J. Phys. Chem. 1985,89, 3335-3337
3335
Flash Photolysis-Laser Induced Fluorescence Study of the Rate Constant for NH2 -I-O2 between 245 and 459 K J. V. Michael: R. B. Klemm: W. D. Brobst,*t S. R. BOSCO,~ and D. F. Nava Astrochemistry Branch, Laboratory for Extraterrestrial Physics, NASAIGoddard Space Flight Center, Greenbelt, Maryland 20771 (Received: November 1 , 1984)
Flash photolysis-laser induced fluorescence experiments were performed to measure the rate constant for the reaction of NH2 with O2as a function of temperature (245, 298, and 459 K). Only upper limits at the three temperatures could be cm3 molecule-' s-I. These results inferred, their respective values being less than 4.6 X lo-", 7.7 X lo-'*, and 5.6 X were obtained under conditions where secondary reactions of NH2 were unimportant. The present work is compared to earlier investigations, and the implications of this result to the oxidation of ammonia are discussed.
Introduction The mechanistic details of the photooxidation of ammonia' are not well understood. Product quantum yields have been measured in two steady-state photolysis studies.z3 There is no question that NHz radicals, which are produced photochemically with unit primary quantum yield, play an important role in the oxidation. Yet a fundamental question that remains is whether N H 2 reacts with O2 a t a significant rate: NH2
+ O2
-
products
(1)
Following Bacon and Duncan,lb Gesser2 assumed that NO + H20 were the products of reaction 1. The observed N z was then explained by the subsequent reaction, NH2 + NO to give N2 + H20! This scheme was questioned in later steady-state photolysis work by Jayanty et al.,3 who suggested that a third-order reaction occurred giving NH202. The Gesser mechanism2 was tested by Levine and Calvert using computer simulations5 for the conditions of the Jayanty et al.3 experiments, and it was concluded that the Gesser scheme gave best agreement with experiment. Ammonia is recognized to be an important atmospheric component. Concentration profiles in the planetary atmosphere are in part dictated by solar photodissociation followed by NH2 oxidation in a large excess of Reaction 1 then may be significant. It may also be important in the high-temperature oxidation of ammonia where a direct bimolecular reaction of NH2 with O2may occur. In flash photolytically induced explosion limit studies, Husain and Norrish* suggested the products HNO + OH, since a branching chain reaction was indicated. Numerous subsequent ammonia studiesg-14 generally have agreed on these products; however, N H HOz production15 was once thought to be important. To date there have been four room-temperature studies1619 that indicate a small rate constant (3.3 X lo-'* Ikl I8.3 X cm3 molecule-' s-' at 298 K) for reaction 1. Three studies1618 utilized either flash photolysis or pulse radiolysis of ammonia to generate NHz, and absorption techniques were used to follow [NH,] temporal behavior. Decay profiles do show oxygen dependence at low [O,],but as [02] is increased, the rate of NH2 removal reaches a maximum value that ultimately becomes independent of [O,]. Computer simulation studies that include the reactions of H atoms (the other primary product from photodissociation) suggest that the most important NH2removal process is NHz H 0 2 and not reaction 1.20 Patrick and Goldenlg photolyzed O3in the presence of O2and NH3 giving O(lD) which subsequently reacted with NH3 yielding N H 2 + OH. Additional 02.637
+
+
Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973. *Departmentof Chemistry, Catholic University of America, Washington, D.C. 20064. Present address: Department of Chemistry, The Johns Hopkins University, Baltimore, MD 20707. 'Visiting Scientist from U S . Air Force Academy, Colorado Springs, CO 80840. Present address: F. J. Seiler Research Laboratory USAFA, CO 80840.
NH2 resulted from the fast reaction (on their time scale) of O H NH3. NH2 was then produced in the absence of H atoms, but the competitive reaction of NH2 + O3 had to be considered. Hence, the earlier rate constant determinations are obtained in complex kinetic systems, and this led Hack et al.z' to carry out the only measurement of kl to date where the kinetic isolation of reaction 1 was seriously attempted. The method used was discharge flow-laser induced fluorescence (DF-LIF). In this study NH2 was produced from the F + NH3 reaction, and thus, H atoms were not present in the system. Secondary reactions of NH2 radicals with themselves or reaction products were also eliminated due to the high sensitivity for N H 2 detection, but wall reaction might have been complicating. Hack et al.21 measured a significant rate constant for the NH2 + O2reaction. They also found reaction 1 to be third order and therefore agreed with the Jayanty et aL3 suggestion. Because of the above-mentioned disagreements on reaction 1 kinetic behavior, we elected to study the reaction with an alternative technique, namely the flash photolysis-laser induced fluorescence (FP-LIF) method. The present experiments could be carried out a t [NH2] levels where reaction 1 was kinetically isolated; Le., secondary reactions involving NH2 were negligible.
