Pressure dependence of methane dissociation

Abstract

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 (β_c) for CH₄, He, Ar of 0.12, 0.088 and 0.079, respectively, were obtained.