J. Phys. Chem. 1995,99, 3201-3205
3201
Atmospheric Chemistry of CF3O Radicals: Reaction with CH4, CD4, CH#, CFsH, 13C0, C~HSF,C2D6, C2H6, CH30H, i-C4HS, and C2H2 Timothy J. Wallington* and James C. Ball Research StafJ; SRL-3083, Ford Motor Company, P.O. Box 2053, Dearbom, Michigan 48121 -2053 Received: September 29, 1994; In Final Form: December 12, 1994@
A relative rate technique has been used to study the title reactions at 296 f 2 K. Using a reference rate constant of k(CF30 C&) = (2.2 f 0.2) x cm3 molecule-' s-', rate constants (in 700 Torr of air diluent) for the reactions of CF30 radicals with the following reactants were established; CD4, (5.1 f 1.6) x CH3F, (2.4 f 0.3) x CF3H, < 6 x IO-'$ 13C0, (7.2 f 0.7) x C Z H ~ F(3.2 , f 0.5) x 10-13; C2D6, (6.4 f 1.1) x C2H6, (1.4 f 0.3) x IO-',; CH30H, (2.5 f 0.4) x 10-l2; i-CJ-Ig, (6.1 f 1.3) x lo-',; C2H2, (1.7 f 0.3) x lo-'' cm3 molecule-' s-'. The rate of the reaction of CF30 radicals with 13C0was observed to be dependent on the total pressure. In 100 Torr of total pressure of air diluent, k(CF30 13CO) = (4.6 f 0.5) x cm3 molecule-' s-'. The reaction of CF30 with 13C0 gives 13C02in a yield of 96 f 8%. Implications for the atmospheric chemistry of CF3O radicals are discussed.
+
+
Introduction
CF30
Recognition of the adverse effect of chlorofluorocarbon (CFC) release into the atmosphere has led to an international effort to replace CFCs with environmentally acceptable alternatives. Hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are under consideration as CFC substitutes. For example, HFC-134a is a replacement for CFC-12 in automative air-conditioning systems. With their large-scale industrial use, the environmental consequence of release of HFCs and HCFCs into the atmosphere has come under scrutiny. CF3 radicals are produced during the oxidation of HFC-l34a, HFC-125, HFC-23, and HCFC-123.' In the atmosphere, CF3 radicals react with 0 2 to give CF3O2 radicals which, in turn, react rapidly with NO to form CF30
+ 0, + M - CF302+ M CF30, + NO - C F 3 0 + NO,
CF,
(1) (2)
+ NO - COF, + FNO
(5)
Kelly et al.16 measured the ratio k3/k4 directly and report k3/k4 = 0.010 & 0.001. There have been a number of absolute rate studies of k3 and h. The results from the absolute studies are in good agreement with k3 = 2.2 x cm3 molecule-' s-l (average from Saathoff and Zellner,15Bednarek et al.,18 Barone et al.,17and Jensen et aLZ2)and k4 = 1.2 x cm3 molecule-' s-' (Saathoff and Zellner15 and Barone et al.17). The ratio of k3/k4 obtained from the absolute methods is then k3/k4 = 0.018. The significant difference between the results obtained by the absolute and relative rate methods is puzzling. To study this discrepancy and to increase our general understanding of the atmospheric chemistry of CF3O radicals, we have used a relative rate method to study the reactions of CF3O radicals with a variety of compounds. A novel source of CF30 radicals was used in the present work, namely, the photolysis of hexafluoroazomethane, CF3NzCF3, in air diluent.
Experimental Details The atmospheric chemistry of CF3O radicals has received considerable attention recently. Interest in CF30 chemistry stems from speculation regarding the ability of this radical to participate in catalytic ozone-destructioncycles? Recent studies of the reactivity of CF3O radicals with ozone have demonstrated that no such cycles are ~ i a b l e . ~The - ~ atmospheric fate of CF30 radicals is reaction with NO,lo-l3 hydrocarbon^,'^-^^ and possibly water vapor.,' Despite the substantial recent improvement in our understanding of the atmospheric chemistry of CF30 radicals, significant uncertainties still remain. For example, consider the relative reactivity of CF3O radicals with methane and ethane. There have been two relative rate studies of k3/k4:
+ CH, - CF30H + CH, CF30 + C,H6 -.CF30H 4-C,H, CF30
(3) (4)
Chen et al.14 measured k3/k5 5 and k5lk4 = 50 & 15 from which a ratio of k3/k4 < 0.0065 can be deduced:
* Author to whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, February 15, 1995.
