Molecular structure–reactivity relationships for the oxidation of sulfur dioxide over supported metal oxide catalysts

Abstract

The catalytic oxidation of sulfur dioxide to sulfur trioxide over several binary (M xO y/TiO 2) and ternary (V 2O 5/M XO Y/TiO 2) supported metal oxide catalysts was systematically investigated. The supported metal oxide components were essentially 100% dispersed as surface metal oxide species, as confirmed by Raman spectroscopy characterization. The sulfur dioxide oxidation turnover frequencies of the binary catalysts were all within an order of magnitude (V 2O 5/TiO 2>Fe 2O 3/TiO 2>Re 2O 7/TiO 2 ∼ CrO 3/TiO 2 ∼ Nb 2O 5/TiO 2>MoO 3/TiO 2 ∼ WO 3/TiO 2). An exception was the K 2O/TiO 2 catalysts, which is essentially inactive for sulfur dioxide oxidation. With the exception of K 2O, all of the surface metal oxide species present in the ternary catalysts (i.e., oxides of V, Fe, Re, Cr, Nb, Mo and W) can undergo redox cycles and oxidize SO 2 to SO 3. The turnover frequency for sulfur dioxide oxidation over all of these catalysts is approximately the same at both low and high surface coverages. This indicates that the mechanism of sulfur dioxide oxidation is not sensitive to the coordination of the surface metal oxide species. A comparison of the activities of the ternary catalysts with the corresponding binary catalysts suggests that the surface vanadium oxide and the additive surface metal oxide redox sites act independently without synergistic interactions. The V 2O 5/K 2O/TiO 2 catalyst showed a dramatic reduction in the catalytic activity in comparison to the unpromoted V 2O 5/TiO 2 catalyst. The ability of K 2O to significantly retard the redox potential of the surface vanadia species is primarily responsible for the lower catalytic activity of the ternary catalytic system. The fundamental insights generated from this research can potentially assist in the molecular design of the air pollution control catalysts: (1) the development of catalysts for low temperature oxidation of SO 2 to SO 3 during sulfuric acid manufacture (2) the design of efficient SCR DeNO x catalysts with minimal SO 2 oxidation activity and (3) improvements in additives for the simultaneous oxidation/sorption of sulfur oxides in petroleum refinery operations.