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Rate Coefficients for the Gas-Phase Reaction of the Hydroxyl Radical with CH2dCHF and CH2dCF2 Munkhbayar Baasandorj,†,‡ Gary Knight,†,‡,| Vassileios C. Papadimitriou,†,‡,§ Ranajit K. Talukdar,†,‡ A. R. Ravishankara,† and James B. Burkholder*,† Earth System Research Laboratory, Chemical Sciences DiVision, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, Colorado 80305-3328, CooperatiVe Institute for Research in EnVironmental Sciences, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: January 19, 2010
Rate coefficients, k, for the gas-phase reaction of the OH radical with CH2dCHF (k1) and CH2dCF2 (k2) were measured under pseudo-first-order conditions in OH using pulsed laser photolysis to produce OH and laser-induced fluorescence (PLP-LIF) to detect it. Rate coefficients were measured over a range of temperature (220-373 K) and bath gas pressure (20-600 Torr; He, N2). The rate coefficients were found to be independent of pressure. The measured rate coefficient for reaction 1 at room temperature was k1(296 K) ) (5.18 ( 0.50) × 10-12 cm3 molecule-1 s-1, independent of pressure, and the temperature dependence is given by the Arrhenius expression k1(T) ) (1.75 ( 0.20) × 10-12 exp[(316 ( 25)/T] cm3 molecule-1 s-1; the rate coefficients for reaction 2 were k2(296 K) ) (2.79 ( 0.25) × 10-12 cm3 molecule-1 s-1 and k2(T) ) (1.75 ( 0.20) × 10-12 exp[(140 ( 20)/T] cm3 molecule-1 s-1. The quoted uncertainties are 2σ (95% confidence level) and include estimated systematic errors. The fall-off parameters for reaction 2 of k∞ ) 3 × 10-12 cm3 molecule-1 s-1 and k0(296 K) ) 1.8 × 10-28 cm6 molecule-2 s-1 with Fc ) 0.6 reproduce the room temperature data obtained in this study combined with the low pressure rate coefficient data from Howard (J. Chem. Phys. 1976, 65, 4771). OH radical formation was observed for reactions 1 and 2 in the presence of O2, and the mechanism was investigated using 18OH and OD rate coefficient measurements with CH2dCHF and CH2dCF2 over a range of temperature (260-373 K) and pressure (20-100 Torr, He). Quantum chemical calculations using density functional theory (DFT) were used to determine the geometries and energies of the reactants and adducts formed in reactions 1 and 2 and the peroxy radicals formed following the addition of O2. The atmospheric lifetimes of CH2dCHF and CH2dCF2 due to loss by reaction with OH are approximately 2 and 4 days, respectively. Infrared absorption spectra of CH2dCHF and CH2dCF2 were measured, and global warming potentials (GWP) values of 0.7 for CH2dCHF and 0.9 for CH2dCF2 were obtained for the 100 year time horizon. 1. Introduction Under the Montreal Protocol and its subsequent amendments and adjustments, chlorofluorocarbons (CFCs) have been phased out of production due to their detrimental effect on the stratospheric ozone layer. Hydrochloroflourocarbons (HCFCs) were used as first generation (transitional) replacements for CFCs in many commercial applications. Second-generation replacements have included hydrofluorocarbons (HFCs), which do not contain chlorine or bromine and therefore have negligible ozone depletion potentials (ODPs) and therefore are less harmful to the ozone layer. HFCs have a relatively short emission history, but the atmospheric abundance of several HFCs, such as HFC134a (CF3CH2F), are increasing rapidly as a result of increased commercial usage.1,2 In addition HFCs are also potent greenhouse gases, climate forcing agents, with significant global warming potentials (GWP). HFCs are currently considered under * To whom correspondence should be addressed. E-mail:
[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. | Current Address: Edwards Ltd, Kenn Business Park, Kenn Road, Clevedon, Somerset, BS21 6TH, U.K.
