9518
J, Phys, Chcm, 1994,98, 9518-9525
Atmospheric Chemistry of HFC=143a: Spectrokinetic Investigation of the CF3CH202' Radical, Its Reactions with NO and NO2, and the Fate of CF3CH20 Ole J. Nielsen,' Elisabeth Gamborg,? and Jens Sehested Section for Chemical Reactivity, Environmental Science and Technology Department, Rise National Laboratory, DK-4000 Roskilde, Denmark
Timothy J. Wallington' and Michael D. Hurley Ford Research Laboratory, SRL-E3083, Ford Motor Company, P.O.Box 2053, Dearborn, Michigan 48121 -2053 Received: January 21, 1994; In Final Form: June 6, 1994@
The ultraviolet absorption spectrum of CF3CH202 radicals, the kinetics of their self-reaction, and their reactions with NO and NO2 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 fate of CF3CH20 radicals. Absorption cross sections were quantified over the wavelength range 220-300 nm. At 250 nm, o(CF3CH202) = (2.73 molecule-'. By monitoring the rate of NO2 formation, k4 = (1.2 f 0.3) x lo-" cm3 f 0.31) x molecule-' s-' was found for the reaction of CF3CH202 radical with NO. The reaction of CF3CH202 radicals with NO gives CF3CH20 radicals. In the atmosphere, '99.3% of the CF3CH20 radicals react with 0 2 to cm3 molecule-' s-' was give CF3CHO. By monitoring the rate of NO2 decay, ks = (5.8 f 1.1) x found for the reaction of CF3CH202 radical with NO2. The results are discussed with respect to the atmospheric chemistry of CF3CH3 (HFC-143a). As a part of the present work, relative rate techniques were used to and (2.0 f 0.5) x for the reaction of F measure the following rate constants: (2.6 f 0.7) x atoms with CF3CH3, (5.5 f 0.3) x lo-" for the reaction of F atoms with CF3CH20H, and (3.6 f 0.2) x lo-'' for the reaction with C1 atoms with CF3CH3 (units of cm3 molecule-' s-').
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 substitutes. 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 CF3CH3,
+ OH -CF3CH, + H,O CF,CH, + 0, + M - CF,CH202 + M CF3CH3
(1)
(2)
As part of a joint program between our two laboratories to survey the atmospheric fate of HFCs,,-* we have conducted an experimental study of the atmospheric chemistry of CF3CH3. A pulse radiolysis technique was used to determine the UV absorption spectrum of CF3CH202 radicals and the kinetics of the reaction of CF3CH202 with CF3CH202, NO, and NO,:
+ CF3CH202- products CF3CH,02 + NO - products CF,CH,O, + NO, + M - CF3CH,0,N0, + M CF3CH,0,
(3)
(4)
(5)
The atmospheric fate of CF3CH20 radicals produced in reaction
* Authors to whom correspondence may be addressed. Present address: Institute of Life Sciences and Chemistry, Roskilde University, P . 0 Box 260, DK-4000Roskilde, Denmark. Abstract published in Advance ACS Abstracts, August 15, 1994. +
@
0022-3654/94/2098-9518$04.50/0
4 was determined by using a FTIR spectrometer coupled to an atmospheric reactor. The results are reported herein.
Experimental Section Two different experimental system were used; both have been described in detail in previous publication^^^^^ and will only be discussed briefly here. Pulse Radiolysis System. CF3CH202 radicals were generated by the radiolysis of SFd02ICF3CH3 gas mixtures in a l-L stainless steel reactor with a 30-11s pulse of 2-MeV electrons from a Febetron 705B field emission accelerator. SFg was always in great excess and was used to generate fluorine atoms: SF,
+ e- - F + products
-
F f CF3CH3
CF3CH,
+ HF
+ 0, + M - CF3CH202+ M
CF3CH2
(6) (7) (2)
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 path length). A McPherson grating spectrometer, a Hamamatsu R 955 photomultiplier, and a Biomation 8100 waveform digitizer were used to detect and record the light intensity at the desired wavelength. The spectral resolution used was 0.8 nm. Reagent concentrations used were: SF6,920-960 mbar; 0 2 , 0-50 mbar; NO, 0-0.6 mbar; NO,, 0-0.7 mbar; and CF3CH3, 0-50 mbar. All experiments were performed at an ambient temperature of 296 K. Ultrahigh purity 0 2 was supplied by L' Air Liquide, sF6 (99.97%) was supplied by Gerling and Holz. NO (99.8%) was obtained from Messer Griesheim. NO2 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 38, 1994 9519
Atmospheric Chemistry of HFC-143a ('98%) was provided by Linde Technische gase, and CF3CH3 ('99%) was provided by PCR Inc. All reagents were used as received. Four sets of experiments were performed by using the pulse radiolysis system. First, the ultraviolet absorption spectrum of CF3CH202 radicals were determined by observing the maximum in the transient UV absorbance at short times (40 ps). Second, the decay of CF3CH202 was observed at long times (1000 ps) following the radiolysis of SFdOzICF3CH3 mixtures. Third, NO was added to the reaction mixtures, and the rate of NO2 formation following the radiolysis pulse was monitored to provide information about the kinetics of reaction 4. Fourth, NO2 was added to SFd02ICF3CH3 mixtures, and the decay of NO2 was monitored to determine the rate of reaction 5. FTIR-Smog Chamber System. The FTIR system was interfaced to a 140-L Pyrex reactor. Radicals were generated by the UV irradiation of mixtures of 101-102 mTorr of CF3CH3, 100-120 mTorr of Clz, and 1-147 Torr of 0 2 in 700 Torr of total pressure with N2 diluent at 296 K using 22 blacklamps (760 Torr = 1013 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-'. Infrared spectra were derived from 32 coadded spectra, CF3CH3 and CF3CHO were monitored using their characteristic features over the wavenumber ranges 800-1000 and 13501410 cm-', respectively. Reference spectra were acquired by expanding known volumes of reference materials into the reactor.
