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摘要
Vanadia-silica catalysts with different vanadium loadings were prepared by sol-gel process. UV-vis diffuse-reflectance spectroscopy was used to elucidate the effect of drying mode (i.e., xerogel vs. aerogel), vanadium loading and calcination on the molecular structure of supported vanadium species. The results indicate that for vanadium loading ranging from 2.8 to 11.2 wt.%, the band-gap energies of all catalysts varying from 2.28 to 2.68 eV which demonstrate that vanadium oxides are predominantly in octahedral structure with the presence of tetrahedral species. The discrimination of different surface VOx species has been based on their characteristic Ligand to Metal Charge Transfer (LMCT) O -> V(V) and d-d transition. It was found that the LMCT band position of V=O bond is not affected by calcination either in xerogels or in aerogels but the position and the shape of bands relative to bridging V-O-V bonds are affected by vanadium loading, calcination and drying mode. For the same V/Si ratio, band-gap energy of xerogel is lower than that of aerogel which indicate that vanadium species are more dispersed in aerogels than in xerogels. Drying and calcination led to rearrangement, dehydration, cleavage and crystallization of vanadium species which explain the presence of some amount of crystalline V2O5 in calcined samples.
- 出版日期2008-12-30
