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Temperature dependent kinetics of the reaction OH + CF3CH2CF3
12 January 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 248 (1996) 296-300
Temperature dependent kinetics of the reaction OH + CF 3CH 2CF3 Nancy L. Garland, H.H. Nelson Chemistry Division, Code 6110, Naval Research Laboratory, Washington, DC 20375-5342, USA
Received 4 August 1995; in final form 12 October 1995
Abstract We have re-examined the kinetics of the reaction of OH with CF3CH2CF 3 (FC 236fa) between 281 and 337 K using highly purified reactant. After accounting for the contribution to the measured rate of all measured impurities, we derive a room temperature rate constant k = (4.15 + 0.33) × 10 16 cm 3 s - l, a factor of two smaller than our earlier report [J. Geophys. Res. 98 (1993) 23, 107] and in general agreement with other recent measurements.
I. Introduction The production of chlorofluorocarbons (CFCs) and halons is being phased out due to their role in the destruction of the ozone layer [1]. Because of the crucial role these compounds play as refrigerants and fire suppression agents, a concerted search is underway to find suitable replacements. Although the first test for a potential replacement compound must be its refrigerant or fire suppression properties, the atmospheric fate of potential replacements must be considered early in the selection process. For many applications, hydrofluorocarbons (HFCs) are being recognized as the replacement compounds of choice. These compounds do not contain chlorine or bromine atoms and therefore have essentially no ozone depletion potential [2]. They do, of course, contain numerous C - F bonds which result in infrared absorption around 1100 c m - 2, the middle of the 8 to 12 txm atmospheric transmission window. Thus, the HFCs have the potential to be potent
greenhouse gases if their atmospheric concentrations become large enough. HFCs have at least one C - H bond and are therefore susceptible to OH attack in the atmosphere. Particularly for the saturated compounds, the rate of reaction with OH limits the HFCs atmospheric lifetime and thus their m a x i m u m concentration. Knowledge of the temperature dependent rate of reaction with the OH radical is an important component in assessing the atmospheric consequences of the use of a particular HFC. In an earlier publication [3], we reported measurements of the OH reaction rates and integrated infrared absorption of three hexafluoropropane isomers. Since that work appeared, Zhang et al. [4], Hsu and DeMore [5], Nelson et al. [6] and Gierczak et al. [7] have also reported measurements on these species. For the isomer C F a C H F C H F 2 (FC 236ea), there is general agreement on the OH reaction rate constant [3-5]. For the isomer C F a C H 2 C F 3 (FC 236fa) the various measurements are not in as good agreement.
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0009-261 4(95)01288-5
N.L. Garland, H.H. Nelson / Chemical Physics Letters 248 (1996) 296-300
We used a laser flash photolysis/laser-induced fluorescence technique to study the reaction of OH with FC 236fa [3] and obtained a rate constant at room temperature of 9.6 × 1 0 - 1 6 c m 3 s - 1 and an activation energy of 1.8 _+ 0.3 kcal mol- i. Hsu and DeMore [5] used a competitive reaction technique with FTIR detection of the reactants to obtain a room temperature rate constant for OH + FC 236fa of 3.4)< 1 0 - 1 6 c m 3 s - l and E a = 4.5 kcal mol - l . Nelson et al. [6] used a discharge flow/laser-induced fluorescence technique to obtain a room temperature rate constant for this reaction of 5.9 × 1 0 - 1 6 c m 3 s - I . Most recently, Gierczak et al. [7] used a flash photolysis/laser-induced fluorescence technique to study the reaction between 269 and 413 K and they report results similar to those of Hsu and DeMore. Our earlier measurements of this reaction rate were made using a limited amount of the FC 236fa, of unknown purity, obtained directly from Dupont. This, in conjunction with the nearly factor of 3 spread in the reported room temperature rate constants and the fact that the US Navy is in the final stages of a decision to adopt FC 236fa as the replacement for R 114 in the fleet, prompts us to reexamine the reaction rate of OH + FC 236fa. We report here our measurements on this reaction using two different samples of FC 236fa, a commercial sample from PCR, Inc. and an ultrapure sample prepared for toxicological analysis by Dupont.
2. Experimental The experiments reported here are carried out in a similar apparatus to that used in our earlier work [3]. Briefly, the rate constants are measured using a laser pump-and-probe method. OH radicals are generated by 248 nm photolysis of HNO 3 using the unfocused output ( = 50 mJ per pulse) of a Lambda Physics 201MSC excimer laser. Care is taken to keep the photolysis beam diameter as large as possible ( = 1.8 c m 2 ) t o minimize the observed OH signal decay rate in the absence of added reactant. The radicals are probed by laser-induced fluorescence (LIF) excited using the frequency-doubled output of an excimerpumped dye laser, Lambda Physics model 101E/FL2002. The Q11 line of the A E ~ + - X 2[I i
297
(1, 0) band near 281.91 nm is used in most experiments. The photolysis and probe laser beams counterpropagate through the stainless-steel reaction cell that is contained in a convection oven for temperature control. To cool the cell for the lowest temperature experiment, the interior of the oven was packed with dry ice. Fluorescence is collected at fight angles to the beam paths using a two-lens telescope and focused through filters onto a photomultiplier tube, RCA 31000M. A bandpass filter ( A 0 = 3 1 1 nm, fwhm = 14 nm) blocks scattered dye laser light while transmitting OH fluorescence and a liquid CC14 filter blocks scattered photolysis light. Reaction cell temperature is measured using a copper-constantan thermocouple outside of the chamber and is found to be steady to 1 K. Dry HNO3, prepared using a standard procedure [8], is stored between runs in the dark under vacuum at 195 K. During kinetics experiments, the HNO 3 is contained in a glass saturator placed in an ice water bath. Nitric acid is introduced into the reaction cell by passing a slow flow (0.5-1.5 SCCM) of argon buffer gas over the liquid. This flow is combined with 2000 to 3000 SCCM of argon buffer gas and 0 to 1000 SCCM of reactant and introduced into the cell through one of the side arms. In addition, 50 to 75 SCCM of argon flows over the reaction cell windows to prevent deposition of photolysis products. Gas flows are measured upstream of the mixing point using calibrated gas flow meters and controllers. The cell pressure is measured using a manometer connected to the center of the cell and is 100 Torr for all experiments. Ar (Air Products, 99.997%), CH 4 (Matheson UHP, 99.97%), FC 236fa (PCR, 99%) and FC 236fa (Dupont, 99.88%) were used as received. Both hydrofluorocarbon samples were analyzed by gas chromatography with flame ionization detection [9]. Mass spectrometry was used to distinguish compounds whose peaks overlapped in the chromatogram. We measure OH decay rates by recording the OH LIF intensity as a function of time following the photolysis laser pulse. A minimum delay of 10 Ixs allows thermalization of the OH radicals. In the absence of added reactant, the OH decay is dominated by diffusion from the reaction zone with a small contribution from reaction with the HNO 3
N.L. Garland, H.H. Nelson / Chemical Physics Letters 248 (1996) 296-300
298
Table 1 Measured rate constants for the reactions of OH with CF3CH2CF 3 and CH 4 T (K)
Reactant
Purity (%)
k . . . . Jr 1 o" (10 -16 cm 3 s - t )
281
FC 236fa
99.88 99.97 99 99.88 99.97 99.88 99.88 99.97
3.93 + 0.15 50.7 + 1.8 17.3 + 2.6 4.55 + 0.12 63.8 -1-4.3 8.62 + 0.68 9.22 + 0.38 122 + 3
CH 4
298
325 337
FC 236fa FC 236fa CH 4 FC 236fa FC 236fa CH 4
precursor. Rate constants are obtained f r o m a plot o f O H d e c a y rate versus reactant partial pressure. All e x p e r i m e n t s are carried out under pseudo-first order conditions with the reactant H F C c o n c e n t r a t i o n at least f o u r orders o f m a g n i t u d e greater than the initial O H concentration.