Results and Discussion Absorption Spectrum of CF3CH202. Measurement of the absolute absorption spectrum for CF3CH202 radicals requires calibration of the initial F atom concentration. Additionally, experimental conditions are needed for stoichiometric conversion of F atoms to CF3CH202 radicals. The yield of F atoms was established by monitoring the transient absorbance at 260 nm due to methylperoxy radicals produced by radiolysis S F d C W 0 2 mixtures as described previously.11 prior to the present series of experiments, the yield of F atoms at 1000 mbar of SF6 was measured to be (2.77 & 0.30) x 1015 cm-3 at full irradiation dose, using a value of 3.18 x cm2 molecule-' for a(CH3O2) at 260 nm.12 As discussed previo~sly,'~ the quoted error on the F atom calibration includes both statistical (2 standard deviations) and potential systematic errors associated with a 10% uncertainty in a(CH3Oz). Errors are propagated by using conventional error analysis methods and are equal to f 2 standard deviations. Following the pulsed radiolysis of mixtures of 50 mbar of CF3CH3, 20 mbar of 0 2 , and 930 mbar of SFs, a rapid increase (complete within 4-7 ps) in W absorbance in the region 220300 nm was observed followed by a slower decay. Figure 1 shows the transient absorption at 250 nm. It seems reasonable to ascribe the UV absorbance resulting from radiolysis of SFd CF3CH3/02 mixtures to CF3CH202 radicals. Control experiments were performed in which mixtures of 40 mbar of CF3CH3 and 960 mbar of SFs were radiolyzed. A small absorbance, 0.02 absorbance unit, at 220-230 nm was observed upon radiolysis of SFdCF3CH3 mixtures which we ascribe to the CF3CH2 radical. There was no significant absorbance upon radiolysis of SFdCF3CH3 mixtures at wavelengths of 240 nm or longer. By analogy to the reaction of other alkyl radicals with 0 2 , the rate constant for reaction 2 is expected to be approximately cm3 molecule-' s-'.14 In the presence of 20 mbar of 0 2 , the lifetime of CF3CH2 radicals with respect to reaction with 0 2 will be approximately 2 ps. Consistent with
n Q
1
0 00 0
10
20
30
40
T i m e , gs
Figure 1. Transient absorbance at 250 nm following the pulsed radiolysis (full dose) of mixtures of 50 mbar of CF3CH3, 20 mbar of 02,and 930 mbar of SFs.
this calculation, the transient absorbance observed on radiolysis of SFdCF3CH3IOz mixtures with 20 mbar of 0 2 reached a maximum in 4-7 ps. To work under conditions where the F atoms are converted stoichiometrically into CF3CH202 radicals, it is necessary to consider potential interfering secondary chemistry. Potential complications include (i) competition for the available F atoms by reaction with molecular oxygen
F
+ 0, + M -F02 + M
(8)
and (ii) unwanted radical-radical reactions such as
+ CF3CH2- products F + CF3CH202- products F
+
CF3CH2 CF3CH,02
-
(9) (10)
+
CF3CH20 CF3CH20 (1 1)
To minimize the complications caused by FOz radicals, experiments were performed using [CF3CH3] = 50 mbar and [OZ] = 20 mbar. Using rate constants for reactions 7 and 8 measured in our laboratory, k7 = (2.6 f 0.7) x 10-l2 cm3 molecule-' (see subsequent section) and kg = (1.9 f 0.3) x cm3 molecule-' s-',l5 we calculate that 2.8% percent of the F atoms are converted into FOz and 97.2% into CF3CH202. Corrections for the presence of 2.8% FO2 radicals were calculated by using the expression u(CF3CH202) = (a(observed) - 0.028 x a(F02))/0.972. Values of a(F02) are taken from the l i t e r a t ~ r e . ' ~ , ' ~ There are no literature data concerning the kinetics of reactions 9-1 1 hence, we cannot calculate their importance. To check for these unwanted radical-radical reactions, the transient absorbance at 250 nm was measured in experiments using [CF3CH3] = 50 mbar, [02] = 20 mbar, and [SF61 = 930 mbar with the radiolysis dose varied over an order of magnitude. The UV path length was 80 cm. Figure 2 shows the observed maximum of the transient absorbance of CF3CH202 at 250 nm as a function of the dose. As seen from Figure 2, the maximum absorbance is linear with the radiolysis dose up to about 40% of the maximum dose. At maximum dose, the maximum transient absorbance falls below that expected from a linear extrapolation of the low-dose results. We ascribe the curvature in Figure 2 to incomplete conversion of F atoms into CF3CH202 caused by secondary radical-radical reactions (9 - 11) and significant self-reaction of CF3CH202 radicals at high initial radical concentrations.