0022-3654/95/2099-3201$09.00/0
The experimental setup used has been described previouslyz3 and consists of a Mattson Instruments Inc. Sirius 100 FT-IR spectrometer interfaced to a 140-L, 2-m-long evacuable Pyrex chamber. White-type multiple reflection optics were mounted in the reaction chamber to provide a total path length of 26.6 m for the IR analysis beam. The spectrometer was operated at a resolution of 0.25 cm-'. Infrared spectra were derived from 32 coadded interferograms. CF3O radicals were generated by the photolysis of CF3N2CF3 using the output of 22 W fluorescent lamps (GTEF4OBLB):
+ hv ( A > 300 nm) - 2CF3 + N, CF, + 0, + M - CF,O, + M CF302+ CF302- CF30 + CF30 + 0,
CF3N,CF3
(6)
(1) (7)
Reaction mixtures consisting of the reactant and reference organics and CF3NzCF3 diluted in air were admitted to the reaction chamber. In the presence of CF30 radicals, the reactant and reference organics decay via CF30
+ reactant organic - products
0 1995 American Chemical Society
(8)
Wallington and Ball
3202 J. Phys. Chem., Vol. 99, No. 10, 1995 CF30
+ reference organic - products
(9)
Provided that the reactant and reference organics are lost solely by reactions 8 and 9 and that neither organic is re-formed in any process, it can be shown that [reactant o r g a n i ~ ] , ~ k, ln{ [reactant organic],} =
-
c
1.6
-
1.4
-
1.2
-
1.0
-
4
C 0
Y
[reference organic],,
6 1n{ [reference organic], L
where [reactant organic],, and [referenceorganic],, and [reactant organic], and [reference organic], are the cocnentrations of the reactant and reference organics at times to and t , respectively, and kg and k9 are the rate constants of reactions 8 and 9, respectively. The loss of reference and reactant compounds was monitored using their characteristic infrared features over the following wavenumber ranges: C&, 1180-1400 and 2850-3150; CD4, 950-1050; CH3F, 950-1 100; CSH, 1350-1400; 13C0,20002200; C~HSF, 840-900; C2D6,2050-2150; C2&, 800-850 and 2950-3050; CHsOH, 900-1 100; i-CJ-Ig, 2850-3000; and C2H2,700-770 cm-'. Reaction mixtures consisted of 50-150 mTorr of CF3N2CF3 and 1-20 mTorr of the reference and reactant compounds diluted in either 100 or 700 Torr of ultrapure synthetic air. The temperature was 296 f 2 K in all experiments. With the exception of CF3NzCF3, all reactants were purchased with commercial sources at purities > 99% and were used as received. CF3N2CF3 was synthesized by passing cyanogen chloride, diluted with nitrogen or helium, over a bed of silver fluoride (AgF2) and collecting the product in a cold trap (pentanefliquid nitrogen or liquid nitrogen).24 CF3N2CF3 (bp -31 "C) is easily separated from its impurities (unreacted CNC1, bp 13 "C; SiF4, bp -86 "C; CF3N0, bp -81 "C) based on their relative vapor pressures. Cyanogen chloride was prepared by passing chlorine diluted with nitrogen through a solution of zinc sulfate and potassium cyanide and collecting the product in a CO2/acetone bath.25 Condensed chlorine (bp -35 "C) was removed from cyanogen chloride (mp -6 "C; bp 13 "C) by passing nitrogen through the sample cooled to -23 "C using a CCLdC02 bath.
