Studies of heterogeneous oxidation catalysts. Part 1.—The vanadium(V) oxide + titanium(IV) oxide system

Abstract

The mechanism of promotion of vanadium(V) oxide catalysts by titanium(IV) oxide (anatase, brookite and rutile) has been investigated by a variety of physicochemical techniques with particular reference to the thermal behaviour. With binary mixtures of vanadium(V) oxide and anatase or brookite, oxygen evolution from V₂O₅ occurs between 873 and 973 K and this is accompanied by a gradual phase-transformation of both modifications of titanium dioxide to rutile. In the case of brookite, an initial phase-change to anatase is followed by a further phase-change to rutile. In all three systems, 10–15 mole % V₂O₅ represents the optimum concentration for this high temperature solid-state reaction between the components. This composition coincides with the maximum catalytic activity of such mixtures. Studies of other properties of the systems indicate that, during this process, V⁴⁺ ions are incorporated into the rutile lattice and are subsequently stabilised. The results obtained, together with other considerations, suggest that at this composition, there is maximum contact between the two types of oxide particle, so that the effective surface area of the V₂O₅ is increased. Although this appears to be the main role of TiO₂, spectroscopic evidence suggests that the presence of titania also weakens the VO bond in vanadia, thus providing an additional promoting action.