3. Results and discussion Our initial m e a s u r e m e n t o f the rate constant for the reaction O H + F C 236fa was carried out using a c o m m e r c i a l F C 236fa sample with stated purity o f 9 9 % . S u b s e q u e n t e x p e r i m e n t s w e r e p e r f o r m e d using a purified sample f r o m the D u p o n t Haskell T o x i c o l o g y L a b o r a t o r y w i t h an a n a l y z e d purity o f 99.88% [9]. The results o f these m e a s u r e m e n t s are listed in T a b l e 1 a l o n g with error limits that reflect statistical uncertainty only. T o ensure that there are no system-
atic errors in our m e a s u r e m e n t s , we m e a s u r e d the rate constant o f the well-studied reaction o f O H with C H 4. Our results for this reaction are also listed in T a b l e 1. E a c h o f our m e a s u r e d rate constants for the O H + C H 4 reference reaction agree with the recent N A S A - J P L r e c o m m e n d e d v a l u e s [10] within 15%. A s seen in T a b l e 1, the m e a s u r e m e n t at r o o m temperature using the c o m m e r c i a l sample o f FC 236fa resulted in a m e a s u r e d rate constant e v e n larger than our p r e v i o u s l y reported value. A detailed c h e m i c a l analysis o f this sample was p e r f o r m e d after the kinetics m e a s u r e m e n t s [9]. E v e n after accounting for the contribution o f the impurities f o u n d in this analysis, the m e a s u r e d reaction rate constant obtained using this sample is c o n s i d e r a b l y h i g h e r than any o f the p r e v i o u s m e a s u r e m e n t s . E x p e r i m e n t s p e r f o r m e d using the purified sample o f F C 236fa f r o m D u P o n t y i e l d e d m u c h m o r e understandable results. The rate constant we obtain at
Table 2 Measured impurities in the Dupont FC 236fa sample and their contribution to the measured, room temperature rate constant Impurity
Concentration (ppm)
koH,298 (cm 3 s - i)
Contribution ( 1 0 - 16 cm 3 s - i)
C 3 H 4 F 4 (HFC 254)
546 362 134 51 38 26 7
1.9 × 1.6 × 6.3 × 4.3 X 1.6 × 6.8 × ???
0.102 0.058 0.0085 0.22 0.0061 0.0018
CzHaF 3 (HFC 143) C3H2F 6 (HFC 236cb) C2H5C1 (HCC 160) C2H2F3CI (HCFC 133a) C3H2F6 (HFC 236ea) unknown total measured k(OH + HFC 236fa) corrected k(OH + HFC 236fa)
a Extrapolated from HFC 236ea and HFC 245ca [5].