Results The relative rate technique relies on the assumption that both the reactant and reference organics are removed solely by reaction with CF3O radicals. Possible complications could arise in the present work if there were significant loss of either reference or reactant compounds by heterogeneous processes, reaction with CF3N2CF3, photolysis, or reaction with radical species other than CF30. These potential complications are addressed below. To check for heterogeneous loss and reaction with CF3N2CF3, mixtures of CF3N2CF3 with the reference and reactant compounds were prepared and allowed to stand in the dark for 10 min. No loss (< 1%) of reactant or reference compounds was observed. To test for photolysis, mixtures of the reactants in nitrogen, in the absence of CF3N2CF3, were irradiated using the output of all the UV lamps surrounding the chamber for 10 min. No photolysis was observed. Photolysis of CF3NzCF3 is expected to produce CF3 radicals. The formation of other radical species such as F atoms, even in a small yield, would complicate the data analysis. The Pyrex reaction chamber blocks UV radiation with 1 < 300 nm. The energy of a 300-nm photon is 95.3 kcal mol-'. The C-F bond strength in CF3NzCF3 has not been measured, but it is reasonable to assume that it is similar to other C-F bonds in alkanes which have strengths of 105-110 kcal The formation of F
0.8 -
0 4
0
0.6
0
-
a, I _I
0.2 0.c
Ln([CH41to/[CH41t) Figure 1. Plot of ln([reactant],J[reactant],) vs ln([CI-Lt],J[CH&)for the following reactanvreference pairs: CDdCK (A),CH3F/C% (V), and 13CO/CH4(0)in 700 Torr of air in 296 f 2 K. Solid lines are
first-order fits to the data. atoms in the present work is unlikely. CF3 radicals formed by photolysis of CF3N2CF3 react rapidly with 02.233 In all experiments, the 0 2 concentration was at least 2100 times greater than that of the reactant and reference organics. Hence, secondary reactions involving CF3 radicals are not expected to be a significant complication. The reaction of CF3 radicals with 0 2 gives CF3O2 radicals. CF302 radicals are expected to behave like typical alkylperoxy radicals which do not react with any of the reference or reactant species considered hereS2q3If CF302 radicals were to react with any of the saturated compounds studied in the present work, the reaction would be expected to proceed via a H-atom abstraction mechanism. Schneider and Wallington2' have calculated the CF3OO-H bond strength to be 95.1 kcal mol-'. Reactions of CF302 radicals with CD4, CI&, CH3F, C2D6, and C2H6 are thermodynamically unfavorable26and need no further consideration. For CH30H, kC4H8, and possibly C2H5F, reaction with CF3O2 cannot be excluded on thermodynamic grounds. Reaction of CF3O2 radicals with 13C0 and C2H2 can, in principal, proceed via a mechanism that does not involve H-atom transfer. Thus, CF3O2 radicals could react with 13C0 to give CF30 and 13C02,while CF3O2 radicals could add to C2H2. Potential complications caused by CF3O2 radicals in experiments involving CH30H, i-Cag, C2H5F, 13C0, and C2H2 should manifest themselves in curved plots of ln([reactant],,j[reactant],) vs In( [referen~e],,j[reference]~).Figures 1 and 2 show typical plots of ln([rea~tant],,j[reactant]~) vs ln([reference],,j[reference],) obtained in this work. All such plots were linear with intercepts at the origin suggesting the absence of complications due to secondary chemistry involving CF3O2 radicals. As an additional experimental check for complications caused by CF3O2 radicals in experiments involving 13C0,four sets of experiments were performed with initial [ 13CO]/[C&] concentration ratios varying over the range 0.87-3.6. As seen in Figure 1, variation of [13CO]/[CI&]by a factor of 4 had no discernible impact on the results. It seems reasonable to conclude that CF3O2 radicals do not react appreciably with 13C0 in the present experiments and that the loss of 13C0 observed upon irradiation of CF3N2CF3/13CO/CH&ikmixtures is attributable to reaction with CF3O radicals. The reaction of CF3O radicals with CO has been studied by Czarnowski and Schumacher.28 Using the thermal decomposition of 12-200 Torr of CF303CF3 as a source of CF3O radicals
J. Phys. Chem., Vol. 99, No. IO, 1995 3203
Atmospheric Chemistry of CF3O Radicals 10
8 n N
I
2.5
L
+0
N
0
Y
2.0
-
€
v
0' c N
I N 0
Y
v
1.5
6
n N
-
0 0
4
P) 7
1.0
-
Y
c
1
a
0.5 -
Y
0.0 0.0
1
1
1
I
I
0.1
0.2
0.3
0.4
0.5
~
~
~
~
~
I
0
2
0.6
~
,
Figure 2. Plot of the decay of CzHz vs that of CH30H following the irradiation of C F ~ N ~ C F ~ / C Z H ~ / C mixtures H ~ O H at 296 f 2 K in the presence of 700 Torr of air diluent.
