J . Phys. Chem. 1984,88, 3648-3653
3648
Kinetics of the Reaction of Ethyl Radicals with Molecular Oxygen from 294 to 1002 K Irene R. Slagle, Qiao Feng, and David Gutman* Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616 (Received: January 30, 1984)
The kinetics and mechanism of the reaction of ethyl radicals with molecular oxygen have been studied between 294 and 1002 K. The radicals were produced by the rapid C1 + C2H6 reaction following the pulsed production of chlorine atoms from the infrared multiple-photon-induced decomposition of CFC13. Reactant and product concentrations were monitored in real-time experiments using photoionization mass spectrometry. Overall rate constants of the C2H5 + O2 reaction were measured near seven temperatures between 294 and 1002 K. In addition, C2H4product yields were determined at 302, 593, and 688 K. Special emphasis was given to obtaining the density dependence of the overall rate constant at four temperatures: 296,467,640, and 904 K. The results obtained below 500 K are consistent with a reaction mechanism that proceeds primarily by addition to form C2H5O2. The C2H5-C2H502equilibrium was never observed at elevated temperatures, indicating that a second reactive route (that which produces C2H4 + H02) is of major importance above 600 K. The pressure and temperature dependencies of the measured rate constants, the CzH4 product yields, and the results of prior studies of this reaction are discussed in terms of two mechanisms: one which proceeds by parallel uncoupled routes to C2H5O2 and C2H4 formation and another which proceeds by coupled routes via a common C2H502*intermediate. The experimental findings appear to be more consistent with the coupled-path mechanism.
Introduction The reactions of alkyl free radicals (R) with molecular oxygen are important elementary steps in the oxidation of Changes that occur in the mechanisms of these elementary reactions as temperatures increases (particularly between 300 and 500 “C) are largely responsible for important changes that occur in combustion processes as temperature increases, properties such as the ultimate stable products that are produced and the rate of the oxidation process The kinetics and mechanisms of R O2reactions near ambient temperature are now generally well understood because of the many direct and indirect investigations of these reactions performed either at room temperature or under mildly heated cond i t i o n ~ . ~ ,Far ~ - ~less ~ is understood about the mechanism changes that occur above 300 OC. What is known is based largely on a series of indirect investigations of several R + O2 reactions by Baldwin and Walker conducted primarily between 450 and 550
+
0 c.4J5-17
Near ambient temperature alkyl and many other hydrocarbon free radicals react with molecular oxygen principally by a reversible addition process:
R
+0 2
R02
(1)
The role of this addition reaction in low-temperature combustion (1) Minkoff, G. J.; Tipper, C. F. H. “Chemistry of Combustion Reactions”; Butterworths: London, 1962. (2) Pollard, R. T. “Comprehensive Chemical Kinetics”; Bamford, C. H., Tipper, C. F. H. Eds.; Elsevier: New York, 1977; Vol. 17, pp 249-367. (3) McKay G. Prog. Energy Combust. Sci. 1977,3, 105-26. (4) Walker, R. W. “Reaction Kinetics”; Ashmore, P. G., Ed.; Chemical Society: London, 1975; Vol. 1, pp 161-211. ( 5 ) Benson, S . W. Prog. Energy Combust. Sei. 1981,7, 125-34. (6) Dechaux, J. C. Oxid. Combust. Rev. 1973,6,75. (7) Knox, J. H. Combust. Flame 1965,9, 297-310. (8) Benson, S . W. J. Am. Chem. SOC. 1965,87,972-9. (9) Morgan, C. A.; Pilling, M. J.; Tulloch, J. M.; Ruiz, R. P.; Bayes, K. D. J. Chem. SOC.,Faraday Trans. 2 1982,78,1323-30. (10) Plumb, I. C.; Ryan, K. R. Int. J. Chem. Kinet. 1981,13, 1011-28. (11) Selzer, E. A.; Bayes, K. D. J. Phys. Chem. 1983,87,392-4. (12) Nelson, H. H.; McDonald, J. R. J . Phys. Chem. 1982,86,1242-44. (13) Lenhardt, T. M.; McDade, C. E.; Bayes, K. D. J. Chem. Phys. 1980, 72, 304-10. (14) Berman, M. R.; Fleming, J. W.; Harvey, A. B.; Lin, M. C. Symp. (Int.) Combust., [Proc.] 1982,19, 73-9. (15) Walker, R. W. “Gas Kinetics and Energy Transfer”; Ashmore, P. G., Donovan, R. J., Eds.; Chemical Society: London, 1977; Vol. 2, pp 296-330. (16) Baldwin, R. R.; Bennett, J. P.; Walker, R. W. Symp. (Int.) Combust., [Proc.] 1977,16,819-29. (17) Baldwin, R. R.; Bennett, J. P.; Walker, R. W. J. Chem. SOC., Faraday Trans. 1 1980, 76,1075-98.
