J . Phys. Chem. 1994,98, 5686-5694
5686
-
Atmospheric Chemistry of CFsCO, Radicals: Fate of CFJCO Radicals, the UV Absorption Spectrum of CF3C(0)02 Radicals, and Kinetics of the Reaction CF3C(0)02 + NO CFjC(0)O + NO2 Timothy J. Wallington' and Michael D. Hurley Research Stafi, SRL- E3083. Ford Motor Company, Dearborn, Michigan 481 21 -2053
Ole J. Nielsen' and Jens Sehested Section for Chemical Reactivity, Environmental Science and Technology Department, Rise National Laboratory, DK-4000 Roskilde, Denmark Received: February 16, 1994; In Final Form: March 28, 1994"
The atmospheric fate of CF3CO radicals has been studied using a pulse radiolysis technique to provide kinetic data and FTIR-smog chamber system to provide product data. In 1 atm of SF6 a t 296 f 2 K, CF3CO radicals decompose to give CF3 radicals and C O with a rate of (1.2 f 0.8) X lo5s-' and react with 0 2 to form CF3C(0)02 radicals with a rate constant of (7.3 f 1.1) X 10-13 cm3 molecule-' s-l. In 1 atm of N2 at 296 f 2 K, the rate constant ratio k(CF3CO 0 2 - CF3C(0)02)/k(CF3CO CF3 CO) = (7.4 f 0.6) X lo-'* cm3 molecule-'. Reaction with 0 2 accounts for 99.5% of the loss of CF3CO radicals in the atmosphere. The ultraviolet absorption spectrum of C F 3 C ( 0 ) 0 2 radicals has been studied over the wavelength range 220-300 nm, and at 230 nm, UCF3C(0)02 = (3.78 f 0.43) X 10-l8 cm2 molecule-'. Monitoring the rate of NO2 formation at 400 nm allowed a lower limit of k4 > 9.9 X lo-', cm3 molecule-' s-' to be derived for the rate constant of the reaction of C F 3 C ( 0 ) 0 2 radicals with NO. Reaction of CF3C(0)02 radicals with N O produces the alkoxy radical C S C (O)O,which undergoes C-C bond scission rapidly with a rate greater than 6 X lo4 s-'. Results are discussed with respect to the atmospheric chemistry of CF3C0, radicals. As part of the present work, a rate constant kg = (2.3 f 0.4) X lo-" cm3 molecule-' s-' was determined for the reaction of F atoms with CF3CHO.
-
+
Introduction
CF,C(O)O
By international agreement, industrial production of chlorofluorocarbons (CFCs) will be phased out. CFC replacements are being sought. Hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are two classes of potential CFC substitutes. Prior to the large scale industrial use of these compounds, it is important to establish the environmental impact of their release. HCFCs and HFCs of the general type CF3CHYX3, (X = F, C1) can degrade by a variety of different pathways in the atmosphere to produce CF3CO radicals. For example, the atmospheric oxidation of HFC-134a (CF3CFH2) produces CF3C(0)F.1J The oxidation of HFC-143a (CF3CH3) produces CF3CH0.3 In the upper stratosphere, photolysis of CF3C(0)F could produce CF3CO radicals. Reaction of OH radicals with CF3CHO produces CF3CO radicals. Once formed, CF3CO radicals are expected to either decompose to give CF3 radicals and CO or add 0 2 to give CF3C(0)02 radicals. Assessment of the environmental impact of the release of compounds having the general formula CFjCX3 (X = H, F, C1) requires information concerning the atmospheric chemistry of CF3C0, radicals (x = 1-3). As part of a joint program between our two laboratories to survey the atmospheric chemistry of HFCs and HCFCS,'~ we have used pulse radiolysis and long path length Fourier transform infrared techniques to provide kinetic and mechanistic data concerning reactions 1-4. Results are reported herein.
+ M -CF3 + CO + M CF,CO + 0, + M CF3C(0)0, + M CF3C0
~
-
(1) (2)
~~~
* Authors to whom correspondence may be addressed. @
+
Abstract published in Advance ACS Abstracts. May 1, 1994.
0022-3654/94/2098-5686$04.50/0
+M
CF3C(0)0, + N O
-
CF3
+ CO, + M
CF3C(0)0
+ NO,
(3) (4)
Experimental Section Two different experimental systems were used. Both have been described in detail in previous publications'0Jl and will only be discussed briefly here. PulseRadiolysis System. CF3C(0)02 radicals weregenerated by the radiolysis of SF6/02/CF3CHO gas mixtures in a 1-L stainless steel reactor with a 30-11s pulse of 2-MeV electrons from a Febetron 705B field emission accelerator. SF6 was always in great excess and was used to generate fluorine atoms:
-+ -
SF6 + 2 MeV e-
F + CF3CH0 CF3C0
+ 0, + M
F
products
(5)
CF3C0 + H F
(6)
-
CF3C(0)02
+M
(2)
Four sets of experiments were performed using the pulse radiolysis system. First, the rate of the appearance of ultraviolet absorption attributed to CF3CO radicals following the radiolysis of SF6/CF3CHO mixtures was measured as a function of the initial concentration of CFjCHO to determine kg. Second, 0 2 was added to the reaction mixtures, and the rate of increase in ultraviolet absorption attributed to CF$2(0)02 and CF3O2 radicals was measured as a function of the 0 2 concentration to determine kl and k2. Third, initial conditions were selected to ensure that the majority of CF3CO radicals react with 0 2 , and the UV spectrum of CF3C(0)02 radicals was recorded. Fourth, NO was added to the reaction mixtures, and the rate of NO2 0 1994 American Chemical Society
Atmospheric Chemistry of CFSCO, Radicals
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5687
