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

The catalytic oxidation of sulfur dioxide to sulfur trioxide over several b inary (MxOy/TiO2) and ternary (V2O5/MXOY/TiO2) supported metal oxide cataly sts was systematically investigated. The supported metal oxide components w ere essentially 100% dispersed as surface metal oxide species, as confirmed by Raman spectroscopy characterization. The sulfur dioxide oxidation turno ver frequencies of the binary catalysts were all within an order of magnitu de (V2O5/TiO2>Fe2O3/TiO2>Re2O7/TiO2 CrOs/TiO similar to Nb2O5/TiO2>MoO3/TiO 2 - WO3/TiO2). An exception was the K2O/TiO2 catalysts, which is essentiall y inactive for sulfur dioxide oxidation. With the exception of K2O, all of the surface metal oxide species present in the ternary catalysts (i.e., oxi des of V, Fe, Re, Cr, Nb, Mo and W) can undergo redox cycles and oxidize SO 2 to SO3. The turnover frequency for sulfur dioxide oxidation over all of t hese catalysts is approximately the same at both low and high surface cover ages. This indicates that the mechanism of sulfur dioxide oxidation is not sensitive to the coordination of the surface metal oxide species. A compari son of the activities of the ternary catalysts with the corresponding binar y catalysts suggests that the surface vanadium oxide and the additive surfa ce metal oxide redox sites act independently without synergistic interactio ns. The V2O5/K2O/TiO2 catalyst showed a dramatic reduction in the catalytic activity in comparison to the unpromoted V2O5/TiO2 catalyst. The ability o f K2O to significantly retard the redox potential of the surface vanadia sp ecies is primarily responsible for the lower catalytic activity of the tern ary catalytic system. The fundamental insights generated from this research can potentially assist in the molecular design of the air pollution contro l catalysts: (1) the development of catalysts for low temperature oxidation of SO2 to SO3 during sulfuric acid manufacture (2) the design of efficient SCR DeNO(x) catalysts with minimal SO2 oxidation activity and (3) improvem ents in additives for the simultaneous oxidation/sorption of sulfur oxides in petroleum refinery operations. (C) 1999 Elsevier Science B.V. All rights reserved.