101010101010-
t4 a 14 [10] 15 [3] 13 [11] 14 [12] 15 [3]
0.40 4.55 4.15
N.L. Garland, H.H. Nelson/Chemical Physics Letters 248 (1996) 296-300
room temperature is below that we reported in our previous work [3] and in agreement with subsequent measurements [5-7]. A similar chemical analysis was performed on this sample before the kinetics experiments and the impurities found and their contribution to the measured rate constant are listed in Table 2. No olefins were identified in the analysis, however we note that an unidentified peak (approximately 7 ppm) is observed. Accounting for the contribution o f these impurities results in an approximately 9% reduction in the observed rate constant. This yields a room temperature rate constant for the OH + FC 236fa reaction of (4.15 _ 0.33) X 1 0 - 1 6 cm 3 s - i. This rate constant compares favorably with Hsu and D e M o r e ' s [5] measurement of (3.4 + 1.3) X 10-16 cm 3 s - l, Nelson et al.'s [6] value of (5.9T_t21~) X 10 - t 6 cm 3 s - l and Gierczak et al.'s [7] value of (3.2 + 0 . 6 ) × 10 -16 cm 3 s -1. If we assume that the unidentified 7 ppm impurity reacts with O H half as quickly as propylene at room temperature, the rate constant is reduced to 3.3 x 10 -]6 cm 3 s -1. Our corrected values at all the temperatures investigated are listed in Table 3 and shown plotted with other recent measurements in Fig. 1. The error limits listed in Table 3 are adjusted to account for possible systematic errors in our experiment. It appears that the value we measure at 281 K is larger than might be expected from the other data. Our measurement of the rate constant for the reference reaction, OH + C H 4 , is also high by 15% at this temperature. The higher rate constant cannot be explained by secondary reactions as the gas flow velocities (12 cm s -1 ) and laser repetition rate (11 Hz) were carefully chosen to reduce secondary reactions to negligible importance [3]. In addition, we carefully chose the concentration o f the OH precursor (1 mTorr) and the photolytic conditions (photolysis laser energy, beam
Table 3 Corrected rate constants for the reaction OH + CF3CH2CF 3 T (K)
k ...... ted -I- 1 O" (10-16cm3 s -I)
281 298 325 337
3.60 4.15 8.07 8.82
-F 0.27 _ 0.33 + 0.90 + 0.75
299 T (K)
400
350
300
i
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0.4 0.2 I
2.5
,
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i
i
I
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i
i
3.0
i
I
i
3.5
103/'r (K" 1) Fig. 1. Arrhenius plot of our measured rate constants for the reaction of OH with FC 236fa ( t ) . Also plotted are the data of Re/. [5] ( I ) , Re/. [6] ( A ) and Re/. [7] ( © ) .
size) such that the OH decay rate in the absence of reactant was controlled by diffusion out of the viewing region. The higher rate constant we determine in the present experiment may be the result o f incomplete thermalization o f the reaction cell at temperatures below ambient, with the gas at a slightly higher temperature than the outside wall of the reactor. A 6 K temperature differential would be sufficient to explain the discrepancy. It is now clear that the FC 236fa sample used in our earlier measurements had a significant impurity that contributed to our measured reaction rate with OH. Unfortunately, the sample used in the previous work was consumed in a series of experiments measuring its decomposition on a submarine air purification catalyst and so a detailed chemical analysis could not be performed. The results we report here are obtained using a well characterized sample of FC 236fa and have been corrected for the contribution from the low level of impurities. Our present results, both the room temperature value and the temperature dependence, agree well with three other recent measurements of this reaction rate constant [5-7]; all four studies o f this rate constant were carried out using different experimental techniques. These measurements result in a temperature dependent rate constant that is a reliable test for models of the reactivity of HFCs with OH.
300
N.L. Garland, H.H. Nelson / Chemical Physics Letters 248 (1996) 296-300
Acknowledgements We acknowledge the Office of Naval Research for funding this work through the Naval Research Laboratory.
References [1] J.P.D. Abbat and M.J. Molina, Ann. Rev. Energy Environ. 18 (1993) 1. [2] A.R. Ravishankara, A.A. Turnipseed, N.R. Jensen, S. Barone, M. Mills, C.J. Howard and S. Solomon, Science 263 (1994) 71 [3] N.L. Garland, L.J. Medhurst and H.H. Nelson, J. Geophys. Res. 98 (1993) 23, 107. [4] Z. Zhang, S. Padmaja, R.D. Saini, R.E. Huie and M.J. Kurylo, J. Phys. Chem. 98 (1994) 4312.
