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

Abstract Alkali-promoted V 2 O 5 catalysts M-V 2 O 5 (M=Li, Na, K, Rb and Cs) synthesised by impregnation of V 2 O 5 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 V 2 O 5 was used for comparison. Whereas in Li- and Na-promoted catalysts only V 2 O 5 as crystalline phase could be detected by X-ray diffraction (XRD), the K-, Rb-, and Cs-promoted catalysts additionally contain the vanadate phase MV 3 O 8 . The surface acidity (Bronsted- 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 V 2 O 5 at relative low temperatures. The bronze phases stabilise the V 4+ 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 V 4+ in the different bronze phases. The favoured formation of bronze-like phases especially under reducing conditions enhances the release of SO 2 at lower temperatures, the formation of H 2 S can be neglected.