J . Phys. Chem. 1990, 94, 1393-1400
1393
Kinetics of the Reactions of CFpCIOpRadicals with Nitrogen Dioxide Steven B. Moore and Robert W. Cam* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received: January 18, 1989; In Final Form: August 28, 1989)
The kinetics of the reaction of CF2C102radicals with NO2 at 298 K, investigated by flash photolysis with time-resolved mass spectrometry, are reported. The peroxy radical, CF2C102,was formed in broadband flash photolysis of CF2C1Br in the presence of O2and detected mass spectrometrically via the fragment ion, CF202+. With NO2 present, peroxy radical decay occurs by pseudo-first-order kinetics. The decay rates are also first order in NO2. The second-order rate coefficients are pressure dependent over the 1-10 Torr experimental range. The pressure-dependent rate coefficients were fitted by the Troe expression for termolecular reactions. Numerical simulation of the peroxy radical decay rates, using a mechanism consisting of known reactions that also occur in this system, were done to investigate the influence of secondary reactions on the measured rates.
Introduction This paper reports the determination by flash photolysis with time-resolved mass spectrometry of the room temperature rate constant for the termolecular reaction CFzClO2 NO, M CFzC102N02 + M (1)
+
+
+
This reaction must certainly occur in the stratosphere and play a role in the rate of CI release from halocarbons present there. The primary source of chlorine in the stratosphere is man-made chlorofluorocarbons'q2 (mainly CF2CI2and CFC13), which are chemically inert in the troposphere, and are slowly transported into the stratcsphere over a period of many years after their release. In the stratosphere, these compounds are photolyzed by shortwavelength ultraviolet radiation and are removed to a lesser extent by reaction3 with O(ID). During the process initiated by these steps the C1 atoms contained in the molecules are released. A plausible detailed mechanism4-I2 for the breakdown of chlorofluorocarbons in the stratosphere is CF,CI, h v ( O ( * D ) )-+ CF,CI,, + CI(C10) (2)
+
+ 02 + M CF,CI,IOI + N O
CF,Cl,I
+
-+
CF,CI,IO
+
+M
(3)
+ NO2
(4)
CF,C1,102
CF,CI,lO
CF,C1,20
+ C1
(5)
Following the initial photolysis step, the above mechanism predicts the rapid release of another CI atomI3 by reaction 5, with the remaining CI ultimately released by the photodissociation of the carbonyl halides. However, a possible competitor with N O for reaction with CF,C1,102 in the above mechanism is NO2, which can form a haloperoxynitrate via the reaction CF,C1,102 NO2 M -+ CF,Cl,I02N02 M (6)
+
+
+
Depending on the stability of the CF,C1,,02N02 formed, this reaction might effectively sequester the chlorine atoms in a haloperoxynitrate molecule. The reaction of halomethylperoxy ( I ) Molina, M. .; Rowland, F. S. Nature 1974, 249, 810. ( 2 ) Rowland, F. S.; Molina, M. J. Rev. Geophys. Space Phys. 1975, 13, I. (3) Brasseur, G.;Solomon, S. Aeronomy of the Middle Atmosphere; D. Reidel: Boston, 1984. (4) Simonaitis, R. In Proceedings of the NATO Aduanced Study Institute On Atmospheric Ozone, Algarve, Portugal; 1979; p 501. ( 5 ) Suong, J. Y.; Carr, R. W. J. Photochem. 1982, 19, 295. (6) Plumb, I. C.; Ryan, K. R. Chem. Phys. Lett. 1982, 92, 236. (7) Ryan, K. R.; Plumb, 1. C. J . Phys. Chem. 1982, 86, 4678. (8) Ryan, K. R.; Plumb, 1. C. Int. J. Chem. Kinet. 1984, 16, 591. (9) Caralp, F.; Lesclaux, R. Chem. Phys. Lett. 1983, 102(1), 54. (IO) Dognon, A.; Caralp, F.; Lesclaux, R. J . Chim. Phys. 1985,82, 349. ( I 1) Dognon, A. Etude cinttique de I'oxydation d'halomtthanes d'inttr8t atmospherique, Ph.D. dissertation, L'Universit6 de Bordeaux 1. (12) Lesclaux, R.; Caralp, F. Int. J. Chem. Kinet. 1984, 16, 1117. (1 3) Lesclaux, R.; Caralp, F.; Dognon, A,; Cariolle, D. Geophys. Res. Lett. 1986, 13, 933.
