J. Phys. Chem. 1984,88, 1006-1012
1006
Shock Tube and Modeling Study of Acetylene Oxidation Yoshiaki Hidaka, Department of Chemistry, Faculty of Science, Ehime University, Bunkyo-cho, Matsuyama 790, Japan
C . S. Eubank, W. C. Gardiner, Jr.,* and S. M. Hwang Department of Chemistry, University of Texas, Austin, Texas 7871 2 (Received: June 7, 1983)
The oxidation of CzHzwas studied behind incident shock waves in the temperature range 1300-2200 K with laser schlieren techniques. A computer simulation study was performed to determine the important elementary reactions. Data analysis using a 23-reaction mechanism showed that relatively few steps play significant roles in determining ignition behavior. The mechanism was tested against a variety of experimental data in the literature.
Introduction The high-temperature oxidation of acetylene has been studied by many workers with various techniques. Measurements of ignition delays in C2H2-0,-Ar mixtures revealed an activation energy of 75 f 5 kJ and a composition dependence on the O2 concentration alone, except in very lean C2H2mixtures.'-" The underlying reaction mechanism of CzH2ignition has been discussed for many years.2,4,5,7,9,12,13 Bradley and Kistiakowsky2 proposed the following reactions as the main reaction process in C2H2-02 induction zones, based on their observation of C4H2formation and an assumed close analogy to the H2-02 ignition mechanism
+0 0 + C2H2 = CZH + O H OH + C2H2 = CZH + H 2 0 CZH + C2Hz = C4H2 + H H
+02=
OH
(14) (sa) (74 (4)
The reaction numbering follows Table I; the "a" denotes a change in reaction products. Bradley and Kistiakowsky supposed that reaction 14 was rate controlling, which was also the view of Miyama and T a k e ~ a m a .Sullivan ~ and Warneck14 proposed instead that methylene formation is the predominant reaction between C2H2and 0, a view since supported by many authors.'>l8
0 + C2H2 = CH2
+ CO
(5)
For the reaction between OH and C2H2,some authors suggest reaction 7a2,6910*129'4 while others OH
+ C2H2 = C2Hz0 + H
(7)
(1) W. C. Gardiner, Jr., J. Chem. Phys., 35, 2252 (1961). (2) J. N. Bradley and G. B. Kistiakowsky, J. Chem. Phys., 35,264 (1961). (3) G. B. Kistiakowsky and L. W. Richards, J . Chem. Phys., 36, 1707 (1962). (4) R. F. Stubbeman and W. C. Gardiner, Jr., J . Phys. Chem., 68, 3169 (1964). ( 5 ) T. Takeyama and H. Miyama, Bull. Chem. Soc. Jpn., 38,936 (1964). (6) H. Miyarna and T. Takeyama, J. Chem. Phys., 42, 2636 (1964). (7) G. P. Glass, G. B. Kistiakowsky, J. V. Michael, and H. Nib, J. Chem. Phys., 42, 608 (1965). (8) D. R. White,Symp. (Int.) Combust. [Proc.],l l t h , 1966, 147 (1967). (9) W. C. Gardiner, Jr., W. G. Mallard, K. Morinaga, D. L. Ripley, and B. F. Walker, J . Chem. Phys., 44, 4653 (1966). (10) C. J. Jachimowski, Combust. Flame, 29, 55 (1977). (1 1) Y. Hidaka, Y. Tanaka, H. Kawano, and M. Suga, Mass Spectrosc., 29, 191 (1981). (12) W. G. Browne, R. P. Porter, J. D. Verlin, and A. H. Clark, Symp. (Int.) Combust. [Proc.],1968, IZth, 1035 (1969). (13) J. Vandooren and P. J. van Tiggelen, Symp. (Int.) Combust. [Proc.], 16th. 1976, 1133 (1977). (14) J. 0. Sullivan and P. Warneck, J . Phys. Chem., 69, 1749 (1965). (15) D. Saunders and J. Heicklen, J . Phys. Chem., 70, 1950 (1966). (16) J. M. Brown and B. A. Thrush, Trans. Furaday SOC.,63,630 (1967). (17) K.Hoyerrnann, H. Gg. Wagner, and J. Wolfrum, 2.Phys. Chem., 55, 72 (1967). (18) G. S. James and G. P. Glass, J. Chem. Phys., 50, 2268 (1969).
