO_<i>x</i> 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 VO_<i>x</i> Raman bands (920, 1032, and 1060 cm⁻¹) whose assignments have been controversial. The resonance Raman spectra of dehydrated VO_<i>x</i>/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 VO_<i>x</i> species on dehydrated SiO₂ even at submonolayer VO_<i>x</i> loading. Quantitative estimation shows that the two different monomeric VO_<i>x</i> species coexist on silica surface from very low VO_<i>x</i> loadings and transform to crystalline V₂O₅ at VO_<i>x</i> loadings above the monolayer. It is postulated that one of the two monomeric VO_<i>x</i> species has pyramidal structure and the other is in the partially hydroxylated pyramidal mode. The two VO_<i>x</i> species show similar reduction−oxidation behavior and may both participate in redox reactions catalyzed by VO_<i>x</i>/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.