Abstract
Valence states and structures of vanadium species supported on silica, which were synthesized by a sol-gel method, were studied by Electron Spin Resonance (ESR), UV-visible absorption, diffuse reflectance, and photoluminescence spectroscopies for various stages of the process (sols, xerogels and heated xerogels). ESR measurements revealed that V4+ ions were present as [VO(H2O)5]2+ in sols and xerogels for a lower vanadium concentration, whereas a higher vanadium concentration led to the aggregation of V4+ ions in the drying stage of the sol-gel process. Upon calcinations at a higher temperature of. 873K, the V4+ ions were oxidized to V5+ which, for a lower vanadium concentration, were located in isolated pseudotetrahedral silica sites, and with an increase in the vanadium content, were aggregated so that V2O5 was finally formed. Changes of pseudotetrahedral structure to octahedral one by the coordination of water molecules in the atmosphere were observed, especially when. the xerogels was heated at a lower temperature of 973K.
