Experimental and theoretical rate constants for CH_4 + O_2 → CH_3 + HO_2

摘要

In this study, rate constants for the primary initiation process in low to moderate temperature CH_4 oxidation CH_4 + O_2 → CH_3 + HO_2 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 HO_2 yielding H atoms, which are instantaneously converted to OH by H + O_2 → 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 x 10~(12) radicals cm~(-3). Hence, kinetics experiments could be performed at conditions of low [CH_4]_0 (60-70 ppm), thereby substantially reducing secondary chemistry. Possible implications of CH_4 dissociation contributing to the OH formation rates were considered. The present experimental results agree with a priori variational transition state theoretical (VTST) calculations, k_(th) = 3.37 x 10~(-19)T~(2.745) exp(-26, 041 K/T) cm~3 molecule~(-1) s~(-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.