the Kyoto Protocol, which sets targets for industrialized countries to reduce greenhouse gas emissions.3 The contribution of HFCs to radiative forcing is projected to increase significantly as a result of population growth and increased use in developing countries.2,4,5 As a result, the next generation of replacement compounds that have negligible ODPs and GWPs are currently under consideration. Making good decisions for replacement compounds requires knowledge of the environmental impact of a replacement compound through an evaluation of its atmospheric lifetime, global warming potential (GWP), atmospheric degradation mechanism and end-products (and their potential impacts), and photochemical ozone creation potential (POCP). Unsaturated hydrofluorocarbons, compounds that contain carbon-carbon double bonds, are currently being considered as replacement compounds. In general, unsaturated hydrofluorocarbons are expected to have short atmospheric lifetimes due to their reactivity with the OH radical. Short atmospheric lifetimes are desirable and in general lead to lower GWPs for unsaturated hydrofluorocarbons compared to saturated HFCs. Therefore, the chemistry and impact of unsaturated hydrofluorocarbons on the environment is a current topic of scientific and societal interest. The primary atmospheric loss process for unsaturated hydrofluorocarbons is expected to be via gas-phase
10.1021/jp100527z 2010 American Chemical Society Published on Web 03/12/2010
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reaction with the OH radical. The reaction of OH with CH2dCHF and CH2dCF2
OH + CH2dCHF f products
(1a)
OH + CH2dCF2 f products
(2a)
the two simplest unsaturated HFCs provides valuable insight to the reactivity of larger and more complex fluoroalkenes. The reaction mechanism involves the formation of a reactive OHHFC adduct as the product with hydrogen atom abstraction expected to be a minor process under typical atmospheric conditions. The OH radical can add to either carbon atom, and the branching ratio for the reaction pathways is currently unknown. The branching ratio is most likely more important for more complex unsaturated HFCs as it may influence the yields of stable end-products and subsequent impacts on the environment. In the atmosphere, the OH-HFC adducts formed in reactions 1a and 2a react rapidly with O2 to form a reactive hydroxyperoxy radical, HORO2, where R ) CH2CHF and CH2CF2 for the molecules in our study. The reaction mechanism is outlined in Figure 1. The database for the reaction of OH with fluoroalkenes is increasing, but only two kinetic studies, performed over a limited range of experimental conditions, are currently available for the reaction of OH with CH2dCHF and CH2dCF2. The ability to predict the reactivity of fluoroalkenes, which is influenced by a combination of energetic, hydrogen bonding, and geometric factors, is limited, although increased reactant fluorination typically leads to lower reactivity for reactions that proceed via H atom abstraction. Perry et al.6 measured rate coefficients for reaction 1a at 299, 347, and 426 K and pressures of 50 and 100 Torr (Ar) and report a pressure-independent rate coefficient with an Arrhenius expression of k1a(T) ) 1.48 × 10-12 exp[(390 ( 150)/T] cm3 molecule-1 s-1. Howard7 reported a pressuredependent rate coefficient for reaction 2a at room temperature over the pressure range 0.7-7.0 Torr (He) with k2a(296 K) ≈ 2 × 10-12 cm3 molecule-1 s-1 at 7 Torr. There are no temperature-dependent or high-pressure studies of k2a currently available. In this work, kinetic studies of reactions 1a and 2a as a function of temperature (220-373 K) and pressure (20-600 Torr (He) and 20-500 Torr (N2)) using pulsed laser photolysis to produce the OH radical and laser-induced fluorescence (PLP-LIF) to detect it are presented. A regeneration of OH radicals was observed for both reactions in measurements made at low pressure. Even though the OH regeneration did not significantly interfere with the determination of k1a(T) and k2a(T), it provided an opportunity to explore the mechanism of these reactions in more detail. Reactions 1a and 2a were also studied using the 18OH and OD isotopes
OH + CH2dCHF f products
18
OD + CH2dCHF f products OH + CH2dCF2 f products
18
OD + CH2dCF2 f products
Figure 1. Reaction scheme for reactions 1a and 2a showing the reaction pathways for the addition of OH to CH2dCHF and CH2dCF2, where two addition sites are possible. The optimized geometries and energies, given in parentheses, of the OH-HFC adducts are shown. In the atmosphere the OH-HFC adducts will rapidly react with O2 to form stable hydroxyperoxy radicals, HORO2, which are also included. The optimized geometries and energies were calculated using density functional theory as described in the text.
