Environ. Sci. Technol. 1993, 27, 1448-1452
Atmospheric Chemistry of HC(0)F: Reaction with OH Radicals Timothy J. Walllngton' and Michael D. Hurley
Research Staff, SRL-E3083, Ford Motor Company, P.O. Box 2053, Dearborn, Michlgan 48 121-2053
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the method of generation of OH radicals. In the first technique, methyl nitrite was photolyzed to produce OH radicals. In the second technique, we employzed ozone photolysis to generate OH radicals. Both sets of experiments are discussed separately.
Introduction
Experiments Using Methyl Nitrite Precursor. The apparatus and experimental techniques used have been described in detail elsewhere (7)and are only briefly mentioned here. The FTIR system was interfaced to a 140-L Pyrex reactor. Hydroxyl radicals were generated by the photolysis of methyl nitrite (CHsONO) in air:
Relative rate techniques have been used to establish an cm3 molecule-l s-l for the rate upper limit of 4 X constant of the reaction of OH radicals with HC(0)F: OH + HC(0)F products. This result translates into alower limit of 10 years for the atmospheric lifetime of HC(0)F with respect to reaction with OH radicals. Reaction of OH radicals with HC(0)F is of negligible atmospheric importance. This finding is discussed with respect to the atmospheric chemistry of hydrofluorocarbons.
By international agreement, large-scale industrial production of chlorofluorocarbons (CFCs) will be phased out. Replacement compounds for CFCs are being considered at this time. Hydrofluorocarbons (HFCs) are one class of potential CFC substituents. The atmospheric chemistry of replacements is an important consideration in their use. In our laboratory we have investigated the products of the simulated atmospheric oxidation of a series of HFCs (1-4). For HFC-134a (CF~CFHZ) and HFC-41 (CH3F), we find that HC(0)F is a major oxidation product. For example, the OH radical initiated oxidation of HFC-134a in the atmosphere in the presence of NO, produces HC(0)F with a yield of 70% (in molar terms) (5):
+ + + + - +
OH + HC(0)F CF,CFH,(HFC-134a) CF,CFH
OH
NO
CF,CFHO
+ 0,
-
CF,CFH
+
(1) H,O (2)
+M
(3)
CF3CFH0 + NO,
(4)
0, M
CF,CFHO,
CF,CFHO
products
CF,CFHO,
CF,
HC(0)F
CF,C(O)F
+ HO,
(5a) (5b)
In the lower atmosphere, HC(0)F is expected to either undergo a gas-phase reaction with OH radicals to give FCO radicals or be incorporated into cloudwater-rainwater-seawater where hydrolysis will lead to HF and HCOOH, or both (6). Photolysis of HC(0)F is expected to be unimportant because of the high C-H bond dissociation energy (6). Assessment of the relative importance of reaction with OH radicals and incorporation into water as possible atmospheric loss mechanisms of HC(0)F is hampered by the lack of kinetic data for either process. To elucidate the atmospheric chemistry of HC(0)F and, hence, to improve the technical background relating to discussions of the environmental impact of CFC replacements, we have studied the kinetics of the gas-phase reaction of OH radicals with HC(0)F. Relative rate techniques were used a t 296 f 2 K and 700-740 Torr total pressure of air or oxygen. Results are reported herein. Experimental Section
Two different relative rate approaches were used in this work. The main difference between these approaches was ~
~~~
* Author to whom correspondence may be addressed. 1448 Envlron.
Scl. Technol., Vol.
27, No. 7, 1993
CH,ONO
--
+ hv
+ 0, HO, + NO
CH,O
CH30 + NO
(6)
+ HCHO
(7)
HO,
OH + NO,
(8)
Methyl nitrite was prepared as described previously (8). Prior to use, the purity of the methyl nitrite was checked using FTIR spectroscopy; no observable impurities were detected. All other reactants were obtained from commercial sources at purities of at least 99 % and were used as received. HC(0)F is unavailable commercially, however it can be produced in situ in approximately 90% yield by the oxidation of CH3F in air (I). In the present work, we have studied the rate of reaction of OH radicals with HC(0)F by monitoring the rate of HC(0)F formation following the OH-initiated oxidation of CH3F in 700 Torr of air. A reference compound whose reactivity with OH radicals is well-known was added to the reaction mixtures to provide a calibration of the exposure of the organics in the chamber to OH radicals. Reaction mixtures consisting of CH3F, acetylene, and methyl nitrite were diluted in synthetic air and introduced into the chamber. The reaction mixture was irradiated using the output from 22 UV fluorescent lamps (GTE F40BLB) for 10-60 min. In the presence of OH radicals there is a competition between reactions 9 and 10: OH
+ CH3F
--
products
(9)
OH + acetylene products (10) Providing that CH3F and acetylene are lost solely by reactions 9 and 10 and that neither organic is reformed in any process, then
where [CH3Flt, and [a~etylene]~, and [CHsFIt and [acetylenelt are the concentrations of CH3F and acetylene at times to and t, respectively, and kg and klo are the rate constants for reactions 9 and 10. The oxidation of CH3F is known to give HC(0)F as a major product (I). Hence, expression I can be rewritten 0013-936X/93/0927-144S$04.00/0
