Atmospheric Chemistry of CF3CFHCF3 (HFC-227ea): Spectrokinetic

and the fire suppressant, 2-H heptafluoropropane. E.P.F. Lee , J.M. Dyke , W.-K. Chow , F.-T. Chau , D.K.W. Mok. Chemical Physics Letters 2006 417...
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8882

J. Phys. Chem. 1996, 100, 8882-8889

Atmospheric Chemistry of CF3CFHCF3 (HFC-227ea): Spectrokinetic Investigation of the CF3CFO2•CF3 Radical, Its Reactions with NO and NO2, and Fate of the CF3CFO•CF3 Radical Trine E. Møgelberg, Jens Sehested,* and Merete Bilde Section for Chemical ReactiVity, EnVironmental Science and Technology Department, Risø National Laboratory, DK-4000 Roskilde, Denmark

Timothy J. Wallington* Ford Research Laboratory, SRL-E3083, Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121-2053

Ole J. Nielsen* Ford Forschungscenter Aachen, Dennewartstrasse 25, D-52068 Aachen, Germany ReceiVed: December 6, 1995; In Final Form: February 21, 1996X

The ultraviolet absorption spectrum of the CF3CFO2•CF3 radical, the kinetics of its self-reaction and reactions with NO and NO2 have been studied in the gas phase at 296K using a pulse radiolysis technique. A longpath-length Fourier transform infrared technique was used to study the fate of the CF3CFO•CF3 radical. Absorption cross sections for the CF3CFO2•CF3 radical were quantified over the wavelength range 220-270 nm. At 230 nm, σ(CF3CFO2•CF3) ) (351 ( 73) × 10-20 cm2 molecule-1. The observed rate constant for the CF3CFO2•CF3 self-reaction was (1.8 ( 0.4) × 10-12 cm3 molecule-1 s-1. The rate constants for the reaction of CF3CFO2•CF3 radicals with NO and NO2 were k3 ) (2.1 ( 0.9) × 10-11 and k4 ) (4.8 ( 1.4) × 10-12 cm3 molecule-1 s-1. The atmospheric fate of CF3CFO•CF3 radicals is decomposition via C-C bond scission to give CF3 radicals and CF3C(O)F. In 1000 mbar of SF6 at 296 K decomposition of CF3CFO•CF3 radicals proceeds at a rate greater than 1 × 105 s-1. As part of this work relative rate techniques were used to measure k(Cl+CF3CFHCF3) ) (4.5 ( 1.2) × 10-17 and k(F+CF3CFHCF3) ) (1.5 ( 0.5) × 10-13 cm3 molecule-1 s-1. This compares well with the value of k(F+CF3CFHCF3) ) (1.9 ( 0.4) × 10-13 cm3 molecule-1 s-1 obtained by pulse radiolysis. The results are discussed in the context of the atmospheric chemistry of HFC-227ea.

Introduction Recognition of the adverse impact of chlorofluorocarbons (CFCs) on stratospheric ozone has led to an international effort to replace CFCs with environmentally acceptable alternatives.1 Hydrofluorocarbons (HFCs) are an important class of CFC replacements used in refrigeration, air conditioning, foam blowing, and cleaning applications. The choice of HFCs is motivated by a number of considerations, not least of which is that unlike CFCs, the HFCs contain at least one C-H bond which makes them susceptible to attack by OH radicals in the lower atmosphere and hence to degradation in the troposphere. As part of a collaborative study of the atmospheric chemistry of HFCs we have undertaken an investigation of HFC-227ea (CF3CFHCF3). HFC-227ea is presently undergoing evaluation as a Halon replacement in firefighting applications and as a replacement refrigerant for CFC-114 in centrifugal chillers.2 The main atmospheric loss mechanism for CF3CFHCF3 is reaction with OH radicals in the troposphere. This reaction produces a fluorinated alkyl radical which will react rapidly with O2 to give a CF3CFO2•CF3 radical:

CF3CFHCF3 + OH f CF3CF•CF3 + H2O •

•

CF3CF CF3 + O2 + M f CF3CFO2 CF3 + M

(1) (2)

By analogy to other peroxy radicals,3 CF3CFO2•CF3 radicals are expected to react with NO, NO2, HO2, or other peroxy * To whom correspondence may be addressed. X Abstract published in AdVance ACS Abstracts, April 15, 1996.

S0022-3654(95)03614-8 CCC: $12.00

radicals in the atmosphere:

CF3CFO2•CF3 + NO f CF3CFO•CF3 + NO2

(3a)

CF3CFO2•CF3 + NO + M f CF3CFONO2CF3 + M (3b) CF3CFO2•CF3 + NO2 + M f CF3CFO2NO2CF3 + M (4) CF3CFO2•CF3 + HO2 f products

(5)

CF3CFO2•CF3 + R′O2 f products

(6)

A pulse radiolysis technique combined with UV-visible spectroscopy was used to determine the UV absorption spectrum of CF3CFO2•CF3 radicals and to study the kinetics of reactions 3, 4, and 6. In the case of reaction 6 we studied the self reaction of the peroxy radical (R′ ) CF3CFO2•CF3). The fate of the alkoxy radical produced in reaction 3a, CF3CFO•CF3, was determined using a FTIR spectrometer coupled to a smog chamber. The results are reported herein. Experimental Section Two different experimental systems were used, both have been described in detail in previous publications4-6 and will only be discussed briefly here. Pulse Radiolysis System. CF3CFO2•CF3 radicals were generated by the radiolysis of SF6/O2/CF3CFHCF3 gas mixtures © 1996 American Chemical Society

