Gas-phase reaction of OH radicals with phenol

Abstract

The gas-phase reaction of OH radicals with phenol was investigated in a flow tube in the temperature range of 266–364 K and a pressure of 100 mbar. The product formation was followed by on-line FT-IR spectroscopy and GC-MS measurements. Newly formed particles were detected by means of a low-pressure CPC (condensation particle counter). In the presence of O₂, OH radicals were generated via the reaction sequence H + O₂ + M → HO₂ + M, HO₂ + NO → OH + NO₂ and in the absence of O₂via H + NO₂ → OH + NO. For evaluation of a possible competing process, the rate constant for H + phenol was measured, k(H + phenol) = (2.5 ± 1.5) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ (295 ± 2 K, 25 mbar He). Under the experimental conditions used the H-atom reaction does not compete with the reaction of OH radicals with phenol. At 295 K, the product distribution was studied for different O₂, NO and NO₂ concentrations. Identified products were catechol, o-nitrophenol and p-benzoquinone. Under all experimental conditions catechol represented the main product. The measured dependence of the catechol yield on NO and NO₂ for constant O₂ concentrations allowed an estimate of the reactivity of the OH/phenol adduct towards O₂, NO and NO₂, k(adduct + O₂)/k(adduct + NO) > 10⁻³ and k(adduct + O₂)/k(adduct + NO₂) = (1.4 ± 0.5) × 10⁻⁴. For constant gas composition, in the absence of additional NO₂, the product distribution was measured for different temperatures. With increasing temperature the catechol yield increased from 0.37 ± 0.06 (266 K) to 0.87 ± 0.04 (364 K). The yields of o-nitrophenol and p-benzoquinone were nearly constant. Below 295 K, with decreasing temperature enhanced formation of newly formed particles was observed. For realistic atmospheric conditions, a catechol yield of 0.73–0.78 (295 K) can be recommended from this study.