Determination of the rate of H + O2 + M → HO2 + M (M = N2, Ar, H2O) from ignition of syngas at practical conditions

In H₂ and H₂/CO oxidation, the H + O₂ + M termination step is one of the most important reactions at elevated pressures. With the recent, increased interest in synthetic fuels, an accurate assessment of its rate coefficient becomes increasingly important, especially for real fuel/air mixtures. Ignition delay times in shock-tube experiments at the conditions selected in this study are only sensitive to the rates of the title reaction and the branching reaction H + O₂ = OH + O, the rate of which is known to a high level of accuracy. The rate coefficient of the title reaction for M = N₂, Ar, and H₂O was determined by adjusting its value in a detailed chemical kinetics model to match ignition delay times for H₂/CO/O₂/N₂, H₂/CO/O₂/Ar, and H₂/CO/O₂/N₂/H₂O mixtures with fuel/air equivalence ratios of ? = 0.5, 0.9, and 1.0. The rate of H + O₂ + N₂ = HO₂ + N₂ was measured to be 2.7 (−0.7/+0.8) × 10¹⁵ cm⁶/mol² s for T = 916-1265 K and P = 1-17 atm. The present determination agrees well with the recent study of Bates et al. [R.W. Bates, D.M. Golden, R.K. Hanson, C.T. Bowman, Phys. Chem. Chem. Phys. 3 (2001) 2337-2342], whose rate expressions are suggested herein for modeling the falloff regime. The rate of H + O₂ + Ar = HO₂ + Ar was measured to be 1.9 × 10¹⁵ cm⁶/mol² s for T = 932-965 K and P = 1.4 atm. The rate of H + O₂ + H₂O = HO₂ + H₂O was measured to be 3.3 × 10¹⁶ cm⁶/mol² s for T = 1071-1161 K and P = 1.3 atm. These are the first experimental measurements of the rates of the title reactions in practical combustion fuel/air mixtures.