Determination of the rate constant for HO2+ CH4→ H2O2+ CH3 at 443 °C

Abstract

Studies have been made of the addition of CH₄ to mixtures of tetramethylbutane (TMB)+ O₂ at 443 °C in KCl-coated Pyrex vessels. From measurements of isobutene and formaldehyde, formed virtually as sole products from TMB and CH₄, respectively, a value of k₁₀/k^1/2₇= 1.00×10^–2(dm³ mol^–1 s^–1)^1/2 has been obtained. HO₂+ CH₄→ H₂O₂+ CH₃(10), HO₂+ HO₂→ H₂O₂+ O₂(7). Computer analysis shows that this value is very sensitive to the value of k₁₈/k₉ which controls the proportion of CH₄ removed by OH attack, and the optimum conditions for studying reaction (10) were carefully selected. Use of A₇= 10^9.48 dm³ mol^–1 s^–1 and E₇= 5.8 kJ mol^–1, obtained recently, gives k₁₀=(3.39 ± 0.78)×10² dm³mol^–1s^–1 at 443 °C. No other experimental determinations of this rate constant have been reported. Based on the reasonable assumption of equal A factors per C—H bond for HO₂+ CH₄ and HO₂+ C₂H₆, then Arrhenius parameters of A₁₀= 10.05 ± 0.3 dm³mol^–1s^–1 and E₁₀=103.1 ± 3.5 kJ mol^–1 are suggested for the temperature range 350–1000 °C. OH + TMB → H₂O + C₈H₁₇(9), OH + CH₄→ H₂O + CH₃(18)