Second limit of hydrogen + oxygen mixtures: the reaction H + HO2

Abstract

Measurements of the second limit of H₂+ O₂ mixtures at very low O₂ concentrations have shown that the results at 500°C cannot be interpreted on the assumption that the only reaction between H atoms and HO₂ radicals is the quadratic branching reaction H + HO₂→2OH (8) whereas a precise interpretation is possible by introducing the additional reaction H + HO₂→H₂+ O₂, (8a) with the ratio k_8a/k₈= 0.17. An alternative explanation in terms of destruction of H atoms at the surface has been eliminated by measurement of the first limit. Comparison of the above estimate at 500°C with the mean of k_8a/(k₈+k_8b)≈1, obtained at 20°C suggests that E₈–E_8a≈1725 cal mol^–1; if E_8a= 0, (k₈+k_8b)= 3.1 × 10¹¹ exp(–1725 cal mol^–1/RT) dm³ mol^–1 s^–1. The present work does not distinguish between (8) and the kinetically equivalent reaction H + HO₂→H₂O + O (8b) but other workers have suggested that k_8b/k₈≈0.1. A higher activation energy and higher preexponential factor for the equivalent reaction, involving O—O bond fission rather than H abstraction, has also been found in the reaction H + H₂O₂.