EPR studies of caesium-doped V2O5–Fe2O3 catalysts: new evidence for active centres

Abstract

EPR studies of unsupported V₂O₅–Fe₂O₃ catalysts revealed that by doping with caesium sulfate a new phase is formed in the solids which apparently increases their selectivity in the catalytic oxidation of polycyclic hydrocarbons. The percentage of this phase rises with increasing iron content. As indicated by XRD and Mössbauer spectroscopy, the new phase is amorphous. Its composition is non-stoichiometric and varies slightly depending on the iron content. For catalysts with V : Fe : Cs atomic ratios of 1 : 1 : 0.06 and 1 : 1.4 : 0.06, approximate formal compositions of Fe_0.70VO_3.04 and Fe_0.65VO_3.14, respectively, were calculated. A number of in situ EPR measurements performed with variation of temperature and atmosphere under mechanical stress and during treatment of the catalysts with fluorene, revealed that the active centres probably consist of oxygen lattice vacancies in the coordination spheres of Fe³⁺ ions which are occupied by an electron.