Experimental and theoretical rate constants for CH4 + O2 → CH3 + HO2

In this study, rate constants for the primary initiation process in low to moderate temperature CH4 oxidationCH4 + O2 → CH3 + HO2 have been measured in a reflected shock tube apparatus between 1655 and 1822 K using multipass absorption spectrometric detection of OH radicals at 308 nm. After rapid dissociation of HO2 yielding H atoms, which are instantaneously converted to OH by H + O2 → OH + O, the temporal concentration of OH radicals was observed as the final product from the rate-controlling title reaction. The present work utilizes 18 optical passes corresponding to a total path length of 1.6 m. This configuration gives a signal to noise ratio of unity at ∼3×1012∼3×1012 radicals cm−3. Hence, kinetics experiments could be performed at conditions of low [CH4]0 (60–70 ppm), thereby substantially reducing secondary chemistry. Possible implications of CH4 dissociation contributing to the OH formation rates were considered. The present experimental results agree with a priori variational transition state theoretical (VTST) calculations, kth=3.37×10−19T2.745exp(−26,041K/T) cm3molecule−1s−1, clearly showing overlap of experiment and theory, within experimental error. The new rate constant values obtained in this study are 8–10 times higher than the values used in the popular mechanisms GRI-Mech 3.0 and Leeds Methane Mechanism, version 1.5.