The temperature dependence of the C1+ propylene rate coefficient and the Arrhenius fit are shown in Figure 1.

Transcription

1 t ' i!, ' nfrared Absorption Measurements of the Kinetics of C1 atom Reactions with C,H, (n=4,6)unsaturated Hydrocarbons between K. John T. Farrell, Jeffrey S. Pilgrim, and Craig A. Taatjes Combustion Research Facility Mail Stop 9055 Sandia National Laboratory Livermore, CA USA ca. sandia.gov COflF The reactions of C1 atoms with the unsaturated C,H, (n=4,6) hydrocarbons propylene, allene, and methyl acetylene have been investigated as a function of temperature ( K) and pressure (3 10 Torr) using the Laser Photolysisfinfrared LongPath Absorption technique. The reaction of C1 atoms with these hydrocarbons proceeds via abstraction to form HCl and addition to form either a chloropropyl or chloroallyl radical. The contribution of these channels to the total rate can be separated by careful measurement of the temporal evolution and yield of the HC product. For example, at T =293 K, the C1+ propylene rate coefficient is pressure dependent (between 3 to 10 Torr) and the HCl time evolution is biexponential. The data indicate that abstraction and addition contribute equally to the total rate coefficient at about 5 Torr. The abstraction channel becomes increasingly important at higher temperature, and at T 2 500K is the only channel that contributes significantly the the total rate, as evidenced by unity HC1 yield, pressure independent rate coefficients, and simple exponential temporal evolution of the HC1. The rate coefficient for HC production is well described over the temperature range of 292<T<800 K by the simple Arrhenius expression: kp,,py,,,,=(5.2rt0.5) x 10'" exp[(138 f 40)a] cm3 molecule' s" The temperature dependence of the C1+ propylene rate coefficient and the Arrhenius fit are shown in Figure 1. The C1 t allene reaction exhibits slightly different behavior. At 292 K, almost no HCl is generated (QHa = 0.03), indicating that addition (without subsequent HC1 loss) is the dominant reaction channel. The HC yield increases at higher temperature more slowly b

2 DSCLAMER This report was prepared as an account of work spomored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or impied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or pnxmss disclosed, or represents that its use would not infringe privately owned rights Reference herein to any specific conunercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessany constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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4 than with propylene, and doesn't reach unity until T K. Between K, the rate coefficient for the HC producing channels is given by the simple Arrhenius expression: k,,,,,=( ) exp[( )/T]cm3molecule" s" X This expression reproduces the data well except at low temperature, where slight curvature is apparent. A modified Arrhenius expression, k=a '(TB98)2eELJRT is invoked, which gives: 1.14H.14) x lo" (T/298)2exp[( )/T]cm3 molecule" s" k,,=( The rate coefficients (total and abstraction ) vs temperature and the Arrhenius fits are shown in Figure 2..h v) r a 3 0 a, 2 5 m v Y k=(5.2 k0.5) x 10' 2x101' (138MO)n e 2.0 cm3molecule' s' 1OOO/T 2.5 (K') Figure 1: Arrhenius plot for C1 + C,H, HC t C,H,

5 3L C1+ Allene 2, e 10l h r u), E!i 2 a a, m v Y 101' A: Total Rate 5Torr Total Rate 10 Torr A Abstraction = (PHC, x hot@ 0 \. k= (4.1t1.2) x 10'" crn3 rnoleculd' 5.' k= (1.04kO.14)x 16" (T/298)2e(386"04)rT~rn3molecule~' <' T x \ 2.5 \ 3.0 (K1) Figure 2: Arrhenius plot for C1 + C3H, + HC 3.5~11c3 + C3H3 The reactions of C1 with propylene and allene are both sufficiently energetic to produce vibrationally excited HCl, and indeed a significant fraction of the HC produced in both reactions is vibrationally excited. Measurement of the HC production in Ar buffer, which is extremely inefficient at quenching HC vibrational excitation, permits the branching fraction into v=l and the rate coefficient for vibrational relaxation of HC by the hydrocarbon to be estimated. With both propylene and allene, about half the HCl is generated in v=l. The fraction of vibrationally excited HC1 produced in each reaction, along with the rate coefficient for HC1 relaxation, are given in Table 1. Results will also be presented for the C1+ methyl acetylene reaction, which has recently been studied in our laboratory.

6 c TABLE 1. Branching fractions into v,,,=l and vibrational relaxation rate coefficients. f"=la c1 Propylene C1 + Allene k 0.8 a The fraction of vibrationally excited HC1 produced in the C1+ propylene and C1+ allene reactions. Units of 1012 cm3 molecule1 s1. The numbers in the parentheses indicate experimental uncertainties (k 20 precision only) in the last digit. Presentation mode: We prefer to present this paper in a poster session. URL's for the Author's nstitutions on the World Wide Web: