Shock tube study on the thermal decomposition of CH₃OH

H atom produced in the thermal decomposition of CH₃OH highly diluted in Ar (0.48-10 ppm) was monitored behind reflected shock waves by atomic resonance absorption spectrometry (ARAS) at fixed temperatures (and pressures), that is, 1660 (1.73 atm), 1760 (2.34 atm), 1860 (2.04 am), 1950 (2.18 atm), and 2050 K (1.76 atm) (±10 K, respectively). High sensitivity for the H atom has been attained by signal averaging of the ARAS signals down to the concentrations of ∼1 x 10¹¹ atoms/cm³ and enables us to determine the brandling fraction for the direct H atom production channel, CH₃OH -→ CH₂OH + H (channel 1c) in a mixture of 1 ppm CH₃OH. Channel 1c is confirmed to be minor, that is, branching fraction for channel 1c is expressai by Log(k_1c/k₁) = (- 2.88 ± 1.88) x 10³/T - (0.23 ± 1.02), which corresponds to k_1c/k₁ < 0.03 for the present temperature range. By using 0.48 and 1.0 ppm CH₃OH with (100-1000) ppm H₂, the total decomposition rate k₁ for CH₃OH -products is measured from the time dependence of H atom, where the radical products of main channels 1a and 1b, that is, OH, CH₃, and CH₂, were converted rapidly into H atoms. The experimental result is summarized as Log(k₁/cm³molecule^-1s^-1) = (-12.82 ± 0.71) x 10³/T - (8.5 ± 0.38). A theoretical study based on ab initio/TST calculations with high accuracy has been conducted for the reaction: ³CH₂ + H2 →CH₃ + H (reaction 3). The rate is given by k₃/cm³molecule ^-1s^-1 = (7.32 x 10^-19)T^2,3 exp (-3699/7). This result is used for numerical simulations to evaluate k ₁. Present experimental results on the thermal decomposition rate of CH₃OH are found to be consistent with previous works. It is also found that time dependence of [H] observed in the 10 ppm CH₃OH in Ar can be reproduced very well by kinetic simulations by using a reaction mechanism composed of 36 elementary reactions.