J. Phys. Chem. A 2010, 114, 5493–5502
5493
Shock Tube Study on the Thermal Decomposition of CH3OH Ku-We Lu, Hiroyuki Matsui,* Ching-Liang Huang, P. Raghunath, Niann-Shiah Wang,* and M. C. Lin Department of Applied Chemistry, National Chiao Tung UniVersity, 1001, Ta Hsuch Road, Hsinchu 30010 Taiwan ReceiVed: January 19, 2010; ReVised Manuscript ReceiVed: March 15, 2010
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 ∼1 × 1011 atoms/cm3 and enables us to determine the branching fraction for the direct H atom production channel, CH3OH f 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(k1c/k1) ) (- 2.88 ( 1.88) × 103/T - (0.23 ( 1.02), which corresponds to k1c/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 f 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) × 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 f CH3 + H (reaction 3). The rate is given by k3/cm3molecule-1 s-1 ) (7.32 × 10-19)T2.3 exp (-3699/T). This result is used for numerical simulations to evaluate k1. 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. 1. Introduction Alcohol fuels are recognized as the most promising renewable energy resources. Therefore, it is important to investigate the reaction mechanisms and kinetics for their thermal decomposition and combustion. Thermal decomposition of CH3OH has been studied extensively1-14,57 but understanding the details of the reaction mechanism of pyrolysis at an elevated concentration still remains to be below an acceptable level. It is suggested in some of the theoretical studies13,14 that CH3OH decomposition has the following possible product channels.
CH3OH + M f products + M CH3OH + M f CH3 + OH + M
(1) (1a)
f 1CH2 + H2O + M
(1b)
f CH2OH + H + M
(1c)
f CH2O + H2 + M
(1d)
f cis-HOCH + H2 + M
(1e) f trans-HOCH + H2 + M (1f) The main reaction channel is concluded in most of the experimental and theoretical studies to be reaction 1a, and some contribution from 1b is also indicated under the ambient * To whom correspondence should be addressed.
conditions. Direct production of the H atom from scission of a C-H bond (reaction 1c) has been indicated to be minor by observing time dependence of H atom production,4,5 but some of the bulk experimental studies indicated its non-negligible contribution in the pyrolysis of methanol.7,8 Trials in the previous experimental studies to examine the branching fractions have been obscured more or less by the secondary reactions because of the high concentration of CH3OH employed. Time dependence of the main product OH from (reaction 1a) was measured in recent shock tube studies at relatively low concentrations of CH3OH ( 0.888
where absorbance is given by A ) ln(I0/I) and F(T) denotes temperature dependence given as F(T) ) 1889/T - 0.0004. It is worth noting that these equations are derived from the summary of the single shot data in the thermal decomposition of C2H5I. As shown in Figure 1, they include large uncertainties because the range of the [H] covers almost three orders of magnitude compressed in the limited range on the vertical scale; the overlapped noise level is too large compared with the logarithmic response for the wide range of concentrations to evaluate [H] accurately. Accuracy becomes very poor for [H] > 5 × 1013 atoms cm-3 because the absorption is highly saturated. Profiles of H atoms produced in the mixtures of CH3OH + Ar are also shown in Figure 1 for comparison. It is clearly demonstrated that the production of H atoms is completely controlled by the secondary reactions; that is, sample A (1 ppm CH3OH + Ar) produces a very small number of H atoms (1850 K), where final steady levels of [H]/[CH3OH]0 can be accurately predicted. Blank tests in shock-heated pure Ar were conducted for all series of runs to evaluate the background level. No detectable H was observed in the blank tests of pure Ar nor 100 ppm H2/ Ar mixtures. For the 1000 ppm H2/Ar mixture, the H atom of about (1 to 2) × 1011 atoms cm-3 was detected, probably from impurities contained in H2. In this case, the signal of H in the background test using H2/Ar mixtures was averaged over three shots and simply subtracted from those of the 0.48 ppm CH3OH + 1000 ppm H2 mixtures under the same experimental condition. Correction of [H] due to the background H was