0 1993 American Chernlcal Soclety
as [CHSFlto
Gin(
[acetylene]
kg
1n(([CH3Fl,o- X[HC(O)F],,)=
1
[acetylene], (11)
where [HC(O)FItis the concentration of HC(0)F at time t and X is a numerical factor which relates the yield of HC(0)F to the loss of CH3F. Expression I1 holds only if the reaction of OH radicals with HC(0)F is of negligible importance. If reaction 1 competes for OH radicals then data plotted according to expression I1 will be markedly nonlinear. The approach used here was to search for such nonlinear behavior to quantify reaction 1. The loss of acetylene and the formation of HC(0)Fwere monitored by FTIR spectroscopy, using an analyzing path length of 28 m and a resolution of 0.25 cm-l. Infrared spectra were derived from 32 to 128 co-added spectra. Reference spectra were acquired by expanding known volumes of a reference material into the reactor. Initial concentrations of the gas mixtures and were 179-219 mTorr of CHsF, 1.6-3.0 mTorr of acetylene, and 0.1-0.3 Torr of CH30NO diluted in 700 Torr of air. Experiments Using Ozone Precursor. The photolysis of ozone at 254 nm in the presence of Hz (or HzO) was also used to generate OH radicals. This method is conceptually similar to that described recently by DeMore (9) and mimics the atmospheric photooxidation process. Hydroxyl radicals were formed by the following reactions
+ + - + + + + -
0,+ hv O(lD) H
O('D)
H2
OH
0 3 +OH
0,
(11)
H
(12)
0,
(13)
O(lD) H,O 20H (14) Once formed, the OH radicals react with CH3F in the reaction mixture to produce HC(0)F. The rate constant ratio kllkg can then be deduced by observing the dependency of the measured yield of HC(0)F on the consumption of CH3F. The greater the ratio k1/k9, the greater will be the curvature in a plot of the observed HC(0)F formation versus CH3F. Reactions 12 and 14 produce vibrationally excited OH radicals. The presence of 1atm of molecular oxygen diluent rapidly relaxes the OH radicals to a thermal distribution. The oxygen diluent will also relax a significant fraction of the electronically excited O(lD) atoms to the ground state O(3P). Ground-state oxygen atoms are scavenged by molecular oxygen to reform ozone. Experiments were performed in a 300-cm3cylindrical Pyrex reactor equipped with NaCl windows, enabling passage of both the IR analysis and the UV photolysis beams. Photolysis was achieved using a low-pressure Hg lamp, and typical irradiation periods were 10-45 min. The loss of CH3F and the formation of HC(0)Fwere monitored by FTIR spectroscopy, using an analyzing path length of 0.17 m and a resolution of 0.25 cm-l. Infrared spectra were derived from 32 co-added spectra. Reference spectra were acquired by expanding known volumes of a reference material into the reactor. The reaction cell was filled by following a stream of ozonized oxygen through the cell for 10-30 min. Gas-tight syringes were used to inject CH3F and Hz. Concentrations of CH3F and 0 3 in the reaction cell were determined by FTIR spectroscopy. The concentration of H2 was calculated by multiplying the measured CH3F concentration in the cell by the ratio of
700
720
740
760
1800
1840
1880
WAVEN U M B ER (c rn-' )
Flgure 1. Spectra taken before (panel A) and after (panel B) UV irradiation of a mixture of CHsF, C2H2, and CHoONO in 700 Torr air. The spectrum in panel C is a reference spectrum of HC(0)F.
the volumes of H2 and CH3F added. Initial concentrations in the reaction cell were as follows: 0 3 , 0.1 Torr; CH3F, 6 in 02 diluent 1-4 Torr; H2,10-40 Torr (or H ~ 0 , Torr); a t 740 Torr total pressure, and 295 K. In a limited series of experiments 1-2 Torr of CHI was added to the reaction mixtures. In all cases, the concentration of Hz employed was 10 times that of the added organics. Results
ExperimentsEmploying Methyl Nitrite Photolysis. Figure 1 shows infrared spectra in the regions 700-760 and 1800-1880 cm-' taken before (panel A) and after (panel B) a 20-min UV irradiation of a mixture of 185 mTorr of CH3F, 1.62 mTorr of acetylene, and 105 mTorr of CHaON0 in 700 Torr air diluent. The loss of acetylene, monitored using the feature at 730 cm-l, was 0.49 mTorr (30% of the initial concentration). The infrared product feature at 1836 cm-l in Figure 1B is attributed to the formation of 1.25 mTorr of HC(0)F. For comparison, our reference spectrum for HC(0)F is also given as panel C in Figure 1. HC(0)F is formed by the oxidation of CH3F by the following reactions:
---
+ CH3F CH,F + 0, + M CH,FO, + NO CH,FO, + NO OH
CH,FO
+ 0,
-
+ CH,F CH,FO, + M CH,FO + NO,
(16a)
CH,FONO,
(16b)
H,O
HC(0)F + HO,
(9) (15)
(17)
To relate the observed increase in the HC(0)F product to the loss of CHsF, we need to determine the yield of HC(0)F in such systems. To provide this information, experiments were performed in which mixtures of CH3F Environ. Sci. Technol., VoI. 27, No. 7, 1993
1449
possible loss of CH3F and CHI by reaction with ozone, reaction mixtures were made up and allowed to stand for 2 h in the dark. No loss (