formation following the radiolysis pulse was monitored to provide information about the kinetics of reactions 3 and 4. To monitor the transient UV absorption, the output of a pulsed 150-W Xenon arc lamp was multipassed through the reaction cell using internal White cell optics (80- or 120-cm path length). A McPherson grating monochromator, Hamamatsu R 955 photomultiplier and Biomation 8 100 waveform digitizer were used to detect and recotd the light intensity a t the desired wavelength. The spectral resolution used was 0.8 nm. Reagent concentrations used were SF6, 945-995 mbar, 0 2 , 0-50 mbar, NO, 0-1.0 mbar, and CF3CH0, 0-5 mbar. All experiments were performed at 296 K. Ultrahigh purity 0 2 was supplied by L'Air Liquide. SF6 (99.97%) was supplied by Gerling and Holz. N O (99.8%) was obtained from Messer Griesheim. CF3CH0 was synthesized by thedropwise addition of trifluoroacetaldehyde methyl hemiacetal to a H~S04/P205slurry. IR analysis did not reveal any observable impurities. All other reagents were used as received. 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 CF3CHO (14-28 mTorr), Cl2 (100120 mTorr), N O (0-40 mTorr), and 0 2 (0-40 Torr) a t 80-700 Torr total pressure of N2 diluent at 296 K using 22 blacklamps (760Torr = 1013mbar). ThelossofCF3CHOandtheformation of products were monitored by FTIR spectroscopy using an analyzing path length of 25 m and a resolution of 0.25 cm-I. Infrared spectra werederived from 32 coadded spectra. Reference spectra were acquired by expanding known volumes of reference materials into the reactor. The IR spectrum of CF3CHO was consistent with that reported by Shechter and Conrad.I2 The IR feature of CF3CHO at 1305 cm-I is broad and essentially devoid of structure at the spectral resolution (0.25 cm-I) used in this work. The IR absorption cross section at 1305 cm-I was (4.9 f 0.3) X 10-19 cm2 molecule.-l This feature was a convenient reference point to calibrate the CF3CHO concentrations used in the FTIR experiments. N2 and 0 2 (both 99.999%) and Clz (99.999%) were obtained from Airco. N O and C12 (both research purity) were obtained from Matheson. All reagents were used as received. Pulse Radiolysis Results
+
Study of F CF3CHO. Following the pulse radiolysis of a mixture of 990 mbar of SF6 and 1-5 mbar of CF$HO, a small (absorbance of 0.08 f 0.01 using full dose) transient UV absorption was observed at 230 nm. N o absorption was observed when either 5 mbar of CF3CHO or 990 mbar of SF6 were radiolyzed separately. We ascribe the absorption observed upon radiolysis of SFb/CF3CHO mixtures to the formation of CF3CO radicals via reaction 6. The increase in absorption displayed first-
-
F + CF3CH0
+
CF3C0 HF
(6)
order kinetics. Figure 1 shows a plot of the observed pseudofirst-order rate of appearance of the absorption ascribed to CF3CO radicals as a function of the concentration of CF3CHO. The solid line is a linear least-squares fit. The y-axis intercept, (2.1 f 2.6) X 104 s-1, is not statistically significant. The slope gives k6 = (2.3 f 0.4) X lo-" cm3molecule-'s-l. This result is in good agreement with a recent determination of k6 = (2.7 f 0.1) X 10-11 cm3 molecule-1 s-1 using a relative rate technique.13 Unless otherwise specified, all errors in the present manuscript are 2 standard deviations. Study of CFJCO + 0 2 + M CFJC(O)OZ+ M and C F F O M CF3 + CO + M. To investigate the kinetics of reactions 1 and 2 , 0 2 was added to the reaction mixtures. Following the radiolysis of SF6/CF3CH0/02 mixtures, the observed transient absorption was substantially greater (by a factor of 4 a t 230 nm) than that observed in the absence of 0 2 . We conclude that
+
-
-
3.5 3.0 2.5
;1..-
a0
2.0
1
I
A
F iCF,CHO
i I
0.0
0
1
2
[CF,COH]
3
5
4
, mbar
Figure 1. Plot of the first-order appearance of absorption attributed to CF$O radicals following radiolysis of SF@FaCHO mixtures (kl%ersus [CF$HO]). The line is a linear regression to the data. o.20
0.15 0 ?
'
1
D
0.05
I
I
I
0.0
10 0
20.0
30 0
40 0
Time, ps
Figure 2. Transient absorption at 230 nm observed following pulsed radiolysis of a mixture of 5 mbar of CFICHO, 40 mbar of 02,and 955 mbar of SFs. The solid line is a first-order rise fit which gives kI8I = 9.23
x 105 SI.
CF3C(0)02 radicals absorb more strongly than CF3CO radicals over the wavelength region of interest (220-300 nm). Figure 2 shows the transient absorption observed a t 230 nm for the first 40 ps following the radiolysis of a mixture of 5 mbar of CFjCHO, 40 mbar of 02, and 955 mbar of SF6 (0.527 dose, 80-cm UV analysis path length). The solid line is a first-order fit using the expression Abs(t) = (Ainf- C)[1 - e ~ p ( - k ' ~ ~ t ) ]C, where Abs( t ) is the absorbance as a function of time, Ainfis the absorbance at infinite time, k*st is the pseudo-first-order appearance rate of absorption, and Cis the extrapolated absorbance at t = 0. For the data shown in Figure 2, kist = 9.23 X los s-I. In all cases, the rise in absorption followed first-order kinetics. Control experiments were performed in which CF$H0/02 mixtures or just SF6 were radiolyzed; no transient absorption was observed. A relevant question at this point is "What radical(s) cause the transient absorption shown in Figure 2?". There are several possibilities; CFjCO, F02, CF&(0)02, and CF3O2 (produced following reaction 1). As previously noted, the absorption due to CF3CO radicals is small at 230 nm and does not explain the absorption seen in Figure 2. The rate constant for the reaction of F atoms with 0 2 (1.9 X 10-13 cm3 molecule-' s-1)14 is 121 times less than that for the reaction of F atoms with CF3CHO (2.3 X 10-1' cm3 molecule-' s-1, this work). The concentration ratio [02]/[CF3CHO] used in the experiment shown in Figure 2 was 8. Hence, 6.2% of the F atoms react with 0 2 to give FOz radicals. For the experiment shown in Figure 2, the radiolysis dose was 0.527 and the SF6 pressure was 955 mbar. The initial F atom concentration is linearly proportional to the radiolysis dose and SF6 pressure. The F atom yield a t full radiolysis dose and 1000 mbar of SF6 was 2.77 X lOI5 ~ m - ~The . initial F atom con-
+
5688
Wallington et al.