Atmospheric Chemistry of CF3CFHCF3

J. Phys. Chem., Vol. 100, No. 21, 1996 8883

in a 1 L stainless steel reactor with a 30 ns 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: 2 MeV e-

SF6 98 F + products

(7)

F + CF3CFHCF3 f CF3CF•CF3 + HF

(8)

CF3CF•CF3 + O2 + M f CF3CFO2•CF3 + M

(2)

The absolute yield of F atoms was determined using the absorbance of CH3O2 radicals at 260 nm following the radiolysis of CH4/O2/SF6 gas mixtures and σ260nm ) 3.18 × 10-18 cm2 molecule-1.7 The derived number for the F atom yield was (3.2 ( 0.3) × 1015 cm3 molecule-1. To monitor the transient UV absorbance, the output of a pulsed 150 W xenon arc lamp was multipassed through the reaction cell using internal White cell optics (80-120 cm pathlength). A McPherson grating spectrometer, a Hamamatsu R 955 photomultiplier, and a Le Croy 9450A digitized oscilloscope were used to detect and record the light intensity at the desired wavelength. The spectral resolution used was 0.8 nm. To obtain the spectrum of the CF3CFO2•CF3 radical, a Princeton Applied Research OMA-II diode array spectrophotometer was used in place of the photomultiplier. The system consisted of the diode array, an image amplifier (type 1420-1024HQ), a controller (type 1421) and a conventional PC computer for handling and storage of the data. Spectral calibration was obtained using a Hg pen ray lamp. Reagent concentrations used were as follows: SF6, 897995 mbar; O2, 0-5 mbar; NO, 0-0.72 mbar; NO2, 0-0.5 mbar; CF3CCl2H, 0-5 mbar; CF3CFHCF3, 0-105 mbar. All experiments were performed at an ambient temperature of 296 K. Ultrahigh-purity O2 was supplied by L’Air Liquide. SF6 (99.9%) was supplied by Gerling and Holz. NO (99.8%) was obtained from Messer Griesheim. NO2 (>98%) was provided by Linde Technische gase, CF3CFHCF3 (99%) and CF3CCl2H (99%) were provided by Fluorochem. All reagents were used as received. Five sets of experiments were performed using the pulse radiolysis system. First, to determine the rate of the reaction of F atoms with CF3CFHCF3, the maximum transient absorbance was observed following the radiolysis of SF6/CF3CCl2H/CF3CFHCF3/O2 mixtures. By variation of the [CF3CCl2H]/[CF3CFHCF3] concentration ratio, the rate of reaction of F atoms with CF3CFHCF3 was measured relative to the reaction of F with CF3CCl2H. Second, the absorption spectrum for CF3CFO2•CF3 radicals was recorded by observing the transient absorbance as a function of wavelength using a diode array camera following the radiolysis of SF6/CF3CFHCF3/O2 mixtures. Third, the rate constant for reaction 6 was determined by observing the rate of the decay of absorption at 230 nm attributed to CF3CFO2•CF3 radicals using long time scales (0500 µs) following the radiolysis of SF6/CF3CFHCF3/O2 mixtures. Fourth, by monitoring NO2 formation at 400 nm following radiolysis of SF6/CF3CFHCF3/O2/NO mixtures a rate constant for the reaction of CF3CFO2•CF3 radicals with NO was determined. Fifth, the rate constant for the reaction of CF3CFO2•CF3 radicals with NO2 was measured by monitoring the rate of NO2 decay at 400 nm using radiolysis of SF6/CF3CFHCF3/O2/NO2 mixtures. FTIR Smog Chamber System. The FTIR system was interfaced to a 140 L Pyrex reactor. Radicals were generated by the UV irradiation of mixtures containing CF3CFHCF3/O2/

Figure 1. Transient absorbance at 230 nm following the pulsed radiolysis of a mixture of 10 mbar of CF3CCl2H, 20 mbar of CF3CFHCF3, and 985 mbar of SF6. The UV path length was 80 cm, and the radiolysis dose was 32% of maximum.

Cl2 or CF3CFHCF3/O2/F2 in 700 Torr total pressure of either air or N2 diluent. The O2 partial pressure was varied over the range 5-700 Torr. Reagent concentrations used were as follows: CF3CFHCF3, 4.7-6.8 mTorr; CF3CF2H, 5.9-15.7 mTorr; CF3H, 2.1-17.6 mTorr; CF3CF2CF2H, 14.1-18.4 mTorr; Cl2, 580-766 mTorr; F2, 621-807 mTorr (760 Torr ) 1013 mbar). UV light (λ > 300 nm) was provided by 22 blacklamps. The loss of reactants and the formation of products were monitored by FTIR spectroscopy, using an analyzing pathlength of 26 m and a resolution of 0.25 cm-1. Infrared spectra were derived from 32 coadded spectra. CF3CFHCF3, CF3C(O)F, and CF3O3CF3 were monitored using their characteristic features over the wavenumber range 1000-1500 cm-1. Reference spectra were acquired by expanding known volumes of reference materials into the reactor. Results and Discussions Reaction of F Atoms with CF3CFHCF3. The kinetics of the reaction of F atoms with CF3CFHCF3 were measured in a series of experiments where CF3CFHCF3/CF3CCl2H/SF6 mixtures were subject to pulse radiolysis. The CF3CCl2H and SF6 concentrations were 1-10 mbar and 990 mbar, respectively, and the CF3CFHCF3 concentration was varied over the range 0-103.7 mbar. Following the radiolysis pulse reactions 8 and 9 compete for the available F atoms:

