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

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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.

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