Pressure dependence of methane dissociation

The very early stages of the pryolysis of methane (0.003–0.05% reaction) have been studied in a quartz-lined flow reactor, over the pressure and temperature ranges 0.5 ⩽p/MPa ⩽ 5 and 1073 ⩽T/K ⩽ 1148, in pure methane and in the diluents helium and argon. The rate constant k1 of the dissociation, CH4→ CH3+ H, has been obtained directly from the product yield data at the lower extents of reaction and by including correction for autocatalysis using a 36-step model at larger extents. The unimolecular rate constant is in the fall-off region under all conditions studied but approaches the high-pressure limit at 5 MPa. Comparison with low-pressure data and with RRKM calculations has enabled the short extrapolation to the high-pressure limit, k∞1, to be made reliable. k∞1 values are a factor of two higher than those previously obtained from the long RRKM extrapolation of low-pressure data. Arrhenius parameters based on a zero activation energy for the reverse reaction are k∞1/s–1=(1.75 ± 0.25)× 1016 exp (–440.7 kJ mol–1/RT). Pyrolysis of mixtures of 10% methane in helium and argon gave fall-off curves for these bath gases from which, by comparison with strong collision curves, the weak collision efficiencies (βc) for CH4, He, Ar of 0.12, 0.088 and 0.079, respectively, were obtained.