+
(1) (a) D. Berthelot and H. Gaudechon, C. R. Hebd. Seances Acad. Sci., 150, 1327 (1910); (b) H. E. Bacon and A. B. F. Duncan, J . Am. Chem. SOC., 56, 336 (1934). (2) H. Gesser, J . Am. Chem. SOC.,77, 2626 (1955). (3) R. K. M. Jayanty, R. Simonaitis, and J. Heicklen, J . Phys. Chem., 80, 433 (1976). (4) C. H. Bamford, Trans. Faraday SOC.,35, 568 (1939). ( 5 ) S. Z. Levine and J. G. Calvert, Chem. Phys. Lett., 46, 81 (1977). (6) J. C. McConnell, J. Geophys. Res., 78, 7812 (1973). (7) M. Nicolet, Can. J. Chem., 52, 1381'(1974). (8) D. Husain and R. G. W. Norrish, Proc. R. SOC.London, A , 273, 145 (1963). (9) D. C. Bull, Combust. Flame, 12, 603 (1968), and references cited therein. (10) W. E. Kaskan and D. E. Hughes, Combust. Flame, 20, 381 (1973), and references cited therein. (11) A. M. Dean, J. E. Hardy, and R. K. Lyon, Symp. (Inr.) Combust. [Proc.],19rh, 1982, 97, (1983), and references cited therein. (12) A. M. Dean, A. J. DeGregoria, J. E. Hardy, and R. K. Lyon, Paper D 41 presented at the 7th International Symposium on Gas Kinetics, Gottingen, Aug 1982. (13) J. A. Miller, M. D. Smooke, R. M. Green, and R. J. Kee, Paper D 42 presented at the 7th International Symposium on Gas Kinetics, Gottingen, Aug 1982. (14) N. Fujii, H. Miyama, M. Koshi, and T. Asaba, Symp. (Inr.) Combusr., [Proc.],18th, 1980, 873 (1981), and references cited therein. (15) T. Takeyama and H. Miyama, J. Chem. Phys., 42, 3737 (1965). (16) R. Lesclaux and M. Demissy, Nouu. J . Chim., 1, 443 (1977). (17) P. B. Pagsberg, J. Eriksen, and H. C. Christensen, J. Phys. Chem., 83, 582 (1979). (18) S. G. Cheskis and 0.M. Sarkisov, Chem. Phys. Lett., 62,72 (1979). (19) R. Patrick and D. M. Golden, J . Phys. Chem., 88, 491 (1984). (20) H. Kurasawa and R. Lesclaux, Paper P4 presented at the 14th Informal Conference on Photochemistry, Newport Beach, CA, March 1980; R. Lesclaux, private communication. Value reported in ref 18 is (2.5 k 0.5) X lo-!' cm3 molecule-' s-!. (21) W. Hack, 0. Horie, and H. Gg. Wagner, J . Phys. Chem., 86, 765 (1982).
This article not subject to US. Copyright. Published 1985 by the American Chemical Society
Michael et al.
3336 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985
- kd Values' as a Function of [O,] and Flash Energy
Experimental Section The experimental results of this study were obtained with the flash photolysis-laser induced fluorescence (FP-LIF) technique. The apparatus has been described in detail previously22 as well as the specific modifications to it for the production and detection of the N H 2 radical.23 This equipment has been used to study the reactions of N H 2 with NO, PH3, C2H2, and C2H4.23 Therefore, only those details specific to the present study are given here. Briefly, an Ar-ion pump laser (Spectra Physics Model No. 170) coupled with a dye laser (Spectra Physics Model No. 375) and tuned precisely to 568.2 or 570.3 nm23 was used to induce the fluorescence of NHz radicalsz4that were produced flash photolytically from NH3. In order to selectively photolyze ammonia and not oxygen (or ethylene used in corroborative studies),25a 206.0-nm interference filter (Acton Research Corp., 35% peak transmittance, fwhm = 40.0 nm) was mounted over the flash lamp for spectral isolation. Fluorescence photons were detected at right angles to both the flash lamp and the incident laser beam through a quartz lens-collimator-polarized window-interference filter (577.7 nm) system with a photomultiplier operating in the multiscaling photon counting mode. For peak operational efficiency, the photomultiplier tube was cooled to about 248 K by flowing chilled dry N2 through a copper coil around the tube. Complications due to secondary product buildup were unimportant even though the repetitive experiments were carried out with static mixtures of premixed gases. This point was tested by varying the number of flashes per fill. Other gas handling techniques used here have been previously described.2z Ammonia (Ideal Gas Products Co., 99.999%) was bulb-to-bulb distilled at 163 K with the middle third of the distillate being retained. Ethylene (Airco, 99.5%) was thoroughly and repeatedly outgassed at 77 K before usage. Oxygen (Matheson, 99.98%) and argon (Scientific Gas Products Inc., 99.9995%) were used without further purification. Reaction gas mixtures were prepared daily.
TABLE I: ka
Results and Discussion Detection of NH2 was accomplished as in other reports from this laboratory23 by LIF under conditions where [NHz]
