An ESR Study on the Working States of a Pure V2O5 Crystal under the CO-oxidation

The V2O5 crystal, prepared from the pure V2O5 powder by a zone-melting method in the air, did not show any ESR signal. When the crystal came into contact with a mixture of CO and O2 ((CO/O2)≥4), an ESR spectrum with the hfs of 15-lines (g⁄⁄=1.932, g⊥=1.978; 〈g〉=1.963), which has been previously ascribed to an oxygen defect, was found at the beginning, but after prolonged contact a sharp spectrum (〈g〉=1.954) appeared. The intensity of the hf-lines was constant but that of the sharp spectrum increased with the contact time. The g-value and line shape of both spectra did not change with the contact time. The same spectra were also found in the crystal grown under a mixture of CO and O2. On the thermal decomposition of pure NH4VO3in vacuo a similar spectrum with a sharp line was found. The g-value became lower in the sample reduced further by SO2 after the thermal decomposition. With reference to the data of TGA, DTA, magnetic susceptibility (χmol), and X-ray analysis of the vanadium oxides, the sharp spectrum was ascribed to a lower oxide or oxide state such as V3O7. Based on the ESR results, the working state of pure V2O5 crystal under CO-oxidation and its catalytic property are discussed.