First In Situ Raman Study of Vanadium Oxide Based SO2 Oxidation Supported Molten Salt Catalysts

Abstract

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 V^V complex V^VO₂(SO₄)₂ ³⁻ (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 V^V dimer (V^VO)₂O(SO₄)₄ ⁴⁻ (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 V^V to V^IV (with the ν(V=O) shifting to 1024 cm⁻¹) and to V^IV precipitation below 420 °C.