The kinetics of the homogeneous gaseous oxidation of sulphur dioxide

Abstract

Studies of the gaseous oxidation of sulphur dioxide show that in the absence of added catalysts an apparently homogeneous reaction can be observed only over a rather narrow temperature range (900 to 1050°C). On the basis of the dependence of the rate of this reaction on the concentration of the reactants and of the value of the activation energy (75 kcal/mole), a tentative reaction mechanism is proposed which involves initially the collisional activation of sulphur dioxide. In the presence of nitric oxide, reaction takes place at an appreciable rate at temperatures as low as 400°C. The catalysed reaction is essentially homogeneous and its rate is unaffected by nitrogen and water vapour. At fairly high concentrations of nitric oxide, the rate is approximately proportional to the square of the catalyst concentration and to the first power of the oxygen concentration, but is almost independent of the concentration of sulphur dioxide and of temperature. In the presence of lower concentrations of the additive, the rate is approximately proportional to the first power of both the nitric oxide and oxygen concentrations, increases slightly with the sulphur dioxide concentration, and is appreciably dependent on temperature. The observed kinetic relationships can be satisfactorily explained in terms of a mechanism involving the interaction of sulphur dioxide with the species, NO 2 and NO 3 , the relative contribution of the two reactions: NO 2 + SO 2 = NO + SO 3 and NO 3 + SO 2 = NO 2 + SO 3 varying with the concentrations of the reactants and with temperature.