F + CF3CFHCF3 f CF3CF•CF3 + HF

(8)

F + CF3CCl2H f CF3CCl2 + HF

(9)

The maximum transient absorbance was monitored at 230 nm, and a radiolysis dose of 32% was used. Figure 1 shows an example of an absorption transient. At 230 nm the CF3CCl2 radical absorbs substantially, and a given concentration of CF3CCl2 radicals absorbs more strongly than the same amount of CF3CF•CF3 radicals. Increasing the [CF3CFHCF3]/[CF3CCl2H] ratio results in a decrease in the maximum absorbance. In Figure 2 the measured maximum absorbance is plotted as a function of [CF3CFHCF3]/[CF3CCl2H] ratio. The y-axis intercept in Figure 2 is the absorbance when all F atoms are converted into CF3CCl2 radicals. The maximum transient absorbance decreases until the ratio is about 50. Further increase does not affect the absorbance significantly. The measured

8884 J. Phys. Chem., Vol. 100, No. 21, 1996

Figure 2. Maximum transient absorbance at 230 nm as a function of the [CF3CFHCF3]/[CF3CCl2H] concentration ratio.

Møgelberg et al.

Figure 4. Maximum transient absorbance at 230 nm following the pulsed radiolysis of mixtures of 50 mbar of CF3CFHCF3, 5 mbar of O2, and 945 mbar of SF6 as a function of the radiolysis dose. The UV path length was 80 cm. The solid line is a linear regression of the low dose data (b). The dotted line is a second-order regression fit to the entire data set to aid visual inspection of the data trend.

To work under conditions where the F atoms are converted stoichiometrically into CF3CFO2•CF3 radicals, it is necessary to consider potential interfering secondary chemistry. Potential complications include competition for the available F atoms by reaction with molecular oxygen:

F + O2 + M f FO2 + M

(10)

and unwanted radical-radical reactions such as:

Figure 3. Transient absorbance at 230 nm following the pulsed radiolysis of a mixture of 50 mbar of CF3CFHCF3, 5 mbar of O2, and 945 mbar of SF6. The UV path length was 80 cm and the radiolysis dose was full dose.

maximum absorbance, Amax, is a function of the rate constants for reactions 8 and 9 and the expected absorbances if only CF3CCl2 or CF3CF•CF3 radicals were produced, A(CF3CCl2) and A(CF3CF•CF3), as expressed in the following equation:

Amax ) {A(CF3CCl2) + A(CF3CF•CF3)(k8/k9)[CF3CFHCF3]/ [CF3CCl2H]}/{1 + (k8/k9)[CF3CFHCF3]/[CF3CCl2H]} The rate constant ratio k8/k9 was determined by performing a three-parameter fit of this expression to the data in Figure 2 in which the parameters A(CF3CCl2), A(CF3CF•CF3), and k8/k9 were varied simultaneously. The best fit is shown in Figure 2 and gives k8/k9 ) (0.15 ( 0.03). Using k9 ) 1.2 × 10-12 cm3 molecule-1 s-1 (ref 8) gives k8 ) (1.85 ( 0.37) × 10-13 cm3 molecule-1 s-1 which is in agreement with the value of k8 ) (1.5 ( 0.5) × 10-13 cm3 molecule-1 s-1 obtained using the FTIR relative rate technique described in a subsequent section of this paper. Absorption Spectrum of CF3CFO2•CF3. Following the pulsed radiolysis of mixtures of 50 mbar of CF3CFHCF3, 5 mbar of O2, and 945 mbar of SF6 a rapid increase (completed within 10-20 µs), in UV absorbance in the region 220-270 nm was observed followed by a slower decay, as shown in Figure 3. It seems reasonable to ascribe the UV absorbance resulting from radiolysis of SF6/CF3CFHCF3/O2 mixtures to CF3CFO2•CF3 radicals.

F + CF3CFO2•CF3 f products

(11)

CF3CF•CF3 + CF3CFO2•CF3 f products

(12)

To minimize complications caused by FO2 radicals, experiments were performed with [CF3CFHCF3] ) 50 mbar and [O2] ) 5 mbar. Using k8 ) 1.85 × 10-13 and k10 ) 1.9 × 10-13 cm3 molecule-1 s-1,9 we calculate that 9.1% of the F atoms are converted into FO2 and 90.9% into CF3CFO2•CF3 radicals. Corrections for the presence of 9.1% of FO2 radicals were calculated using σFO2(230nm) ) 508 × 10-20 cm2 molecule-1.9,10 There are no literature data concerning the kinetics of reactions 11 and 12, and hence we cannot calculate their importance. To check for these unwanted radical-radical reactions the transient absorbance at 230 nm was measured in experiments using [CF3CFHCF3] ) 50 mbar, [O2] ) 5 mbar, and [SF6] ) 945 mbar with the radiolysis dose varied over an order of magnitude. The UV path length was 80 cm. Figure 4 shows the observed maximum of the transient absorbance of CF3CFO2•CF3 at 230 nm as a function of the dose. As seen from Figure 4, the maximum absorbance is linear with the radiolysis dose up to about 40% of the maximum dose. At full dose the maximum transient absorbance falls below that expected from a linear extrapolation of the low dose results. We ascribe the curvature in Figure 4 to incomplete conversion of F atoms into CF3CFO2•CF3 radicals caused by secondary radical-radical reactions (eqs 11 and 12) at high initial F atom concentrations. The solid line drawn through the data in Figure 4 is a linear least-squares fit of the low-dose data. The slope is (0.380 ( 0.025). From this value and three additional pieces of information, (i) the yield of F atoms of (3.2 ( 0.3) × 1015 molecules cm-3 (full dose and [SF6] ) 1000 mbar),11 (ii) the conversion