0022-3654/84/2088-3648$01.50/0
processes was quantitatively described by both Knox and Benson in 1965.738 More recent kinetic and thermochemical studies of R + O2reactions have provided direct determinations of the rate constants for several hydrocarbon free radicals (including, in some instances, their pressure and temperature dependencie~).~-’~ A direct measurement of the equilibrium constant of one of these reactions has also been reported (C,H5 + 0, e C3H50,).9All of these investigations have essentially confirmed the basic description of the low-temperature mechanisms of R + 0, reactions presented by Knox and Benson. Above 400 “C, the mechanism of R + 0, reactions has changed significantly. The equilibrium in reaction 1 is far to the left, favoring reactants at the oxygen pressures that are encountered in most combustion p r o c e ~ s e s . ~Yet - ~ alkyl radicals continue to react with molecular oxygen. Measurements of product yields near 480 “C reveal that there is nearly complete conversion of the alkyl radical to the corresponding olefin (R+) at this temp e r a t ~ r e . ~ ~The ’ ~ mechanism J~ of this high-temperature process is still unknown. Two possibilities have been suggested that are consistent with experimental observation^.^^^ The first (mechanism A) is that there exists a parallel uncoupled reactive route, reaction 2, R+OZ+R+ +HO2 (2) that is of negligible importance at ambient temperature because of its activation energy (estimated to be 3-10 kcal mo1-1),18-20but which becomes increasingly important as temperature increases. This second route becomes the dominant R O2 reactive path above 400 OC due to the increase in its rate constant with rising temperature as well as to the shift in the R-R02 equilibrium as temperature increa~es.~-~ Mechanism A is presumed in combustion modeling studies where rate constants for R O2 reactions at the high temperatures of combustion must be obtained by extrapolation procedures from the results of studies conducted at much lower t e m p e r a t ~ r e s . ~ l - * ~ The second possible high-temperature R + 0, mechanism (mechanism B) involves coupled reactive paths. Olefin production
+
+
(18) Benson, S. W. Adu. Chem. Ser. 1968,76, 143-53. Benson, S . W. “Thermochemical Kinetics”; Wiley-Interscience: New York, 1976; p 241. (20) Knox,J. H.; Wells, C. H. J. Trans. Faraday SOC.1963,59,2801-12. (21) Westbrook, C. K.; Dryer, F. L. Report UCRL-88651; Lawrence Livermore National Laboratory: Livermore, CA, 1983 (and references therein). (22) Warnatz, J. Report SAND-8606; Sandia Laboratories: Albuquerque, NM 1983 (and references therein). (23) Cathonnet, M.; Boettner, J. C.; James, H. Symp. (In?.)Combust. [ P ~ o c .1977, ] 18,903-13. ( 19)
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3649
Reaction of Ethyl Radicals with Molecular Oxygen results from a second decomposition channel of the R 0 2 adduct: 2,4 R
+ 0 2 s RO2
+
R H
+ HO2
(3)
The minor importance of this second decomposition pathway at ambient temperature is due to the stability of the R 0 2 radical and to the low rate constant for the alternate decomposition path of the energy-rich R 0 2 * adduct compared to the rate constant for dissociation back to the original reactants. However, as the temperature increases, the importance of the alternate decomposition path does also. Although decomposition of R 0 2 * to form an olefin and H 0 2 is never a rapid process compared to dissociation back to reactants, it can account for the increasing importance of olefin formation (as a fraction of the R + O2encounters that result in the formation of new products). This mechanism, although consistent with the product yield and rate constant determinations of Baldwin and Walker near 480 OC, can result in entirely different extrapolations to higher and lower temperatures. With mechanism B, it is possible for the overall rate constant to continue to decrease with increasing temperature in spite of the change in mechanism. If this is the case, then current extrapolations of rate constants obtained at 480 "C to combustion temperatures or to room temperature that are based on mechanism A are in error. There is a need for more quantitative studies of R O2 reactions that will provide additional knowledge of the kinetics and mechanisms of these reactions at elevated temperatures. As part of our continuing interest in the kinetics of polyatomic free radicals, we have begun a series of studies of R O2 reactions that are capable of undergoing mechanism changes of the type described above. We have now studied the simplest of these, the C2H5 O2 reaction, over a 17-fold density range ((1.4-24) X 10l6 molecules ~ m - and ~ ) from 294 to 1002 K. The results of this investigation are reported here and are discussed in terms of both mechanisms A and B.