Nielsen et al,
9520 J. Phys. Chem., Vol. 98, No. 38, 1994
-ua
i
E
t z
500 400 300 200
100 0 200
Relative dose
Figure 2. Maximum transient absorbance at 250 nm following the pulsed radiolysis of mixtures of 50 mbar of CF3CH3, 20 mbar of 02, and 930 mbar of SFs as a function of the radiolysis dose. The W path length was 80 cm. The solid line is a linear regression of the low-dose data (0).The dotted line is a second-order regression fit to the entire data set to aid in visual inspection of the data trend. TABLE 1: Measured UV Absorption Cross Sections wavelength, nm 1O2Ou,cm2molecule-' 220 46 1 230 240 250 260 270 280 290 300
448 316 273
220
The solid line drawn through the data in Figure 2 is a linear least-squares fit of the low-dose data. The slope is 0.242 f 0.008. From this value and three additional pieces of information, (i) the yield of F atoms of (2.77 f 0.30) x 1015molecules cm-3 (full dose and [SF61 = 1000 mbar), (ii) the conversion of F atoms into 97.2% CF3CH202 and 2.8% FOz, and (iii) the absorption cross section for F02 at 250 nm ( a = 1.30 x cm2 molecule-' 15),we derive a(CF3CH202) at 250 nm = (2.73 f 0.31) x cm2 molecule-'. The quoted error includes both statistical and potential systematic errors and so reflects the accuracy of the measurement. To map out the spectrum of the CF3CH202 radical, experiments were performed to measure the initial absorbance between 220 and 300 nm following the pulsed irradiation of SFdCF3CH3/O2 mixtures. The initial absorbancies were scaled to that at 250 nm and corrected for FO2 to obtain absolute absorption cross sections. Absorption cross sections are given in Table 1 and shown in Figure 3. The absorption spectrum of CF3CH202 is compared to those of the peroxy radicals derived from several HFCs in Figure 3. As expected, the spectra of these closely related peroxy radicals are very similar in shape. There is some evidence for a slight shift of the red on progressing along the series CF302,4CF3C F Z O ~CHFZOZ,~ ,~ C F ~ C F H O ZCH2F0z3 ,~ to CF3CH202 (investigated in this work). This shift is consistent with the literature spectral7 concerning halogenated alkyl peroxy absorption spectra where substitution of increasingly electronwithdrawing groups on the carbon bearing the 0-0' group results in a shift of the spectrum to the blue. Reaction of F Atoms with CFJCH~. To measure k7, experiments were performed in which the maximum in the transient absorbance at 250 nm was observed following the radiolysis of SFdCF3CH3/02 mixtures. The radiolysis dose and
280
300
Figure 3. Absorption cross-sectional data for CF3CH202 radicals measured in this work ( 0 ) .The solid line is a fifth-order regression. For comparison, literature cross-sectional data for CF302, CF3CF202, CHF202, CH2F02, and CFsCFHO2 are shown. I
0.10
0
Iu 0
t U
0.08
n L
$
n
006
Q
171 90 42 20 11
240 260 Wavelength, nm
0.04
t
0.02
00
05
1 0
1.5
20
2 5
Figure 4. Plot of maximum absorbance as a function of the concentration ratio [CF3CH#[02]. the SF.5 concentration were held fixed, and the concentrations of CF3CH3 and 0 2 were varied over the ranges 0-20 and 2050 mbar, respectively. Figure 4 shows the observed variation of the maximum absorbance as a function of the concentration ratio [CF3CH3]/ [02]. At 250 nm, FOz absorbs less than CF3CHz02; hence, the absorbance is less at lower CF3CH3 concentrations. As the CF3CH3 concentration is increased, CF3CH202 radicals are formed at the expense of FO2 and the absorption increases. A,, increases until the [CF3CH3]/[02] ratio is about 1. Further increase in the [CF3CH3]/[02] ratio does not affect the maximum absorbance. The solid line in Figure 4 represents a three-parameter fit of the following expression to the data:
1 + (k7/k,)[CF,CH,l/[O,l} where A,, is the observed maximum absorbance, AFO, is the maximum absorbance expected if only FO2 were produced, and A C F ~ C His, the ~ ~ maximum absorbance expected if CF3CH202 were the sole absorbing species. Parameters AFO,, ACF~CH,O~, and k7/k8 were simultaneously varied. The best fit was obtained with k7/k8 = 13.7 & 3.2. Using k8 = 1.9 x lopi3 cm3 molecule-' s-l gives k7 = (2.6 f 0.7) x cm3 molecule-' s-l. This value is used for the calculations in this article. Self-Reaction of the CF3CHz02 Radical. Following the pulsed radiolysis of mixtures of 50 mbar of CH3CH3, 20 mbar of 0 2 , and 930 mbar of SF6, a rapid increase in UV absorbance in the region 220-300 nm was observed followed by a slower
Atmospheric Chemistry of HFC-143a
1
J. Phys. Chem., Vol. 98, No. 38, 1994 9521
0.03
m
(0
0
r'
\ 7
O.O1 0.00
c
//J 0.05
0.00
0.10
Y
0.20
0.15
Absorbancelo
Figure 5. Plot of lltl12as a function of A,, 0.04
0.03
,
,
.
,
,
,
,
[NOIco,,.
at 250 nm. ,
.
,
,
t
-20
+
0
20
40
60
80
mbar
Figure 7. Plot of kist vs [NO]. The closed symbols are the observed and open symbols are data corrected for the influence values for kLSL, of the self-reaction of CF3CH202radicals; see text for details.
100
Time, ps
Figure 6. Transient absorbance at 400 nm observed following pulsed radiolysis (dose = 42% of maximum) of a mixture of 0.31 mbar of NO, 50 mbar of CFjCH3,20 mbar of 0 2 , and 930 mbar of SF36. The UV path length was 120 cm. The initial F atom concentration was . solid line is a 0.42 x 2.77 x loL5x 0.93 = 1.1 x loL5~ m - ~The first-order rise fit which gives kist = 9.13 x 104 s-I.