Atmospheric Chemistry of CF3CFHCF3

J. Phys. Chem., Vol. 100, No. 21, 1996 8885

TABLE 1: UV Absorption Cross Sections for CF3CFO2•CF3 Radicals wavelength (nm)

σ × 1020 (cm2 molecule-1)

wavelength (nm)

σ × 1020 (cm2 molecule-1)

220 225 230 235 240

415 394 351 285 227

250 260 270 280

114 44 17 13

of F atoms into 90.9% CF3CFO2•CF3 and 9.1% FO2, (iii) the absorption cross section of FO2 at 230 nm (σ ) 508 × 10-20 cm2 molecule-1 (ref 9)), we derive σ(CF3CFO2•CF2) at 230 nm ) (351 ( 73) × 10-20 cm2 molecule-1. The quoted error includes both statistical uncertainties in the analysis of the data in Figure 4 and potential systematic errors associated with calibration of the F atom yield11 and so reflects the accuracy of the measurement. To map out the spectrum of the CF3CFO2•CF3 radical, experiments were performed to measure the initial absorbance between 220 and 300 nm following the pulsed radiolysis of SF6/ CF3CFHCF3/O2 mixtures. The absorption between 220 and 300 nm was detected with a diode array. The delay was 15 µs, the gate width was 5 µs, and a spectral resolution of 2 nm was applied. The initial absorbances were scaled to that at 230 nm and corrected for FO2 to obtain absolute absorption cross sections. Absorption cross sections are given in Table 1 and shown in Figure 5. The absorption spectrum of CF3CFO2•CF3 is compared to those of the peroxy radicals from two structurally similar HFCs: CF3CF2CF2H (HFC-227ca)12 and CF3CH2CF3 (HFC236fa)13 in Figure 5. As expected, the spectra of these closely related peroxy radicals are very similar in shape. There is some evidence for a slight shift to the blue with increasing F atom substitution. Self-Reaction of the CF3CFO2•CF3 Radical. Following the pulse radiolysis of SF6/CF3CFHCF3/O2 mixtures a fast increase in the absorbance at 230 nm followed by a slower decay was observed. We ascribe this to the self-reaction of the CF3CFO2•CF3 radicals via the reaction

CF3CFO2•CF3 + CF3CFO2•CF3 f products

(13)

The decay followed second-order decay kinetics and was fitted with the expression A(t) ) (A0 - Ainf)/(1+2k13obs(A0 - Ainf)t) + Ainf, where A(t) is the measured absorbance at time t, A0 is the absorbance at time zero, Ainf is the absorbance at infinite time, and k13obs is the observed second-order rate constant for reaction 13. From the fit the half-life t1/2 for the self-reaction was calculated. A series of experiments was performed using SF6/CF3CFHCF3/O2 mixtures with 50 mbar of CF3CFHCF3, 5 mbar of O2, and 945 mbar of SF6. The CF3CFO2•CF3 radical concentration was varied. In addition, a limited number of experiments were performed using full radiolysis dose with the SF6 pressure varied over the range 700-900 mbar. Figure 6 shows the reciprocal of the values of t1/2 obtained as a function of the measured maximum absorbance. A linear least-squares fit through the data in Figure 6 gives a slope of (0.290 ( 0.030) × 105 s-1. Inserting the slope into the following expression:

0.290 × 105 s-1 ) 2k3obs ln 10/(80 cm × σ(230 nm)) where σ(230 nm) ) 351 × 10-20 cm2 molecule-1 and 80 cm is the optical path length, gives k13obs ) (1.8 ( 0.4) × 10-12 cm3 molecule-1 s-1.

Figure 5. Absorption cross section data for CF3CFO2•CF3 radicals measured in this work; the line through the data is a third-order fit through the data to aid visual inspection. Literature data for CF3CHO2CF313 (hollow circles) and CF3CF2CF2O212 (filled circles) are given for comparison.

Figure 6. Plot of 1/t1/2 as a function of Amax at 230 nm.

The self-reaction of CF3CFO2•CF3 radicals gives CF3CFO•CF3 radicals which, as we will show in a latter section, decompose rapidly to give CF3 radicals and CF3C(O)F. CF3 radicals will add O2 to give CF3O2 radicals. The formation of CF3O2 radicals introduces two problems in the interpretation of k13obs. First, since they absorb at the monitoring wavelength of 230 nm, the formation of CF3O2 radicals masks the loss of CF3CFO2•CF3 radicals. Second, CF3O2 radicals will react with CF3CFO2•CF3 radicals, hence complicating the kinetic analysis. At the present time we are unable to correct k13obs for such complications and so we are unable to derive a value for the true bimolecular rate constant k13. Kinetic Data for the Reaction CF3CFO2•CF3 + NO f Products. An increase in the absorbance at 400 nm following the radiolysis of SF6/O2/CF3CFHCF3/NO mixtures was observed. We ascribe this to the formation of NO2 via reaction 3. A series of experiments was performed with the NO concentration varied between 0.24 and 0.72 mbar and the concentrations of O2, CF3CFHCF3, and SF6 fixed at 50, 5, and 945 mbar, respectively. The radiolysis dose used was 40% of maximum. In Figure 7A a typical absorption transient following the radiolysis of a mixture of 0.35 mbar NO, 50 mbar CF3CFHCF3, 5 mbar O2, and 945 mbar SF6 is shown. To obtain a rate constant for reaction 3, each experimental trace was fitted using the Chemsimul Numerical Integration

8886 J. Phys. Chem., Vol. 100, No. 21, 1996

Møgelberg et al.

