Semiconductive and redox properties of V2O5/TiO2 catalysts

The de electrical conductivities of a series of V2O5/TiO2 samples were measured at 250 degrees C and observed to be sensitive to the V2O5 loadings: 1, 2, 3, 4 and 6 V2O5 wt.% corresponding to 0.9, 1.8, 2.65, 3.5 and 5.3 V atom%. Up to 3% V2O5 loading (half monolayer), the electrical conductivity, slightly but constantly, increased whereas, for greater than or equal to 4% vanadium oxide loading, the electrical conductivity increased dramatically by over one order of magnitude. It was concluded that, for low vanadia loadings, well dispersed fractions of a monolayer could be obtained with a better dispersion than that of Eurocat ones, previously studied with the same technique. It appears that the electrical conductivity provides a sensitive method to determine the total amount of V5+ ions incorporated in titania during the preparative calcination. In situ measurements of the electrical conductivity during oxygen and methanol cycles suggest that methanol reduces both vanadia and titania. Reduction by the introduction of methanol strongly increases the electrical conductivity of the samples, whereas oxygen decreases the conductivity to the original value. The reproducible conductivity levels indicate the reversible redox process occurring during the exposure of the samples to methanol (reduction) and to oxygen (oxidation). No quantitative relationship exists, between the amounts of dissolved V5+ ions detected by the variations in electrical conductivity and the methanol oxidation activity, for this series of V2O5/TiO2 samples. TiO2 appears as a reducible, non-inert support in electronic interaction with supported V2O5.