The Journal of Physical Chemistry, Vol. 98, No. 22. 1994
of CF3C(0)02radicals and minimize fragmentation of CF3CO radicals into CF3 and CO. Clearly, to study the UV spectrum of CF3C(0)02 radicals, it is desirable to work under conditions of high [02]. However, there is a limit to the amount of 02 that is desirable, as with increasing [OZ],reaction 8 competes with reaction 6 for the available F atoms.
F
+ 0, + M
.--+
FO,
+M
(8)
To investigate the UV spectrum of CF3C(0)02 radicals, the experimental conditions used were 5 mbar of CF3CHO,40 mbar of 02,and 955 mbar of SF6. Measurement of the absolute absorption spectrum of the CF3C(0)02 radical requires calibration of the initial F atom concentration. The yield of F atoms 0 10 20 30 40 50 60 was established by monitoring the transient absorption at 260 [ O 2 I , mbar nm due to methylperoxy radicals produced by radiolysis of SF6/ CH4/02 mixtures as described previo~s1y.l~ In the present series Figure 3. Plot of the first-order appearance of absorption attributed to of experiments, based upon a value of 3.1 8 X cm2molecule-1 CF3C(0)02and CF302 radicals following radiolysis of SF~/CFICHO/ 0 2 mixtures (klStversus[OZ]).The line is a linear regression to the data. for u(CH302) a t 260 nm,I8 the yield of F atoms a t 1000 mbar The intercept provides a value of k l , and the slope gives k2. of SF6 was (2.77 f 0.30) X lOI5~ m a t- full ~ irradiation dose. The quoted error on the F atom calibration includes both statistical centration in the experiment shown in Figure 2 was (0.527 X 2.77 (2 standard deviations) and potential systematicerrors associated X 10'5 X 0.955) = 1.39 X l O I 5 ~ m - hence ~ ; the F02 concentration with a 10% uncertainty in u(CH302). Errors are propagated was 8.6 X lOI3 cm-3. At 230 nm, u(F02) = 5.48 X 10-18 cm2 using conventional error analysis methods. molecule-' l4 and the absorbance expected from FO2 radicals Following the pulsed radiolysis of mixtures of 5 mbar of CF3will then be 0.016, which is a factor of 10 less than observed in CHO, 40 mbar of 02, and 955 mbar of sF6, a rapid increase Figure 2. The transient absorption shown in Figure 2 must then (complete within 5 p s ) in UV absorption in the region 220-300 arise from either CF,C(0)02 or CF3O2, or both. nm wasobserved, followed by a slower decay. Control experiments For the purpose of studying the kinetics of reactions 1 and 2, were performed in which 1000 mbar of SF6 or 5 mbar of CF3it is irrelevant as to whether the absorption is due to either CF3CC H O were radiolyzed. As mentioned in the previous section ( 0 ) 0 2or CF3O2 radicals, or both. The rationale for this statement dealing with the kinetics of reaction 6, no significant absorption is as follows. Assume that reactions 6 and 7 proceed rapidly on (C0.02 absorbance units) was observed upon radiolysis of SF6or the time scale of the present experiments (0-40 p s ) . CF3CHO. As discussed above, the reaction of CF3CO radicals with 02 proceeds with a rate constant of k2 = (7.3 f 1.1) X 10-13 CF3C0 M --, CF, CO M (1) cm3 molecule-I s-l at 1000 mbar total pressure of SF6 diluent. In the presence of 40 mbar of 02, the lifetime of CF3CO radicals CF3C0 0, M CF,C(O)O, M (2) with respect to reaction with 0 2 is then 1.4 ps. Consistent with this calculation, the transient absorption observed on radiolysis F CF3CH0 CF3C0 HF (6) of SF6/CF3CH0/02 mixtures reached a maximum in 5 ps. To measure the absorption spectrum of CF3C(0)02 radicals, CF, 0, M --, CF302 M (7) it is necessary to consider potential secondary reactions that could interfere with the conversion of F atoms into CF3C(0)02 radicals. Then, d[CF3CO]/dt = - ( k ~+ ~~[OZ])[CF~CO],~[CF~C(O)O~]/ Potential complications include (i) competition for the available dt = k2[02][CF3CO], and d[CF302]/dt = kl[CFsCO]. If kall F atoms by reaction with molecular oxygen, = (ki + k2[021), then [cF&ol(t) = [CF3COlt, exp(-kaiit), [CF3C(0)02l(t) = (k2[021 [cF3COlta/%11)(1-exP(-kaIlt)), and F 0, M * FO, M (8) [CF302](t)= (kl[CF3CO],,/k,ll)(l -exp(-kal$)). Theimportant point to note is that both CF3C(0)02 and CF3O2 are formed with and (ii) unwanted radical-radical reactions such as a rateconstant kall = ( k l + k2[02]). Hence,regardlessofwhether the absorption is due to CF3C(0)02 or CF3O2 or both, the firstF + CF3C0 products (9) order rate constant for the appearance of UV absorption is kalr. If kall is measured as a function of [O,], then a plot of kall versus F CF,C(O)O, products (10) [02] will give a slope of k2 and an intercept of kl. Such a plot is shown in Figure 3. The intercept gives kl = (1.2 f 0.8) X los CF3C0+ CF,C(O)O, CF,C(O)O + CF,C(O)O s-1 and the slope gives k2 = (7.3 f 1.1) X lO-I3 cm3 molecule-' (1 1) SKIat 1000 mbar total pressure of SF6 diluent and 296 f 2 K. To minimize