J. Phys. Chem. 1995,99, 16301-16306
16301
OH Reaction Kinetics and Atmospheric Lifetime Estimates for Several Hydrofluorocarbons David D. Nelson, Jr.,* Mark S. Zahniser, and Charles E. Kolb Center for Chemical and Environmental Physics, Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821
Hillel Magid Buffalo Research Laboratory, Allied Signal Corp., 20 Peabody St., Buffalo, New York 14210 Received: June 22, 1995; In Final Form: September 5, I995@
The room-temperature rate constants for the reaction of the OH radical with 10 hydrofluorocarbons have been measured using the discharge flow technique with laser-induced fluorescence detection of the OH radicals. ~ = (42 f 4.3) x cm3 molecule-' s-I), CF3CH2The rate constants at T = 298 K are C F ~ C H Z C H(k" cm3 molecule-' s-I), CF~CHFCHZF (k" = (14.8 f 1.7) x CHF'z (k" = (6.6 f0.7) x cm3 molecule-' s-I), CHFZCHFCHFZ(k" = (16.0 f 2.3) x cm3 molecule-' s-I), C F ~ C H Z C (k" F ~ = 0.572;;' x cm3 molecule-' s-'), CF3CHFCHF2 (k" = (5.3 f 0.7) x cm3 molecule-' s-l), C F ~ C H Z C H ~ C (k" F ~= (7.0 f 0.7) x cm3 molecule-' s-I), CF~CFZCHZCHZF (k" = (42.2 f 3.1) x cm3 molecule-' s-'), C F ~ C H ~ C F Z C H Z(k" C F= ~ (2.62 f 0.19) x cm3 molecule-' s-'), C F ~ C F ~ C H Z C H ~ C F(k" Z C= F ~(8.3 f 0.9) x cm3 molecule-' s-'). In addition, the temperature dependence of the rate coefficient was measured for two of the above hydrofluorocarbons. In both cases the temperature dependence is well described by a cm3 molecule-' s-' simple Arrhenius expression, k(T) = A exp[-E/(RT)]. We find that A = 1.74 x and EIR = 1108 K for CF~CFZCHZCHZF and that A = 1.23 x lo-'' cm3 molecule-' s-' and EIR = 1833 K for C F ~ C H Z C F ~ C H Z CThe F ~ . temperature dependent rate coefficients are used to derive atmospheric lifetimes for C F ~ C F ~ C H Z C H and ~ FC F ~ C H Z C F Z C H ~of C F1.2 ~ and 23 years, respectively. We estimate the atmospheric lifetimes of the remaining HFCs based on their estimated OH reaction rates at T = 277 K. The rates at T = 277 K are derived from the room temperature reaction rates and estimates of the activation energies for these reactions. The OH reaction rates of this group of closely related compounds provide a data base for the testing of several OH reaction rate estimation schemes.
I. Introduction The production of chlorofluorocarbons(CFCs) and halons is being eliminated due to their adverse effects on stratospheric o ~ o n e . l - In ~ many applications previously served by CFCs and halons, hydrochlorofluorocarbons (HCFCs) will be used as interim substitutes because of their shorter atmospheric lifetimes and consequent smaller (though still significant) impact on stratospheric ozone! Long term replacements for the CFCs, halons, and HCFCs should have negligible impact on stratospheric ozone. In this regard hydrofluorocarbons (HFCs) are very attractive candidates as permanent replacements since they contain no halogens other than fluorine and are therefore thought to have essentially zero ozone d e p l e t i ~ n . ~There . ~ are many individual applications that require replacement compounds to be utilized as refrigerants, chemical solvents, thermal insulators, fire extinguishers, and cleaning agents. Hence, the chemical and physical properties of a broad range of HFCs are currently of great interest. Although the HFCs are not thought to be a threat to stratospheric ozone, it is still important that we understand the ultimate fate of these molecules since they are released to the atmosphere. A particular point of concern is that the HFCs strongly absorb infrared radiation and contribute to "greenhouse" radiative forcing. A more general motivation is that it is prudent to evaluate the atmospheric fate and the atmospheric lifetime of any species that is released to the atmosphere in significant
* To whom correspondence should be addressed. E-mail: ddn @ aerodyne.com. Abstract published in Advance ACS Abstracts, October 15, 1995. @
0022-3654/95/2099-16301$09.00/0
quantities. The atmospheric lifetimes of the HFCs will be controlled by their rates of reaction with the OH radical. This process will, in general, be much faster than other potential mechanisms such as ultraviolet photolysis or dissolution in the oceans or clouds. For example, the lifetimes of the fluorocarbons, which are otherwise similar to the HFCs, are thought to be thousands of years because they do not react significantly with OHS6 We have measured reaction rate constants for 10 hydrofluorocarbons with the OH radical in order to estimate the atmospheric lifetimes of these species. For eight of these species (CF3CH2CH3, CF3CH$XF2, CF~CHFCHZF, CHFzCHFCHFz, CF3CHzCF3, CF$XFCHFz, C F ~ C H ~ C H Z Cand F ~ CF~CF~CHZ, CHzCFzCF3) we report room temperature rate measurements and for two of the HFCs (CF~CF~CHZCHZF and CF~CHZCFZCHzCF3) we report temperature dependent rate coefficients. These rate constants are important for at least two reasons. First, they are used to estimate atmospheric lifetimes for these HFCs. The lifetimes of these species in the atmosphere is one important variable determining the viability of these particular compounds as candidates for industrial use. Second, these rate constants add to a growing data-base of OH rate constants with HFCs and HCFCs. This database is being used to develop rate constant estimation methods that will allow us to predict OH rate constants with additional HFCs and HCFCs. This information is very important to the efficient selection of replacement compounds for the wide variety of applications mentioned above. In section II we describe the experimental technique employed and provide details concerning the operating conditions and the 0 1995 American Chemical Society