decay, as shown in Figure 1. As previously discussed, we ascribe the absorbance to the CF3CH202 radical (reaction 3). The observed self-reaction rate constant for reaction 3 is defined as -d[CF3CH~02]ldt = 2k30&F3CH20212. Figure 5 shows the reciprocal half-life for the decay of the absorption at 250 nm as a function of the initial absorbance due to CF3CH202 radicals. The absorbance is corrected for the absorption of FO2. A linear least-squares fit of the corrected data in Figure 5 gives a slope of (0.176 f 0.006) x lo6 s-l = (k3obs x 2 ln(10))/ (c~(CF3CH202)x 80 cm). The intercept of the linear regression of the data is zero within the noise. Using o(CF3CH202) = 2.73 x cm2 molecule-' at 250 nm, k3obs = (8.4 x 1.1) x cm3 molecule-' s-' was derived. k3obs may be an overestimate of the true bimolecular rate constant for reaction 3; as the CF3CH202 radical might react with the reaction products, as discussed in ref 8. Kinetic Data for the Reaction CF3CH202 NO Products. The kinetics of reaction 4 were studied by monitoring the rate of increase in absorbance at 400 nm, attributed to the formation of NOz, following the radiolysis of mixtures of 0.20-0.60 mbar of NO, 50 mbar of CF3CH3, and 20 mbar of 0 2 and SF6 to a total pressure of 1000 mbar. This method of measuring the kinetics of the reaction of peroxy radicals with NO has been used extensively in our laboratory and is discussed 6 shows the results from a in detail e l s e ~ h e r e . ~ Figure ,'~ mixture with [NO] = 0.31 mbar. After radiolysis, CF3CH202 will form within a few microseconds. The smooth curve in Figure 6 is a first-order fit starting at 3 p. The transient is fitted by using the expression A(t) = (A, - Ao)[l -
+
-
exp(-k%)] Ao, where A(t) is the absorbance as a function of time, A, is the absorbance at infinite time, kist is the pseudofist-order appearance rate of NO2, and A0 is the extrapolated absorbance at t = 0. For the data shown in Figure 6, kist = 9.13 x lo4 s-l. In all cases, the rise in absorbance was well fitted by first-order kinetics. It seems reasonable to conclude that NO2 is the species responsible for the absorbance change following radiolysis of SFdCF3CH3INO mixtures. As seen from Figure 7, the pseudo-first-order rate constant, kist, increased linearly with [NO]. The initial F atom concentration employed in the present experiments (1.1 x ~m-~) is a significant fraction (7-22%) of that of the initial NO concentration, and deviations from pseudo-first-order kinetics may be expected. However, no such deviations were discernible within the experimental data scatter. Corrections have been applied to the [NO] values given in Figure 7 to account for consumption of NO during the reaction. Corrections were computed by using the expression [NO],,, = [NO], - [ q J 2 and were in the range 4- 11%. Linear least-squares analysis gives k4 = (1.13 k 0.10) x cm3 molecule-' s-'. The y-axis intercept in Figure 7 is (1.2 f 1.O) x lo4 s-l. The origin of this intercept may be a small contribution to the CF3CH202 decay caused by the self-reaction of CF3CH202 radicals. To assess the impact of the CF3CH202 on the measured value of k4, the formation of NO2 was modeled by using the Acuchem chemical kinetic modeling packageI8 with a mechanism consisting of reactions 3 and 4 with k3 = 8.5 x lo-'* and k4 = 1.1 x cm3 molecule-' s-'. A first-order fit was made to the simulated NO2 results. In all cases, the simulated data were well fit by first-order kinetics. The pseudo-first-order rates of NO2 formation were slightly highly than the expected values given by k4[NO],,,; the difference increased with decreasing [NO],,,. Corrections for the effect of the self-reaction on our measured kinetics for the NO2 formation were computed by detailed modeling of each data point. Corrected data are shown in Figure 7. Linear least-squares analysis of the corrected data gives k4 = (1.19 f 0.10) x lo-" cm3 molecule-' s-l with no significant y-axis intercept. We estimate that potential systematic errors could add an additional 20% to the uncertainty range of k4. Propagating this additional 20% uncertainty gives k4 = (1.19 0.26) x lo-" cm3 molecule-' s-'. The increase in absorbance at 400 nm can be combined with the literature value of a(N02)(400 nm) = 6.0 x cm2 molecule-' l9 to calculate the yield of NO2 in this system. The yield of NO2 in the five experiments given in Figure 7, expressed as molecules of NO2 produce per mole of F atoms consumed, was 73% f 13%. In the calculation, corrections were made
Nielsen et al.
9522 J. Phys. Chem., Vol. 98, No. 38, 1994
for the loss of F atoms, via reaction 8 using ks = 1.9 x lowi3 cm3 molecule-' SKI and via reaction with NO producing FNO, using k(F NO-FNO) = 5.5 x lo-', cm3 molecule-' s-'.13 The absorption of FNO at 400 nm is negligible.,O The fact that the yield of NO2 is less than 100% is not unexpected. The self-reaction of CF3CH202 radicals is sufficiently rapid that a significant fraction of the CF3CH202 radicals undergo selfreaction. Using a chemical mechanism consisting of reactions 3 and 4 with k3 = 8.4 x and k4 = 1.1 x lo-" cm3 molecule-' s-l and assuming that reaction 4 gives exclusively NO,, then the NO2 yield, calculated using the Acuchem modeling packaging,IS varies from 79% to 92% as the NO is varied from 0.2 to 0.6 mbar. While no experimental evidence was observed that the NO2 yield was dependent on the initial NO concentration, our experimental uncertainties were sufficiently large that such a relatively small effect would go undetected. Within the experimental uncertainties, the yield of NO2 was independent of the initial NO concentration, suggesting that CF3CH20 radicals do not scavenge NO2 on the time scale of the present experiments. While the results from the present work do not preclude the possibility of the presence of a minor channel leading to products other than N02, it is clear that the majority of the reaction of CF3CH202 radicals with NO gives NO2 and (by inference) CF3CH20 radicals. The kinetics and products of the reaction of CF3CH202 radicals with NO measured in the present work are consistent with the available literature data base for the reaction of halogenated peroxy radicals with NO.I4 Kinetic Data of the Reaction CF3CHzOz NO2 M Products M. The kinetics of reaction 5 were studied by monitoring the absorbance at 400 nm following the radiolysis of mixtures of 50 mbar of CF3CH3, 20 mbar of 0 2 , and 0.30.7 mbar NO2 and SF6 to 1000 mbar of total pressure. Figure 8 shows the absorbance as a function of time after radiolysis of a mixture with [NO,] = 0.5 mbar. The absorbance before radiolysis is due solely to the N02. The rate of the decay of absorbance was shown to increase with increasing NO2 concentration. It seems reasonable to explain the transient in Figure 8 by loss of NO2. Three reactions could be responsible:
+
+
+
+
+ NO, + M - FNO,/FONO + M CF3CH, + NO2 - products CF,CH,O, + NO, + M - CF,CH,O,NO, + M F
-
(12) (13)
(5)
By using high concentrations of CF3CH3 and 0 2 . reactions 12 and 13 will have negligible influence, as discussed previously. The smooth curve in Figure 8 is a frrst-order fit of the transient from 4 ys. The curve gives a pseudo-first-order rate constant, kist. The initial F atom concentration employed in the present experiments (1.1 x ~ m - is ~ )a significant fraction (615%) of that of the initial NO:! concentration, and deviations from pseudo-first-order kinetics may be expected; no such deviations were discemible within the experimental data scatter. Corrections have been applied to the [NO21 values given in Figure 9 to account for consumption of NO2 during the reaction. Corrections were computed using the expression [ N O Z ] ~= ,~ [NO,], - [F]d2 and were in the range 3-8%. Linear leastsquares analysis of the data in Figure 9 gives k5 = (5.63 f 1.12) x lo-'* cm3 molecule-' s-l. To assess the impact of the CF3CH202 self-reaction on the measured value of k5, the loss of NO;! was modeled by using the Acuchem chemical kinetic program18 with a mechanism consisting of reactions 3 and 5 with k3 = 8.5 x lo-', and k5 = 5.6 x lo-', cm3
0.00
-0.01 P)
V
; C
-0.02
0
vr
13 Q
-0.03
-0.04
-0.05 -20
0
40
20
Time,
60
80
'00
JLS
Figure 8. Maximum transient absorbance at 400 nm observed following pulsed radiolysis (dose = 42% of maximum) of a mixture of 0.5 mbar of NOz, 50 mbar of CF3CH3, 20 mbar of 0 2 , and 930 mbar of SFs. The W path length was 120 cm. The initial F atom concentration was 0.42 x 2.77 x loi5 x 0.93 = 1.1 x loi5~ m - The ~. solid line is a first-order rise fit which gives kist = 8.01 x lo4 s-l.
