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Atmospheric Chemistry of CF3CFdCH2 and (Z)-CF3CFdCHF: Cl and NO3 Rate Coefficients, Cl Reaction Product Yields, and Thermochemical Calculations Vassileios C. Papadimitriou,†,‡,| Yannis G. Lazarou,§ Ranajit K. Talukdar,†,‡ and James B. Burkholder*,† Earth System Research Laboratory, Chemical Sciences DiVision, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, Colorado, 80305, United States, CooperatiVe Institute for Research in EnVironmental Sciences, UniVersity of Colorado, Boulder, Colorado, 80309, United States, and Institute of Physical Chemistry, National Centre for Scientific Research “Demokritos”, Aghia ParaskeVi 153 10, Attiki, Greece ReceiVed: October 19, 2010; ReVised Manuscript ReceiVed: NoVember 23, 2010
Rate coefficients, k, for the gas-phase reactions of Cl atoms and NO3 radicals with 2,3,3,3-tetrafluoropropene, CF3CFdCH2 (HFO-1234yf), and 1,2,3,3,3-pentafluoropropene, (Z)-CF3CFdCHF (HFO-1225ye), are reported. Cl-atom rate coefficients were measured in the fall-off region as a function of temperature (220-380 K) and pressure (50-630 Torr; N2, O2, and synthetic air) using a relative rate method. The measured rate coefficients are well represented by the fall-off parameters k0(T) ) 6.5 × 10-28 (T/300)-6.9 cm6 molecule-2 s-1 and k∞(T) ) 7.7 × 10-11 (T/300)-0.65 cm3 molecule-1 s-1 for CF3CFdCH2 and k0(T) ) 3 × 10-27 (T/300)-6.5 cm6 molecule-2 s-1 and k∞(T) ) 4.15 × 10-11 (T/300)-0.5 cm3 molecule-1 s-1 for (Z)-CF3CdCHF with Fc ) 0.6. Reaction product yields were measured in the presence of O2 to be (98 ( 7)% for CF3C(O)F and (61 ( 4)% for HC(O)Cl in the CF3CFdCH2 reaction and (108 ( 8)% for CF3C(O)F and (112 ( 8)% for HC(O)F in the (Z)-CF3CFdCHF reaction, where the quoted uncertainties are 2σ (95% confidence level) and include estimated systematic errors. NO3 reaction rate coefficients were determined using absolute and relative rate methods. Absolute measurements yielded upper limits for both reactions between 233 and 353 K, while the relative rate measurements yielded k3(295 K) ) (2.6 ( 0.25) × 10-17 cm3 molecule-1 s-1 and k4(295 K) ) (4.2 ( 0.5) × 10-18 cm3 molecule-1 s-1 for CF3CFdCH2 and (Z)-CF3CFdCHF, respectively. The Cl-atom reaction with CF3CFdCH2 and (Z)-CF3CFdCHF leads to decreases in their atmospheric lifetimes and global warming potentials and formation of a chlorine-containing product, HC(O)Cl, for CF3CFdCH2. The NO3 reaction has been shown to have a negligible impact on the atmospheric lifetimes of CF3CFdCH2 and (Z)-CF3CFdCHF. The energetics for the reaction of Cl, NO3, and OH with CF3CFdCH2 and (Z)-CF3CFdCHF in the presence of O2 were investigated using density functional theory (DFT). 1. Introduction Chlorofluorocarbons (CFC) and long-lived hydrochlorofluorocarbons (HCFC) are important anthropogenic sources of reactive chlorine to the stratosphere.1 The Montreal Protocol and subsequent amendments have phased out manmade production of many ozone-depleting substances necessitating the development of environmentally acceptable alternatives. Hydrofluorocarbons (HFC), which have essentially zero ozone depletion potential, have been extensively used as replacements for CFCs, e.g., CF3CH2F (HFC-134a) which was a replacement compound used in mobile air conditioning units. However, HFC134a, like most HFCs and HCFCs, is a potent greenhouse gas and is subject to use restrictions due to its potential contribution to climate change.2-5 Two alternative refrigerants currently under consideration are the unsaturated HFCs (hydrofluoroolefins, HFOs) CF3CFdCH2 (2,3,3,3-tetrafluoropropene, HFO* To whom correspondence should be addressed. E-mail: James.
