Structural characterization of the V2O5/TiO2 system obtained by the sol–gel method

The influence of the vanadium load and calcination temperature on the structural characteristics of the V2O5/TiO2 system was studied by X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. Samples of the V2O5/TiO2 system were prepared by the sol–gel method under acid conditions and calcined at different temperatures. The rutile phase was found to predominate in pure TiO2 calcined at 450 8C as a result of the reduction of phase transition temperature promoted by the sol–gel method under acid conditions. The anatase phase became predominant at 450 8C as the amount of vanadium increased from 6 to 9 wt%. A structural change in the TiO2 phase from predominantly anatase to totally rutile with increased calcination temperature was observed in 6 wt% samples. An analysis of the vanadium X-ray Absorption Near Edge Structure (XANES) spectra showed that the oxidation state of vanadium atoms in the samples containing 6 and 9 wt% of vanadium and calcined at 450 8C was predominantly V4þ. However, the presence of V5þ atoms cannot be ruled out. A qualitative analysis of extended Xray absorption fine structure (EXAFS) spectra of the samples containing 6 and 9 wt% of vanadium calcined at 450 8C showed that the local structure around vanadium atoms is comparable to that of VO2 crystalline phase, in which vanadium atoms are fourfold coordinated in a distorted structure. For the sample after calcination at 600 8C, the EXAFS and XANES results showed that a significant portion of vanadium atoms were incorporated in the rutile lattice with a VxTi(12x )O2 solid solution formation. The conditions of sample preparation used here to prepare V2O5/TiO2 samples associated with different amounts of vanadium and calcination temperatures proved to be useful to modifying the structure of the V2O5/TiO2 system. q 2002 Elsevier Science Ltd. All rights reserved.