molecule-' s-'. A first-order fit was made to the simulated NO2 results. In all cases, the simulated data were well fit by first-order kinetics. The pseudo-first-order rates of NO2 loss were larger than the expected values given by ks[N02],,,; the difference increased with decreasing [N021com Corrections for the effect of the CF~CHZOZ self-reaction were computed by detailed modeling of each data point. Corrected data are shown in Figure 9. Linear least-squares analysis of the corrected data gives k5 = (5.81 f 1.14) x cm3 molecule-' s-'. Kinetics of the Reactions of F and C1 Atoms with CF3CH3. Prior to investigating the products arising from the F atom initiated oxidation of CF3CH3 in air, a series of relative rate experiments were performed using the FTIR setup to investigate the kinetics of the reactions of F and C1 atoms with CF3CH3. The relative rate technique is described in detail elsewhere.21~22 Photolysis of the molecular halogen was used as a source of C1 or F atoms. C1, (F,)
+ hv - 2C1(2F)
(14)
The kinetics of reaction 15 were measured relative to reaction 16. The kinetics of reaction 7 were measured relative to those of reactions 17 and 18.
-
C1+ CF3CH3 C1+ CF,ClCH,
F
F
+ CF3CH3 F + CH,F,
+ CF3CFH2
+ HC1 CF,ClCH, + HC1 CF3CH, + HF CHF, + HF CFJFH + HF CF3CH,
(15) (16)
(7) (17)
(18)
The observed loss of CF3CH3 vs that of CF2CICH3 following the W irradiation of CF3CH3/CFZClCH3/C12 mixtures in 700 Torr of total pressure of N2 or air diluent is shown in Figure 10. Linear least-squares analysis of the data in Figure 10 gives k15/k16 = 0.094 f 0.006. Using k16 = 3.8 x 1O-'6 cm3 molecule-' s-l (average from refs 21 and 23) gives k15 = (3.6 f 0.2) x lo-'' cm3 molecule-' s-'. We estimate that potential systematic errors associated with uncertainties in the reference rate constant could add an additional 20% to the uncertainty range. Propagating this additional 20% uncertainty gives k15 = (3.6 f 0.8) x cm3 molecule-' s-l. This result can be
J. Phys. Chem., Vol. 98, No. 38, 1994 9523
Atmospheric Chemistry of HFC-143a
ratios of k7/k17 = 0.44 f 0.02 and k7/k18 = 1.55 k 0.06 are derived. The reactivity of both CHzF2 and CF3CFH2 toward F atoms has been the subject of a recent investigation in our laboratories.22 The rate constants k17 and k1g were measured relative to k19; k17/k19 = 0.063 f 0.004 and klgIk19 = 0.019 & 0.002.22
12
10
-
8
*
01
0
-
6
F
c yi
+ CH, -.CH3 + HF
(19)
x- 4 2
I 0.0
0.2
0.4
0.6
0.8
[ N O 2 l c o r r ' mbar
Figure 9. Plot of kist vs [NOz]. The closed symbols are the observed values for k'", and open symbols are data corrected for the influence of the self-reaction of CF3CH202 radicals; see text for details. 0.20
,
I
1
n
m
I
0.16
Om LL
0
-> Y
0.12
m
I
Om
0.08
G
0 v Y
0.04
c J
0 00
Figure 10. Plot of the loss of CF3CH3 vs that of CFzClCH3 following H ~ C ~ Zin the continuous W irradiation of C F ~ C H ~ / C F ~ C ~ Cmixtures 700 Torr of either air (0)or NZ(0)diluent. The line is a linear leastsquares analysis. 1.2
,--. --
1.0
LL
0.8
0
> Y
n
0.6
m
I 0
m 0.4
LL
0 Y v
c
J
+
+
I Om
These rate constants ratios can be combined with the determinations of k7/k17 = 0.44 f 0.02 and k7/k18 = 1.55 f 0.06 in the present work to derive values of k7/k19 = 0.028 f 0.002 and 0.029 f 0.003, respectively. Errors were propagated by using conventional error analysis. It is gratifying to note the excellent internal consistency in the body of relative rate data. To arrive at a final value, we choose to quote the average of the two independent relative rate determinations of k7/k19 with error limits which encompass the extremes of both ranges; k7/k19 = 0.029 f 0.003. Using k19 = (6.8 f 1.4) x lo-" cm3 molecule-' s-1,22 we derive k7 = (2.0 f 0.5) x 10-l2 cm3 molecule-' s-'. Quoted errors reflect statistical uncertainties together with potential systematic errors associated with the uncertainty in k19. This result is in good agreement with the value of k7 = (2.6 f0.7) x cm3 molecule-' s-l measured by using the pulse radiolysis technique as part of the present work. The kinetics of reaction 7 have been studied previously by Manning et al.,26 Williams and R ~ w l a n dand , ~ ~Kaiser28using relative rate methods. Manning et a1.26used H2 as the reference compound, Williams and R ~ w l a n dused ~ ~ C2H2, and Kaiser28 used HFC-152a. To the best of our knowledge, there are no independent kinetic data available concerning the kinetics of the reaction of F atoms with C2H2, so it is not possible to place the rate constant ratio measured by Williams and R ~ w l a n d ~ ~ on an absolute basis. In contrast, kinetic data for the reaction of F atoms with H2 and HFC-152a are available. Using k(F H2) = 2.7 x lo-" cm3 molecule-' s-' l9 to place the relative data from Manning et al.26on an absolute basis gives k7 = 2.5 x cm3 molecule-' s-l, in good agreement with the results from the present work. By using k(F HFC-152a) = 1.7 x lo-" cm3 