Gas-phase reaction of OH radicals with phenol

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 O-2, OH radicals were generated via the reaction sequence H+ O-2 + M --> HO2 + M, HO2 + NO --> OH+ NO2 and in the absence of O-2 via 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) cm(3) 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 O-2, 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 O-2 concentrations allowed an estimate of the reactivity of the OH/phenol adduct towards O-2, NO and NO2, k (adduct + O-2) / k (adduct + NO) > 10(-3) and k (adduct + O-2) / 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.