Comparative study of the thermal and redox behaviour of alkali-promoted V2O5 catalysts

Alkali-promoted V2O5 catalysts M-V2O5 (M=Li, Na, K, Rb and Cs) synthesised by impregnation of V2O5 with alkali sulfate solution have been investigated under inert and reducing atmosphere using thermoanalytical methods (TG/DTA, differential scanning calorimetry (DSC) and temperature-programmed reduction (TPR)). Pure V2O5 was used for comparison. Whereas in Li- and Na-promoted catalysts only V2O5 as crystalline phase could be detected by X-ray diffraction (XRD), the K-, Rb-, and Cs-promoted catalysts additionally contain the vanadate phase MV3O8. The surface acidity (Brønsted- and Lewis-sites) as well as the starting temperature of the hydrogen consumption decrease with increasing size of the alkali cation. The reduction of the K-, Rb-, and Cs-promoted catalysts leads to the formation of bronze-like phases besides V2O5 at relative low temperatures. The bronze phases stabilise the V4+ oxidation state and improve the redox properties. A characteristic splitting and shifting of the ν(VO) mode in the FTIR spectrum indicates the formation of V4+ in the different bronze phases. The favoured formation of bronze-like phases especially under reducing conditions enhances the release of SO2 at lower temperatures, the formation of H2S can be neglected.