Surface Characterisation of V2O5/TiO2 Catalytic System
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Samples of the V2O5/TiO2 system were prepared by the sol-gel method and calcined at different temperatures. Surface species of vanadium, their dispersion, as wefl as the structural evolution of the system were analysed by XRD, Raman, EPR, and XPS techniques. The results of XRD showed the evolution of TiO2 from anatase phase to rutile phase. The Raman spectra for calcination temperatures up to 500°C showed a good dispersion of vanadium over titania in the form of monomeric vanadyl groups (V4+) and polymeric vanadates (V5+). At least three families of V4+ ions were identified by EPR investigations. Two kinds of isolated V4+ species are placed in sites of octahedral symmetry, substituting Ti4+ in the rutile phase. The third is formed by pairs of V4+ species on the surface of titania. Above 500°C part of superficial V4+ is inserted into the matrix of titania and part is oxidized to V5+. The XPS results showed that the V/Ti ratio rises with increasing calcination temperature, indicating a smaller dispersion of vanadium.