First in situ Raman study of vanadium oxide based SO2 oxidation supported molten salt catalysts

In situ Raman spectroscopy at temperatures up to 500 °C is used for the first time to identify vanadium species on the surface of a vanadium oxide based supported molten salt catalyst during SO2 oxidation. Vanadia/silica catalysts impregnated with Cs2SO4 were exposed to various SO2/O2/SO3 atmospheres and in situ Raman spectra were obtained and compared to Raman spectra of unsupported “model” V2O5–Cs2SO4 and V2O5–Cs2S2O7 molten salts. The data indicate that (1) the VV complex VVO2(SO4)23− (with characteristic bands at 1034 cm−1 due to ν(V=O) and 940 cm−1 due to sulfate) and Cs2SO4 dominate the catalyst surface after calcination; (2) upon admission of SO3/O2 the excess sulfate is converted to pyrosulfate and the VV dimer (VVO)2O(SO4)44− (with characteristic bands at 1046 cm−1 due to ν(V=O), 830 cm−1 due to bridging S–O along S–O–V and 770 cm−1 due to V–O–V) is formed and (3) admission of SO2 causes reduction of VV to VIV (with the ν(V=O) shifting to 1024 cm−1) and to VIV precipitation below 420 °C.