(1b)
tions have been employed to evaluate the thermochemistry of the possible OH-HFC adducts formed in reactions 1a and 2a and for the hydroxyperoxy radicals formed following the addition of O2. Geometries and energetics were calculated using density functional theory (DFT) methods. Infrared absorption cross sections for CH2dCHF and CH2dCF2 were measured between 500 and 4000 cm-1 at 296 K. The atmospheric lifetime, radiative efficiency, and global warming potentials for CH2dCHF and CH2dCF2 are presented.
(1c)
2. Experimental Details
(2b) (2c)
in the absence and presence of O2 to identify the source and mechanism of the OH regeneration. Quantum chemical calcula-
Rate coefficients for reactions 1a and 2a were measured as a function of temperature, pressure, and bath gas using pulsed laser photolysis (PLP) to produce OH radicals and laser-induced fluorescence (LIF) to detect it. The PLP-LIF apparatus shown in Figure 2 has been used in our laboratory8,9 previously, and only the details relevant to the present study are described here. The key components of the experimental apparatus include (1) a LIF reactor where the OH radicals
Reaction of the OH Radical with CH2dCHF and CH2dCF2
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(CH3)3COOH + hV f product + OH
Figure 2. Diagram of the pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) experimental apparatus used for the OH + CH2dCHF and OH + CH2dCF2 rate coefficient measurements. The Fourier transform spectrometer had a 485 cm path length absorption cell and could be positioned either before the LIF reactor, as shown, or after. P: pressure gauge; PM: power meter.
were produced and detected, (2) a pulsed excimer laser photolysis source used to produce OH radicals, (3) a pulsed Nd:YAG pumped dye laser used to excite OH, and (4) a Fourier transform infrared (FTIR) spectrometer and UV absorption cells that were used to determine the reactant concentration in the LIF reactor. 2.1. OH Rate Coefficient Measurements. Reaction rate coefficients were measured under pseudo-first-order conditions in OH, [HFC] . [OH]. A jacketed 150 cm3 Pyrex cell housed in a vacuum chamber was used as the LIF reactor. The temperature of the reactor was controlled by circulating fluid from a temperature-regulated reservoir through its jacket. The temperature of the gas in the LIF reactor was measured directly with a retractable thermocouple and was accurate to within (1 K. Over the course of our study OH radicals were produced using several different 248 nm (KrF excimer laser) photolytic precursors including
H2O2 + hV f 2OH
(3)
HNO3 + hV f OH + NO2
(4)
and the reaction system
O3 + hV f O(1D) + O2
(5a)
f O(3P) + O2
(5b)
O(1D) + H2O f 2OH
(6a)
where k6 ) 2 × 10-10 cm3 molecule-1 s-1.10 The various OH radical sources were used to evaluate possible systematic errors in the rate coefficient measurements. H2O2 was the primary OH radical precursor used for rate coefficient measurements at temperatures >250 K. For temperatures below 250 K, the photolysis of HNO3 was used as the primary OH radical source. The O3 photolysis/reaction source was only used at 296 K. The UV photolysis of (CH3)3COOH (t-butylhydroperoxide)
(7)
was also used at temperatures in the range 220-296 K. (CH3)3COOH photolysis has not been used extensively in our laboratory prior to this study. Therefore, this study was also used to provide a comparison of this OH radical source with the more commonly used photolytic OH radical sources. The initial OH radical concentration, [OH]0, was estimated from the precursor concentration, its absorption cross section at 248 nm, and the photolysis laser fluence. The absorption cross section and the quantum yield of the precursors except (CH3)3COOH were taken from Sander et al.10 The absorption cross section and OH quantum yield in the photolysis of (CH3)3COOH at 248 nm were determined in our laboratory and are described elsewhere.11 The photolysis laser power was measured at the exit of the LIF reactor with a calibrated power meter. Photolysis laser fluences between 1 and 18 mJ cm-2 pulse-1 were used. The concentrations of H2O2, HNO3, and (CH3)3COOH in the LIF reactor were estimated from the pseudo-first-order rate coefficients measured in the absence of the reactant as described later. 18 OH and OD radicals were produced using 248 nm pulsed laser photolysis of O3 in a He bath gas to produce O(1D) followed by the reaction
