33 Kinetics of CH Radical Reactions Important Laser Probes for Combustion Chemistry Downloaded from pubs.acs.org by NANYANG TECHNOLOGICAL UNIV on 09/25/17. For personal use only.
to Hydrocarbon Combustion Systems J. E. BUTLER, J. W. FLEMING, L. P. GOSS, and M. C. LIN Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375
One of the important hydrocarbon combustion reaction intermediates is the CH radical. Although CH chemiluminescence ( A Δ --> Χ π ) has been observed in many hydrocarbon flames, the mechanism of CH formation and its reaction kinetics have been difficult to unravel in situ due to the low steady-state concentrations and the com plex nature of combustion reactions. This project was undertaken to investigate a means of CH radical production and to study its reactions with various important species so that an overall picture of the oxidation processes, particularly with regard to the mechanism of NO formation, may be better understood. 2
2
x
Production and Detection One of the most effective methods of CH production is the multiphoton dissocia tion of CHBr (1). A high-power ArF excimer laser (193 nm) was used to dissociate a CHBr :Ar gas mixture (~1:10 ) slowlyflowingthrough the reaction cell at pressures of 30-100 torr. A high-power tunable dye laser pumped by a tripled Nd:YAG laser was employed to monitor the production and decay of the CH radical formed in the dissoci ation process via laser inducedfluorescenceof the CH (A -> X) transition near 430 nm. The ArF beam, dye laser probe beam andfluorescencecollection optics were mutually perpendicular as shown in a schematic diagram given in Figure 1. Figure 2 shows a typical laser excitation spectrum and the rotational assignments. 3
5
3
Kinetics Kinetic measurements were made by monitoring the laser-induced fluorescence of C H following the excitation in the (0-0) band of the X —> A transition as a function of the time delay after the A r F laser dissociation. In the absence of any added reactants, C H had a decay time of 100 to 300 /usee at a total pressure of 30 to 100 torr ( C H B r pressures of 1 to 10 mtorr) which can be attributed mainly to the C H -I- C H reaction. The addition of the reactants listed in Table I shortened the C H radical decay times considerably, indicative of some removal process involving a bimolecular mechanism since the total pressure was always maintained constant. Least squares plots of the inverse lifetimes of C H radicals versus the partial pressure of the added reactant yielded 3
This chapter not subject to U . S . copyright. Published 1980 A m e r i c a n Chemical Society
LASER PROBES FOR COMBUSTION
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398
CHART RECORDER!
CHEMISTRY
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PULSE GENERATOR! MOLECTRON DL242 SCAN CONTROL
To Pr««tur« Gaug« Vacuum Pump
PHOTO DIODE 355n
TACHISTO TAC II ArF EXCIMER LASER
DOUBLE PULSE DELAY GENERATOR
QUANTA| RAY YAG LASER
ILamp Trigger
Figure 1.
Apparatus for the productions and detection of CH radicals and the measurement of their reaction kinetics
33.
Kinetics
BUTLER ET AL.
of CH Radical
399
Reactions
a second order rate constant for each reactant. These results are summarized in Table I and compared with previously published values for selected molecules.
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Table I. Rate Constants for C H Radical Reactions at Room Temperature with Selected Molecules Relevant to Hydrocarbon Combustion
k x 10 (cmVmolecule · sec) 11
Reactant I H 0 NO CO
2.6±0.5 5.9±0.8 29±7 2.1 ± 0 . 3 0.093 ± 0 . 0 1 0.19 ± 0 . 0 4 10±3* 40±1 58±5
2
2
N C0 CH C H
6
C H
1 0
2
2
4
2
4
II 0.10 — —
Ill 1.74±0.20 4.0 —
—
0.0071 —
0.48 0.10±0.02 —
0.25
3.3 ± 0 . 0 8
—
—
-
13±1
a. I— This work (100 T o r r total system pressure ) ; II — Braun et al. (2); III — Bosnali and Perner (3). b. T h e value reported in Ref. ( l ) was too high by a factor of 3 due to errors in CH4 concentration calculations.
Previous studies on the reactions of C H employed either the vacuum ultra-violet photodissociation (2) or the electron beam dissociation (3) of C H to generate the radi cal. The formation and decay of the C H was monitored by U V absorption spectroscopy on the C «— X transition at 314 nm. The results of the former study (2), which relied partly on final product analysis, are considerably smaller (by a factor of 10 to 40) than the values of Bosnali and Perner (3) and our present data for the reactions with H , N and C H . The agreement between ours and those of Bosnali and Perner, although significantly better, is only fair and lies within a factor of 2 to 5. Further work is cer tainly needed in order to reconcile these two sets of data. 4
2
4
Comments on C H + No The C H + N reaction has now been generally considered as one of the most important precursor reactions for "prompt" N O formation in high temperature hydrocar bon combustion systems. The rate constant of this reaction at room temperature was found to be pressure-dependent (see Figure 3) and is considerably higher than the value extrapolated from the expression, 1.3 χ 10~ exp ( - 1 1 , 0 0 0 / R T ) cmVmolecule · sec, obtained from the rate of N O production in several hydrocarbon flame fronts (4). These findings could be understood by the thermochemistry of the C H + N CHN H C N + Ν system (5). Since the production of H C N + Ν is endothermic by 3 kcal/mole, it probably occurs with a relatively high activation energy (such as the value, 11 kcal/mole, obtained from the high temperature flame studies mentioned above). The formation of the C H N radical adduct, which is expected to be pressure-dependent as was found experimentally, probably can proceed with little or no activation energy. The following mechanism can at least qualitively account for the overall reaction: 2
12
2
2
2
2
LASER PROBES FOR COMBUSTION
400
CHEMISTRY
CH (Α Δ - — - Χ Π, O-O)
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2
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ι
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7 6 5 i. A1.3 I ' l l
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L 432
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-λ (nm) Figure 2.
Laser-induced fluorescence spectrum of the Α Δ H C - N - N + M 2
HCN + N
(high Τ )
Although the formation of H C N + Ν is not spin-conserved, the formation of the C H N radical intermediate is expected to overcome and facilitate the doublet —• quartet conversion. A t room temperature, the C H N adduct probably disappears by secondary reactions with active species present in the system. T h e effects of both temperature and pressure on the rate of this important reaction will be thoroughly investigated in the near future.
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2
2
Conclusions In this work, we have demonstrated that the C H radical can be generated with sufficiently high concentrations by means of the multiphoton dissociation of C H B r at 193 n m for kinetic measurements. T h e formation and decay of the C H radical was monitored by the laser-induced fluorescence technique using the (A Δ «— Χ π ) transi tion at 430 n m . Several rate constants for the reactions relevant to high temperature hydrocarbon combustion have been measured at room temperature. One of the key reactions, C H + N , has been shown to be pressure-dependent, presumably due to the production of the C H N radical at room temperature. 3
2
2
2
2
Literature Cited 1.
Butler, J.E., Goss, L.P., Lin, M.C. and Hudgens, J.W., Chem. Phys. Lett. (1979) 63, 104.
2.
Braun, W., McNesby, J.R. and Bass, A.M., J. Chem Phys. (1967) 46, 2071.
3.
Bosnali, M.W. and Perner, D., Z. Naturforsch. (1971) 26a, 1768.
4.
Blaurvens, J., Smets, B. and Peeters, J., Sixteenth Symposium (International) Combustion (The Combustion Institute, 1977) p. 1055.
5.
Benson, S.W., Sixteenth Symposium (International) on Combustion (The Combustion Institute, 1977) p. 1062.
RECEIVED
February 1, 1980.
