EPR Investigation of VPO_x Catalysts

EPR and chemical analysis were used to investigate the changes of oxidation numbers of Vanadium ions in seven VPO_x catalysts with P/V ratio of 0.9——1.1.
VOSO₄ was applied as a standard of spin density. The percentage of (VO)²⁺ in VOSO₄ was determined by chemical analysis, and then the ratio of unit doubly integrated EPR signals of VOSO₄ and (VO)²⁺(%) were obtained. The relative content of V³⁺, V⁴⁺, and V⁵⁺ ions in catalysts were also obtained by combination of EPR and chemical analysis. According to the characters of EPR signals, the catalysts were devided into two kinds. The V⁴⁺/V⁵⁺ > 0.7, V³⁺(%) < 9% were found for freshly calcined catalysts in air. They exhibited a broad but intense EPR signals. After reaction, the ΔH_pp increased, and EPR intensity decreased. But the other kind of freshly calcined catalysts in N₂ were quite different. The V⁴⁺/V⁵⁺ < 0.3, V³⁺(%) > 22% were found. A quasidouble peak EPR signal appeared. In fact, it was broadening, low resolution hyperfine structure due to the (VO)²⁺(S=1/2, I=7/2) ions were apart from each other and dispersed both on the surface and in the bulk. The hfs was vanished, when P/V decreased to 0.9. In addition it should be noted that ΔH_pp decreased and EPR intensity increased after reaction.
Besides, the linear relation was found between formation velocity of MA and ratio of V⁴⁺/V⁵⁺ in catalysts apart from one best catalyst.