HO2 formation from the OH + benzene reaction in the presence of O2

Abstract

In this study we investigated the secondary formation of HO₂ following the benzene + OH reaction in N₂ with variable O₂ content at atmospheric pressure and room temperature in the absence of NO. After pulsed formation of OH, HO_x (= OH + HO₂) and OH decay curves were measured by means of a laser-induced fluorescence technique (LIF). In synthetic air the total HO₂ yield was determined to be 0.69 ± 0.10 by comparison to results obtained with CO as a reference compound. HO₂ is expected to be a direct product of the reaction of the intermediately formed OH–benzene adduct with O₂. The HO₂ yield is slightly greater than the currently recommended yield of the proposed HO₂ co-product phenol (∼53%). This hints towards other, minor HO₂ forming channels in the absence of NO, e.g. the formation of epoxide species that was proposed in the literature. For other test compounds upper limits of HO₂ yields of 0.10 (isoprene) and 0.05 (cyclohexane) were obtained, respectively. In further experiments at low O₂ concentrations (0.06–0.14% in N₂) rate constants of (2.4 ± 1.1) × 10⁻¹⁶ cm³ s⁻¹ and (5.6 ± 1.1) × 10⁻¹² cm³ s⁻¹ were estimated for the OH–benzene adduct reactions with O₂ and O₃, respectively. The rate constant of the unimolecular dissociation of the adduct back to benzene + OH was determined to be (3.9 ± 1.3) s⁻¹. The HO₂ yield at low O₂ was similar to that found in synthetic air, independent of O₂ and O₃ concentrations indicating comparable HO₂ yields for the adduct + O₂ and adduct + O₃ reactions.