Figure 8. Plot of k1st versus [NO2].

TABLE 2: Results for k3

Figure 7. (A) Transient absorbance at 400 nm observed following pulsed radiolysis of a mixture of 0.35 mbar of NO, 50 mbar of CF3CFHCF3, 5 mbar of O2, and 945 mbar of SF6. The UV path length was 120 cm. The solid line is a result of a simulation described in the text. (B) Transient absorbance at 400 nm observed following pulsed radiolysis of a mixture of 0.25 mbar of NO2, 50 mbar of CF3CFHCF3, 5 mbar of O2, and 945 mbar of SF6. The UV path length was 120 cm. The solid line is a first-order decay fit which gives k1st ) 3 × 104 s-1.

Program, CHEMSIMUL,14 with a chemical mechanism consisting of reactions 2, 4, 6, 8, and 10, with k2 ) 4 × 10-12 cm3 molecule-1 s-1 (assumed equivalent to the rate for CF3 + O2 (ref 17)), k4 ) 4.8 × 10-12, k6 ) 1.8 × 10-12, k8 ) 1.9 × 10-13 (this work), and k10 ) 1.9 × 10-13.9 The reaction of CF3CFO2•CF3 radicals with NO was assumed to proceed via channel 3a, giving NO2. The reaction of NO with F atoms was included using kF+NO ) 5.1 × 10-12 (average from refs 15 and 16). To reproduce the NO2 yield in the experiments, we had to include rapid decomposition of the CF3CFO•CF3 radical. Without this assumption the model could account only for approximately half of the measured NO2 yield. As shown in a following section, the decomposition of CF3CFO•CF3 radicals gives CF3 radicals. The following reactions of CF3 radicals were included in the model k(CF3 + O2) ) 4 × 10-12,17 k(CF3O2 + NO) ) 1.6 × 10-11,18 k(CF3O2 + NO2) ) 5 × 10-12,7 and k(CF3 + NO) ) 2 × 10-11. In each case the trace obtained from the simulation was fitted to the experimental trace by adjusting k3 to get the best fit by eye. The results are listed in Table 2 and they give an average of k3 ) 2.1 × 10-11 cm3 molecule-1 s-1. We choose to cite an uncertainty that encompasses the extremes in these values giving k3 ) (2.1 ( 0.9) × 10-11 cm3 molecule-1 s-1. For 3 values of the NO concentration, [NO] ) 0.35, 0.52, and 0.61 mbar, the sensitivity of k3 to changes in the rate constants for reactions 2 and 8 was checked. Simulations were performed

[NO], mbar

k3, 10-11 cm3 molecule-1 s-1

[NO], mbar

k3, 10-11 cm3 molecule-1 s-1

0.24 0.32 0.35 0.45

2.2 2.2 2.5 ( 0.5 2.5

0.52 0.58 0.61 0.72

1.9 ( 0.3 2.5 1.5 ( 0.3 1.5

with k2 and k8 simultaneously varied over the ranges (4-8) × 10-12 and (1.3-2.2) × 10-13, respectively. The error limits given in Table 2 show the maximum changes in k3 necessary to maintain adequate fits to the experimental data. As seen from Table 2, k3 is relatively insensitive to changes in k2 and k8. The value of k3 determined here is typical for a reaction of this type.7,18 As discussed above, the observed NO2 yield shows that reaction 3 proceeds essentially entirely via channel 3a to give NO2 and a CF3CFO•CF3 radical that decomposes to give radical species which generate more NO2. From a consideration of the uncertainty on the experimental traces and the quality of the fits to these traces we deduce that k3a/(k3a + k3b) > 0.8. In addition, the decomposition of the CF3CFO•CF3 radical proceeds faster than the rise time of NO2. Hence, the decomposition rate of the CF3CFO•CF3 radical is greater than 1 × 105 s-1. Kinetic Data of the Reaction CF3CFO2•CF3 + NO2 + M f (CF3)2CFO2NO2 + M. Following the radiolysis of NO2/ CF3CFHCF2/O2/SF6 mixtures a decrease in the absorbance was observed at 400 nm. We ascribe this to consumption of NO2 via

CF3CFO2•CF3 + NO2 + M f (CF3)2CFO2NO2 + M

(4)

In Figure 7B the transient absorption observed in an experiment using 0.25 of mbar of NO2, 50 mbar of CF3CFHCF3, 5 mbar of O2, and 945 mbar of SF6 is shown. Reaction 4 was investigated by monitoring the decay at 400 nm following the pulsed radiolysis of mixtures of 50 mbar of CF3CFHCF3, 5 mbar of O2, 945 mbar of SF6, and 0-0.50 mbar of NO2 using a radiolysis dose which was 42% of full dose. Using k8 ) 1.9 × 10-13 and assuming a value of k2 ) 4.0 × 10-12 cm3 molecule-1 s-1 by analogy to the CF3 + O2,17 it follows that in the presence of 50 mbar of CF3CFHCF3 the formation lifetime of CF3CFO2•CF3 is 4 µs. All the transients were fitted using a firstorder decay expression with the fit starting 4 µs after the radiolysis pulse to avoid complications from reactions 2 and 8. The pseudo-first-order rate constants as a function of NO2 concentration are shown in Figure 8. A linear least-squares fit through the data gives a slope of k4 ) (4.8 ( 1.4) × 10-12