[email protected]. † National Oceanic and Atmospheric Administration. ‡ University of Colorado. § Current address: Laboratory of Photochemistry and Chemical Kinetics, Department of Chemistry, University of Crete, Vassilika Vouton, 71003, Heraklion, Crete, Greece. | National Centre for Scientific Research “Demokritos”.
1234yf) and (Z)-CF3CFdCHF (1,2,3,3,3-pentafluoropropene, HFO-1225ye). The high gas-phase reactivity of HFOs, in general, leads to low global warming potentials, GWPs, for this class of compounds and, therefore, a desirable reduced climate forcing. Making informed decisions for commercial use of HFOs with regard to their environmental impact, however, requires a thorough understanding of their atmospheric processing on a case-by-case basis. Rate coefficients for the gas-phase reactions of CF3CFdCH2 and (Z)-CF3CFdCHF with the OH radical, a major atmospheric loss process for these compounds, were recently reported from this laboratory6 and elsewhere.7,8 The gas-phase reaction of these compounds with Cl atoms was suggested to be an important atmospheric loss process in certain locations.6 The fact that Clatom reaction rate coefficients are significantly greater than the analogous OH rate coefficients6-8 partially offsets the lower abundance of Cl atoms in most regions of the atmosphere.9,10 In this study, gas-phase rate coefficients for the reaction of Cl atoms with CF3CFdCH2 (k1) and (Z)-CF3CFdCHF (k2)
Cl + CF3CFdCH2 f products
10.1021/jp110021u 2011 American Chemical Society Published on Web 12/15/2010
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Figure 1. Schematic of the experimental apparatus used in the Cl-atom reaction relative rate study. FT-IR: Fourier transform infrared spectrometer (FT-IR).
Cl + (Z)-CF3CFdCHF f products
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were measured as a function of temperature (220-380 K) and pressure (50-630 Torr). Yields for several key stable degradation end products following reactions 1 and 2 in the presence of O2 were also measured. Rate coefficients for the gas-phase NO3 reactions
NO3 + CF3CFdCH2 f products
(3)
NO3 + (Z)-CF3CFCHF f products
(4)
were also measured, at 296 K, using a relative rate method. Finally, quantum mechanical calculations (density functional theory, DFT) were employed to further investigate the degradation mechanisms of reactions 1-4 and determine the thermochemically favorable reaction pathways. Calculations for the analogous atmospherically important OH addition reaction are also included for comparison. 2. Experimental Details Rate coefficients for the gas-phase reactions of Cl atoms and NO3 radicals with CF3CFdCH2 and (Z)-CF3CFdCHF were measured using separate experimental setups. Cl-atom rate coefficients were measured using a relative rate method as a function of temperature (220-380 K) and pressure (50-630 Torr). Yields of the stable CF3C(O)F, HC(O)F, and HC(O)Cl degradation products were measured following reactions 1 and 2 in the presence of O2. Rate coefficients for the NO3 reactions, reactions 3 and 4, were determined using relative rate (296 K) and absolute methods (233-353 K). The experimental apparatus and methods used in the Cl and NO3 studies are described separately below. 2.1. Cl Atom Reaction Rate Coefficient Measurements. A schematic of the experimental setup, which has been used previously in our laboratory, is shown in Figure 1.11 The apparatus consists of (1) a temperature-regulated Pyrex reactor, (2) a pulsed 351 nm excimer laser photolysis source used to produce Cl atoms, (3) a Fourier transform infrared spectrometer (FT-IR) used to monitor the reactant and product concentrations, and (4) a Teflon diaphragm pump used to circulate the reaction mixture between the reactor and the FT-IR absorption cell. Note
