EPR and TPR investigation of the redox properties of vanadia based ceria catalysts

Vanadium cerium oxides, with different V/Ce atomic ratios, were prepared using the impregnation method and calcined under air at 500°C. Physicochemical studies have shown that at low vanadium content, polymeric V-O-V chains are stabilized on the ceria surface. Increasing the vanadium content tends to favor the formation of the CeVO4 and V2O5 phases. The redox properties of these oxides have been simultaneously investigated by TPR/TPO and EPR techniques. V-O-V chains and V2O5 species are more easily reducible than the CeVO4 phase. The reduction of V2O5 to V2O3 proceeds in several steps, the intermediate species being V6O13, VO2 and V5O9. The reduction of V2O5 species interacting with ceria support leads to VO oxide. EPR measurements performed at T = −269°C have permitted to observe progressively different signals of V4+ in addition to vanadium ions in V2+ (3d3) paramagnetic configuration. This attribution is based on an EPR signal at g = 3.956 with eight well resolved hyper fine lines (A = 96 Gauss), which may be attributed to the perpendicular components of one of the fine transitions corresponding to the V2+ spectrum. At high reduction temperature, CeVO4 phase leads in one step to CeVO3 and a continuous and partial reduction of CeO2 into Ce2O3 is observed. Re-oxidation process shows that polymeric V-O-V chains, easily reducible, are hardly re-oxidized whereas V2O5 species, present in the high vanadium loading samples, are easily re-oxidized at low temperatures. However, redox processes seem to be reversible.