in the presence of 100-500 Torr of CO at 42-70 "C, it was deduced that reaction of CF30 radicals with CO proceeds via an addition mechanism to give CF30CO radicals.28 The addition of 0 2 to the chemical system used by Czarnowski and Schumacher led to an uncontrolled reaction and thermal explosion. The reactions operative in such a system are
+ CO + M - C F 3 0 C 0 + M (loa) C F 3 0 C 0 + 0, CF,OC(O)O, (11) CF,OC(O)O, + CF30C(0)0, C F 3 0 C ( 0 ) 0 + CF,OC(O)O (12) CF,OC(O)O + M CF30 + CO, (13) CF30
-
-
~
~
CF30
-
+ CO
+ CO
+ CO, COF, + FCO CF,
To provide information on the products formed following reaction of CF3O radicals with CO in air, experiments were performed in which CF3NzCF3/l3CO/airmixtures were irradiated. In such experiments, 13C02 was observed as the major product in a molar yield (i.e., moles of 13C02formed per mole of 13C0 consumed) of 96 f 8% (see Figure 3). Under the present experimental conditions, FCO radicals formed in reaction 1Oc are expected to be converted into FC(O)OOC(O)F and not C02.29 Hence, the data in Figure 3 show that channel 1Oc is not important. While the results from the present work do not exclude (lob) as a significant reaction channel, when viewed together with the data of Czarnowski and Schumacher,28it seems likely that under atmospheric conditions reaction 10 proceeds predominately via an addition mechanism to give CF3OCO radicals. In the atmosphere, CF30CO radicals will add 0 2 to give the corresponding peroxy radical (CF30C(O)Oz), which will presumably react with NO, NOz, H02 radicals, and other peroxy radicals. Reaction with NO2 will give a thermally unstable peroxy nitrate, which will decompose to regenerate the peroxy radical. Reaction of CF30C(0)02 radicals with other peroxy
,
~
4
A
/
6
~
[l3C0]
8
(mTorr) ~ ~
10
,
~
"CO/air mixtures; the line is a linear least-squares fit.
radicals and NO is expected to give the alkoxy radical CF3OC(O)O, which will decompose to give CF3O radicals and CO2. Unless reaction of CF30C(0)0~with H02 radicals gives CF3OC(0)02H or CF30C(O)OH as products, there will be no loss of CF30, radicals in the atmosphere as a result of reaction of CF3O radicals with CO. Table 1 lists the rate constant ratios measured in the present work. Experiments were performed to establish the reactivity of CF30 radicals toward CF3H. Upon irradiation of CF3NzCF3/CF3H/CDdair mixtures, there was no discernible CF3H loss, while CD4 was observed to decay. An upper limit of k(CF3O CF3H)/k(CF30 CD4) -= 0.07 was derived. As a check of the internal consistency of the data set, the reactivities of CzHsF, C2D6, and C2H6 were measured relative to each other:
+
+
+ C,H,F - CF30H + C,H4F CF30 + C,D6 - CF30D + C,D, CF30 + C,H, - CF30H + C,H,
CF30
( 1Ob)
(lOC>
l
~
Figure 3. Formation of 13C02following the irradiation of C F ~ N Z C F ~ /
In addition to channel loa, two other reaction pathways are possible: CF30
2
(14) (15) (16)
From the measured values of kldk15 = 0.50 f 0.04 and k1dk16 = 0.22 & 0.02 (quoted errors reflect 2 standard deviations), a value of k15/k16 = 0.44 f 0.05 can be deduced (errors were calculated using conventional error propagation techniques). This value is in good agreement with the measured value of k&16 = 0.47 f 0.03. It is gratifying to note the internal consistency of the data set. Similar consistency is evident upon inspection of the data given in Table 1 for CHq, CH3F, and 1 3 ~ 0 .