that the FT-IR absorption cell and circulating pump were operated at room temperature. The jacketed reactor was 85 cm long (4 cm i.d.) with quartz windows. The reactor temperature was controlled by circulating fluid from a temperature-regulated bath through its jacket. The reactor temperature was measured using a calibrated thermocouple with an accuracy of 0.5 K. The reactor window setup eliminated H2O condensation on the window exterior when the reactor was operated at low temperatures and confined the photolysis to the temperature-regulated region of the reactor. Cl atoms were generated by pulsed laser photolysis of Cl2 using a XeF excimer laser (351 nm) where the reactants and reference compounds have negligible absorption. The laser fluence was typically ∼10 mJ cm-2 pulse-1 but was varied between 5 and 20 mJ cm-2 pulse-1 over the course of the study. Infrared spectra were recorded over the range 500-4000 cm-1 at a resolution of 0.5 cm-1. The FT-IR absorption cell was 15.2 cm long with KBr or Ge windows. The volume of the infrared cell was significantly smaller than the reactor volume (∼1:30). Spectral subtractions were performed using a least-squares fitting routine on various infrared absorption bands for each compound. Reaction products were identified and quantified using measured reference spectra. Kinetic data were obtained using a conventional relative rate method in which the loss of the reactant compound was measured relative to the loss of a reference compound
(
ln
)
( )
kHFO [ref]0 [HFO]0 ) ln [HFO]t kref [ref]t
(I)
where [HFO]0 and [ref]0 are the initial concentrations of the HFO and reference compound, respectively, [HFO]t and [ref]t are the concentrations at time t, and kHFO and kref are the rate coefficients for the reaction of the reactant and reference compound. Equation I is valid provided that the HFO and reference compound are lost due to reaction with the same species, in this case, reaction with Cl atoms. Rate coefficients were determined using CH2dCH2 and CH3CH3 as reference compounds
Cl + CH2dCH2 + M f products + M
(5)
Chemistry of CF3CFdCH2 and (Z)-CF3CFdCHF
Cl + CH3CH3 f HCl + CH3CH2
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where reaction 5 is pressure dependent and the recommended rate coefficient is given by the fall-off parameters k0(T) ) 1.6 × 10-29 (T/300)-3.3 and k∞(T) ) 3.1 × 10-10 (T/300)-1.0.12 The rate coefficient for reaction 6 is well established with a recommended value of 7.2 × 10-11 exp(-70/T) cm3 molecule-1 s-1.12 Two independent reference compounds were used in this study to better define the uncertainty in the measured rate coefficients for reactions 1 and 2. CH3CH3 was used in the majority of the experiments, while CH2dCH2 was used in measurements at 298 K to test for possible systematic errors in our experiments. Several test measurements were also performed using CH3CH2CH3 as described in the Error Analysis section. Kinetic measurements were performed by first adding the HFO, reference compound, Cl2/N2, bath gas, and O2 to the reactor and mixing thoroughly. The homogeneity of the mixture was verified by FT-IR measurements. The reaction mixture composition was varied throughout the course of this study with initial pressures in the range 0.4-2.0 Torr (Cl2), 0.3-3.0 Torr (HFO), 0.6-2.7 Torr (CH3CH3 or CH2dCH2), 0-580 Torr (O2), and 50-600 Torr (N2, Synthetic Air, or N2 + O2). Next, an infrared absorption spectrum of the initial mixture was recorded. Then, the mixture in the reactor was photolyzed to produce Cl atoms in the presence of the reactants. Following photolysis, the sample was mixed and an infrared absorption spectrum recorded. The number of photolysis laser pulses and the initial Cl2 concentration determined the extent of reaction. Typically, the sample was exposed to ∼1000 photolysis pulses corresponding to a reactant loss of ∼5-10%. The sequence of photolysis and infrared measurement was repeated until the sample or the reference compound was significantly depleted. The loss of reactants and reference compounds under dark conditions, i.e., no laser photolysis, was measured in separate experiments and found to be negligible,