molecule-' s-1,22 the data from Kaiser28 gives k7 = (1.1 f 0.5) x cm3 molecule-' s-l. This result is a factor of 2 lower than measured in our work. However, it should be noted that the two rate constants agree within the maximum of their stated error limits. As discussed by Kaiser,28measurement of the reactivity of F atoms toward CF3CH3 relative to the reactivity of CH3CHF2 (HFC-152a) is complicated by the production of CF3CH3 from reaction of CH3CF2 radicals (produced from F HFC-152a) with molecular fluorine. Kaiser28 noted that an accurate determination of k7 requires use of a reference compound other than HFC-152a. The present work fulfills this requirement and so supersedes the previous determination by Kaiser.28 Study of the Atmospheric Fate of CF3CH20 Radicals. To determine the atmospheric fate of the alkoxy radical CF3CH20 formed in reaction 4 (the RO2 NO reaction), experiments were performed in which F2/CF3CH3/02 mixtures at a total pressure of 700 Torr made up with N2 diluent were irradiated in the FTIR-smog chamber system. The loss of CF3CH3 and the formation of products were monitored by FTIR spectroscopy. Three sets of experiments were performed with 0 2 partial pressures of 1, 5,and 147 Torr. In all cases, CF3CHO was the major product. Figure 12 shows a plot of the observed yield of CF3CHO vs the loss of CF3CH3. For all of the experiments shown in Figure 12, [CF3CH3]0 was 101-102 mTorr. As shown in Figure 12, the observed molar yield of CF3CHO decreased
0.2
+
0.0 C 1
0.5
1 .o
1.5
2.0
Ln ([Reterence],,/[Reference],)
Figure 11. Plot of the loss of CF3CH3 vs that of CF3CFHz and CH2F2 and following the continuous U V irradiation of CF~CH~/CF~CFH~/FZ CF3CH3/CHzF2/F2 mixtures in 700 Torr of air diluent. Solid lines are linear least-squares analyses. compared to previous determinations of k15 = (2.3 f 1.3) x and k15 = 1.3 x cm3 molecule-' s-' by TschuikowRoux et aL2, and Cadman et al.,25 respectively. Within the combined experimental uncertainties, the results from the present work are consistent with those reported by Tschuikow-Roux et aL2, but are substantially lower than the value of Cadman et al.25
Figure 11 shows the observed decay of CF3CH3, CH2F2, and CFsCFH2 when mixtures of these compounds were exposed to F atoms in air diluent. From the data in Figure 11, rate constant
+
9524 J. Phys. Chem., Vol. 98, No. 38, 1994
Nielsen et al.
with increasing consumption of CF3CH3. Such a trend is indicative of secondary reactions consuming the CF3CHO product. This behavior is expected based on the fact that CF3CHO reacts considerably more rapidly with F atoms than does CF3CH3. In the present work, a rate constant ratio of k7/k19 = 0.029 zt 0.003 was derived.
F
+ CF3CH0 - CF3C0 + HF
(20)
The rate constant ratio kzdk19 = 0.388 & 0.012 has been reported recently,29 so kzdk7 = 13 & 2. This ratio was used together with the Acuchem chemical kinetic modeling program*8 to calculate appropriate corrections for the data shown in Figure 12. Correction factors applied to the data ranged from 1.1 to 3.2. Corrected and uncorrected data are shown in Figure 12. At this point, it is relevant to consider other possible losses of CF3CHO in the chamber. Potential loss mechanisms include photolysis, heterogeneous loss to the chamber walls, and reaction with molecular fluorine. To test for such complications, mixtures of CF3CHO in air were prepared and irradiated for 2 min in the absence of Fz. Also, mixtures of CF3CHO with F2 were prepared and left to stand in the dark for 10 min. In both cases, there was no observable loss of CF3CHO (el%). There are several points of interest regarding the data shown in Figure 12. First, within the experimental errors, the corrected data all lie on one straight line. As noted above, the corrections applied range from 10% to a factor of 3.2. The fact that the data obtained at large CF3CH3 consumptions are consistent with a linear extrapolation of the data obtained using low conversions lends confidence to the rate constant ratio k2dk7 used to compute the corrections. Second, CF3CHO is a major product of the F atom initiated oxidation of CF3CH3. Linear least-squares analysis of the composite data set in Figure 12 gives a molar CF3CHO yield of 101% zt 8% (the y-axis intercept is -0.7 k 0.8 and is not significant). Third, there is not discemible effect of the 0 2 partial pressure on the yield of CF3CHO. There are two possible sources of CF3CHO in the present chemical system; reaction 3b and reaction 21. CF3CH,0,
+ CF3CH,0, - CF3CH,0 + CF3CH,0 + 0,
(34
CF3CH20,
+ CF3CH20, -CF3CH,0H + CF3CH0 + 0, CF3CH,0
+ 0, -CF3CH0 + HO,
(3b) (21)
The observation of a CF3CHO yield which is significantly in excess of 50% suggests but does not prove that reaction 21 is the major source of CF3CHO. It is possible that channel 3b together with reactions 22 and 23 could contribute significantly to the observed CF3CHO yield.