0022-3654/84/2088-1006$01.50/0
Gehring et a1.21also suggested the possible formation of the methyl radical by OH CzH2 = CH3 CO (7b)
+
+
It can be seen that even the basic framework of the ignition mechanism is still vague. This difficulty is especially important to combustion chemistry because C2H2is formed as an intermediate in the oxidation of other hydrocarbons. In the present experimental work the high-temperature oxidation of acetylene was characterized with the laser-schlieren technique. The goal was to provide a set of ignition delay data based on overall heat release rather than on profiles of any specific reaction product or intermediate. For interpretive purposes, we were particularly interested in studying an ignition mechanism that is also consistent with the two other families of C2H2combustion data, exponential growth rates and laminar flame profiles, as well as with related results on C2H2pyrolysis and individual elementary reactions. A final mechanism (Table I) capable of achieving good agreement between experiment and computation is discussed with reference to earlier shock tube and other results.
Experimental Section A shock tube with 7.62-cm id., previously described in was used. Briefly, the deflection of a 1-mW, 633-nm He-Ne laser beam traversing the shock tube was detected by a quadrant photodiode with a time resolution of about 300 ns. The experiments covered the temperature range of 1300-2200 K. The composition of the reaction mixtures used, which were allowed to mix for 48 h before use, were as follows: (A) 0.5% C2H2,1.25% 02,98.25% Ar; (B) 0.5% C2H2,5% 02, 94.5% Ar. The C2H2was specified by the manufacturer (Matheson) to be 99.6% pure. It was passed through a dry iceacetone cold trap packed with glass beads to remove possible acetone contamination. The O2and Ar were Matheson research grade (99.99%) and Matheson prepurified grade (99.99%), respectively. A starting pressure of 1.33 a& ' ! (10 torr) was used for all experiments reported here. Data interpretation was performed with the aid of computer modeling, which entailed numerical integrations of the set of differential equations describing steady variable-area reactive flowsZ3 (Constant density reaction was assumed for modeling results obtained by other authors using reflected shock waves.) Thermochemical properties were computed from polynomial fits to J A N A F or other published data except for C 2 H 0 and C4H3. (19) R. P. Porter, A. H. Clark, W. E. Kaskan, and W. G. Browne, Symp. (Int.) Combust. [Proc.],l l t h , 1966, 907 (1967). (20) J. R. Kanofsky, D. Lucas, F.Pruss, and D. Gutman, J . Phys. Chem., 78, 311 (1974). (21) M. Gehring, K. Hoyermann, H. Gg. Wagner, and J. Wolfrum, Nafurforschung A, 25, 675 (1970). (22) D. B. Olson, T. Tanzawa, and W. C. Gardiner, Jr., Inr. J . Chem. Kinet., 11, 23 (1979). (23) W. C. Gardiner, Jr., C. B. Wakefield, and B. F. Walker, "Shock Waves in Chemistry", A. Lifshitz, Ed., Marcel Dekker, New York, 1981, Chapter 7, p 319.
0 1984 American Chemical Society
Modeling Study of Acetylenc Oxidation
?'he Journal o/Physical Chemistry, Vol. 88, Nu. 5, 1984
1007
Kcaction Mcclianirnr and llrte ('ocflicienl I ~ . i [ ) r e & h i ~ i i s ~
'TAB1,F I:
~-
~. . I 7
3
4 5 b
4.1 i I i X 4.3 t I4
I .i
1 . 2 t 17 i 14 9 i i
11 Y
4.0 + I4 3.6 t 15 1.1) t 13 .?.I1 t 1 3
7 K 9 111
11
12 I3 14
1s 16 17 18 19
xi ?I 22 23
I 1 t 0, . O H 0 If, 011
+
f
(ill t 11,
-0 i
.11,o
.
11 I
II
0 1 1 t OH 11,o T 0 l l t 0,i hl 110, + M 110, t II 0 1 1 + Oli IIO, i I1 11, i- 0 ,
-
~
~
110.
