J. Phys. Chem. 1995, 99, 5373-5378
5373
Atmospheric Chemistry of HFC-236fa: Spectrokinetic Investigation of the CF3CH02*CF3 Radical, Its Reaction with NO, and the Fate of the CF3CHWCF3 Radical Trine E. Mogelberg, Jesper Platz, Ole J. Nielsen," and Jens Sehested Section for Chemical Reactivity, Environmental Science and Technology Department, Risl National Laboratory, DK-4000 Roskilde, Denmark
Timothy J. Wallington" Ford Research Laboratory, SRL-3083, Ford Motor Company, P.O. Box 2053, Dearbom, Michigan 48121-2053 Received: October 19, 1994; In Final Form: January 18, 1995@
A pulse radiolysis technique was used to investigate the UV absorption spectrum of CF3CHOiCF3 over the wavelength range 220-290 nm. At 250 nm the absorption cross section for CF3CH02'CF3 was (T = (186 f 27) x cm3 molecule-'. The rate constant for the self-reaction rate of CF3CH02TF3 was determined to be (5.6 f 0.7) x lo-', cm3 molecule-' s-I. By following the increase in NO2 at 400 nm, the rate constant for the reaction of CF3CHO,'CF3 with NO was found to be (1.1 f 0.3) x lo-" cm3 molecule-' s-I. The reaction of CF3CHO,'CF3 with NO gives CF3CHO'CS. A Fourier transform infrared technique was used to show that in the atmosphere >99% of CF3CHOCF3 react with 0 2 to give CF3COCF3. The atmospheric fate of CF3COCF3 is photolysis or incorporation into rain-cloud-sea water followed by rapid hydrolysis. As part of the present work relative rate techniques were used to measure rate constants at 295 f 2 K for the reactions of C1 and F atoms with CF3CH2CF3 of 2 x lo-'' cm3 molecule-' at 296 K in 700 Torr total pressure of N2 diluent. At 1-atm pressure of air the partial 0 2 pressure is 160 Torr; under such conditions reaction 20 will account for >99% of the loss of CF3CHDCF3. Finally, it is of interest to consider the carbon balance in the present experiments. As noted above, CF3COCF3 accounts for 81 f 5% of the loss of CF3CH2CF3. In addition to CF3COCF3, trace amounts (2-5% molar yield) of COF2 product was observed. After subtraction of IR features ascribed to CF3COCF3 and C O S , unidentified IR features remained at 1184, 1198, 1295, 1370, and 1924 cm-'. The hydroperoxide, (CF3)2CHOOH, formed in the reaction of (CF&CH02 radicals with
5378 J. Phys. Chem., Vol. 99, No. 15, 1995
HO2 may be a candidate for at least some of these IR features. The aim of the present experiments is to establish the relative importance of reactions 20 and 23 in the atmospheric chemistry of CF3CHOCF3, not to conduct a detailed study the products of reaction 12. Product identification was not pursued further.
4. Discussion Following release to the atmosphere, HFC-236fa is expected to react with OH radicals, with a lifetime greater than 72 years,Is thereby leading to the formation of CF~CHOZ'CF~. It is shown herein that these peroxy radicals react rapidly with NO to give NO2 and, by inference, CF3CHOCF3. Also the peroxy radical may react with H02 in the atmosphere to produce the corresponding hydroperoxide or CF3COCF3. The hydroperoxide will either react with OH to give back the peroxy radical or photolyze to give the alkoxy radical, CF3CHOCF3. As shown in the present work, in the presence of 1 atm of air at 296 K, '99% of CF3CHOCF3 react with 0 2 to give CF3COCF3. Temperature and total pressure both decrease with increasing altitude in the earth's atmosphere. Of these factors temperature is the most important. Reaction 23 is a unimolecular decomposition reaction, and its rate will be very sensitive to changes in temperature. Decreasing temperature will further suppress the importance of pathway 23 relative to 20. We conclude that in the atmosphere >99% of CF3CHOCF3 react with 0 2 to give CF3COCF3. CF3COCF3 displays a weak absorption in the region 300-340 nmI93*O with 4 3 3 4 nm) = 1 x cm2 In the absence of added diluent gases, the quantum yield for CO formation (Le., photolysis of CF3COCF3) following UV irradiation (,I = 313 nm) of CF3COCF3 'is substantial (up to 0.5 at 27 OC).I9 However, the addition of diluent gases such as C02 and CF4 was found to suppress the photolysis quantum yield. At 78 "C hexafluoroacetone molecules excited by absorption of a 3 13 nm photon were found to decompose at a rate of 3 x lo7 s-I.l9 