Semiconductive and redox properties of V₂O₅/TiO₂ catalysts

Abstract

The dc electrical conductivities of a series of V₂O₅/TiO₂ samples were measured at 250°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 ⩾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.

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