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

V-Zr and V-Nb oxide systems were prepared by two methods, impregnation of t he support (zirconia or niobia) with a salt precursor of vanadium, or by me chanically mixing vanadia with both supports. The solids obtained were char acterized following several techniques. The state of vanadium on the surfac e of the support depends on the preparation method. In the mechanical mixtu res the predominant species is slightly distorted crystalline V2O5. In samp les prepared by impregnation the species detected are isolated vanadyl, pol yvanadates and crystalline V2O5, depending on the vanadium content; the coo rdination of V ions in these species is tetrahedral, distorted octahedral o r pentacoordinated, respectively. Formation of crystalline ZrV2O7 and V2O5 is observed in the case of the V-Zr sample with 7.5 wt.% vanadium. All thes e species are more easily reducible than bulk V2O5 (i.e., the onset tempera ture for reduction is lower), and show surface acidity, both Lewis and Bron sted, the Bronsted acidity increasing as the dispersion increases.