Inhibiting effect of CF3I on the reaction between CH4 and O-2 in a jet-stirred reactor

This is an experimental and theoretical study of the inhibiting effect of the addition of 1-5% (relative to the amount of methane) of CF3I on methane oxidation under the following conditions: equimolecular CH4-O-2 mixtures, temperature from 970 to 1150 K, total pressure 1 bar, space times ranging from 1 to 8 s corresponding to methane conversions of 1-80%. The main observed effects of the addition of this halogenated compound were a large decrease of the conversions of methane and oxygen, a reduction of the selectivity in ethane formation, and an important increase of the selectivity in ethylene formation. In the experimental range studied, the inhibiting effect slightly increased when the amount of additive decreased. Methane oxidation and the influence of halogenated compounds on this reaction were modelled using a detailed kinetic reaction mechanism, including the recent values for the rate constants of reactions of iodinated compounds. The proposed mechanism was able to reproduce the experimental data. Sensitivity analyses led to a determination of the dominant steps that account for the inhibiting effect of CF3I. The inhibiting influence of CF3I on methane oxidation seems to be mainly due to the two following steps: I + CH4 --> HI + CH3 and I + HO2 --> O-2 + HI. These two steps form a cycle which acts as a termination reaction between HO2 and CH3 radicals to give CH4 and O-2. (C) 1997 by The Combustion Institute.