Oxidation of sulfur dioxide to sulfur trioxide over supported vanadia catalysts

Abstract

The objectives of this research are to establish the fundamental kinetics and mechanism of sulfur dioxide oxidation over supported vanadia catalysts and use these insights to facilitate the design of SCR DeNO x catalysts with minimal sulfur dioxide oxidation activity. A series of supported vanadia catalysts were prepared on various metal-oxide supports: ceria, zirconia, titania, alumina and silica. Raman spectroscopy was used to determine the coordination of surface species. At low vanadia loadings, vanadia preferentially exists on oxide support surfaces as isolated tetrahedrally coordinated (M–O) 3V +5=O species. At higher vanadia loadings, the isolated (M–O) 3V +5=O species polymerize on the oxide support surface breaking two V–O–M bonds and forming two V–O–V bridging bonds. The turnover frequency for sulfur dioxide oxidation was very low, 10 −4 to 10 −6 s −1 at 400°C, and was independent of vanadia coverage suggesting that only one vanadia site is required for the oxidation reaction. As the support was varied, sulfur dioxide oxidation activity of the supported vanadia catalysts varied by one order of magnitude (Ce>Zr, Ti>Al>Si). The basicity of the bridging V–O–M oxygen appears to be responsible for influencing the adsorption and subsequent oxidation of the acidic sulfur dioxide molecule. Over the range of conditions studied, the rate of sulfur dioxide oxidation is zero-order in oxygen, first-order in sulfur dioxide and inhibited by sulfur trioxide. The turnover frequency for sulfur dioxide oxidation over WO 3/TiO 2 was an order of magnitude lower than that found for V 2O 5/TiO 2, and no redox synergism between the surface vanadia and tungsten oxide species was evident for a ternary V 2O 5/WO 3/TiO 2 catalyst. This suggests that WO 3 promoted catalysts may be suitable for low-temperature SCR where minimal sulfur dioxide oxidation activity is required.