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

• Published in: Journal of materials science 2006-03, Vol.41 (6), p.1827-1833
• Main Authors: Abi-Aad, E, Matta, J, Courcot, D, Aboukaïs, A
• Format: Article
• Language: English
• Subjects: air; catalysts; ceric oxide; cerium; Cerium oxide; Cerium oxides; Chains (polymeric); ions; Materials science; Olefinic thermoplastic elastomers; Oxidation; Oxides; Phases; Reduction; temperature; Vanadium; Vanadium oxides; Vanadium pentoxide
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/s10853-006-3160-3

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.