LASER RAMAN-SPECTROSCOPY OF SUPPORTED VANADIUM-OXIDE CATALYSTS

Supported vanadia catalysts were studied by Raman spectroscopy to establish the effects of vanadia loading and support composition on the structure of the dispersed vanadia species. Three types of vanadia species were found to be present on SiO2, TiO2, and Al2O3 supports: monomeric vanadyls, one- and two-dimensional vanadate chains, and crystallites of V2O5. Monomeric vanadyls are characterized by a narrow Raman band in the region of 1026-1042 cm-1, the position being sensitive to the identity of the support. The terminal V=O groups of polyvanadate chains give rise to a broad feature in the region of 800-1000 cm-1 which, in the case of TiO2-supported V2O5, shifts to higher frequency as the V2O5 loading increases. Crystalline V2O5 was detected at higher loadings on all three supports. Evidence from Raman spectroscopy and O2 chemisorption indicates that the dispersion of vanadia increases in the order SiO2 < Al2O3 ∼ TiO2. Raman spectra taken after the catalysts were exposed to ethanol and water show that only the monomeric species interact with the adsorbates. For SiO2-supported V2O5, these interactions result in a reduction in the extent of bonding between the vanadyl groups and the support. Similar experiments conducted with TiO2- and Al2O3-supported V2O5 show no evidence of a reduction in the extent of vanadyl bonding to the support. © 1990 American Chemical Society.