On the Behavior of Paramagnetic Species Formed in the Pure V2O5 Crystal under SO2-Oxidation

Two types of ESR spectra were found in the highly purified V2O5 crystal under conditions of SO2 oxidation. The shapes and the intensities of the spectra were investigated in detail in the temperature range from 470 to 656 °C under various mixing ratios of SO2 to O2. Below ca. 600 °C a sharp and asymmetric spectrum existed stably, while above that temperature it was replaced by a spectrum with an hfs of 29 lines, the intensities of which always were very weak. The low-temperature-type spectrum was reasonably assigned to the VOSO4 phase, and the high-temperature-type, to an oxygen defect surrounded equivalently by four vanadium ions in the V2O5 phase. The interchange between the two spectra near 600 °C occurred reversibly and was related favorably to the following process in the surface layer of the V2O5 crystal: 2VOSO4=V2O5+SO3+SO2. The change in the working states of a pure V2O5 crystal according to the conditions of SO2 oxidation was discussed.