(568c) Initiation in Ch4/O2: High Temperature Rate Constants for Ch4 + O2-> Ch3 + Ho2

About five years ago, we measured rate constants at high temperature for the initiation reaction in H2/O2,

H2 + O2 -> HO2 + H, (1)

using O-atom atomic resonance absorption spectrometry (ARAS) as the method of detection.(Ref.1) In this study, rate constants for the primary initiation process in CH4 oxidation

CH4 + O2 -> CH3 + HO2, (2)

has been measured in a reflected shock tube apparatus between temperatures of 1667-2018 K using multi-pass absorption spectrometric detection of OH-radicals at 308 nm. [OH]t was observed as a product of reaction (2) after rapid dissociation of HO2, yielding H-atoms which are instantaneously converted to OH via H + O2 -> OH + O. The present work utilizes 32 optical passes corresponding to a total path length of 2.8 m. This configuration gives a signal to noise ratio of unity at 5 x 10^11 radicals cm-3. Hence, kinetics experiments could be performed at conditions of low [CH4]0 (~ 75-100PPM) thereby reducing secondary chemistry substantially. Possible implications due to CH4 dissociation contributing to the OH formation rates at high-T were considered. This reaction has been accurately measured under similar conditions in our laboratory (Ref.2) and minor perturbations to that rate do not affect our conclusions on reaction (2). Over the temperature range, 1667-2018 K, the rate constants for the title reaction can be represented by the Arrhenius expression,

k2=2.86 x 10^-10 exp (-27194 K/T) cm3 molecule-1 s-1. (3)

The present experimental results for k2, compared with a recent variational Transition State Theory calculation using the VARIFLEX code for the reverse reaction transformed through equilibrium constant, clearly overlap each other within experimental error. The new values for k2 obtained in this study are 8-10 times higher than the values used in the popular kinetics codes GRI-Mech 3.0 and Leeds Methane mechanism version 1.5.

This work was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under Contract No. W-31-109-ENG-38.