[5] K.-J. Hsu and W.B. DeMore, J. Phys. Chem. 99 (1995) 1235. [6] D.D. Nelson Jr., M.S. Zahniser and C.E. Kolb, submitted for publication. [7] T. Gierczak, R.K. Talukdar, J.B. Burkholder, R.W. Portmann, J.S. Daniel, S. Soloman and A.R. Ravishankara, submitted for publication. [8] H.S. Johnston, S.-G. Chang and G.Z. Whitten, J. Phys. Chem. 78 (1974) 1. [9] T. Kegelman, Dupont Haskell Laboratory for Toxicology and Industrial Medicine, Newark, DE, private communication (1995). [10] W.B. DeMore, S.P. Sander, D.M. Golden, M.J. Molina, R.F. Hampson, M.J. Kurylo, C.J. Howard and A.R. Ravishankara, Chemical kinetics and photochemical data for use in stratospheric modeling, Evaluation No. 11, JPL 94-26, Pasadena (1990) p. 1. [11] J.H. Kasner, P.H. Taylor and B. Dellinger, J. Phys. Chem. 94 (1990) 3250. [12] V. Handwerk and R. Zellner, Ber. Bunsenges. Physik. Chem. 82 (1978) 1161.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 248 (1996) 296-300
Temperature dependent kinetics of the reaction OH + CF 3CH 2CF3 Nancy L. Garland, H.H. Nelson Chemistry Division, Code 6110, Naval Research Laboratory, Washington, DC 20375-5342, USA
Received 4 August 1995; in final form 12 October 1995
Abstract We have re-examined the kinetics of the reaction of OH with CF3CH2CF 3 (FC 236fa) between 281 and 337 K using highly purified reactant. After accounting for the contribution to the measured rate of all measured impurities, we derive a room temperature rate constant k = (4.15 + 0.33) × 10 16 cm 3 s - l, a factor of two smaller than our earlier report [J. Geophys. Res. 98 (1993) 23, 107] and in general agreement with other recent measurements.
I. Introduction The production of chlorofluorocarbons (CFCs) and halons is being phased out due to their role in the destruction of the ozone layer [1]. Because of the crucial role these compounds play as refrigerants and fire suppression agents, a concerted search is underway to find suitable replacements. Although the first test for a potential replacement compound must be its refrigerant or fire suppression properties, the atmospheric fate of potential replacements must be considered early in the selection process. For many applications, hydrofluorocarbons (HFCs) are being recognized as the replacement compounds of choice. These compounds do not contain chlorine or bromine atoms and therefore have essentially no ozone depletion potential [2]. They do, of course, contain numerous C - F bonds which result in infrared absorption around 1100 c m - 2, the middle of the 8 to 12 txm atmospheric transmission window. Thus, the HFCs have the potential to be potent
greenhouse gases if their atmospheric concentrations become large enough. HFCs have at least one C - H bond and are therefore susceptible to OH attack in the atmosphere. Particularly for the saturated compounds, the rate of reaction with OH limits the HFCs atmospheric lifetime and thus their m a x i m u m concentration. Knowledge of the temperature dependent rate of reaction with the OH radical is an important component in assessing the atmospheric consequences of the use of a particular HFC. In an earlier publication [3], we reported measurements of the OH reaction rates and integrated infrared absorption of three hexafluoropropane isomers. Since that work appeared, Zhang et al. [4], Hsu and DeMore [5], Nelson et al. [6] and Gierczak et al. [7] have also reported measurements on these species. For the isomer C F a C H F C H F 2 (FC 236ea), there is general agreement on the OH reaction rate constant [3-5]. For the isomer C F a C H 2 C F 3 (FC 236fa) the various measurements are not in as good agreement.