complications caused by FO2 radicals, experiments At this point, it is germane to consider the validity of the two were performed using [CFSCHO]= 5 mbar and [OZ]= 40 mbar. assumptions made above, namely that reactions 6 and 7 proceed Using rate constants for reactions 6 and 8 measured in our rapidly on the present experimental time scales. Using kg = 2.3 X 10-11 cm3 molecule-' s-l (this work) gives the lifetime of F laboratory (k6 = 2.3 x 10-11 cm3 molecule-' s-1 (see previous section), ka = 1.9 X l0-l' cm3 molecule-1 s-I l4), we calculate that atoms in the presence of 5 mbar of CF3CHO as 0.35 ps. On the 6.2% percent of the F atoms are converted into F02 and 93.8% basis of the work of Caralp et al.,l5 reaction 7 should be close to the high-pressure limit of k7 = 8.5 X 10-12 cm3 molecule-l s-1 16 percent into CF3CO radicals. Using the rate constant ratio k2/ at 1000 mbar of SF6. At the lowest [O,] of 10 mbar, the lifetime kl = 6 X 10-18 cm3 molecule-' (see previous section), it can be calculated that, in the presence of 40 mbar of 0 2 , 86% of the of CF3 radicals with respect to reaction 7 is 0.5 p s . The time scale of the experimental observations was 3-1 1 p s . It appears CF3CO radicals produced in reaction 6 are converted into CF3Cthat both assumptions made above are justified. (0)02and 14% decompose to give CF3 radicals and CO. CF, radicals will be rapidly converted into CF302 radicals. Corrections Study of the UV Absorption Spectrum of CF&(0)02 Radicals. forthepresenceof6.2%of F02and (0.938)(0.14)(100) = 13.1% After the absoluterates for reactions 1 and 2 have been established, experimental conditions can be chosen to maximize the production of CF3O2 radicals were calculated using the expression of
+
+ +
+
+ +
-
+
+
+
+
+
+ +
+
-
+
-
-
Atmospheric Chemistry of CF3C0, Radicals
0.30
-
0.25
-
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5689
W 0
&
0.20 1
2
0.15:
e n
a 0.10
0.05
-
1
0.00 0.0
0.2
0.4
0.8
0.6
1.0
Dose Figure 4. Maximum transient absorption at 230 nm following :-.2 pulsed radiolysisof mixtures of 5 mbar CFsCHO, 40 mbar of 02, and 955 mbar SF6as a function of the radiolysisdose. The solid line is a linear regression to the data (full- and half-dose experiments excepted). 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 u ( 10”
cm2 molecule-1)
wavelength (nm)
CSC(O)O2
220 230 240
50 1 378 257 185 133 78 45 16
250 260 270 280 290
r
,
,
.
,
,
,
,
0.05 .
0
0.04 .
P)
u
6
e : n
0.03 . 0.02
.
0.01
.
Q
0.00 .
+
a(CF$(0)02) = (a(0bserved) - ((0.062)u(F02) (0.131) X a(CF302))/0.807. Values for a(CF302) and a(FO2) were taken from the l i t e r a t ~ r e . ~ J ~ As mentioned above, it is necessary to ensure that radicalradical reactions 9-1 1 do not complicate the data analysis. 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 absorption at 230 nm was measured with the radiolysis dose varied by over 1 order of magnitude. The UV path length was 80 cm. Figure 4 shows the observed maximum of the transient absorption a t 230 nm as a function of the dose. As seen from Figure4, the maximum absorption is linear with the radiolysis dose up to a dose of 0.40 of maximum. At maximum dose (and possibly at half dose) the maximum transient absorption falls below that expected from a linear extrapolation of the low-dose results. We ascribe the curvature in Figure 4 to the importance of secondary radicalradical reactions 9-1 1 at high initial F atom concentrations. The solid line drawn through the data in Figure 4 is a linear least-squares fit (maximum- and half-dose data expected). The slope is 0.331 f 0.013 (errors are 2 standard deviations). From this value and three additional pieces of information, (i) the yield of F atoms of (2.77 f 0.30) X 1015 molecules cm-3 (full dose and [SFs] = 1000 mbar), (ii) the conversion of F atoms into 80.7% CFjC(0)02,6.2% FO2, and 13.1% CF3O2 radicals, and (iii) the absorption cross sections for FOz and CF3O2 at 230 nm (a = 5.48 X 10-18 l 4 and 2.06 X 10-18 cm2 molecule-1,’ respectively), we derive a(CF3C(0)02) at 230 nm = (3.78 0.43) X cmz molecule-’. The quoted error includes statistical uncertainties from the linear regression of the data in Figure 4 and the potential systematic uncertainties associated with the calibration of the initial F atom yield. Errors have been propagated using standard error analysis. To map out the spectrum of the CF$2(0)02 radical, experiments were performed to measure the initial absorption between 220 and 300 nm following the pulsed irradiation of SF6/CF3-
*
.
0.06
0.0
10.0
20.0
30.0
40.0
Time, ps
Figure 6. Transient absorption at 400 nm observed following pulsed radiolysis of a mixture of 0.6 mbar of NO, 5 mbar of CFICHO, 40 mbar of 02, and 955 mbar of SF6. The solid line is a first-order rise fit which gives kist = 2.24 X 105s-I.