-
+ CF3CH20H CF3CHOH + HF CF3CHOH + 0, - CF3CH0 + HO,
F
(22) (23)
There was no observable CF3CH20H product in experiments conducted in the presence of 147 or 5 Torr of 0 2 . Interestingly, CF3CH20H was observed as a product in experiments using 1 Torr of 0 2 . At the lowest conversions of CF3CH3, the yield of CF3CH20H dropped substantially. This dependence of yield on conversion of CF3CH3 is suggestive of loss of CF3CHzOH via reaction 22. To investigate this possibility, a relative rate method was used to measure the rate constant ratio k22/k19 = 0.81 f 0.04. Combining this ratio with k19 = 6.8 x lo-" 22 gives k22 = (5.5 f 0.3) x lo-" cm3 molecule-' s-l. F atoms
O
I &" I
L
G
'
5
10
A[CF,C.I,]
'5
iC
25
(mtorr)
Figure 12. Plot of the observed yield of CF3CHO (open symbols) vs the loss of CF3CH3 following the irradiation of mixtures of 101-102 mTorr of CF3CH3,Fz and 1 (diamonds), 5 (triangles), or 147 (circles) Torr of 0 2 at 700 Torr of total pressure in N2 diluent at 296 K. Filled symbols are data corrected for the reaction of F atoms with CFXHO; see text for details.
react approximately 30 times more rapidly with CF3CH20H than with CF3CH3. Hence, it is not surprising that where CF3CH2OH is an observed product, the measured yields of this species are sensitive to the degree of conversion of CF3CH3 over the range used (2-20%). The aim of the present experiments was to establish the relative importance of reactions 21 and 24 in the atmospheric chemistry of CF3CH20 radicals.
+ 0, - CF3CH0 + HO, CF3CH,0 + M CF3 + HC(0)H + M CF3CH,0
-
(21) (24)
The CF3CHO product serves as a tracer for the importance of reaction 21. However, as discussed above, it is possible that CF3CHzOH is a significant product of reaction 3. For the experimental conditions pertaining to the data shown in Figure 12, any CF3CH20H formed will rapidly react with F atoms and probably give CF3CHO. To evaluate this potential complication, the relative importance of channels 3a and 3b needs to be determined. To provide such information, experiments were performed in which a mixture of 1 Torr of CF3CH3 and 0.15 Torr of F2 in 700 Torr of air was irradiated. The resulting product mixtures were analyzed for CF3CHO and CF3CH20H. While CF3CHO was observed, no CF3CH20H was detected. The CF3CH20H yield was less than 6% of that of CF3CHO. The conversion of CF3CH3 (as measured by the observed yield of CSCHO) was '0.5%. With such a low conversion of CF3CH3, any loss of CF3CHzOH (or CF3CHO) via reaction with F atoms will be of minor importance. It seems reasonable to ascribe the relative yields of CF3CHO and CF3CH20H to the relative importance of channels 3a and 3b. Taking into account the small loss of CF3CHO and possible loss of CF3CH20H via reaction with F atoms gives a branching ratio of k3dk3a < 0.077. In this calculation, it is implicitly assumed that all CF3CH20 radicals formed in reaction channel 3a react with 0 2 and do not decompose. This assumption does not impact the upper limit derived above. As seen from Figure 12, the yield of CF3CHO following irradiation of C F ~ C H ~ / F ~ / Omixtures ~ / N Z with [02] = 5 or 147 Torr is indistinguishable from 100%. If we assume that 100% of reaction 3 proceeds via channel 3a and that the combined random and systematic uncertainty of the determination of the CF3CHO yield is f20%, then in the presence of 5 Torr of 0 2 at least 80% of the CF3CH20 radicals generated in the system must
Atmospheric Chemistry of HFC-143a
J. Phys. Chem., Vol. 98, No. 38, 1994 9525
react with 0 2 in preference to decomposition via reactionz4. Hence, at 700 Torr in N2 diluent at 296 f 2 K, k21/k24 > 2.5 x cm3 molecule-'. Having established that reaction 3b is of no consequence, it is interesting to consider the origin of the CF3CH2OH product observed in the experiments in which 1 Torr of 0 2 was used. It is well established that CF3O radicals readily abstract H atoms from H-containing compounds to form CF30H.6,30 The same reaction is possible for CF3CH20 radicals. CF3CH3 is the most likely H atom donor in the present experiment system.
+
-
CF3CH20 CF3CH3
+
CF3CH20H CF3CH, (25)
Reaction 25 has to compete with reaction 21 for the available CF3CH20 radicals. The rate of reaction 21 will be directly proportional to [02]. Hence, with [02] = 147 and 5 Torr, it is possible that reaction 21 dominates the loss of CF3CH20 radicals. However, by using [02] = 1 Torr, reaction 21 is sufficiently slow that an appreciable fraction of the CF3CH20 radicals react via reaction 25. If this hypothesis is correct, then the CF3CH20H yield in the experiments employing an oxygen partial pressure of 5 Torr should be approximately 5 times less than observed in the corresponding 1-Torr experiments, i.e., 4%. Such a small CF3CH20H yield is below the experimental detection limit. The experimental evidence suggests, but does not prove, that reaction 25 is the source of the CF3CH20H observed in the lowest [02] experiments. To resolve this point would take further product studies using a range of different initial [HFC-l43a]/[02] ratios and is beyond the scope of the present work. The observation that even with an 0 2 partial pressure of 5 Torr the unimolecular decomposition reaction 24 does not compete effectively with reaction 21 serves to define *e atmospheric fate of CF3CH2O radicals. Compared to our 5-Torr 0 2 partial pressure experiment, in the atmosphere the 0 2 concentration is higher (at least for altitudes below 25 km19), the temperature is lower, and in general the total pressure is lower. All these factors will further suppress the importance of reaction 24 relative to reaction 21. Using k21Ik24 > 2.5 x lo-'' cm3 molecule-', it can be calculated that >99.3% of the CF3CH20 radicals formed in the atmosphere react with 0 2 to give CF3CHO. Reaction 24 is of negligible atmospheric importance. As mentioned above, evidence has been obtained that under conditions of low 0 2 concentration, CF3CH20 radicals can abstract H atoms from H-containing compounds such as CF3CH3 to form CF3CHzOH. In the atmosphere, 0 2 is abundant and H-containing organic compounds are scarce. Hence, reaction of CF3CH20 radicals with H-containing organic compounds will be of no atmospheric importance.
