Investigation of the radical product channel of the CH3C(O)O2 + HO2 reaction in the gas phase

Abstract

The reaction of CH₃C(O)O₂ with HO₂ has been investigated at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl₂/CH₃CHO/CH₃OH/air mixtures. The branching ratio for the reaction channel forming CH₃C(O)O, OH and O₂ (reaction (3c)) has been determined from experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The dependence of the phenol yield on benzene concentration was found to be consistent with its formation from the OH-initiated oxidation of benzene, thereby confirming the presence of OH radicals in the system. The dependence of the phenol yield on the initial peroxy radical precursor reagent concentration ratio, [CH₃OH]₀/[CH₃CHO]₀, is consistent with OH formation resulting mainly from the reaction of CH₃C(O)O₂ with HO₂ in the early stages of the experiments, such that the limiting yield of phenol at high benzene concentrations is well-correlated with that of CH₃C(O)OOH, a well-established product of the CH₃C(O)O₂ + HO₂ reaction (via channel (3a)). However, a delayed source of phenol was also identified, which is attributed mainly to an analogous OH-forming channel of the reaction of HO₂ with HOCH₂O₂ (reaction (17c)), formed from the reaction of HO₂ with product HCHO. This was investigated in additional series of experiments in which Cl₂/CH₃OH/benzene/air and Cl₂/HCHO/benzene/air mixtures were photolysed. The various reaction systems were fully characterised by simulations using a detailed chemical mechanism. This allowed the following branching ratios to be determined: CH₃C(O)O₂ + HO₂ → CH₃C(O)OOH + O₂, k_3a/k₃ = 0.38 ± 0.13; → CH₃C(O)OH + O₃, k_3b/k₃ = 0.12 ± 0.04; → CH₃C(O)O + OH + O₂, k_3c/k₃ = 0.43 ± 0.10: HOCH₂O₂ + HO₂ → HCOOH + H₂O + O₂, k_17b/k₁₇ = 0.30 ± 0.06; → HOCH₂O + OH + O₂, k_17c/k₁₇ = 0.20 ± 0.05. The results therefore provide strong evidence for significant participation of the radical-forming channels of these reactions, with the branching ratio for the title reaction being in good agreement with the value reported in one previous study. As part of this work, the kinetics of the reaction of Cl atoms with phenol (reaction (14)) have also been investigated. The rate coefficient was determined relative to the rate coefficient for the reaction of Cl with CH₃OH, during the photolysis of mixtures of Cl₂, phenol and CH₃OH, in either N₂ or air at 296 K and 760 Torr. A value of k₁₄ = (1.92 ± 0.17) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ was determined from the experiments in N₂, in agreement with the literature. In air, the apparent rate coefficient was about a factor of two lower, which is interpreted in terms of regeneration of phenol from the product phenoxy radical, C₆H₅O, possibly via its reaction with HO₂.