O(1D) + H218O f
OH + OH
18
(6b)
or
O(1D) + D2O f 2OD
(6c)
The water vapor concentration (H2O, D2O, or H218O) was estimated from gas flow rates and pressures to be ∼2 × 1016 molecules cm-3. The H2O vapor concentration was sufficient to remove 99% of the O(1D) within 1 µs following the photolysis pulse. The O3 concentration in the LIF reactor was estimated from the measured gas flow rates and total pressure to be ∼1 × 1013 molecules cm-3. OH radical fluorescence was detected following pulsed laser excitation in the A2Σ+ r X2Π(V ) 0) transition near 282 nm with the frequency doubled output from a pulsed Nd:YAG pumped dye laser. 18OH and OD were excited using transitions near 282.07 and 287.6 nm, respectively. The probe laser beam propagated through the LIF reactor at a right angle to the photolysis laser beam. The photolysis and probe laser beams intersected in the middle of the reactor. The OH fluorescence from the reaction zone was detected by a photomultiplier tube (PMT) mounted orthogonal to the plane of the photolysis and probe laser beams. A band-pass filter (308 nm, fwhm ) 10 nm) mounted in front of the PMT was used to isolate the OH fluorescence. The PMT signal was averaged for 100 laser shots with a gated charge integrator. OH temporal profiles were measured by varying the delay between the photolysis and the probe lasers (i.e., the reaction time) between 10 and 50 000 µs. OH temporal profiles were measured under pseudo-first-order conditions
( ) ()
ln
[OH]t St ) ln ) -(k[HFC] + kd)t ) -k't [OH]0 S0
(I)
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where St is the measured OH signal at time t, which is proportional to [OH]t; [HFC] is the fluoroalkene concentration; and k′ and kd are the first-order rate coefficients for loss of OH in the presence and absence of the fluoroalkene, respectively. k′ values were obtained as the slope of a nonlinear least-squares fit of St versus time. kd represents the loss of OH due primarily to its reaction with the OH precursor and diffusion out of the detection volume. The actual values of kd depended on the OH radical precursor used and were in the range 50-500 s-1. OH temporal profiles were measured over a range of fluoroalkene concentrations at each temperature and pressure. Rate coefficients, k(T), were determined as the slope of k′ versus [HFC] using a weighted linear least-squares fit with the data weighted by the precision of the measurement. Gas flows were measured with calibrated electronic mass flow meters, and pressures were measured using 10, 100, and 1000 Torr capacitance manometers. The photolysis and probe lasers were operated at 10 Hz repetition rate. The gas flow velocity used was in the range 6-16 cm s-1 and ensured a fresh sample of gas in the LIF reaction volume for each photolysis pulse. 2.2. Absorption Measurements. The fluoroalkene (HFC) concentration in the LIF reactor was determined using in-line infrared and UV absorption measurements of the compound in the gas flow as well as from the measured gas flows and pressures. Infrared absorption spectra were recorded using a Fourier transform infrared spectrometer (FTIR) equipped with a multipass cell (485 cm optical path length, 750 cm3 volume) with KBr windows. Infrared absorption spectra of the sample were recorded either before the sample entered the LIF reactor or after it exited the LIF reactor. Infrared absorption cross sections for CH2dCHF and CH2dCF2 were measured as part of this work (see Atmospheric Implications section) between 500 and 4000 cm-1 at a spectral resolution of 1 cm-1. UV absorption measurements at 184.9 nm were made using a Hg pen-ray lamp light source and either a 15 cm or a 100 cm long absorption cell and a solar blind photodiode with a 185 nm bandpass filter. All absorption measurements were performed at room temperature, 296 K. The HFC concentration determined from the absorption measurements was scaled for the differences in pressure and temperature between the absorption cell and the LIF reactor to obtain the concentration