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0009-261 4(95)01288-5
N.L. Garland, H.H. Nelson / Chemical Physics Letters 248 (1996) 296-300
We used a laser flash photolysis/laser-induced fluorescence technique to study the reaction of OH with FC 236fa [3] and obtained a rate constant at room temperature of 9.6 × 1 0 - 1 6 c m 3 s - 1 and an activation energy of 1.8 _+ 0.3 kcal mol- i. Hsu and DeMore [5] used a competitive reaction technique with FTIR detection of the reactants to obtain a room temperature rate constant for OH + FC 236fa of 3.4)< 1 0 - 1 6 c m 3 s - l and E a = 4.5 kcal mol - l . Nelson et al. [6] used a discharge flow/laser-induced fluorescence technique to obtain a room temperature rate constant for this reaction of 5.9 × 1 0 - 1 6 c m 3 s - I . Most recently, Gierczak et al. [7] used a flash photolysis/laser-induced fluorescence technique to study the reaction between 269 and 413 K and they report results similar to those of Hsu and DeMore. Our earlier measurements of this reaction rate were made using a limited amount of the FC 236fa, of unknown purity, obtained directly from Dupont. This, in conjunction with the nearly factor of 3 spread in the reported room temperature rate constants and the fact that the US Navy is in the final stages of a decision to adopt FC 236fa as the replacement for R 114 in the fleet, prompts us to reexamine the reaction rate of OH + FC 236fa. We report here our measurements on this reaction using two different samples of FC 236fa, a commercial sample from PCR, Inc. and an ultrapure sample prepared for toxicological analysis by Dupont.
2. Experimental The experiments reported here are carried out in a similar apparatus to that used in our earlier work [3]. Briefly, the rate constants are measured using a laser pump-and-probe method. OH radicals are generated by 248 nm photolysis of HNO 3 using the unfocused output ( = 50 mJ per pulse) of a Lambda Physics 201MSC excimer laser. Care is taken to keep the photolysis beam diameter as large as possible ( = 1.8 c m 2 ) t o minimize the observed OH signal decay rate in the absence of added reactant. The radicals are probed by laser-induced fluorescence (LIF) excited using the frequency-doubled output of an excimerpumped dye laser, Lambda Physics model 101E/FL2002. The Q11 line of the A E ~ + - X 2[I i
297
(1, 0) band near 281.91 nm is used in most experiments. The photolysis and probe laser beams counterpropagate through the stainless-steel reaction cell that is contained in a convection oven for temperature control. To cool the cell for the lowest temperature experiment, the interior of the oven was packed with dry ice. Fluorescence is collected at fight angles to the beam paths using a two-lens telescope and focused through filters onto a photomultiplier tube, RCA 31000M. A bandpass filter ( A 0 = 3 1 1 nm, fwhm = 14 nm) blocks scattered dye laser light while transmitting OH fluorescence and a liquid CC14 filter blocks scattered photolysis light. Reaction cell temperature is measured using a copper-constantan thermocouple outside of the chamber and is found to be steady to 1 K. Dry HNO3, prepared using a standard procedure [8], is stored between runs in the dark under vacuum at 195 K. During kinetics experiments, the HNO 3 is contained in a glass saturator placed in an ice water bath. Nitric acid is introduced into the reaction cell by passing a slow flow (0.5-1.5 SCCM) of argon buffer gas over the liquid. This flow is combined with 2000 to 3000 SCCM of argon buffer gas and 0 to 1000 SCCM of reactant and introduced into the cell through one of the side arms. In addition, 50 to 75 SCCM of argon flows over the reaction cell windows to prevent deposition of photolysis products. Gas flows are measured upstream of the mixing point using calibrated gas flow meters and controllers. The cell pressure is measured using a manometer connected to the center of the cell and is 100 Torr for all experiments. Ar (Air Products, 99.997%), CH 4 (Matheson UHP, 99.97%), FC 236fa (PCR, 99%) and FC 236fa (Dupont, 99.88%) were used as received. Both hydrofluorocarbon samples were analyzed by gas chromatography with flame ionization detection [9]. Mass spectrometry was used to distinguish compounds whose peaks overlapped in the chromatogram. We measure OH decay rates by recording the OH LIF intensity as a function of time following the photolysis laser pulse. A minimum delay of 10 Ixs allows thermalization of the OH radicals. In the absence of added reactant, the OH decay is dominated by diffusion from the reaction zone with a small contribution from reaction with the HNO 3
N.L. Garland, H.H. Nelson / Chemical Physics Letters 248 (1996) 296-300
298
Table 1 Measured rate constants for the reactions of OH with CF3CH2CF 3 and CH 4 T (K)
Reactant
Purity (%)
k . . . . Jr 1 o" (10 -16 cm 3 s - t )
281
FC 236fa
99.88 99.97 99 99.88 99.97 99.88 99.88 99.97
3.93 + 0.15 50.7 + 1.8 17.3 + 2.6 4.55 + 0.12 63.8 -1-4.3 8.62 + 0.68 9.22 + 0.38 122 + 3
CH 4
298
325 337
FC 236fa FC 236fa CH 4 FC 236fa FC 236fa CH 4
precursor. Rate constants are obtained f r o m a plot o f O H d e c a y rate versus reactant partial pressure. All e x p e r i m e n t s are carried out under pseudo-first order conditions with the reactant H F C c o n c e n t r a t i o n at least f o u r orders o f m a g n i t u d e greater than the initial O H concentration.
