Browsing by Author "Lee, Pei-Fang"

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Production of H and O(P-3) Atoms in the Reaction of CH2 with O-2
(2012-09-20)
Lee, Pei-Fang
;
Matsui, Hiroyuki
;
Chen, Wei-Yu
;
Wang, Niann-Shiah
;
應用化學系
The reaction CH2 + O-2 -> 4 products has been studied by using atomic resonance absorption spectrometry (ARAS) of H and O(P-3) atoms behind reflected shock waves over 1850-2050 K and 2.1-1.7 atm. Measurements of [H] and [O] are conducted using mixtures of highly diluted CH2I2 (0.2 and 0.4 ppm) with excess O-2 (100 and 262 ppm) in Ar; comparative measurement of [H] in the 0.2 and 0.4 ppm CH2I2 + 300 ppm H-2 mixture has been conducted simultaneously to confirm the initial concentration of CH2. The apparent reaction rate of CH2 + O-2 (k(2)'), including the contributions of (CH2)-C-3 + O-2 -> products (2) and (CH2)-C-1 + O-2 -> products (3), has been measured from the evolutions of [H] and [O] and summarized as k(2)'/cm(3) molecule(-1) s(-1) = (1.90 +/- 0.31) x 10(-11). The contribution of (CH2)-C-1 + O-2 reaction on the measured k(2)' has been evaluated as 0.15 +/- 0.04, with an assumption that k(3) is independent of temperature and given by the result measured at room temperature [Langford, A. O.; Petek, H.; Moore, C. B. J. Chem. Phys. 1983, 78, 6650-6659]. The net rate for the (CH2)-C-3 + O-2 reaction is given as k(2)/cm(3) molecule(-1) s(-1) = (1.69 +/- 0.31) x 10(-11). The result on k(2) in this study is found to be consistent with the extrapolation of the previous work at lower temperature range of 295-600 K. [Vinckler, C.; Debruyn, W. J. Phys. Chem. 1979, 83, 2057-2062]; these results can be summarized as k(2)/cm(3) molecule(-1) s(-1) = 2.74 x 10(-11) exp(-874/T), (T = 295-2050 K). The apparent production yields of H and O atoms for the reaction channels (CH2)-C-1,3 + O-2 -> H + products (2a, 3a) and (1)'(CH2)-C-3 + O-2 -> O(P-3) + products (2h, 3b) have been evaluated as phi(2a)' = 0.59 +/- 0.06 and phi(2b)' = 0.23 +/- 0.06, respectively. The contributions of (CH2)-C-1 + O-2 reaction on measured phi(2a)' and phi(2b)' are indicated to be minor; the net branching fractions for the (CH2)-C-3+O-2 reaction are estimated as phi(2a) = 0.58 +/- 0.06 and phi(2b) = 0.25 +/- 0.06. No obvious temperature dependence is indicated in the measured rate constant nor in the branching fractions of H and 0 atoms. The mechanism of the reaction of CH2 with O-2 is discussed based on the result of the present study together with those of the previous theoretical/experimental studies.
Study on the Reaction of CH2 with H-2 at High Temperature
(2012-03-01)
Lee, Pei-Fang
;
Matsui, Hiroyuki
;
Wang, Niann-Shiah
;
應用化學系
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Department of Applied Chemistry
Thermal decomposition of CH2I2 [sequential C-I bond fission processes, CH2I2 + Ar -> CH2I + I + Ar (1a) and CH2I + Ar -> (CH2)-C-3 + I + Ar (1b)], and the reactions of (CH2)-C-3 + H-2 -> CH3 H (2) and (CH2)-C-1 + H-2 -> CH3 + H (3) have been studied by using atomic resonance absorption spectrometry (ARAS) of I and H atoms behind reflected shock waves. Highly diluted CH2I2 (0.1-0.4 ppm) with/without excess H-2 (300 ppm) in Ar has been used so that the effect of the secondary reactions can be minimized. From the quantitative measurement of I atoms in the 0.1 ppm CH2I2 + Ar mixture over 1550-2010 K, it is confirmed that two-step sequential C-I bond fission processes of CH2I2, (la) and (1b), dominate over other product channels. The decomposition step (1b) is confirmed to be the rate determining process to produce 3CH2 and the least-squares analysis of the measured rate gives, In(k(1b)/cm(3) molecule(-1) s(-1)) = -(17.28 +/- 0.79) - (30.17 +/- 1.40) x 10(3)/T. By utilizing this result, we examine reactions 2 and 3 by monitoring evolution of H atoms in the 0.2-0.4 ppm CH2I2 + 300 ppm H2 mixtures over 18502040 K. By using a theoretical result on k(2) (Lu, K. W.; Matsui, H.; Huang, C.-L.; Raghunath, P.; Wang, N.-S.; Lin, M. C. J. Phys. Chem. A 2010, 114, 5493), we determine the rate for (3) as k(3)/cm(3) molecule(-1) s(-1) = (1.27 +/- 0.36) x 10(-10). The upper limit of k(3) (k(3max)) is also evaluated by assuming k(2) = 0, i.e., k(3max)/cm(3) molecule(-1) s(-1) = (2.26 +/- 0.59) x 10(-10). The present experimental results on k(3) and k(3max) is found to agree very well with the previous frequency modulation spectroscopy study (Friedrichs, G.; Wagner, H. G. Z. Phys. Chem. 2001, 215, 1601); i.e., the importance of the contribution of (CH2)-C-1 in the reaction of CH2 with H-2 at elevated temperature range is reconfirmed.
Thermal Decomposition and Oxidation of CH3OH
(2013-01-24)
Lee, Pei-Fang
;
Matsui, Hiroyuki
;
Xu, Ding-Wei
;
Wang, Niann-Shiah
;
應用化學系
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 k(1) for CH3OH + M -> products obtained in this study is expressed as, ln(k(1)/cm(3) molecule(-1) s(-1)) = -(14.81 +/- 1.22) - (38.86 +/- 1.82) X 10(3)/T, over 1359-1644 K. The present result on k(1) 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 O-2 + Ar. For the low concentration CH3OH (below 10 ppm) + O-2 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 H-2 diluted in Ar. The branching fraction for CH3OH + Ar -> (CH2)-C-1 + 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 (CH2)-C-1,3 + O-2 [J. Phys. Chem. A 2012, 116, 9245-9254]; i.e., the branching fraction for the above reaction is evaluated as, phi(1a) = 0.20 +/- 0.04 at T = 1880-2050 K, in the 1.3 and 3.5 ppm CH3OH + 100 ppm O-2 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 O-2) of methanol.