Thermal Decomposition and Oxidation of CH₃OH

Thermal decomposition of CH₃OH diluted in Ar has been studied by monitoring H atoms behind reflected shock waves of 100 ppm CH₃OH + Ar. The total decomposition rate k₁ for CH₃OH + M → products obtained in this study is expressed as, ln­(k₁/cm³ molecule^–1 s^–1) = −(14.81 ± 1.22) – (38.86 ± 1.82) × 10³/T, over 1359–1644 K. The present result on k₁ is indicated to be substantially smaller than the extrapolation of the most of the previous experimental data but consistent with the published theoretical results [Faraday Discuss. 2002, 119, 191−205 and J. Phys. Chem. A 2007, 111, 3932–3950]. Oxidation of CH₃OH has been studied also by monitoring H atoms behind shock waves of (0.35–100) ppm CH₃OH + (100–400) ppm O₂ + Ar. For the low concentration CH₃OH (below 10 ppm) + O₂ mixtures, the initial concentration of CH₃OH is evaluated by comparing evolutions of H atoms in the same concentration of CH₃OH with addition of 300 ppm H₂ diluted in Ar. The branching fraction for CH₃OH + Ar → ¹CH₂ + H₂O + Ar has been quantitatively evaluated from this comparative measurements with using recent experimental result on the yield of H atoms in the reaction of ^1,3CH₂ + O₂ [J. Phys. Chem. A 2012, 116, 9245−9254]; i.e., the branching fraction for the above reaction is evaluated as, ϕ_1a = 0.20 ± 0.04 at T = 1880–2050 K, in the 1.3 and 3.5 ppm CH₃OH + 100 ppm O₂ samples. An extended reaction mechanism for the pyrolysis and oxidation of CH₃OH is constructed based on the results of the present study combined with the oxidation mechanism of natural gas [GRI-Mech 3.0]; evolution of H atoms can be predicted very well with this new reaction scheme over a wide concentration range for the pyrolysis (0.36–100 ppm CH₃OH), and oxidation (0.36–100 ppm CH₃OH + 100/400 ppm O₂) of methanol.