Kinetic modeling of the CO/ H₂O/O₂/NO/SO₂ system: Implications for high-pressure fall-off in the SO₂ + O(+M) = SO₃(+M) reaction

Flow reactor experiments were performed to study moist CO oxidation in the presence of trace quantities of NO (0-400 ppm) and SO₂ (0-1300 ppm) at pressures and temperatures ranging from 0.5-10.0 atm and 950-1040 K, respectively Reaction profile measurements of CO, CO₂, O₂, NO, NO₂, SO₂, and temperature were used to further develop and validate a detailed chemical kinetic reaction mechanism in a manner consistent with previous studies of the CO/H₂/O₂/NO_x and CO/H₂O/N₂O systems In particular, the experimental data indicate that the spin-forbidden dissociation-recombination reaction between SO₂ and O-atoms is in the fall-off regime at pressures above 1 atm The inclusion of a pressure-dependent rate constant for this reaction, using a high-pressure limit determined from modeling the consumption of SO₂ in a N₂O/SO₂/N₂ mixture at 10.0 atm and 1000 K, brings model predictions into much better agreement with experimentally measured CO profiles over the entire pressure range Kinetic coupling of NO_x and SO_x chemistry via the radical pool significantly reduces the ability of SO₂ to inhibit oxidative processes. Measurements of SO₂ indicate fractional conversions of SO₂ to SO₃ on the order of a few percent, in good agreement with previous measurements at atmospheric pressure Modeling results suggest that, at low pressures, SO₃ formation occurs primarily through SO₂ + O(+M) = SO₃(+M), but at higher pressures where the fractional conversion of NO to NO₂ increases, SO₃ formation via SO₂ + NO₂ = SO₃ + NO becomes important. For the conditions explored in this study, the primary consumption pathways for SO₃ appear to be SO₃ + HO₂ = HOSO₂ + O₂ and SO₃ + H = SO₂ + OH Further study of these reactions would increase the confidence with which model predictions of SO₃ can be viewed.