ELECTRICAL-CONDUCTIVITY CHARACTERIZATION OF EUROCAT TITANIA-SUPPORTED VANADIA CATALYSTS

Two Eurocat V2O5/TiO2 Catalysts (EL10V1 and EL10V8), were characterized by electrical conductivity measurement at 300-degrees-C. It was demonstrated that vanadium is partially dissolved as V5+ ions into the lattice of TiO2 during the preparative calcination. The concentration of dissolved V5+ ions increases with the amount of deposited vanadia and concordant results indicate that 2.5 times more V5+ ions are dissolved in the sample supporting 8 wt.-%/V2O5 than in that supporting only 1 wt.-% V2O5. The major part Of V2O5 is present as deposited particles. There is an indirect evidence (i) for the absence of a regular and continuous V2O5 surface layer model and (ii) for an increase in the mean particle size as the percentage of deposited V2O5 increases. At a higher temperature (450-degrees-C), equal to the final calcination temperature during catalyst preparation, anionic vacancies were detected at the surface of TiO2 and are in equilibrium with gaseous oxygen. Their concentration in TiO2 increases with the content of deposited V2O5. Consequently, TiO2 appears as a non-inert support and should intervene in the catalytic performance of the solid. Etching in boiling isobutanol selectively removes V2O5-supported particles but does not destroy the solid solution of V5+ in TiO2. A schematic representation is proposed for the two catalysts tested: this consists of a solid solution (V5+ in TiO2) supporting V2O5 particles. The solubility of V5+ and the mean particle size (with the assumption of homodispersed particles) both increase with the percentage of deposited vanadia (2-3 nm and 15-20 nm for V2O5 particles in EL10V1 and EL10V8 samples, respectively).