Surface structure and reactivity of vanadium oxide supported on titanium dioxide. V2O5/TiO2(rutile) Catalysts prepared by Hydrolysis

Abstract

V–Ti–O (rutile) catalysts have been prepared by hydrolysis of V^IV–Ti^IV solutions and low-temperature calcination (673 K). The catalysts were characterized by chemical analyses, X.r.d., e.s.r. and F.t.i.r. spectroscopies, and thermogravimetry and tested in the oxidation of o-xylene to phthalic anhydride and in the ammoxidation of toluene to benzonitrile. Different vanadia species were identified: (i) a V^IV species present in substitutional solid solution within the rutile lattice, which is stable as regards both oxidation and reduction, (ii) a V^V species, chemically interacting with the rutile surface and constituting the so-called monolayer which is easily reducible to V^IV, (iii) surface VO²⁺ ions formed by reduction during the catalytic tests from the V^V monolayer species and (iv) highly disordered and poorly crystalline V₂O₅. The VO²⁺ surface ions formed during the catalytic tests are thought to be the active sites of hydrocarbon activation during both oxidation and ammoxidation. A modification of the chemical structure occurs during permanence in the reaction environment, this modification depending on the reductive power of the gas phase. In the case of the ammoxidation of toluene a change in the relative amounts of the different vanadia species occurs, while in the case of o-xylene oxidation only an higher average degree of reduction is observed, as compared to the samples before reaction. A comparison with V–Ti–O (anatase) samples suggests that the crystalline structure of TiO₂ does not influence the activity of the V–Ti–O catalysts, but possibly the number of surface VO²⁺ ions does, and is strongly related to the method of preparation and to the activation procedure.