Studies of heterogeneous oxidation catalysts. Part 2.—The vanadium (V) oxide + molybdenum (VI) oxide and other binary oxide systems

Abstract

Studies of vanadium (V) oxide + molybdenum (VI) oxide mixtures have shown that, in contrast to the V2O5+ TiO2 system, molybdenum (VI) oxide can scarcely be described as a promoter for V2O5, since the molybdenum compound itself exerts a significant catalytic effect on oxidation reactions. Although 25–30 % w/w MoO3 represents the optimum concentration for the evolution of reactive oxygen from binary mixtures of this compound with V2O5, and this constitutes the most selective catalyst for the oxidation of certain hydrocarbons, spectroscopic studies indicate the involvement of both components. The observed formation of compounds closely related to Mo6V9O40 is thought to be of little significance catalytically.In contrast, a study of vanadia + zirconia mixtures has revealed that, like titanium (IV) oxide, ZrO2 acts as a true promoter. The preferred composition of catalyst for the selective oxidation of o-xylene also represents the optimum composition (5–10 mole % V2O5) for a high temperature solid-state reaction between V2O5 and ZrO2. During this reaction a small proportion of V5+ ions (19 atom %) is reduced to a lower valency state.Finally, thermal studies of other oxide pairs (V2O5+ ZnO, Nb2O5+ TiO2(anatase), MnO2+ MoO3 and CuO + Cr2O3) have shown that the existence of a maximum in the weight loss against composition curves obtained at high temperatures constitutes a good diagnostic test of their catalytic activity. In isolation, however, such measurements give little information regarding the role in catalysis of the separate constituents.