3. Results and discussion Our initial m e a s u r e m e n t o f the rate constant for the reaction O H + F C 236fa was carried out using a c o m m e r c i a l F C 236fa sample with stated purity o f 9 9 % . S u b s e q u e n t e x p e r i m e n t s w e r e p e r f o r m e d using a purified sample f r o m the D u p o n t Haskell T o x i c o l o g y L a b o r a t o r y w i t h an a n a l y z e d purity o f 99.88% [9]. The results o f these m e a s u r e m e n t s are listed in T a b l e 1 a l o n g with error limits that reflect statistical uncertainty only. T o ensure that there are no system-
atic errors in our m e a s u r e m e n t s , we m e a s u r e d the rate constant o f the well-studied reaction o f O H with C H 4. Our results for this reaction are also listed in T a b l e 1. E a c h o f our m e a s u r e d rate constants for the O H + C H 4 reference reaction agree with the recent N A S A - J P L r e c o m m e n d e d v a l u e s [10] within 15%. A s seen in T a b l e 1, the m e a s u r e m e n t at r o o m temperature using the c o m m e r c i a l sample o f FC 236fa resulted in a m e a s u r e d rate constant e v e n larger than our p r e v i o u s l y reported value. A detailed c h e m i c a l analysis o f this sample was p e r f o r m e d after the kinetics m e a s u r e m e n t s [9]. E v e n after accounting for the contribution o f the impurities f o u n d in this analysis, the m e a s u r e d reaction rate constant obtained using this sample is c o n s i d e r a b l y h i g h e r than any o f the p r e v i o u s m e a s u r e m e n t s . E x p e r i m e n t s p e r f o r m e d using the purified sample o f F C 236fa f r o m D u P o n t y i e l d e d m u c h m o r e understandable results. The rate constant we obtain at
Table 2 Measured impurities in the Dupont FC 236fa sample and their contribution to the measured, room temperature rate constant Impurity
Concentration (ppm)
koH,298 (cm 3 s - i)
Contribution ( 1 0 - 16 cm 3 s - i)
C 3 H 4 F 4 (HFC 254)
546 362 134 51 38 26 7
1.9 × 1.6 × 6.3 × 4.3 X 1.6 × 6.8 × ???
0.102 0.058 0.0085 0.22 0.0061 0.0018
CzHaF 3 (HFC 143) C3H2F 6 (HFC 236cb) C2H5C1 (HCC 160) C2H2F3CI (HCFC 133a) C3H2F6 (HFC 236ea) unknown total measured k(OH + HFC 236fa) corrected k(OH + HFC 236fa)
a Extrapolated from HFC 236ea and HFC 245ca [5].