C H 0 / 0 2 mixtures. The initial absorptions were scaled to that at 230nmand thencorrectedfor FO2and CF302toobtainabsolute absorption cross sections. Absorption cross sections are given in Table 1 and shown in Figure 5. The absorption spectrum of CF3C(0)02 measured in the present workis compared to that recorded by Maricq and Szentelg using a flash photolysis technique in Figure 5 . With the exception of the data point at 230 nm, the absorption cross sections from both studies are indistinguishable within the experimental uncertainties. For reasons which are unclear, the value of u(CF~C(O)OZ) measured a t 230 nm in the present work is 35% larger than that observed by Maricq and Szente.19 Kinetic Data for the Reaction CF$(O)O2 NO CF3C( 0 ) O + N01. The kinetics of reaction 4 were studied by monitoring the rateof increase in absorption at 400 nm (attributed to the formation of NOz) following the radiolysis (dose 0.41 times that of maximum) of mixtures of 0.3 1-1 .OOmbar of NO, 5 mbar of CF3CHO,40 mbar of 02, and 940 mbar of SF6. This method of measuring the kinetics of the reaction of peroxy radicals with NO has been used extensively in our laboratory and is discussed in detail e l ~ e w h e r e . ~ ~ *Figure ~ ~ * ~6~ shows ~ 5 the results from a mixture with [NO] = 0.6 mbar. The UV pathlength was 120 cm. The smooth curve in Figure 6 is a first-order fit using the expression Abs(t) = (& - C)[1 - exp(-kIStt)] + C, where Abs(t) is the absorbance as a function of time, Ainf is the absorbance at infinite time, kist is the pseudo-first-order appearance rate of N02, and C is the extrapolated absorbance at t = 0. Control experiments were performed in which SF6/CFjCHO, SF6/CF3CH0/02, SF6/02,or just SFa were radiolyzed; no change
+
-
5690
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
Wallington et al. TABLE 2 Chemical Mechanism Used To Model the NO2 Formation Traces reaction rate constant" ref F + CF3CHO CF3CO + HF 2.3 X lO-" this work 1.9 x 10-13 14 F + 0 2 + M FO2 + M F+NO-FNO 5.5 X 1O-I2 25 CFsCO + 0 2 + M CF$(0)02 + M 7.3 X this work 1.2 x 105 this work CF,CO+ M+CF3 + CO + M CF3C(0)02 + NO CF3C(O)O + NO2 1.0-2.5 X 1O-I1 see text CF,C(0)02 + NO2- CFsC(O)02N02 6.6 X 1O-l2 26 CFsC(0)O + M CF3 + COS 0.01-1.0 x 1086 see text CF3 + 0 2 + M CF302 8.5 X 16 CF302 + NO CF30 + NO2 1.8 X 10-I' 18 CF3O2 + NO2 + M CF302N02 + M 6.0 X 1O-I2 18 CF3O + NO COF2 + FNO 5.2 X 10-I' 24 CF30 + NO2 + M CFsON02 1.5 X 10-I' 23 a Units of cm3 molecule-' s-1. 6 Units of s-1.
-
I
m
+
Lo
0
-
-
7
Y
-+
-+
0.0
0.2
0.6
0.4
0.8
1.0
1.2
-C
[NO] , mbar Figure 7. Plot of kist versus [NO]. The solid line is a linear regression to the experimentaldata. The dotted line is the behavior predicted using the chemical mechanism in Table 2; see text for details.
in absorption at 400 nm was observed. It seems reasonable to conclude that NO2 is the species responsible for the absorption change following radiolysis of SF6/CF3CH0/02/NO mixtures. In the presence of 40 mbar of 02, the lifetime of CF&O radicals with respect to conversion into CF3C(O)O2 radicals is 1.4 ps. The transient absorption a t 400 nm was fitted from 3 ps after the pulse. This delay was used to ensure adequate separation of the time scale for CF3C(0)02 radical formation from that of the subsequent reaction with NO. For the data shown in Figure 6, kist = 2.24 X lo5 s-I. As seen from Figure 7, the pseudo-first-order rate constant, kist, increased linearly with [NO]. Linear least-squares analysis gives k4 = (1.14 f 0.15) X lo-" cm3 molecule-' s-I. The y-axis intercept in Figure 7 is (3.9 f 2.4) X lo4 s-' and is statistically significant. The increase in absorbance at 400 nm can be combined with the literature values of UNO2 (400nm) = 6.0 X 10-19 cm2 molecule-' 16 to calculate the yield of N02. The yield of NO2 in the seven experiments performed, expressed as moles of NO2 produced per mole of CF3C(0)02 radicals consumed, was 174 f 30%, suggesting that the majority of reaction 4 proceeds to give NO2 and, by implication, CF3C(O)O radicals. In this calculation, allowance was made for loss of F atoms via reaction with O2and N O using k6 = 1.9 X cm3molecule-l s-l l 4 and k(F + N O FNO) = 5.5 X 10-12 cm3 molecule-' s-l 25 and for the fact that 14% of the CF3CO radicals formed in the system decompose via reaction 1 (see discussion in the previous two sections). The fact that theNO2yieldexceeds 100%shows that thealkoxy radical CF3C(O)O formed in reaction 4 rapidly decomposes to give CF3 radicals which add 0 2 to form CF3O2 radicals which then react with N O to give more NO2. The presence of additional processes forming NO2 subsequent to reaction 4 increases the time taken for the NO2 to reach a maximum. Hence, the rate constant derived from the data in Figure 7, k4 = (1.14 f 0.15) X 10-1' cm3 molecule-' s-l, is actually a lower limit. Thus, we report k4 > 9.9 X 10-12 cm3 molecule-' s-1. As seen in Figure 6 , the NO2 concentration reaches a limiting value 15 ps after the radiolysis pulse. Thereafter, the NO2 concentration is unchanged. This behavior places an upper limit of 15 ps for the lifetime of C F 3 C ( 0 ) 0 radicals with respect to decomposition via reaction 10; hence k12 > 6 X lo4 s-1.
