Shock Tube Study on the Thermal Decomposition of CH3OH

Abstract

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 atm), 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 similar to 1 x 10(11) atoms/cm(3) and enables us to determine the branching 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 expressed by Log(k(1c)/k(1)) = (- 2.88 +/- 1.88) x 10(3)/T - (0.23 +/- 1.02), which corresponds to k(1c)/k(1) < 0.03 for the present temperature range. By using 0.48 and 1.0 ppm CH3OH with (100-1000) ppm H-2, the total decomposition rate k(1) 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(k(1)/cm(3)molecule(-1)s(-1)) = (-12.82 +/- 0.71) x 10(3)/T - (8.5 +/- 0.38). A theoretical study based on ab initio/TST calculations with high accuracy has been conducted for the reaction: (CH2)-C-3 + H-2 -> CH3 + H (reaction 3). The rate is given by k(3)/cm(3)molecule(-1) s(-1) = (7.32 x 10(-19))T-2.3 exp (-3699/T). 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.