Temperature dependence of the heterogeneous reaction of carbonyl sulfide on magnesium oxide.

The experimental determination of rate constants for atmospheric reactions and how these rate constants vary with temperature remain a crucially important part of atmosphere science. In this study, the temperature dependence of the heterogeneous reaction of carbonyl sulfide (COS) on magnesium oxide (MgO) has been investigated using a Knudsen cell reactor and a temperature-programmed reaction apparatus. We found that the adsorption and the heterogeneous reaction are sensitive to temperature. The initial uptake coefficients (gammat(Ini)) of COS on MgO decrease from 1.07 +/- 0.71 x 10-6 to 4.84 +/- 0.60 x 10-7 with the increasing of temperature from 228 to 300 K, and the steady state uptake coefficients (gammat(SS)) increase from 5.31 +/- 0.06 x 10-8 to 1.68 +/- 0.41 x 10-7 with the increasing of temperature from 240 to 300 K. The desorption rate constants (kdes) were also found to increase slightly with the enhancement of temperature. The empirical formula between the uptake coefficients, desorption rate constants and temperature described in the form of Arrhenius expression were obtained. The activation energies for the heterogeneous reaction and desorption of COS on MgO were measured to be 11.02 +/- 0.34 kJ.mol-1 and 6.30 +/- 0.81 kJ.mol-1, respectively. The results demonstrate that the initial uptake of COS on MgO is mainly contributed by an adsorption process and the steady state uptake is due to a catalytic reaction. The environmental implication was also discussed.