-
CF,C(O)O
+M
-
CF,
+ C 0 2+ M
(12)
To provide insight into the origin of the positive intercept in the plot of kist versus [NO] shown in Figure 7, the chemical system was modeled using the Acuchem program22 with the chemical mechanism given in Table 2. Kinetic data were taken from the literat~re.l4.16.'~,23,2~26 The modeling exercise was an
iterative process consisting of three steps. First, the mechanism in Table 2 was used to simulate the formation of N02. Second, a regression analysis was used to fit a first-order rise to the NO2 formation. The resulting pseudo rate constant was then compared to that expected from the product k4[NO]o. Third, values of k4 and kI2were then varied and the sequence repeated. Several insights were provided by the modeling exercise. First, consistent with the experimental observations, the simulated NO2 formation trace was well fit by first-order kinetics even though NO2 is produced by the reaction of both CF,C(0)02 and CF302 radicals with NO. Second, the pseudo-first-order NO2 rise time was in all cases slower than the product k4[NO]o. Third, the NO2 yields (expressed relative to the formation of CF$(0)02 radicals) were consistent with the experimental observation of 173 f 30%. Fourth, the pseudo-first-order rate constant for the formation of NO2 was dependent on the values of both k4 and k12used in the model. k4 obviously determines the rate of NO2 production from CF3C(0)02 directly. kl2 determines the rate at which CF3 and hence CF3O2 radicals are formed and, hence, determines the subsequent NO2 formation. If values of kl2 are chosen such that the time scale for CF3C(O)O radical decomposition is comparable to that of the experimental time scale (ps), then plots of the simulated kist values versus [NO] gave positive intercepts. As an example, the dotted line in Figure 7 shows the behavior predicted using k4 = 2.5 X 1 cm3 molecule3 s-' and k12 = 1.5 X 106 s-1. The intercept, 2 X lo4 s-l, is indistinguishable from thevalue of (3.9 f 2.4) X lo4 s-l observed experimentally. It seems likely that the cause of the positive intercept observed in Figure 7 is that decomposition of CF3C(0)Oradicals occurs on a time scale which is comparable to that of the experimental observations. While it is pleasing that the chemical mechanism in Table 2 reproduces the experimentally observed NO2 formation profiles, the derivation of a precise value of k4 is clearly complex. In view of the complexities in the present system, we choose to quote a lower limit of k4 > 9.9 X 10-12 cm3 molecule-' s-l. The kinetics and products of the reaction of CF3C(0)02 radicals with N O measured in the present work are consistent with the available literature data base for the reaction of halogenated peroxy radicals with NOe20 FITIR Results Study of theRateConstant Ratiokzlkl. In a set ofexperiments designed to complement those described above, the rate constant ratio k2/ kl was determined using an FTIR-smog chamber system. In these experiments, C12/CF3CH0/02 mixtures in 80-700Torr of N2 diluent with, and without, added N O were irradiated using UV blacklamps. The loss of CF3CHO and the formation of products were monitored by FTIR spectroscopy. Following the irradiation of CF3CHO/C12/02/N2 mixtures, five carbon-containing products were identified: CF303CF3,CF3-
Atmospheric Chemistry of CF3C0, Radicals
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5691
OH, CO, C02, and COF2. The observation of these products is
-
n
6 1
I
consistent with the following reactions occurring in the chamber:
+ (13) CF3C0 + M CF, + CO + M (1) CF,CO + 0, + M CF,C(O)O, + M (2) CF, + 0, + M CF30, + M (7) CF,C(O)O, + CF,C(O)O, CF,C(O)O + CF,C(O)O + 0, (14a) CF,C(O)O CF, + CO, (12) CF302+ RO, ( R CF,, CF3CO) CF30 + RO + 0, C1+ CF3CH0
CF3C0 HC1
-
-
-
- + + - +
CF30 + CF3CH0
CF30H
CF30+ CF302 M CF30H
COF,
(15)
CF3C0
(16)
CF3000CF3
(17)
HF
(18)
In addition to the identified carbon-containing products, an unidentified product which we will label as "X" was observed with IRfeaturesat 762,1053,1298,and 1859cm-1. Theunknown product X decayed rapidly with a lifetime of approximately 100 s when reaction mixtures were allowed to stand in the dark. The CF3OH product also decayed when reaction mixtures were left tostand in thedark. TherateofdecayofCF30Hwassubstantially slower than that of the unknown X. CF30H decay giving COF2 (and presumably HF) was first order with a rate in the range 2-3 X 10-4 s-I. HF absorbs in the infrared at frequencies above 3800 cm-I and is not detected by our spectrometer. The decay of CF3OH in glass reaction chambers is well documented and probably heterogeneous in n a t ~ r e . ~ J 7During the decay of the unknown X, the concentration of CF30H in the reaction chamber increased slightly while that of CFJCHOdecreased slightly. When X had decayed completely, there was no further loss of CF3CHO. CF3OH is a product of the reaction of CF30radicals with saturated organic compound^.^^^^ It seems reasonable to suppose that the unknown X is a compound that can decompose to generate CF30 radicals which would then give CF3OH via reaction 16. There are several possibilities for X that need consideration: CF3C(O)OOC(O)CF3, CF3C(O)OOOCF3, and CF3C(0)OOCF,. These compounds could be formed in reaction 14b, 19, or 20
CF,C(O)O, CF,C(O)O, CF,C(O)O
+ CF,C(O)O,
-
+ CF30+ M + CF30 + M
-
CF,(O)OOC(O)CF,
+ 0, (14b)
CF3C(0)OOOCF3+ M
CF,C(O)OOCF,
(19)
+M
(20)
In the previous section a lower limit for the lifetime of CFsC(0)Oof 15 ps with respect to dissociation via reaction 12 was derived. The radical-radical reaction (20) cannot compete with reaction 12 for CF3C(O)Oradicals, and hence X cannot be CF3C(O)OOCF3. In previous studies of the products following the self-reaction of CF302, CF3CF202, and CF3CFH02radicals, the trioxides formed by the association reaction of CF30 with RO, radicals (R = CF3,7 CF3CF2,28 CF3CFH2) were important products. By analogy to these previous studies, it seems likely that "X" is the trioxide CF3C(O)OOOCF3. The 762-cm-1 IR
./
5:
I
't
/
--0
1
I
i 2
3
A [CF,CHO],
4
5
5
(mTorr)
Figure 8. Plot of the observed yield of CO as a function of the loss of CFpCHO following the irradiation of mixtures of 25-30 mTorr of CF3CHO and 110-1 15 mTorr of C12 at 700 Torr total pressure of N2 with (A)and without ( 0 )the addition of 34 mTorr of NO.
