The oxidation of SO2 on platinum in sulfuric acid solution is shown here to be extremely complex. The reaction is highly irreversible and is kinetically second order for both water and SO2 (at pressures below 1 atm). The pH dependence (0.5 M to 0.005 M acid) is zero. The suggested rate-determining step, apparently under Temkin conditions, is:. 2H2SO+3→H2S206+2H+ which is followed by a disproportionation step. H2S2O6→S02+H2S04 In acids of high water activity (less than 50% acid concentration), the effect of temperature on current density at constant potential E vs. HE. is small. In more concentrated acids, where wide variations of water activity with temperature occur, the effect is more pronounced, even in the limiting-current region. At high convection rates, the rate-determining step in the limiting-current region changes from complex coupled diffusion of SO2 and H2O to a chemical step involving SO2 and H2O that is potential-independent. It is suggested that this step is. SO2+0ads→SO3 At high partial pressures the reaction order for SO2 falls to low values. © 1979 Elsevier Sequoia S.A.
