Abstract
The structural conversion in the course of synthesis and thermal activation of silica-supported vanadium oxide catalysts has been investigated by means of ESR, O2 chemisorption and XPS techniques over wide temperature ranges and low vanadium concentrations. Three ESR signals (A, assigned to the reduction V2O3 phases; B, due to the V4+ ions surrounded by an axial distorted ligand field in amorphous V2O3; and C, assigned to VO2+ ions in some static disordered states), their relative appearance as a function of pretreatments, concentration of vanadium, and chemical route of preparation, are discussed. Moreover, the O2 chemisorption data show that the number of V4+ centres able to adsorb oxygen varied with the vanadium content and the preparation method, XPS and ESR results have also shown that the chemical species in the surface, and their structure, are more affected by the pretreatment in highly dispersed vanadium oxide than in silica-supported vanadium oxide catalysts.
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