In-Situ Raman Spectroscopic Study of Supported Molten Salt Catalysts During SO2 Oxidation

The catalytic oxidation of SO2 to SO3 plays a key role in a number of industrial processes, which due to the associated sulfur oxide emissions have significant environmental impact. Although the main source of SO2 emissions to the atmosphere is the coal-fired power generation, large amounts of SO2 are also emitted from sulfuric acid manufacturers and smelters of non-ferrous metals. Production of sulfuric acid is currently performed not only from traditional sulfuric acid manufacturers but also from NOx and SOx removal stations, combined with SCR technology like e.g. in the so-called Haldor-Topsoe SNOX process. The catalyst used for sulfuric acid production catalyzing the reaction SO2 + 1⁄2O2 → SO3 contains its active phase in a molten salt, which is distributed in the pores of an inert sili ca support and is the most important supported-liquid-phase (SLP) catalyst. During SO2 oxidation, large amounts of SO3 are taken up by the catalyst, of which the active phase is best simulated by vanadium oxide dissolved in alkali pyrosulfate thereby giving rise to formation of vanadium oxosulfato complexes (1). In-situ real-time spectroscopic characterization of catalytic active centers in vanadium oxide based SO2 oxidation supported molten salt catalysts under gas and temperature conditions of practical importance has been a long-sought goal in catalysis (1). In the present study, in-situ Raman spectroscopy at temperatures up to 500C 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/sili ca 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. Figure 1 compares a representative in-situ Raman spectrum of a supported molten salt catalyst with the Raman spectrum of a V2O5Cs2SO4 molten salt. The data indicate that the V V complex VO2(SO4)2 3[bands a-e, of which the most characteristic at 1034 cm (band b) due to (V=O) and 940 cm (band c) due to sulfate] and Cs2SO4 (bands A,B) dominate the catalyst surface after calcination (2). Upon admission of SO3/O2 the excess sulfate is converted to pyrosulfate (SO4 2+ SO3 → S2O7) and the V dimer (VO)2O(SO4)4 4[with characteristic bands at 1046 cm due to (V=O), 830 cm due to bridging S-O along S-OV and 770 cm due to V-O-V] is formed (3,4) V2O5 + 2S2O7 → (VO)2O(SO4)4 Admission of SO2 causes reduction of V V to V IV and to V IV precipitation below 420C (4). Figure 2 shows the proposed structural models for the V species present in the liquid (molten) phase supported on the carrier.