Combustion chemistry of COS and occurrence of intersystem crossing

This contribution combines results of experiments with kinetic modelling to probe the unusual behaviour of carbonyl sulfide (COS), a sulfur species that frequently arises in fuel systems. The experiments identified CO and SO₂ as the primary oxidation products, with no formation of CO₂. The low ignition temperature (<600 K) of COS observed in prior experiments conflicts with the high activation barrier for the reaction COS + O₂ → CO₂ + SO of 211.3 kJ mol⁻¹ on the traditional triplet reaction surface. We proposed that, this kinetic barrier prompts the reaction to transfer onto the singlet surface through intersystem crossing that allows the process to surmount lower-energy hurdles. By considering the oxidation of COS as a single step reaction, we fitted the Arrhenius parameter for the reaction COS + O₂ → CO + SO₂ directly from our experimental measurements. The fitted activation energy of 70.1 kJ∙mol⁻¹ agrees with that of 85.4 ± 20.0 kJ∙mol⁻¹ as calculated in literature at the Hartree-Fock level of theory, indicating the appearance of the intersystem crossing process in the oxidation of COS. The reaction mechanism based on this comportment leads to excellent agreement between the kinetic model and the experimentally measured quantities, such as the onset temperature and the conversion profiles of detected species. The proposed kinetic model for the oxidation of COS provides a tool to design both the SO_x mitigation processes and industrial systems for safe handling of sulfur impurities in fossil fuels.