Formation of HO2 from OH and C2H2 in the presence of O2

Abstract

Pulsed production of OH in a gas-phase system containing acetylene, O₂ and NO resulted in biexponential OH-decay curves, indicating formation of HO₂ in secondary reactions. Production and detection of OH were performed by 248 nm photolysis of H₂O₂ and cw-laser long-path absorption at 308 nm, respectively. Measurements were made at room temperature in O₂ or N₂–O₂ mixtures containing 5% O₂ at total pressures between 10 and 100 kPa. Analysis of the decay curves resulted in effective rate constants for the removal of OH and the formation of HO₂ by acetylene in the presence of O₂ in the range (1.4–3.5)×10^-13 cm³ s^-1, dependent on total pressure and O₂ concentration. HO₂ is thought to be formed from HCO and O₂, with HCO originating in a reaction of an intermediate acetylene–OH adduct with O₂. HO₂ yields were found to vary between 1.13 and 1.01 and tending to higher values at lower total pressures. These yields are higher than the expected value of 1, which can be explained by a dissociation of a small fraction of vibrationally excited glyoxal formed, together with OH in a second channel of the acetylene–OH adduct+O₂ reaction. In order to check whether the increased HO₂ yields are real, CO was used instead of acetylene. In this case, an HO₂ yield of 0.99 was found, in good agreement with expectations, and a rate constant of (1.66±0.25)×10^-13 cm³ s^-1 for the OH+CO reaction in 20 kPa O₂ was determined. In addition, a rate constant for the HO₂+NO reaction of (9.5±1.5)×10^-12 cm³ s^-1, rate constants for the OH+NO reaction in the range (1.3–7.4)×10^-12 cm³ s^-1, depending on total pressure, and upper limits for the rate constants of possible reactions HO₂+C₂H₂ (k⩽5×10^-15 cm³ s^-1) and HO₂+CO (k⩽3×10^-15 cm³ s^-1) were derived. Error limits include statistical (2σ) and possible systematic errors.