A Fourier transform infrared study of the adsorption of SO₂ on n‐hexane soot from −130° to −40°C

Abstract

This paper focuses on the uptake of SO 2 on soot at temperatures below room temperature. Oxidation on soot may provide a mechanism for the oxidation of atmospheric SO 2 under conditions when the standard gas‐phase and aqueous‐phase mechanisms cannot explain the rapid rate of H 2 SO 4 production. An understanding of the uptake of SO 2 under dry conditions provides a useful step toward understanding the uptake and oxidation of SO 2 on soot under wet conditions. We find that rapid, reversible SO 2 adsorption on soot occurs within a few seconds, presumably by adsorption on the outer surfaces of the spherical soot particles. Subsequently, uptake continues slowly for over an hour, presumably by diffusion into micropores within the soot particles. We focused only on the rapid adsorption. An isothermal analysis of rapid SO 2 uptake revealed that a small fraction (<1%) of adsorption sites have a strong binding affinity (ΔH des = 42±4 kJ/mol), while the majority of adsorption sites bind SO 2 more weakly (26±4 kJ/mol). The lower‐limit saturation coverage of SO 2 on soot is 0.3 monolayer, but the more likely value is 0.7 monolayer. The uptake coefficient is 0.002 (plus or minus factor of 2) at low coverages.