Shock-tube atom kinetics of nitric oxide decomposition

Abstract

The rate of formation of ground-state atomic oxygen, O(3P), has been measured behind shock waves in NO−Ar mixtures by atomic resonance absorption spectrophotometry. After experimental calibration of the atomic absorption coefficient, rate measurements were made over a temperature range of 2600°–6300°K, for 0.1%, 1.0%, and 10.0% mixtures of NO in Ar. The specific reaction-rate constants were first calculated, assuming that the dissociation proceeds by a simple bimolecular NO−Ar collision process. On this basis, Arrhenius plots of the data for the mixtures containing the higher concentrations of NO exhibit, at lower temperatures, rate constants which are much greater than the constants corresponding to the lower concentrations. At the higher temperatures, the rate constants converge. A chemical mechanism was then developed and employed to reproduce, by means of a computer calculation, the results obtained experimentally. The experimental data have been explained in terms of six reactions, of which two have been verified as the initial, rate-limiting steps of the dissociation process. These are: NO+NO→N2O+O(3P) and NO+Ar→N+O(3P)+Ar, for which the specific rate-constant expressions k=2.35×1010 exp(−29,000/RT) and k=1.37×1014 exp(−148,400/RT), respectively, have been obtained. The first of these two reactions is dominant at low temperatures and high NO concentrations, while the second predominates at high temperatures and low concentrations.