feature displayed by the unknown X can be assigned to a symmetric 0-0-0stretch. The peroxide CF3C(O)OOC(O)CF3 is not expected to have an I R feature a t this frequency. The IR features displayed by X at 1053, 1298, and 1859 cm-1 are characteristicof C-C, C-F, and C I O stretchesand are consistent with the identification of X as the trioxide CF3C(0)OOOCF3. The results presented above can be compared to an analogous study reported recently by Richer et al.,29 who irradiated Cl2/ CF3CHO mixtures in 720 Torr of air diluent. Observed products were COF,, CF30H, CO,, and an unknown which was believed to be CF3C(O)OOOCF3. Richer et al.29did not report any CF3OOOCF3 or CO products. The high oxygen partial pressure used by Richer et al. explains the absence of CO. There are two possible explanations for the absence of CF3000CF3. Either theinitial CF3CHO concentration was sufficiently high that CF30 radicals were efficiently scavenged by reaction 16 or CF3000CF3 was produced but not identified. The initial conditions used by Richer et al.Z9 are unclear, and it is not possible to distinguish between these two possibilities. Richer et al.29 did not report absolute product yields, and so a quantitative comparison of the results form the present work and those of Richer et al.29 is not possible. The aim of the present FTIR experiments was to establish the relative importance of reactions 1 and 2 as fates for CF3C0 radicals in the atmosphere, and not to conduct an exhaustive study of the products following the self-reaction of CF3C(O)O2 radicals. To measure kl/k,, we need only to consider two products: CO and COz. For each CFJCO radical that decomposes, one CO molecule is formed. For each CF3CO radical that reacts with 02, one CO2 molecule is formed. This statement is true irrespective of the identity of the unknown X, so long as sufficient time is allowed for the X to decompose. In the FTIR experiments the reaction mixtures were analyzed after X had decomposed completely. Assuming that reactions 1, 2, 7, 12, 13, 14a, 15-18, and -19
CF3C(0)OOOCF3+ M
-
CF30+ CF,C(O)O,
+M (-19)
describe the chemistry occurring following the irradiation of CF3CHO/C12/02/N2 mixtures and that CO and CO2 are not lost in any process, then the sum of the molar yields of CO and C02 should be 100%. Experimentally, this was observed to be the case. The relative yields of CO and COz were dependent upon the 0 2 partial pressure present in the reaction chamber. As the 02concentration increased, the CO2 yield increased a t the expense of CO. Figures 8 and 9 show results from the extreme cases where [O,] = 0.0 and 147 Torr. In 700 Torr of N2 diluent with no added 02,the CO yield was 93 f 8%, and traces of CO, were
5692
Wallington et al.
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 I
20,
15
o
I
A L
/10
A [CF,CHO],
o
(mTorr)
Figure 9. Plot of the observed yield of C02 as a function of the loss of CF3CHO following the irradiation of a mixture of 29.5 mTorr of CF3CHO and 112 mTorr of C12 at 700 Torr total pressure of air.
observed corresponding to a yield of 6 f 2%. In 700 Torr of air with [ 0 2 ] = 147 Torr, the yield of C02 was 97 f 4% with traces of CO observed corresponding to a yield of 5 f 2%. With 0 2 concentrations of 2-40 Torr, both CO and C02 were observed products. With the assumptions that (i) C O is produced only from the reaction 1, (ii) reactions 1 and 2 are the sole fate of CF3C0 radicals, and (iii) C O is not lost by any process, the following expression holds:
+ (k*/k,)[021
l/YCO = 1
+ M -,ClCO + M CF30 + CO CF, + C 0 2
-
CF302+ CO
-
CF30 + CO,
(21)
(22) (23)
The rate constant for reaction 21 (2.9 X cm3 molecule-' s-1 at 700 Torr of N2)16 is 62 times slower than for the reaction of C1 atoms with CF3CH0 (1.8 X 10-I2 cm3 molecule-I s-I).l3 For the typical range of CF3CHO conversions used in the present work (2-25%), loss of C O via reaction 21 is negligible. Quantitative assessment of the role of reactions 22 and 23 is hampered by the sparsity of the kinetic data for C F 3 0and CF3O2 radicals. To test for the importance of reactions 22 and 23, CF3CHO/C12/02/N2 mixtures were irradiated in the presence of 20-40 mTorr of added NO. N O reacts rapidly with C F 3 0 and CF302radicals, k24 = (5.2 f 2.7) X 10-11 24and k25 = 1.8 X 10-11 cm3 molecule-' s-1,18 thereby removing any possible complications caused by reactions 22 and 23.
CF30 + NO
+
-
CF302 NO
+ FNO CF30 + NO,
COF,
15,
I
'
1
(24) (25)
As seen from Figure 10, there was no observable difference in results obtained with, and without, N O present in the reaction
'
'
I
A %
(1)
where Ycoisthemolar yieldofCO, kl and kzaretherateconstants for reactions 1 and 2, and [02] is the concentration of 0 2 . Experiments were performed with the partial pressureof 02varied over the range 0-40 Torr at a constant total pressure of 700 Torr madeupwithN2diluent. Figure 10showsaplotof l/Ycoversus [O,]. The experimental data are clearly consistent with the functional form of expresson I. As noted above, in the derivation of expression I it is assumed that there are no significant losses of CO in the chamber. There are three species present in the chamber that may react with CO; C1 atoms, CF30, and possibly CF3O2 radicals.
C l + CO
Figure 10. Plot of the reciprocal of the molar CO yield versus the partial pressure of 0 2 following the irradiation of CF3CHO/Cl2/02 mixtures in 700 Torr total pressure of N2 diluent. Open symbols were obtained in the absence of NO. Filled pints were obtained with 25-30 mTorr of NO initially present in the reaction mixture.
0
100
200
300
400
500
600
700
800
P r e s s u r e (Torr)
Figure 11. Plot of kl/k2 as a function of total pressure of N2 diluent.