Implications for Atmospheric Chemistry Following release into the atmosphere, CF3CH3 will react predominantly with hydroxyl radicals. The atmospheric lifetime of CF3CH3 with respect to reaction with OH is 40 years.31 Reaction with OH gives CF3CH2, which within 1 ps will be converted into the corresponding peroxy radical, CF3CH202. We have shown here that CF3CH202 radicals react rapidly with NO to produce NO2 and (by inference) CF3CH20 radicals. By using k4 = 1.2 x IO-" cm3 molecule-1 s-l together with an estimated background tropospheric NO concentration of 2.5 x lo8 cm-3,32the lifetime of CF3CH202 radicals with respect to reaction 4 is calculated to be 6 min. Reaction 4 is likely to be an important atmospheric loss of CF3CH202 radicals. We have shown here that the atmospheric fate of the alkoxy radical
generated in reaction 4 is reaction with 0 2 to give CF3CHO. The subsequent atmospheric chemistry of CF3CHO is believed to be dominated by photolysis and reaction with OH radicals. The atmospheric lifetime of CF3CHO with respect to reaction with OH is 24 days.33 The atmospheric lifetime of CF3CHO with respect to photolysis is estimated to be comparable to that of reaction with OH.33 The reactions of the CF3CO radical in the atmosphere are discussed in ref 34.
Acknowledgment. We thank Steve Japar (Ford) and Eigil Prmtgaard (Roskilde University) for helpful discussions. Financial support for the work at Riso was provided by the Commission of the European Communities. References and Notes (1) Atkinson, R. J . Phys. Chem. Re& Data 1989, Monograph No. 1. (2) Wallington, T. J.; Nielsen, 0. J. Chem. Phys. Lett. 1991, I87, 33. (3) Wallington, T. J.; Ball, J. C.; Nielsen, 0. I.; Bartkiewicz, E. J . Phys. Chem. 1992, 96, 1241. (4) Nielsen, 0. J.; Ellermann, T.; Bartkiewicz, E.; Wallington, T. J.; Hurley, M. D. Chem. Phys. Lett. 1992, 192, 82. (5) Nielsen, 0. J.; Ellermann, T.; Sehested, J.; Bartkiewicz, E.; Wallington, T. J.; Hurley, M. D. Int. J . Chem. Kinet. 1992, 24, 1009. (6) Sehested, J.; Wallimgton, T. J. Environ. Sci. Technol. 1993,27, 146. (7) Nielsen, 0.J.; Ellerknn, T.; Sehested, J.; Wallington, T. J. J . Phys. Chem. 1992, 96, 10875. (8) Sehested, J.; Ellermann, T.; Nielsen, 0.J.; Wallington, T. J.; Hurley, M. D. Int. J . Chem. Kinet. 1993, 25, 701. (9) Nielsen, 0. J. Risb-R-480, 1984. (10) Wallington, T. J.; Japar, S. M. J . Atmos. Chem. 1989, 9, 399. (1 1) Wallington, T. J.; Ellermann, T.; Nielsen, 0.J. J. Phys. Chem. 1993, 97, 8442. (12) Wallington, T. J.; Dagaut, P.; Kurylo, M. J. Chem. Rev. 1992, 92, 667. (13) Wallington, T. J.; Ellermann, T.; Nielsen, 0.J.; Sehested, J. J . Phys. Chem. 1994, 98, 2346. (14) Sehested, J.; Nielsen, 0. J.; Wallington, T. J. Chem. Phys. Lett. 1993, 213, 457. (15) Ellermann, T.; Sehested, J.; Nielsen, 0. J.; Pagsberg, P.; Wallington, T. J. Chem. Phys. Lett. 1993, 218, 287. (16) Maricq, M. M.; Szente, J. J. J . Phys. Chem. 1992, 96, 4925. (17) Lightfoot, P. D.; Cox, R. A,; Crowley, J. N.; Destriau, M.; Hayman, G. D.; Jenkin, M. E.; Moortgat, G. K.; Zabel, F. Armos. Environ. 1992, 26A, 1805. (18) Braun, W.; Herron, J. T.; Kahaner, D. K. Int. J . Chem. Kinet. 1988, 20, 51. (19) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Jet Propulsion Laboratory Publication 92-20, Pasadena, CA, 1992. (20) Sehested, J.; Nielsen, 0. J. Chem. Phys. Lett. 1993, 206, 369. (21) Wallington, T. J.; Hurley, M. D. Chem. Phys. Lett. 1992,189,437. (22) Wallington, T. J.; Hurley, M. D.; Shi,I.;Maricq, M. M.; Sehested, J.; Nielsen, 0. J.; Ellermann, T. Int. J . Chem. Kinet. 1993, 25, 651. (23) Tuazon, E. C.; Atkinson, R.; Corchnoy, S. B. Int. J . Chem. Kinet. 1992, 24, 639. (24) Tschuikow-Roux, E.; Yano, T.; Niedzielski, J. J . Chem. Phys. 1985, 82, 65. (25) Cadman, P.; Kirk,A. W.; Trotman-Dickenson, A. F. J . Chem. SOC., Faraday Trans. I , 1976, 72, 1027. (26) Manning, R. G.; Grant, E. R.; Menill, J. C.; Parks, N. J.; Root, J. W. Int. J . Chem. Kinet. 1975, 7, 39. (27) Williams, R. L.; Rowland, F. S. J . Phys. Chem. 1973, 77, 301. (28) Kaiser, E. W. Int. J . Chem. Kinet. 1993, 25, 667. (29) Wallington, T. J.; Hurley, M. D. Int. J . Chem. Kinet. 1993, 25, 665. (30) Chen, J.; Zhu, T.; Niki, H.; Mains, G. J. Geophys. Res. Lett. 1992, 19, 2215. (31) Derwent, R. G.; Volz-Thomas, A,; Prather, M. J. World Meteorological Organization Global Ozone Research and Monitoring Project, Report No. 20; Sci. Assess. Stratosph. Ozone 1989, 2, 124. (32) Atkinson, R. World Meteorological Organization Global Ozone Research and Monitoring Project, Report No. 20; Sci. Assess. Stratosph. Ozone 1989, 2, 167. (33) Scollard, D. J.; Treacy, J. J.; Sidebottom, H. W.; Balestra-Garcia, C.; Laverdet, G.; LeBras, G.; MacLeod, H.; Tbton, S. J . Phys. Chem. 1993, 97, 4683. (34) Wallington, T. J.; Hurley, M. D.; Nielsen, 0. J.; Sehested, J. J . Phys. Chem. 1994, 98, 5686.
