Effect of water vapor on the molecular structures of supported vanadium oxide catalysts at elevated temperatures

The effect of water vapor on the molecular structures of V2O5-supported catalysts (SiO2, Al2O3, TiO2, and CeO2) was investigated by in situ Raman spectroscopy as a function of temperature (from 500 degrees C to 120 degrees C). Under dry conditions, only isolated surface VO4 species are present on the dehydrated SiO2 surface, and multiple surface vanadium oxide species (isolated VO4 species and polymeric vanadate species) are present on the dehydrated Al2O3, TiO2, and CeO2 surfaces. The Raman features of the surface vanadium oxide species on the SiO2 support are not affected by the introduction of water vapor due to the hydrophobic nature of the SiO2 support employed in this investigation. However, the presence of water has a pronounced effect on the molecular structures of the surface vanadium oxide species on the Al2O3, TiO2, and CeO2 supports, The Raman band of the terminal V = O bond of the surface vanadia species on these oxide supports shifts to lower wavenumbers by 5-30 cm(-1) and becomes broad upon exposure to moisture. Above 230 degrees C, the small Raman shift of the surface vanadium oxide species in the presence of water suggests that the dehydrated surface VOx species form a hydrogen bond with some of the adsorbed moisture. Upon further decreasing the temperature below 230 degrees C, the hydrogen-bonded surface VOx species are extensively solvated by water molecules and form a hydrated surface vanadate structure (e.g., decavanadate). The broad Raman band at approximate to 900 cm(-1), which is characteristic of the polymeric V-O-V functionality, appears to be minimally influenced by the presence of water vapor and is a consequence of the broadness of this band. Oxygen-18 isotopic labeling studies revealed that both the terminal V=O and bridging V-O-V bonds readily undergo oxygen exchange with water vapor, The current observations account for the inhibiting effect of moisture upon oxidation reactions over supported metal oxide catalysts and are critical for interpreting in situ Raman data during hydrocarbon oxidation reactions where H2O is a reaction product.