Gas-phase reaction of OH radicals with phenol

Physical Chemistry Chemical Physics Pub Date: 2002-12-04 DOI: 10.1039/B208187C

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.

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