in the LIF reactor. UV (184.9 nm) absorption cross sections for CH2dCHF and CH2dCF2 at 296 K were measured as part of this work. Broadband UV absorption spectra of these compounds were also obtained using a diode array spectrometer. Absorption cross sections were determined using absolute pressure measurements under static conditions using calibrated manometrically prepared mixtures of CH2dCHF and CH2dCF2 (0.5 and 1% of CH2dCHF in He and 0.1 and 1% of CH2dCF2 in He). UV (184.9 nm) measurements of CH2dCHF and CH2dCF2 were made using a Hg pen-ray lamp, a 50 cm long absorption cell, and a 185 nm band-pass filter mounted in front of a solar blind photodiode detector. The HFC concentration was varied over the range (5.41-37.3) × 1015 molecules cm3 for CH2dCHF and (0.52-4.46) × 1015 molecules cm3 for CH2dCF2. At least 15 different concentrations of the compound were used to determine the absorption cross section values. The measured absorbance, A, obeyed Beer-Lambert’s Law
()
A ) -ln
I ) σL[HFC] I0
(II)
where I and I0 are the transmitted intensity through the cell with and without the HFC sample, respectively; σ is the HFC
absorption cross section at 184.9 nm in cm2 molecule-1; L is the path length of the cell in cm; and [HFC] is the concentration of the fluorinated ethylene in units of molecules cm-3. A linear least-squares analysis of A versus [HFC] yielded the absorption cross section values (4.80 ( 0.02) × 10-19 cm2 molecule-1 and (6.31 ( 0.05) × 10-18 cm2 molecule-1 at 184.9 nm for CH2dCHF and CH2dCF2, respectively. The quoted uncertainties are at the 2σ level from the precision of the fit. Absorption cross sections obtained using different sample mixtures were identical within the precision of the measurement. The 184.9 nm absorption cross section for CH2dCF2 obtained in this work agrees to within 5% with the value reported by Limao-Wieira et al.,12 ∼6.6 × 10-18 cm2 molecule-1. 2.3. Materials. He (UHP, 99.999%), N2 (UHP, 99.99%), N2 (UHP, O2 < 0.5 ppm), Ar (UHP, O2 < 1 ppm), SF6 (99.999%), and O2 (UHP, 99.99%) were used as supplied. Concentrated H2O2 (>95% mole fraction) was prepared by bubbling N2 for several days through a sample that was initially ∼60% mole fraction. The H2O2 concentration in solution was determined by titration with a standard KMnO4 solution. Pure HNO3 was prepared by mixing KNO3 with concentrated H2SO4 under vacuum. HNO3 was collected from the vapor as a solid in a trap at dry ice temperature. H2O2 and HNO3 were introduced into the gas flow by passing a small flow of He through a bubbler containing the pure liquid samples at 273 K. H2O2 and HNO3 were added to the main gas flow just prior to entering the LIF reactor. CH2dCHF (98% pure) and CH2dCF2 (99% pure) samples were degassed in several freeze-pump-thaw cycles before use. Purity of the CH2dCHF and CH2dCF2 samples were analyzed using gas chromatograph with an Al2O3/KCl column (30 m × 0.53 mm) and a flame ionization detector. No detectable impurities were observed in the CH2dCHF and CH2dCF2 samples. Dilute mixtures of the HFC compounds in a He bath gas (1-5% in He) were prepared manometrically in 12 L Pyrex bulbs and used for the OH rate coefficient measurements. The mixing ratio of the HFC in the bulb was monitored periodically and was found to be stable to within ∼1% over the duration of our experiments. 2.4. Computational Methods. Quantum molecular calculations were used to evaluate the optimized geometries and energetics of the reactants and OH adducts formed in reactions 1a and 2a. Geometries and energies were also calculated for the hydroxyperoxy radicals formed following the addition of O2 to the OH-HFC adducts. Geometry optimization and vibrational frequency calculations were performed using the B3P86 method in conjunction with a AUG-cc-pV(D+d)Z basis set employing Gaussian 98 and Gaussian 03 program suites.13,14 Vibrational frequencies were scaled down by 0.9723. Absolute electronic energies were calculated employing B3P86/AUGcc-pV(T+d)Z level of theory. 3. Results and Discussion Rate coefficients for reactions 1a and 2a were determined between 220 and 373 K over the pressure range 20-600 Torr (He) and 20-500 Torr (N2). The OH temporal profiles measured at total pressures
