ESR,紫外-可視吸収,拡散反射,ホトルミネッセンス分光法によるゾル-ゲルシリカ中のバナジウムイオンの研究

概要

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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 V^<4+> ions were present as [VO(H_2O)_5]^<2+> in sols and xerogels for a lower vanadium concentration, whereas a higher vanadium concentration led to the aggregation of V^<4+> ions in the drying stage of the sol-gel process. Upon calcinations at a higher temperature of 873 K, the V^<4+> ions were oxidized to V^<5+> 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 V_2O_5 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 973 K.
2002-06-01