Open symbols were obtained in the absence of NO. Filled pints were obtained with 25-30 mTorr of NO initially present in the reaction mixture.
chamber, showing that reactions 22 and 23 are not a complication in the present work. Linear least-squares analysis of the data in Figure 10 gives a slope = k2/kl = (7.4 f 0.6) X cm3 molecule-I, and an intercept = 1.3 f 0.4. Quoted errors are 2 standard deviations. Thevalue of k2/kl= (7.4 f 0.6) X 1 k 1 8 cm3 molecule-' obtained at 700 Torr of N2 diluent is consistent with that of k2/kl = (6.1 f 4 . 2 ) X 10-i8cm3molecule-lat 1000mbarofSF6diluentobtained using the pulse radiolysis technique. To investigate the effect of total pressure on the rate constant ratio k2/kl, experiments were performed at total pressures ranging from 80 to 700 Torr (Nzused asdiluent). The results are presented in Figure 11. The data point a t 700 Torr was derived from the slopeof the plot in Figure 10; the error bars represent thestatistical uncertainty (2 standard deviations). All other data points in Figure 11 were derived from single determinations and hence have not been ascribed statistical uncertainties. It should be noted that the single-point determinations will be less certain than the value derived at 700 Torr from the composite data set shown in Figure 10. We estimate that the single-point determinations have a f20% uncertainty. From Figure 11 it can be seen that the rate constant ratio kl/k2 decreases as the total pressure is reduced. Another way to express this is to state that reaction 1 displays a greater pressure dependence than reaction 2. This finding is entirely consistent with the greater molecular complexity of the adduct in reaction 2. As seen from Figure 11, within the experimental uncertainties, there is no distinguishable difference between data acquired with and without NO. The rate constant ratio kl/k2 displays an
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5693
Atmospheric Chemistry of CF3C0, Radicals essentially linear dependence on total pressure over the range studied. The entire data set in Figure 11 have been arbitrarily fit using a linear least-squares regression to give kl/kz = (5.44 f 1.05) X 10-3[M] molecule cm-3 where [M] is the third-body concentration in molecule cm-3. Implicationsfor AtmosphericChemistry. The results presented here substantially improve our understanding of the atmospheric chemistry of CF3C0, radicals. As demonstrated here, CF3CO radicals undergo both decomposition to give CF3 radicals and CO, and reaction with 02 to give CF,C(0)02 radicals.
+ M .-,CF, + CO + M CF3C0 + 0, + M C F 3 C ( 0 ) 0 2+ M CF,CO
-
(1)
(2)
As shown herein, at 296 f 2 K and 700 Torr total pressure of Nzdiluent, k2/kl = (7.4f 0.6) X 10-18cm3molecule-1. Reactions 1 and 2 are of equal importance a t an 0 2 partial pressure of 4 Torr. In the presence of 760 Torr of air (160 Torr 0 2 ) , 97% of CF3CO radicals will react to give CF3C(0)02 with 3% decomposing to CF3 and CO. In the earth's atmosphere both temperature and pressure decrease with increasing altitude. It is important to consider the effect of changes in temperature and pressure upon the relative importance of reactions 1 and 2. A decrease in temperature will cause both reactions 1 and 2 to slow down. Reaction 1 is a unimolecular decomposition; the rate of reaction 1 is expected to decrease substantially as the temperature is lowered. In contrast, the rate of the bimolecular association reaction 2 is not expected to be strongly temperature dependent. Using the value of kl = (1.2 f 0.8) X 105 s-I measured in the presenceof 1 atm of SF6diluent in the present workand assuming a typical A factor for this type of reaction of lo1, s-I gives the activation energy for reaction 1 as 10.8 kcal mol-'. This estimate is consistent with an earlier estimate of E, = 10.0 kcal mol-' by Amphlett and Whittle30 and a recent calculation of E, = 11.4 kcal mol-l by Francisco31 but is inconsistent with the value of E, = 19.8 kcal mol-' deduced by Kerr and Wright.32 As noted by Francisco,31 the value reported by Kerr and Wright may be in error due to large uncertainties in the assumed parameters used in the RRKM modeling of the experimental data. With E, = 10.8 kcal mol-', the rate of reaction 1 is expected to drop by a factor of 1100 on moving from 296 to 215K (corresponding to an altitude of 20 km). In contrast, by analogy to the available data for the association reaction of CF3 radicals with 02, the rate of reaction 2 is not expected to decrease dramatically. Indeed, the high-pressure limiting rate constant for the association reaction between CF3 radicals and 0 2 actually increases by 40% over this temperature range.16 At reduced temperature, reaction 2 is then expected to be strongly favored over reaction 1. At high altitudes the total pressure is substantially less than that at sea level (at 20 km the total pressure is 50 mbar). The rates of both reactions 1 and 2 will decrease as the pressure is reduced. As expected on the basis of the numbers of degrees of freedom in the molecules and as demonstrated in the present work, reaction 1 is more sensitive to pressure than reaction 2. At reduced pressure, reaction 2 is then expected to be favored over reaction 1. The effects of reduced temperatures and pressures with increasing altitude reinforce each other in favoring reaction 2 over reaction 1. To quantify the relative importance of reactions 1 and 2 in the atmospheric chemistry of CF3CO radicals, a simple 1-D model was used with the following input and assumptions. First, temperature and pressure profiles were taken from the U S . Standard Atmosphere33 up to 20 km. Second, CF3CO radicals were assumed to be generated throughout the atmosphere at a rate which was linearly proportional to theatmospheric density (pressure). Third, the activation energy of reaction 1 was taken to be 10.8 kcal mol-', and reaction 2 was assumed to be temperature independent. Fourth, a ratio of k2/kl= (7.4 f 0.6)
X 10-18 cm3 molecule-l at 700 Torr and 296 K was used. Finally, it was assumed that the rate constant ratio kl/ka was linearly dependent on the total pressure. Under these assumptions it was calculated that 99.5%of CF3COradicals formed in the atmosphere are converted into CF,C(0)02 radicals. For all practical purposes the exclusive atmospheric fate of CF3CO radicals is reaction with 0 2 . Using kz = 7 X 10-13 cm3 molecule-1 s-I, we calculate a lifetime of 0.3 ps for C F 3 C 0 radicals with respect to conversion into CF3C(0)02 in 760 Torr of air. It is shown here that CF3C(O)O2 radicals react rapidly with N O to give CF,C(O)O radicals and NO2. Using k4 > 9.9 X 10-12 cm3 molecule-' s-1 together with a background tropospheric N O level of 2.5 X 108 cm-3 gives the lifetime of CF3C(0)02 with respect to reaction with N O as