Atmospheric Chemistry of CF3CFHCF3

J. Phys. Chem., Vol. 100, No. 21, 1996 8887

Figure 9. Loss of CF3CFHCF3 versus those of CF3CF2H (circles) and CF3CF2CF2H (triangles) when mixtures containing these compounds were exposed to Cl atoms in 700 Torr of N2 (open symbols) or air (closed symbols) diluent.

cm3 molecule-1 s-1, the intercept is (-0.04 ( 0.12) × 104 s-1, and it is not statistically significant. The value of k4 obtained here is typical for a peroxy radical with NO2.7,18 Relative Rate Studies of the Reactions of Cl and F Atoms with CF3CFHCF3. Prior to investigating the atmospheric fate of CF3CFO•CF3 radicals, a series of relative rate experiments were performed using the FTIR system to investigate the kinetics of reactions 15 and 8. The techniques used are described in detail elsewhere.19 Photolysis of molecular halogen was used as a source of halogen atoms:

Cl2 (or F2) + hν f 2Cl (or 2F)

(14)

Cl + CF3CFHCF3 f CF3CF•CF3 + HCl

(15)

F + CF3CFHCF3 f CF3CF•CF3 + HF

(8)

The rate of reaction 15 was measured relative to reactions 16 and 17. The rate of reaction 8 was measured relative to reactions 18 and 19:

Cl + CF3CF2H (HFC-125) f products

(16)

Cl + CF3CF2CF2H (HFC-227ca) f products

(17)

F + CF3CF2H (HFC-125) f products

(18)

F + CF3H f products

(19)

The observed losses of CF3CFHCF3 versus those of reference compounds in the presence of either Cl or F atoms are shown in Figures 9 and 10, respectively. Experiments were performed in 700 Torr of N2 or air diluent at 295 K. Linear least-squares analyses give k15/k16 ) 0.18 ( 0.02, k15/k17 ) 0.13 ( 0.02, k8/k18 ) 0.46 ( 0.03, and k8/k19 ) 0.98 ( 0.10. Using k16 ) 2.5 × 10-16,20 k17 ) 3.4 × 10-16,12 k18 ) 3.5 × 10-13,3 and k19 ) 1.4 × 10-13 (ref 8) gives k15 ) (4.50 ( 0.50) × 10-17, k15 ) (4.42 ( 0.68) × 10-17, k8 ) (1.61 ( 0.11) × 10-13, and k8 ) (1.37 ( 0.14) × 10-13 cm3 molecule-1 s-1, respectively. We estimate that potential systematic errors associated with uncertainties in the Cl and F reference rate constants could add an additional 20% uncertainty range for k15 and k8. Propagating this additional uncertainty gives k15 ) (4.50 ( 1.03) × 10-17, k15 ) (4.42 ( 1.12) × 10-17, k8 ) (1.61 ( 0.34) × 10-13, and k8 ) (1.37 ( 0.31) × 10-13 cm3 molecule-1 s-1. We choose to cite final values of k15 and k8 which are averages of those

Figure 10. Loss of CF3CFHCF3 versus those of CF3CF2H (triangles) and CF3H (circles) when mixtures containing these compounds were exposed to F atoms in 700 Torr of N2 diluent.

determined using the different reference compounds together with error limits which encompass the extremes of the individual determinations. Hence, k15 ) (4.5 ( 1.2) × 10-17 and k8 ) (1.5 ( 0.5) × 10-13 cm3 molecule-1 s-1. Quoted errors reflect the accuracy of our measurements. The value of k8 determined using the FTIR technique is in agreement with the determination of k8 ) (1.9 ( 0.4) × 10-13 cm3 molecule-1 s-1 using the pulse radiolysis technique described in the present work. The value of k15 measured here is consistent with the value of 3.8 × 10-17 cm3 molecule-1 s-1 reported by Zellner et al.21 There are no literature data available for k8 with which to compare our result. The results for k8 and k15 reported here are consistent with the data base for other heavily fluorinated alkanes which react slowly with Cl and F atoms.22 Study of the Atmospheric Fate of CF3CFO•CF3 Radicals. To determine the atmospheric fate of the alkoxy radical CF3CFO•CF3 formed in reaction 3a, experiments were performed in which mixtures of 6.5 mTorr of CF3CFHCF3, 711 mTorr of F2, and 5-147 Torr of O2 in 700 Torr of total pressure of N2 diluent were irradiated in the FTIR-smog chamber system. Loss of HFC-227ea and product formation were monitored using FTIR spectroscopy. Following their formation in the chamber by reactions (8) and (2), CF3CFO2•CF3 radicals will undergo self-reaction to give the alkoxy radical CF3CFO•CF3:

CF3CFO2•CF3 + CF3CFO2•CF3 f 2CF3CFO•CF3 + O2 (13) The aim of this study was to determine the relative importance of reactions 20 and 21 in the atmospheric chemistry of CF3CFO•CF3 radicals:

CF3CFO•CF3 + M f CF3 + CF3C(O)F + M

(20)

CF3CFO•CF3 + M f CF3C(O)CF3 + F + M

(21)

While the heat of formation of CF3CFO•CF3 radicals is, to the best of our knowledge, unknown, thermochemical data exist for the products of reactions 20 and 21: ∆Hf(CF3) ) -111,23 ∆Hf(CF3C(O)F) ) -247,24 ∆Hf(CF3C(O)CF3) ) -334,25 and ∆Hf(F) ) +19 kcal mol-1. These data show that reaction 20 is 43 kcal mol-1 more exothermic than reaction 21. On the basis of the thermochemistry alone, reaction 20 would be expected to be favored over reaction 21. At this point it should be noted that reaction 21 involves the breaking of a C-F bond, whereas reaction 20 proceeds via C-C bond scission. The C-F

8888 J. Phys. Chem., Vol. 100, No. 21, 1996

Møgelberg et al.

