OH-initiated oxidation of benzene Part II.Influence of elevated NOx concentrations

Abstract

The present work represents a continuation of part I of this series of papers, in which we investigated the phenol yields in the OH-initiated oxidation of benzene under conditions of low to moderate concentrations of NO_x, to elevated NO_x levels. The products of the OH-initiated oxidation of benzene in 700–760 Torr of N₂/O₂ diluent at 297 ± 4 K were investigated in 3 different photochemical reaction chambers. In situ spectroscopic techniques were employed for the detection of products, and the initial concentrations of benzene, NO_x, and O₂ were widely varied (by factors of 6300, 1500, and 13, respectively). In contrast to results from previous studies, a pronounced dependence of the product distribution on the NO_x concentration was observed. The phenol yield decreases from approximately 50–60% in the presence of low concentrations (<5 ppb) of NO_x to values below 5% in the presence of extremely high (>10 000 ppb) NO_x concentrations. In the presence of high concentrations of NO_x, the phenol yield increases with increasing O₂ partial pressure. The rate constant of the reaction of hydroxycyclohexadienyl peroxyl radicals with NO was determined to be (1.7 ± 0.6) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. This reaction leads to the formation of E,E-2,4-hexadienedial as the main identifiable product (29 ± 16%). The reaction of the hydroxycyclohexadienyl radical with NO₂ gave phenol (5.9 ± 3.4%) and E,E-2,4-hexadienedial (3.4 ± 1.9%), no other products could be identified. The residual FTIR product spectra indicate the formation of unknown nitrates or other nitrogen-containing species in high yield. The results from the present work also show that experimental studies aimed at establishing/verifying chemical mechanisms for aromatic hydrocarbons must be performed using NO_x levels which are representative of those found in the atmosphere.