J. Phys. Chem. 1994, 98, 5435-5440
5435
Atmospheric Chemistry of HFC- 152: UV Absorption Spectrum of CHzFCFHOz Radicals, Kinetics of the Reaction CH2FCFH02 + NO CHzFCHFO + NO2, and Fate of the Alkoxy Radical CH2FCFHO +
Timothy J. Wallington,' Michael D. Hurley, and James C. Ball Ford Research Laboratory, SRL-E3083, Ford Motor Company, P.O.Box 2053, Dearborn, Michigan 48121 -2053
Thomas Ellermann,+Ole J. Nielsen,. and Jens Sehested Section for Chemical Reactivity, Environmental Science and Technology Department, Risr National Laboratory, DK-4000 Roskilde, Denmark Received: November 9, 1993; In Final Form: January 21, 1994"
The ultraviolet absorption spectrum of CH2FCFH02 radicals and the kinetics and mechanism of their reaction with NO have been studied in the gas phase at 296 K using a pulse radiolysis technique. A long path-length Fourier transform infrared technique was used to study the atmospheric fate of CHzFCFHO radicals. Absorption cross sections were quantified over the wavelength range 220-300 nm. At 240 nm, OCHZFCFH02 = (3.28 f 0.40) X 10-18 cm2 molecule-'. Errors are statistical (2 standard deviations) plus our estimate of potential systematic uncertainty (10%). Monitoring the rate of NO2 formation using a monitoring wavelength of 400 nm allowed a lower limit of k3 > 8.7 X lo-', cm3 molecule-' s-' to be derived for the reaction of CH2FCFH02 radicals with NO. The alkoxy radical CH2FCFHO was found to undergo C-C bond scission rapidly with a rate greater than 6 X lo4 s-l. The C1-atom-initiated oxidation of HFC-152 in air in the presence of NO, gave H C ( 0 ) F as the sole carbon-containing product. The carbon balance was 91 f 10%. Results are discussed with respect to the atmospheric chemistry of HFC-152. As part of the present work, a relative rate technique was used to measure rate constants of (6.7 f 0.8) X and (3.8 f 1.1) X lo-" cm3 molecule-' s-' for the reaction of CH2FCH2F with C1 and F atoms, respectively
Introduction By international agreement, industrial production of chlorofluorocarbons (CFCs) will be phased out. CFC replacements are being sought. Hydrofluorocarbons (HFCs) are one class of potential CFC substituents. Prior to their large scale industrial use, it is important to establish the environmental impact of the release of HFCs. Following release, HFCs will react with OH radicals in the lower atmosphere to produce fluorinated alkyl radicals which will, in turn, react with 0 2 to give peroxy radicals.' For example, in the case of CH2FCH2F (HFC-152),
+
+ -
CH2FCH2F O H CH,FCHFH
+ 0,
+ H,O
CH,FCFH
M
CH,FCFHO,
+M
(1) (2)
As part of a joint program between our two laboratories to survey the atmospheric fate of HFCs,Z-' we have conducted an experimental study of the atmospheric chemistry of HFC-152 (CH2FCH2F). HFC-152 is one of two possible isomers of difluoroethane. The other isomer (CH3CHF2, HFC-152a) is a commercially important CFC replacement. The atmospheric chemistry of HFC-152a is under investigation in our laboratories and will be discussed in a future publication. We report herein results of a study of HFC- 152. A pulse radiolysis technique was used to measure the UV absorption spectrum of CH2FCFH02 radicals and the kinetics and products of reaction 3. The CH,FCFHO,
+ NO
-
CH,FCFHO
+ NO,
(3)
atmospheric fate of CHlFCFHO radicals produced in reaction ~~~~~
~~~~
* Authors to whom correspondence may be addressed. f
Permanent address: NERI, DK-4000, Roskilde, Denmark. Abstract published in Advance ACS Abstracts, April 15, 1994.
