The mutual reaction of HO2 radicals and the rate constants of the hydrogen + oxygen reaction

Abstract

The yields of i-butene and H₂ have been measured in the decomposition of tetramethylbutane (TMB) in the presence of O₂. The relative yields are effectively independent of the concentration of TMB and O₂ and of temperature over the range 440–500 °C. If H₂ formation is due to reaction (10_a): HO₂+ HO₂→ H₂+ 2O₂(10a), HO₂+ HO₂→ H₂O₂+ O₂(10), the value of k_10a/k₁₀ is obtained as 0.065 ± 0.005 at 480 °C.

However, introduction of reaction (10a) into the mechanism of the H₂+ O₂ reaction at 500 °C has a marked effect on the calculated reaction rate, and a satisfactory interpretation of the variation of rate with mixture composition requires the ratio k₁₅/k₁ to change from 3.9 ± 0.5 to 1.07: OH + H₂→ H₂O + H (1), OH + H₂O₂→ H₂O + HO₂(15), This latter value is totally inconsistent with values of 3.2 ± 0.5 from studies of the decomposition of H₂O₂ in the presence of H₂, and 2.7–3.6 from independent measurements of k₁ and k₁₅. Further, studies in KCl-coated vessels do not significantly alter the [i–C₄H₈]/[H₂] ratios, in contrast to the large increase predicted by computer treatment, as a consequence of the much lower HO₂ concentration. It is concluded that reaction (10a) is not the source of H₂.