Oxidation of SO2 over Supported Metal Oxide Catalysts

A systematic catalytic investigation of the sulfur dioxide oxidation reactivity of several binary (MxOy/TiO2) and ternary (V2O5/ MxOy/TiO2) supported metal oxide catalysts was conducted. Raman spectroscopy characterization of the supported metal oxide catalysts revealed that the metal oxide components were essentially 100% dispersed as surface metal oxide species. Isolated fourfold coordinated metal oxide surface species are present for most oxides tested at low coverages, whereas at surface coverages approaching monolayer polymerized surface metal oxide species with sixfold coordination are present for some of the oxides. The sulfur dioxide oxidation turnover frequencies (SO2 molecules converted per surface redox site per second) of the binary catalysts were all within an order of magnitude (V2O5/TiO2>Fe2O3/TiO2>Re2O7/TiO2∼ CrO3/TiO2∼Nb2O5/TiO2>MoO3/TiO2∼WO3/TiO2). An exception was the K2O/TiO2 catalyst system, which is inactive for sulfur dioxide oxidation under the chosen reaction conditions. With the exception of K2O, 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 sulfur dioxide to sulfur trioxide. The turnover frequency for SO2 oxidation over all of these catalysts is approximately the same at both low and high surface coverages, despite structural differences in the surface metal oxide overlayers. 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 oxide redox sites act independently without synergistic interactions: the sum of the individual activities of the binary catalysts quantitatively correspond to the activity of the corresponding ternary catalyst. The V2O5/K2O/TiO2 catalyst showed a dramatic reduction in catalytic activity in comparison to the unpromoted V2O5/TiO2 catalyst. The ability of potassium oxide to significantly retard the redox potential of the surface vanadia species is primarily responsible for the lower catalytic reactivity. c © 1999