+
+
+
Experimental Section The experimental apparatus and most procedures used were essentially the same as those described p r e v i o ~ s l y . Radiation ~~~~ from a Lumonics 103-2 C 0 2 TEA laser was mildly concentrated with a 5-m focal length concave mirror. The long beam waist was concentric with and located along the axis of the 35-cm-long, 0.95-cm-i.d. tubular quartz reactor. Gas flowing through the reactor was irradiated with almost uniform intensity along the reactor's length at a repetition rate of 0.5 Hz. Laser fluences near 3.5 J cm-2 were used. The gas flowing through the tube contained CFC13 (typically less than 0.1%), C2H6 (;=2%), O2 (typically 0-6%), and a large excess of He or N2 buffer gas (typically >95%). The C 0 2 laser photolysis decomposed CFC13 (=l%/pulse) to yield chlorine atoms CFCl,
+ nhv(1078.6 cm-')
-
CFC1,
+ C1
(4)
which in turn reacted rapidly with C2H6 to produce C2H5 C1 C2H6 HC1+ C2H5 (5)
+
+
The gas mixture flowed through the reactor at 5 m s-l which assured replacement of the photolyzed gas with a fresh gas mixture between laser pulses. The high percentage of inert gas assures negligible heating of the mixture following the relaxation of the undissociated but internally excited C1-atom source. Experiments to measure the C2H5 0,rate constant were performed at total gas densities from 1.4 X 10l6 to 2.5 X 1017molecules cm-3 and at several temperatures between 294 and 1002 K. Gas was continuously sampled from an orifice located at the apex of a cone-shaped hole in the side of the reactor. The cone did not protrude into the reactor. The orifice diameter was 0.044 cm in those experiments performed at densities below 1017 molecules cm-, and 0.022 cm in those conducted at higher densities. The gas emerging from the reactor was formed into a beam
+
(24) Slagle, I. R.; Yamada, F.; Gutman, D. J. Am. Chem. SOC.1981, 103, 149-53. (25) Slagle, I. R.; Gutman, D. J . Am. Chem. SOC.1982, 104, 4741-8.
by a conical skimmer as it entered the vacuum chamber containing a photoionization mass spectrometer. As the gas traversed the ion source, a portion was photoionized (using an atomic resonance lamp) and mass selected by a quadrupole mass filter. In separate experiments, temporal ion signals profiles of reactants and possible products were recorded with a multichannel scalar from a period just before each laser pulse to about 50 ms following the pulse. The resonance radiation energies used to photoionize the molecular and radical species of interest were 11.6 (for C2H4and C2H6), 10.2 (for CFC1, and C2H,C1), and 9.5 (for C,H,) eV. Heating the Tubular Reactor. The quartz tubular reactor was tightly wrapped with a nichrome ribbon (1 .O cm wide, 0.001 cm thick) in a spiral that extended from 19.5 cm upstream to 7.5 cm downstream from the sampling orifice. The space between turnings was less than 1 mm. A small hole was cut in the turning that covered the gas-sampling cone to provide an unimpeded path for the gas to flow from the reactor. No heat shielding was used. The reactor was heated by passing an alternating current (provided by a Variac autotransformer) through the ribbon heater. At one of the highest temperatures of these studies (904 K) the current used was 9 A, and the power dissipated was 200 W. Measurements were made to determine the temperature uniformity along the tubular flow reactor, particularly in the 10cm-long region upstream from the sampling orifice where the gas elements that are sampled during a typical experiment originate. These experiments were done over the full range of densities, temperatures, and flow velocities used in these studies. Measurements were done by using a thermocouple that could be. moved along the axis of the reactor. Below 200 OC the temperature variation is under f 5 T , and above 300 OC it is under f 1 0 OC. Attempts were made to monitor the reactor temperature during or just prior to an experiment with an in situ thermocouple located 2-3 cm downstream from the sampling orifice in a region that is still part of the uniformly heated zone of the reactor, but which is beyond the portion of the reactor from which gas is sampled. Although the temperature could be accurately measured in this manner, this monitoring