101010101010-
t4 a 14 [10] 15 [3] 13 [11] 14 [12] 15 [3]
0.40 4.55 4.15
N.L. Garland, H.H. Nelson/Chemical Physics Letters 248 (1996) 296-300
room temperature is below that we reported in our previous work [3] and in agreement with subsequent measurements [5-7]. A similar chemical analysis was performed on this sample before the kinetics experiments and the impurities found and their contribution to the measured rate constant are listed in Table 2. No olefins were identified in the analysis, however we note that an unidentified peak (approximately 7 ppm) is observed. Accounting for the contribution o f these impurities results in an approximately 9% reduction in the observed rate constant. This yields a room temperature rate constant for the OH + FC 236fa reaction of (4.15 _ 0.33) X 1 0 - 1 6 cm 3 s - i. This rate constant compares favorably with Hsu and D e M o r e ' s [5] measurement of (3.4 + 1.3) X 10-16 cm 3 s - l, Nelson et al.'s [6] value of (5.9T_t21~) X 10 - t 6 cm 3 s - l and Gierczak et al.'s [7] value of (3.2 + 0 . 6 ) × 10 -16 cm 3 s -1. If we assume that the unidentified 7 ppm impurity reacts with O H half as quickly as propylene at room temperature, the rate constant is reduced to 3.3 x 10 -]6 cm 3 s -1. Our corrected values at all the temperatures investigated are listed in Table 3 and shown plotted with other recent measurements in Fig. 1. The error limits listed in Table 3 are adjusted to account for possible systematic errors in our experiment. It appears that the value we measure at 281 K is larger than might be expected from the other data. Our measurement of the rate constant for the reference reaction, OH + C H 4 , is also high by 15% at this temperature. The higher rate constant cannot be explained by secondary reactions as the gas flow velocities (12 cm s -1 ) and laser repetition rate (11 Hz) were carefully chosen to reduce secondary reactions to negligible importance [3]. In addition, we carefully chose the concentration o f the OH precursor (1 mTorr) and the photolytic conditions (photolysis laser energy, beam
Table 3 Corrected rate constants for the reaction OH + CF3CH2CF 3 T (K)
k ...... ted -I- 1 O" (10-16cm3 s -I)
281 298 325 337
3.60 4.15 8.07 8.82
-F 0.27 _ 0.33 + 0.90 + 0.75
299 T (K)
400
350
300
i
©
2.0
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o} 1.0 ~E 0.8 o 0.6 o
0.4 0.2 I
2.5
,
i
i
i
I
i
i
i
3.0
i
I
i
3.5
103/'r (K" 1) Fig. 1. Arrhenius plot of our measured rate constants for the reaction of OH with FC 236fa ( t ) . Also plotted are the data of Re/. [5] ( I ) , Re/. [6] ( A ) and Re/. [7] ( © ) .
size) such that the OH decay rate in the absence of reactant was controlled by diffusion out of the viewing region. The higher rate constant we determine in the present experiment may be the result o f incomplete thermalization o f the reaction cell at temperatures below ambient, with the gas at a slightly higher temperature than the outside wall of the reactor. A 6 K temperature differential would be sufficient to explain the discrepancy. It is now clear that the FC 236fa sample used in our earlier measurements had a significant impurity that contributed to our measured reaction rate with OH. Unfortunately, the sample used in the previous work was consumed in a series of experiments measuring its decomposition on a submarine air purification catalyst and so a detailed chemical analysis could not be performed. The results we report here are obtained using a well characterized sample of FC 236fa and have been corrected for the contribution from the low level of impurities. Our present results, both the room temperature value and the temperature dependence, agree well with three other recent measurements of this reaction rate constant [5-7]; all four studies o f this rate constant were carried out using different experimental techniques. These measurements result in a temperature dependent rate constant that is a reliable test for models of the reactivity of HFCs with OH.
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N.L. Garland, H.H. Nelson / Chemical Physics Letters 248 (1996) 296-300
Acknowledgements We acknowledge the Office of Naval Research for funding this work through the Naval Research Laboratory.
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