0022-3654/94/2098-5435$04.50/0
3 was determined using a FTIR spectrometer coupled to an atmospheric reactor. Results are reported herein. Experimental Section Two different experimental systems were used. Both have been described in detail in previous publication^^.^ and will only be discussed briefly here. Pulse Radiolysis System. CH2FCFH02 radicals were generated by the radiolysis of SF6/02/CHzFCHzF gas mixtures in a 1-L stainless steel reactor with a 30-11s pulse of 2 MeV electrons from a Febetron 705B field emission accelerator. SF6 was always in great excess and was used to generate fluorine atoms:
SF,
-+ + -
+ e-
F
F + CH,FCH,F CH,FCFH
+ 0,
products
+ HF CH,FCFHO, + M
CH,FCFH
M
(4) (5)
(2)
Two sets of experiments were performed using the pulse radiolysis system. First, the ultraviolet absorption spectrum of CH2FCFHOz radicals was determined by observing the maximum in the transient UV absorption at short times ( 1 0 4 0 ps) following the radiolysis of SF6/02/CH2FCH2F mixtures. Second, NO was added to the reaction mixtures, and the rate of NO2 formation following the radiolysis pulse was monitored over longer time scales (0-100 ps) to provide information about the kinetics of reaction 3. To monitor the transient UV absorption, the output of a pulsed 150-W Xenon arc lamp was multipassed through the reaction cell using internal White cell optics (80-cm path length). A McPherson grating spectrometer, Hamamatsu R 955 photomultiplier, and Biomation 8 100 waveform digitizer were used to detect and record the light intensity at the desired wavelength. 0 1994 American Chemical Society
Wallington et al.
5436 The Journal of Physical Chemistry, Vol. 98, No. 21, 1994
The spectral resolution used was 0.8 nm. Reagent concentrations used were as follows: sF6,92&940 mbar; 02,0-80 mbar; NO, 0-0.99 mbar; CHzFCHzF, 20 mbar. All experiments were performed at 296 K. Ultrahigh purity 0 2 was supplied by L'Air Liquide. SFs (99.9%) was supplied by Gerling and Holz. NO (99.8%) was obtained from Messer Griesheim. HFC-152 (CH2FCH2F) was synthesized by the nucleophilic displacement of the p-toluenesulfonate ester of 2-fluoroethanol by fluoride ion.1OHFC152 prepared in this fashion was analyzed by GC-MS and FTIR and was >99% pure. All reagents were used as received. lTIR-Smog Chamber System. The FTIR system was interfaced to a 140-L Pyrex reactor. Radicals were generated by the UV irradiation of mixtures of C H Z F C H ~ F50-100 , mTorr; Cl2, 100-120 mTorr; and NO, 0-47 mTorr in air at 700 Torr total pressure at 296 K using 22 blacklamps (760 Torr = 1000 mbar). The loss of reactants and the formation of products were monitored by FTIR spectroscopy, using an analyzing path length of 26 m and a resolution of 0.25 cm-I. Infrared spectra were derived from 128 co-added spectra. Reference spectra were acquired by expanding known volumes of reference materials into the reactor. Results Absorption Spectrum of CH2FCFHO2. Measurement of the absolute absorption spectrum of the CH2FCFH02 radical requires calibration of the initial F atom concentration. Additionally, experimental conditions are needed such that there is stoichiometric conversion of F atoms to CHzFCFHO2 radicals. The yield of F atoms was established by monitoring the transient absorption at 260 nm due to methylperoxy radicals produced by radiolysis of SFs/CH4/02 mixtures as described previously.lI In the present series of experiments, the yield of F atoms a t 1000 mbar of SF6 was measured to be (2.92 f 0.33) X loi5 ~ m at- ~ full irradiation dose, using a value of 3.18 X le1* cm2molecule-I for u(CH302) at 260 nm.12 As discussed previously,I3 the quoted error in the F atom calibration includes both statistical (2 standard deviations) and potential systematic errors associated with a 10% uncertainty in a(CH302). Errors are propagated using conventional error analysis methods. Following the pulsed radiolysis of mixtures of 20 mbar of CH2FCH2F, 40 mbar of 02, and 940 mbar of SF.5, a rapid increase (complete within 2 ps) in UV absorption in the region 220-300 nm was observed, which was followed by a slower decay. Control experiments were performed in which 1000 mbar Of SF6,20 mbar of CH2FCH2F, or mixtures of 20 mbar of CHzFCHzF and 940 mbar of SF6 were radiolyzed. No significant absorption (XO.02 absorbance units at 220-230 nm and 8.7 X 10-l2 cm3 molecule-' s-l together with an estimated background tropospheric NO concentration of 2.5 X lo8 cm-3,26we calculate the lifetime of CH2FCFH02 radicals with respect to reaction 3 to be