method could not be used in our experiments. The laser radiation ablated material from the thermocouple which then deposited on the walls of the reactor. This material acted as a catalyst for the heterogeneous destruction of the C2H5 radicals. Temperature was therefore measured on the day before a group of experiments was to be performed (under the exact conditions of each of the experiments to be conducted) by using a thermocouple placed in the center of the 10-cm-long region from which gas is sampled. The thermocouple was then removed. Measurements of the temperatures under the same conditions after the experiments were completed yielded nearly the same values as those recorded before they were conducted. This temperature-measuring procedure has an estimated accuracy of r t l 0 OC below 200 OC and f 1 5 OC above 300 OC. The inside of the tubular reactor was periodically cleaned to minimize heterogeneous free-radical wall-loss processes. The tube was washed with a 5% NH4F.HF solution, rinsed with distilled water, and then dried at 120 OC for 10 h. With this treatment kwall,the first-order C2H5 decay constant in the absence of 02, was typically 60 SI. This rate constant was essentially the same at all temperatures up to 800 K. It actually decreased slightly with increasing temperature probably as a result of the increased removal of adsorbed gases on the reactor wall as the temperature increased. Above 800 K, the first-order rate constant for the radical-removal processes in the absence of O2 increased dramatically with further increases in temperature. This is probably due to the onset of significant unimolecular decomposition of the free radical at these high temperatures. This explanation is indicated not only by the large temperature dependence of kwallat these high temperatures, but also by an observed significant production of C2H4when O2is absent as well as by an increase in kwallwith total gas pressure. Additional wall treatment, e.g., coating the reactor with a thin layer of boric acid, did reduce kwallslightly at low temperatures (to 40-50 s-]) but had a detrimental effect above about 500 K. Above this temperature the apparent wall rate constant increased
3650 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
c 150
--
100
v
:'
Slagle et al.
TABLE I: Conditions and Results of Experiments To Measure the Overall Rate Constant of the CzHS+ Oz Reactiona
c
1013k,
T,
I
K 295 294 295 299 299 384 386 467 467 467 466 466 580 579 640 640 638 640 640 904b 904 904 1002
t 0
0
10
5 [O~IX
15
1uM (rnotec
Figure 1. Plot of first-order rate constants vs. [O,] from the set of experiments conducted at 904 K and [MI = 1.54 X 10I6 molecules The C2HSion signal decay profile is from the experiment with [O,] = 4.1 1 X 1014molecules For conditions and results of experiments, see Table I. more rapidly with time. Inspection of the wall coating after use of the reactor at high temperatures for several days revealed that the boric acid had largely migrated to the very narrow region between turnings of the nichrome ribbon heater. Such coatings were used only in those experiments conducted below 550 K. Source and Purification of Gases. All the carbon-containing compounds used in this study and O2 (Extra Day) were obtained from Matheson. The reactants were condensed at liquid-nitrogen temperature and fractionally distilled (the middle third being retained for these experiments). Helium (Linde, High Purity), nitrogen (Linde, Prepurified), and oxygen were used without additional purification. Results In this study we have measured the overall rate constant ( k ) of the C2H5 O2reaction at several temperatures between 294 and 1002 K. Special emphasis was placed ofl obtaining falloff curves ( k vs. [MI) at four temperatures between 296 and 904 K. Additional experiments were performed to determine the CzH4 product yield from this reaction at three temperatures in order to gain additional information regarding the reaction mechanism. The results of these experiments are presented in this section. Rate Constants of the C2H5 O2 Reaction. Rate constant determinations were done as described p r e v i o u ~ l y . C2H5 ~~~~~ radicals were generated by reactions 4 and 5 in a time that was short (
