Oxidation of SO2 over supported metal oxide catalysts

A systematic catalytic investigation of the sulfur dioxide oxidation reacti vity of several binary (M(x)O(y/)lTiO(2)) and ternary (V2O5/ MxOy/TiO2) sup ported metal oxide catalysts was conducted. Raman spectroscopy characteriza tion of the supported metal oxide catalysts revealed that the metal oxide c omponents were essentially 100% dispersed as surface metal oxide species. I solated fourfold coordinated metal oxide surface species are present for mo st oxides tested at low coverages, whereas at surface coverages approaching monolayer polymerized surface metal oxide species with sixfold coordinatio n 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 > Fe 2O3/TiO2 > Re2O7/TiO2 similar to CrO3/TiO2 similar to Nb2O5/TiO2 > MoO3/TiO 2 similar to WO3/TiO2). An exception was the K2O/TiO2 catalyst system, whic h is inactive for sulfur dioxide oxidation under the chosen reaction condit ions. With the exception of K2O, all of the surface metal oxide species pre sent in the ternary catalysts (i,e., oxides of V, Fe, Re, Cr, Nh, Mo, and W ) can undergo redox cycles and oxidize sulfur dioxide to sulfur trioxide. T he turnover frequency for SO2 oxidation over all of these catalysts is appr oximately the same at both low and high surface coverages, despite structur al differences in the surface metal oxide overlayers. This indicates that t he mechanism of sulfur dioxide oxidation is not sensitive to the coordinati on of the surface metal oxide species. A comparison of the activities of th e ternary catalysts with the corresponding binary catalysts suggests that t he surface vanadium oxide and the additive surface oxide redox sites act in dependently without synergistic interactions: the sum of the individual act ivities of the binary catalysts quantitatively correspond to the activity o f the corresponding ternary catalyst. The V2O5/K2O/TiO2 catalyst showed a d ramatic reduction in catalytic activity in comparison to the unpromoted V2O 5/TiO2 catalyst. The ability of potassium oxide to significantly retard the redox potential of the surface vanadia species is primarily responsible fo r the lower catalytic reactivity. (C) 1999 Academic Press.