Vanadia–niobia and vanadia–zirconia catalysts: preparation and characterization

Abstract

V–Zr and V–Nb oxide systems were prepared by two methods, impregnation of the support (zirconia or niobia) with a salt precursor of vanadium, or by mechanically mixing vanadia with both supports. The solids obtained were characterized following several techniques. The state of vanadium on the surface of the support depends on the preparation method. In the mechanical mixtures the predominant species is slightly distorted crystalline V₂O₅. In samples prepared by impregnation the species detected are isolated vanadyl, polyvanadates and crystalline V₂O₅, depending on the vanadium content; the coordination of V ions in these species is tetrahedral, distorted octahedral or pentacoordinated, respectively. Formation of crystalline ZrV₂O₇ and V₂O₅ is observed in the case of the V–Zr sample with 7.5 wt.% vanadium. All these species are more easily reducible than bulk V₂O₅ (i.e., the onset temperature for reduction is lower), and show surface acidity, both Lewis and Brönsted, the Brönsted acidity increasing as the dispersion increases.