+ 011
~
I1 0 , 0
..
lcvcl o f ~ ~ ~ n i i ~ l oiw c n cin,i) c
)pi
1 , ) 11iC
2.1 ' 1 34
i
1.3
7 5 t
li
2.5
?
I4
1.5
1
I.?
5.1) 4- 13
~~,~rcssi3000 K, and pressures can vary from 10 atm. In the terrestial troposphere and stratosphere, temperatures range from 300 to 200 K and pressures vary from 1 atm to a few torr. Unimolecular reactions and their reverse, combination reactions, are important components of the chemical description of both of these environments, and the extremely wide variation of parameters provides a severe test of our quantitative understanding of such reactions. The pressure dependence of unimolecular and combination reactions depends on ( AE),the average energy transferred per collision. The temperature dependence of (AI?) has been the subject of several recent investigations on several unimolecular reactions, and the results of those investigations have varied widely.'" Thus, the temperature dependence of ( AE) cannot be reliably predicted on the basis of the limited data available. In the experiments reported here, a Ydirect" technique is employed that does not rely on unimolecular reactions.' Thus, there are no complications introduced by secondary reactions or heterogeneous catalysis on reactor walls. Although the "direct" method employed does not depend on chemical reactions, it does ( 1 ) For reviews, see Tardy, D. C.; Rabinovitch, B. S. Chem. Rev. 1977, 77, 326. Quack, M.; Tree, J. Spec. Period. Rep., Chem. SOC.1977, 2, 175. (2) Krongauz, V. V.; Rabinovitch, B. S.J . Chem. Phys. 1983, 78, 3872. Krongauz, V. V.; Rabinovitch, B. S.; Linkaityte-Weiss, E. [bid. 1983, 78, 5643, and references cited therein. (3) Endo, H.; Gllnzer, K.; Troe, J. J. Phys. Chem. 1979, 83, 2083. (4) Nguyen, T. T.; King, K. D.; Gilbert, R. G. J . Phys. Chem. 1983.87, 494. (5) Brown, T. C.; Taylor, J. A.; King, K. D.; Gilbert, R. G. J . Phys. Chem. 1983,87, 5214. (6) Heymann, M.; Hippler, H.; Troe, J . J . Chem. Phys., in press. (7) For a review, see Barker, J. R. J . Phys. Chem. 1984, 88, 1 1 .
0022-3654/84/2088-1012$01.50/0
depend on infrared fluorescence to monitor the energy content of excited molecules. However, as discussed below, a calibration curve and theorys provide a direct connection between infrared fluorescence and vibrational energy and the uncertainties associated with this technique are small. The molecule studied in the present investigationis azulene (CloHs),an aromatic hydrocarbon, and the temperature is varied by about a factor of two. In previous work using excited azulene at constant temperature, we were able to show that (AE)depends on the vibrational energy E.8*9The exact energy-dependence differs for different collider gases. As the temperature increases, the average thermal vibrational energy also increases, and one would predict a concomitant variation in (AE).It is of interest to determine whether there is an "extra" temperature dependence not predicted by the known energy dependence of (AE)and that required by detailed balance. Such an "extra" effect can be due to the temperaturedependent energy distributiuons in the various degrees of freedom of the collider gas and of azulene itself. The average energy transferred per collision, ( AE), is the net result of averaging upward and downward collisional transitions. The upward transitions are related to the downward transitions by detailed balance. Thus, it is common to discuss energy transfer in terms of (AI?),,the average energy transferred in deactivating col1isions.l0 As discussed below, our results indicate that for excited azulene, Az(E), deactivated by unexcited azulene, (AE(E)), is nearly independent of temperature; for Az(E) deactivated by N,, (AE(E))dmay decrease slightly as temperature increases. ~~
~
(8) Rossi, M. J.; Barker, J. R. Chem. Phys. Lett. 1982, 85, 21. (9) Rossi, M . J.; Pladziewicz, J. R.; Barker, J. R. J . Chem. Phys. 1983, 78, 6695. (10) For an interesting discussion of the relative merits of citing (ME)or (m)d, see Gilbert, R. G. Chem. Phys. Lett. 1983, 96, 259.
0 1984 American Chemical Society