The probability that collision with H2, C02, or CF4 molecules deactivates the excited hexafluoroacetone molecules is essentially 0, 0.3 1, and 0.065, respecti~e1y.I~COz is believed to be a particularly efficient third body because like CFsCOCFs it contains a carbonyl group. While the efficiency of air in quenching excited CF3COCF3 has not been studied, it seems reasonable to suppose that air will be substantially less efficient than CF4. If we arbitrarily suppose that air is 50% less efficient than CF4, then using a gas kinetic collision frequency of 5 x lo9 s-l, the pseudo-first-order rate constant for collisional deactivation of excited CF3COCF3 in the presence of 760 Torr of air is 1.6 x lo8 s-l, Le., about an order of magnitude greater than the decomposition rate. This crude analysis suggests that the photolysis quantum yield is on the order of 0.1. Using an average solar flux of approximately 1 x l O I 4 photons cm-2 s-l nm-' (Continental United States and zenith angle = 0') over the wavelength range 300-340 nm at the earth's surface2I and an average value of a(CF3-
Mogelberg et al. COCH3) = 1.0 x cm2molecule-', we can derive a crude estimate of 4 x s-' for the photolysis rate. This translates into a lifetime of approximately 3 days. CF3COCF3 is known to react vigorously with water.22 The time scale of interaction of gas species with cloud-rain-sea water surfaces in the atmosphere is approximately 5- 10 daysz3 With the available information it is not possible to decide whether photolysis or reaction with water dominates the atmospheric chemistry of CF3COCF3. However, this point is moot, as the same ultimate products are expected from both routes. Photolysis will generate CF3 radicals, which will be converted into CF3O radicals. CF30 radicals will either react with H-containing species such as CH4, or possibly H2O vapor, to give CSOH, or react with NO to give COF?;. The atmospheric chemistry of CF3OH and COFz is dominated by incorporation into rain-sea-cloud water followed by hydrolysis to give HF and CO2.l The same products are expected from the hydrolysis of CF3COCF3.
Acknowledgment. O.J.N. thanks the Commission of the European Communities for financial support. We thank Hillel Magid (Allied Signal Corp.) for helpful discussions. References and Notes (1) Wallington, T. J.; Worsnop, D. G.; Nielsen, 0. J.; Sehested, J.; DeBruyn, W.; Shorter, J. A. Environ. Sci. Techno!. 1994, 28, 320. (2) Nielsen, 0. J.; Gamborg, E.; Sehested, J.; Wallington, T. J.; Hurley, M. D. J. Phvs. Chem. 1994, 98. 9518. (3) Sehested, J.; Nielsen, 0. J.; Wallington, T. J. Chem. Phys. Lett. 1993, 213, 457. (4) Magid, H. Personal communication, 1994. ( 5 ) Hansen, K. B.; Wilbrandt, R.; Pagsberg, P. Rev. Sci. Instrum. 1979, 50, 1532. (6) Nielsen, 0. J. Risg-R-480, 1984. (7) Wallington, T. J.; Japar, S. M. J. Atmos. Chem. 1989, 9, 399. (8) Sehested, J.; Sehested, K.; Nielsen, 0. J.; Wallington, T. J. J . Phys. Chem. 1994, 98, 6731. (9) Ellerman, T.; Sehested, J.; Nielsen, 0. J.; Pagsherg, P.; Wallington, T. J. Chem. Phys. Lett. 1994, 218, 287. (IO) Maricq, M. M.; Szente, J. J. J . Phys. Chem. 1992, 96, 4925. (11) Sehested, J.; Ellerman, T.; Bartkiewicz, E.; Wallington, T. J.; Hurley, M. D. Inr. J . Chem. Kinet. 1993, 25, 701. (12) Wallington, T. J.; Nielsen, 0. J. Chem. Phys. Left. 1991, 187, 33. (13) Rasmussen, 0. L.; Bjergbakke, E. B. Ris~-R395,1984. (14) Sehested, J. Inr. J. Chem. Kinet. 1994, 26, 1023. (15) Wallington, T. J.; Ellerman, T.; Nielsen, 0. J.; Sehested, J. J. Phys. Chem. 1994, 98, 2350. (16) Wallington, T. J.; Hurley, M. D. Chem. Phys. Left. 1992, 189,437. (17) Wallington, T. J.; Hurley, M. D.; Shi, J.; Maricq, M. M.; Sehested, J.; Nielsen, 0. J.; Ellermann, T. Int. J. Chem. Kinet. 1993, 25, 65 1. (18) Huie, R. E. Personal communication, 1994. (19) Ayscough, P. B.; Steacie, E. W. R. Proc. R. Soc. London 1959, A234, 476. (20) Giacometti, G.; Okabe, H.; Steacie, E. W. R. Proc. R. SOC.London 1959, A250, 287. (21) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques; Wiley: New York, 1986; p 110. (22) Aldrich Chemical Company Catalog; Aldrich: Milwaukee, WI. (23) Worsnop, D. G. Private communication, 1994.
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