Figure 12. Formation of CF3C(O)F versus loss of CF3CFHCF3 following irradiation of CF3CFHCF3/F2/O2/N2 mixtures. The total pressure was 700 Torr, the O2 partial pressure was either 5 (open symbols) or 700 (filled symbols) Torr.

decomposition of the CF3CFO•CF3 radicals via reaction 20. The yield of CF3O3CF3 was not quantified as it is not relevant to determination of the fate of the CF3CFO•CF3 radical:

Figure 11. Infrared spectra acquired before (A) and after (B) irradiation of a CF3CFHCF3/F2/O2 mixture in N2 diluent (see text for details). Panel C shows the result of subtracting features attributable to CF3CFHCF3 from panel B. Panels D and E show reference spectra of CF3C(O)F and CF3O3CF3.

bond is expected to be substantially stronger than the C-C bond and so the activation energy of reaction 21 will be greater than that of reaction 20. Thermodynamic and kinetic factors favor reaction 20. CF3C(O)F product serves as a marker for the importance of reaction 20 while CF3COCF3 is a marker for reaction 21. Figure 11 shows IR spectra acquired before (A) and after (B) a 22 min irradiation of a mixture of 6.5 mTorr of CF3CFHCF3, 711 mTorr of F2, 5 Torr of O2, and 695 Torr of N2. Panel C shows the product spectrum derived by subtracting the IR features attributable to CF3CFHCF3 from panel B. Comparison of the product spectrum with the reference spectrum of CF3C(O)F given in panel D shows that CF3C(O)F is a product. Panel E shows a reference spectrum of the trioxide CF3O3CF3 which was also detected as a product. Figure 12 shows a plot of the formation of CF3C(O)F versus the loss of CF3CFHCF3 observed in the present experiments. Variation of the O2 partial pressure over the range 5-147 Torr had no discernible impact on the CF3C(O)F yield. Linear leastsquares analysis of the data in Figure 13 gives a molar CF3C(O)F yield of 94 ( 6%. Infrared features attributable to CF3COCF3 were sought but not found (the detection limit was 0.16 mTorr for this species). An upper limit of 4% was established for the yield of CF3COCF3. We can conclude that under the present experimental conditions, 700 Torr of N2 at 295 K, reaction 20 is the dominant fate of CF3CFO•CF3 radicals. CF3 radicals formed in reaction 20 will react with O2 to form CF3O2 radicals, which undergo self-reaction to give CF3O radicals which in turn add to CF3O2 radicals to give the trioxide CF3O3CF3 via reactions 22 and 23.26 Hence the observation of CF3O3CF3 product in the present work is consistent with the

CF3O2 + CF3O2 f CF3O + CF3O + O2

(22)

CF3O2 + CF3O f CF3O3CF3

(23)

The observation in the present work that C-C bond scission dominates over C-F bond scission can be attributed to the greater activation barrier associated with C-F bond scission. Temperatures in the atmosphere are typically less than that employed in our experiments (295 K), and such lower temperatures will further favor decomposition via channel 20. We conclude that the atmospheric fate of all CF3CFO•CF3 radicals is C-C bond scission (reaction 20). This conclusion is the same as that reached by Zellner et al.21 following a product study of the Cl atom initiated oxidation of HFC-227ea in air in the presence of NO. Implications for Atmospheric Chemistry. To access the environmental acceptability of CF3CFHCF3 as a CFC replacement, three issues need to be addressed: stratospheric ozone depletion, potential global warming, and formation of toxic/ noxious degradation products. As with all HFCs, HFC-227ea (CF3CFHCF3) has no impact on stratospheric ozone.27 The atmospheric lifetime of HFC-227ea is determined by its reaction with OH radicals which proceeds with a rate constant of k ) 3.7 × 10-13 exp(1615/T) cm3 molecule-1 s-1.28 On the basis of this result the atmospheric lifetime of CF3CFHCF3 is estimated to be 42 years.28 The direct global warming potential of CF3CFHCF3 has been calculated to be 73% of that of CFC11 (CCl3F) on a 100 year time horizon.29 OH radical attack leads to the formation of a peroxy radical. We have shown here that reaction of this peroxy radical with NO is rapid and that the products are NO2 and (by implication) an alkoxy radical CF3CFO•CF3 which decomposes into a CF3C(O)F molecule and a CF3 radical. In the atmosphere CF3 radicals form CF3O radicals which react with hydrocarbons and NO to give CF3OH and COF2, respectively.3 The atmospheric fates of CF3COF, COF2, and CF3OH are dominated by incorporation into cloud-rain-sea water followed by hydrolysis.3 Hydrolysis of CF3COF gives trifluoroacetic acid, CF3COOH. CF3COOH is not toxic toward animals but does have a mild herbicidal effect.3 The concentration of CF3COOH expected in rainfall as a result of the atmospheric degradation of HFCs is orders of magnitude below that observed to have an impact on plant systems.3 However, it has been speculated that over long time periods

