Identification of Vanadium Oxide Species and Trapped Single Electrons in Interaction with the CeVO₄ Phase in Vanadium−Cerium Oxide Systems. ⁵¹V MAS NMR, EPR, Raman, and Thermal Analysis Studies

Vanadium−cerium oxide catalysts, with different V/Ce atomic ratios, were prepared using vanadyl oxalate (VOC 2 O 4 ) impregnated on ceria (CeO 2 ) as precursors. Subsequently, the freshly prepared solids were calcined under a flow of dried air at different temperatures from 400 to 800 °C. These solids have been characterized with different techniques: Raman spectroscopy, thermal analysis (TG-DSC), specific area measurements (BET), EPR, and solid-state 51 V MAS NMR. Different vanadium species in the vanadium−cerium oxide catalysts have been evidenced. Polymeric V−O−V chains, vanadium tetrahedral surface species, and V 2 O 5 phase are stabilized mainly when solids are calcined in the temperature range 400−500 °C. Their formation also depends on the vanadium content. For higher calcinations temperatures (≥500°C), CeO 2 reacts with vanadium (V) species to form a CeVO 4 mixed phase. Further to that formation, a single electron is trapped in an oxygen vacancy in the CeVO 4 phase and can be considered as its probe.