To place the relative rate data in Table 1 on an absolute scale, we need to choose one, or more, reference compound whose reactivity toward CF3O radicals has been established using absolute techniques. In the present work, the choices are CD4, c&,CzH6, i-CdHg, or co. Of these possible references, c& has been studied the most extensively. Thus, at the present time, CHq is the most appropriate reference compound. With CHq defined as the primary reference, the reactivity of each of the compounds studied needs to be related to that of methane. Where CHq has been used directly as an experimental reference, the measured rate constant ratio can be used. For compounds which were not measured directly against methane, those rate constant ratios which relate the reactivity of the compound to that of methane were multiplied together. The result is shown in Table
l
~
Wallington and Ball
3204 J. Phys. Chem., Vol. 99, No. 10, 1995
TABLE 1: Rate Constant Ratios, kreaerantlkreferenee reference reactant CD4 CH3F
13co
_ _
C7H5F C2D6 C2H2 i-C4H8
CHq 0.23 f 0.02 1.08 f 0.11 3.29 f 0.13 2.09 f 0.12b
CH3F
C2D6
C2H6
CH3OH
0.50 f 0.04
0.22 f 0.02 0.47 f 0.03 11.7 f 1.0
6.7 f 0.2
C2H2
3.22 f 0.21 13.4 f 0.9
0.36 f 0.02
Quoted errors are 2u. At 100 Torr total pressure of air diluent.
TABLE 2: Reactivities Relative to Methane k(CF10 + reactantYk(CF30 + CH4) reactant '0.02 0.23 f 0.02 1 1.08 f 0.11 3.29 f 0.13 2.09 f 0.12" 14.5 f 1.8 29.0 f 4.3 65.9 f 10.1 115 f 20 277 f 51 771 f 135 138 k 27" At 100 Torr total pressure. 2. Quoted errors in Table 1 are 2 standard deviations from statistical analyses of the experimental data. Errors in Table 2 either are taken directly from Table 1 or, if the reactivity was calculated from two or more measured relative rates, were calculated using conventional error propagation analysis. There have been five absolute rate studies of the reactivity of CF30 radicals toward CH4 at 296 f 2 K; the values of k3 reported are as follows (in units of cm3 molecule-' s-l): (2.2 Saathoff and Zellner;15 (3 f 2) x f 0.2) x Bevilacqua et a1.;l1 (2.7 f 0.2) x Bednarek et a1.;18 (1.9 Jensen Barone et al.;" and (2.2 f 0.4) x f 0.1) x et a1.22 With the exception of the result from Bevilacqua et a1.,l1 which is superseded by the study of Jensen et a1.,22there is no obvious reason to prefer any of these studies. Thus, we have chosen to place our relative rate data on an absolute basis by using an average of the above studies (ref 11 excluded) of cm3 molecule-' s-'; the quoted k3 = (2.2 f 0.2) x uncertainty reflects our estimate of a 10%uncertainty in k3 based upon the close agreement of the absolute rate data. In Table 3, the results from the present work are compared to previous studies. Errors quoted in Table 3 reflect statistical uncertainties associated with our measured rate constant ratios together with uncertainties associated with k3. Our measured rate constants are consistent with the literature values where they exist. The rate of reaction of CF30 radicals with 13C0 was studied at 700 and 100 Torr of total pressure and found to decrease as the pressure was decreased. This behavior is similar to that observed in the analogous reaction of OH radicals with C0.30 As discussed above, the reaction of CF30 with I3CO is believed to proceed via the formation of the adduct CF3013C0. With such a mechanism, there should be little 13Ckinetic isotope effect, so the results from the present work involving 13C0 should be valid for unlabeled CO facilitating a direct comparison of our results with those of Zellner and S a a t h ~ f f .The ~ ~ rate constant measured in the present work at 100 Torr of total pressure of air is in good agreement with that measured by Zellner and Saathoff)' in 50 Torr of helium diluent.
TABLE 3: Comparison of Present Data with Literature Values reactant k(296 f 2 K)" ref this work CF3H