Atmospheric Chemistry of CF3CFHCF3 (50-100 years) CF3COOH could accumulate to significant levels in seasonal wetlands.30 The persistence and environmental impact of CF3COOH are uncertain and are the subject of significant research effort.31 Hydrolysis of COF2 and CF3OH give HF and CO2 which are of no environmental concern. Acknowledgment. Financial support for the work at Risø was provided by the Commission of the European Communities. We thank Steve Japar (Ford Motor Company) for a careful reading of the manuscript. References and Notes (1) Alternative Fluorocarbon Environmental Acceptability Study, World Meteorological Organization Global Ozone Research and Monitoring Project, Report No. 20; Scientific Assessment of Stratospheric Ozone, Vols. I and II, 1989. (2) Magid, H. Allied Signal Cor., private communication, 1995. (3) Wallington, T. J.; Schneider, W. F.; Worsnop, D. R.; Nielsen, O. J.; Sehested J.; Debruyn, W. J.; Shorter, J. A. EnViron. Sci. Technol. 1994, 28, 320 (4) Hansen, K. B.; Wilbrandt, R.; Pagsberg, P. ReV. Sci. Instrum. 1979, 50, 1532. (5) Sehested, J. Ph.D. Thesis, Risø-R-804, 1994. (6) Wallington, T. J.; Japar, S. M. J. Atmos. Chem. 1989, 9, 399. (7) Wallington, T. J.; Dagaut, P.; Kurylo, M. J. Chem. ReV. 1992, 92, 667. (8) Wallington, T. J.; Hurley, M. D.; Shi, J.; Maricq, M. M.; Nielsen, O. J.; Ellermann T. Int. J. Chem. Kinet. 1993, 25, 651. (9) Ellermann, T.; Sehested, J.; Nielsen O. J.; Pagsberg P.; Wallington T. J. Chem. Phys. Lett. 1994, 218, 287. (10) Maricq, M. M.; Szente, J. J. J. Phys. Chem. 1995, 96, 4925. (11) Bilde, M.; Møgelberg, T. E.; Sehested, J.; Wallington, T. J.; Nielsen,O. J. J. Phys. Chem., in press. (12) Giessing, A. M. B.; Feilberg, A.; Møgelberg, T. E.; Sehested, J.; Bilde, M.; Wallington, T. J.; Nielsen, O. J. J. Phys. Chem. 1996, 100, 6572. (13) Møgelberg, T. E.; Platz, J.; Nielsen, O. J.; Sehested, J.; Wallington, T. J. J. Phys. Chem. 1995, 99, 5373.

J. Phys. Chem., Vol. 100, No. 21, 1996 8889 (14) Rasmussen, O. L.; Bjergbakke, E. B., Risø-R-395, 1984. (15) Sehested, J. Int. J. Chem. Kinet. 1992, 26, 1023. (16) Wallington, T. J.; Ellermann, T.; Nielsen, O. J.; Sehested, J. J. Phys. Chem. 1994, 98, 2346. (17) Kaiser, E. W.; Wallington, T. J.; Hurley, M. D. Int. J. Chem. Kinet. 1995, 27, 205. (18) Lightfoot, P. D.; Cox, R. A.; Crowley, J. N.; Destriau, M.; Hayman, G. D.; Jenkin, M. E.; Moortgat, G. K.; Zabel, F. Atmos. EnViron. 1992, 26a, 1928. (19) Wallington, T. J.; Hurley, M. D. Chem. Phys. Lett. 1992, 189, 437. (20) Sehested, J.; Ellermann, T.; Nielsen, O. J.; Wallington, T. J.; Hurley, M. D. Int. J. Chem. Kinet. 1993, 25, 701. (21) Zellner, R.; Bednarek, G.; Hoffmann, A.; Kohlmann, J. P.; Mors, V.; Saathoff, H. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 141 (22) Mallard, W. G., Westley, F.; Herron, J. T.; Hampson, R. P. NIST Chemical Kinetics Database, version 6.0, NIST Standard Reference Data, Gaithersburg, MD, 1994. (23) Schneider, W. F.; Wallington, T. J. J. Phys. Chem. 1993, 97, 12873. (24) Dixon, D. A. 1995 private communication, revised slightly from a value of ∆Hf(CF3COF) ) 246 kcal mol-1 reported by Dixon, D. A., Fernandez, R., p 189, University College Dublin, proceedings of STEP Halocside/AFEAS Workshop, Dublin, 1993. (25) Stein, S. E.; Rukkers, J. M.; Brown, R. L., NIST Standard Reference Database 25, Version 1.2, NIST, Gaithersburg, MD, 1991. (26) Nielsen, O. J.; Ellermann, T.; Sehested, J.; Bartkiewicz, E.; Wallington, T. J.; Hurley, M. D. Int. J. Chem. Kinet. 1992, 24, 1009. (27) Wallington, T. J.; Schneider, W. F.; Sehested, J.; Nielsen, O. J. Faraday Discuss. 1996, 100, 55. (28) Nelson, D. D.; Zahniser, M. S.; Kolb, C. E. Geophys. Res. Lett. 1993, 20, 197. (29) Pinnock, S.; Hurley, M. D.; Shine, K. P.; Wallington, T. J.; Smyth, T. J. J. Geophys. Res. 1995, 100, 23227. (30) Tromp, T. K.; Ko, M. K. W.; Rodriguez, J. M.; Sze, N. D. Nature 1995, 376, 327. (31) Visscher, P. T.; Culbertson, C. W.; Oremland, R. S. Nature 1994, 369, 729.

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