16302 J. Phys. Chem., Vol. 99, No. 44, I995 chemical purity of the samples studied. In section I11 we present the scheme used to analyze the data and the resulting bimolecular rate constants. Finally, in section IV we discuss our results and their atmospheric implications. We also discuss the efficacy of several rate constant estimation methods with respect to the compounds studied in this work. 11. Experimental Procedure
The OH reaction rate measurements were made using the discharge flow technique. In this study, the OH radicals were created in a movable source via the reaction of H atoms with NOz. The H atoms were generated by flowing an HdHe mixture through a 2.5 GHz microwave discharge cavity. In all cases the NO2 concentration exceeded the H atom concentration by a factor of 2-20, and the OH formation reaction was completed within the radical source injector. The OH radicals were mixed with the reactant gas and an excess flow of carrier gas (He) in the main flow tube. This tube (diameter = 2.5 cm) and the injector tube were coated with halocarbon wax to minimize OH wall losses. Flow tube temperature was regulated to within 1 K over a 50 cm reaction zone using a circulating fluid cooling/ heating jacket. The temperature in the reaction zone was monitored using an alumellchrome1 thermocouple. Upon exiting the reactor flow tube, the OH radicals were detected with laser induced fluorescence. The radicals were excited at 282 nm using the Qll line of the 1-0 vibrational band in the A-X electronic transition. The ultraviolet light source was a Molectron nitrogen pumped dye laser whose output was frequency doubled. To enhance the sensitivity for OH detection, the UV laser beam executed 32 passes across the detection region using a Herriott cell mirror arrangement. The OH fluorescence was filtered to discriminate against scattered laser light and detected using the 1- 1 and 0-0 band emission between 300 and 325 nm. The fluorescence photons were collected with appropriate imaging optics and focused onto the cathode of a Hamamatsu R212 UH photomultiplier tube. The signal from the photomultiplier was integrated with a Stanford Research Systems gated integrator and sent to a personal computer with a Lab Master AD data acquisition board for signal processing. This arrangement provided a detection limit for OH radicals of -1 x los cm-3 in a 1 Hz bandwidth at room temperature. This detection limit is valid for OH in a low pressure (4 Torr or less) helium carrier gas where quenching of the OH A state is negligible. The flow rates of the He carrier gas and of the reactants were measured using Tylan mass flow meters. These flow meters were calibrated by measuring the rate of pressure rise in a calibrated volume. Pressure measurements in the flow calibrations as well as in the kinetics experiments were made with an MKS Baratron 10 Ton: capacitance manometer which was calibrated with a McLeod gauge. The helium that passed through the discharge region had a stated purity of 99.999'3 (Northeast Cryogenics) and was further purified by passing it through a liquid nitrogen trap with molecular sieves. The main carrier gas was helium which was delivered with a stated purity of 99.995% and used with no further purification. NO2 (M.G. Industries, 99.5%) was also used as provided. H2 (Air Products, 99.995%) was passed through a liquid nitrogen trap before use. Sample purity is of central importance in these studies, particularly for the slowest reacting species. All of the reactants were provided by Allied Signal Corp. and were extensively purified. Each of the samples studied was analyzed by Allied Signal Corp. using flame ionization gas chromatography and tandem gas chromatographic mass spectroscopy to search for
Nelson et al. and identify remaining impurities. The results of the sample analysis for each compolind follow. In each case we estimate the uncertainty in our reported rate constants due to the presence of the impurities. These uncertainties range from 0.24 to 8%; for all compounds except C F I C H ~ C Fthese ~ uncertainty estimates are much smaller than our random error uncertainties. For both C F ~ C H Z C H and ~ CF3CH2CH2CF3, sample analysis found no evidence of any impurities. In these cases, the detection limit for impurities was -10 ppm. The most likely fast-reacting impurity would be a heavily halogenated alkene.? We assume an upper limit for the room temperature rate constant for the impurity with OH of 1 x lo-" ,mi molecule-' s-I, which implies an upper limit for the resulting error in the reported rate constants of 1 x cm3 molecule-' s-I. This generates an uncertainty in the reported rate constants of 0.2% for C F ~ C H ~ C H and I 1.5% for CF3CH2CH2CFj. For C F ~ C H ~ C H Fsample Z analysis identified three impurities: (1) CH3CHFCl (1 1 ppm), (2) CF3CHFCHFl (175 ppm), and (3) a fluorinated alkene ( < l o ppm). The presence of the first two species will not significantly affect the measured rate constant. The possible presence of the alkene generates an uncertainty in the reported rate constant of 1.64. Analysis of the CF3CHFCH2F revealed the presence of CFjCHFCHF, (31 ppm), CF3CH2CH>CF?(250 ppm), and CFiCHFCH3 (380 ppm). These concentrations are too small to be of significance. In addition, there was an unknown species with a concentration of 40 ppm. We take this as the upper limit for the olefin concentration. This implies an uncertainty in the rate constant of 2.9%. For CHFlCHFCHF? sample analysis found 3 perfluorinated alkanes (total concentration = 330 ppm) and 10 heavily halogenated alkanes (total concentration = 270 ppm). The presence of the perfluorinated species is irrelevant. The partially halogenated alkanes also have little effect (
