Kinetics of the OH + C2H2 reaction in the presence of O2

Abstract

Room-temperature rate constants for the removal of OH radicals in the gas-phase reaction with acetylene have been determined using pulsed H₂O₂ photolysis production of OH followed by cw UV-laser long-path absorption detection. The pressure dependences of the rate constants in N₂, O₂ and synthetic air as buffer gases have been investigated in the fall-off range between 1.5 and 100 k Pa. In N₂ the pressure dependence can be described by the empirical expression of Gardiner: k={(k_∞)^a+(k₀[M])^a}^1/a, using the parameter a=–⅓. This results in the rate constants k_∞=(1.07 ± 0.07)× 10^–12 cm³ s^–1, and k₀=(3.9 ± 1.6)× 10^–29 cm⁶ s^–1 for the given pressure range. Smaller effective rate constants were obtained in the presence of O₂, owing to a fast regeneration channel for OH. In terms of the above expression the rate constants k_∞=(4.6 ± 0.2) and (3.1 ± 0.1)× 10^–13 cm³ s^–1 and k₀=(1.8 ± 0.2) and (1.9 ± 0.5)× 10^–29 cm⁶ s^–1 were obtained for O₂ and synthetic air as buffer gases, respectively. Compared with N₂ there is virtually no difference in the fall-off behaviour in O₂ and synthetic air, indicating that the OH regeneration yield is independent of pressure. On the other hand, this yield was found to decrease with the O₂ mixing ratio. A kinetic model is proposed that qualitatively explains this effect with different chemical properties of non-thermalized OH–acetylene adducts with regard to O₂. This model is confirmed by measurements showing a significantly stronger dependence of the regeneration yield on the O₂ mixing ratio in O₂–He mixtures.