0022-3654/90/2094- 1393$02.50/0
radicals with nitric oxide and nitrogen dioxide1*l2 have been studied by flash photolysis using an excimer laser as the light source with time-resolved mass spectrometry. The effect of reaction 6 on the release of C1 has been estimated for the case of CFCl2O2,which has CFC1, as a precursor, but the rate constant for reaction when CF,C1,,02 is CF2C102(which has CF2C12as a precursor) has not been previously reported. Experimental Method The reaction is initiated by broadband flash photolysis, and the concentration of the peroxy radical intermediate is followed by time-resolved mass spectrometry. The experimental apparatus consists of a gas handling vacuum system, a quadrupole mass spectrometer with an electron bombardment ion source and a Daly detector with pulse counting electronics, a flash lamp, and a chemical reactor. A diagram of the experimental apparatus is shown in Figure 1. All of the reaction rate measurements were carried out at 25 OC. The radiation from the capacitive discharge flash lamp (Xenon Corp. Model N-734 with a Suprasil envelope, flash duration approximately 3 X 10" s) is focused into the reactor by means of a Suprasil lens (Suprasil is ultraviolet-grade fused silica manufactured by Amersil, Inc., Hillside, NJ). The optical path was purged with flowing N, to prevent absorption of vacuum ultraviolet radiation by 02.The lamp is pulsed about 6 times per second at a discharge voltage of 7.5 kV. A diagram of the reactor is shown in Figure 2. The reactor consists of a 3.3-cm slot milled in a stainless-steel disk. Gas continuously flows into the reactor through a 5/ 16-h-diameter tube and exits via a 3/ 16h-diameter tube at a rate sufficient to remove the reaction products and renew the reactant mixture between photolysis flashes. A pinhole that admits a continuous sample of the reactor gas into the mass spectrometer ion source was made by drilling a 100-pm-diameter hole in 25-pm-thick gold foil and attaching the foil with lowvapor-pressure epoxy cement (Torr Seal) over a centered 1.6"diameter hole drilled through the reactor bottom. The stainless-steel reactor body was plated with gold to minimize possible catalytc reactions on its surface. A 10-mm-thick Suprasil window covers the reactor and is held in place with low-vaporpressure epoxy cement. A thin aluminum plate masks the epoxy holding the window in place from the ultraviolet light of the flash lamp; it also defines the length of the reactor exposed to ultraviolet light. The flash lamp initiates photochemical reactions only in the exposed region of the reactor. The pressure in the reactor is monitored by two capacitance manometers (Baratron Type 222A and Type 222B), one with a 0-10 Torr range and the other with 0-1000 Torr range. These gauges are also used to measure the pressures in the glass storage system used to prepare the reaction mixtures. The pressure can be controlled within about 1% by adjusting the inlet needle valve. The pinhole leaks a small fraction of the reaction mixture into the ion source of a quadrupole mass spectrometer (Extrel4-270-9). 0 1990 American Chemical Society
Moore and Carr
1394 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
Data are accumulated in the multichannel scaler over many flashes and then transferred to a microcomputer for further analysis. The peroxy radicals were produced by photolyzing bromochlorodifluoromethane in the presence of oxygen:
+ hv CF2C1 + 0, + M CF,CIBr
IHI Uullm.nn.1
Yoocompllw
SC.1.t
Figure 1. Schematic diagram of the flash photolysis with time-resolved
mass spectrometry experimental apparatus. The ultraviolet flash lamp creates reactive species in the reactor and the transient reactor concentrations are mass spectrometrically sampled via a small pinhole leak. The signal from the repetitive lamp flashes is accumulated in a multichannel scaler.
gold toll wllh .1 mm plnholo
1
Figure 2. Diagram of reactor. The reactor body is constructed of stainless steel, and the area in contact with the reactor contents is gold plated to suppress surface reactions.
-
A large oil diffusion pump (pump speed 1500 s-I) backed by a mechanical pump maintains a vacuum of less than 3 X Torr in the ion source during an experiment. Ions created in the ion source are extracted and focused into the mass spectrometer by a series of electrostatic lenses. After being filtered by the mass spectrometer, the ions are focused by another electrostatic lens into a Daly detector. The Daly detectorI4J5produces an output current pulse for each ion that passes through the mass spectrometer. A turbomolecular pump differentially pumps the mass spectrometer and Daly detector regions maintaining the pressure at -5 X 1 0-7 Torr even when gas is entering the vacuum chamber via the pinhole. The Daly detector high-voltage bias, photomultiplier voltage bias, and the discriminator level in the pulsecounting apparatus were adjusted to maximize the signal-to-noise ratio. The Daly output current pulse is amplified, counted, and then stored digitally in the multichannel scaler. The multichannel scaler is triggered by the lamp flash and sequentially records the number of pulses received in each increment of time following the flash.
-
+ Br CF2C102 + M
CF2Cl
+
(7)
(8)
Bromochlorodifluoromethane was chosen as the source of the chlorodifluoromethyl radicals because of its relatively large absorption cross section and the lower reactivity of the bromine radicals (as compared to C1) produced in the photolysis. The excess of O2 present assured that the CF2Cl radicals produced by the flash were converted to peroxy radicals on a much shorter time scale than subsequent reaction with NO,. The typical reaction mixture consisted of CF2CIBr, o,, and NO2. CF2C1Br and O2 form the bulk (greater than 99.9%) of the mixture with the CF2C1Br/02ratio being held constant at 4. The mole fraction of NO, varied from 0.0005 to 0.04 depending on the experiment. The reaction mixture was prepared at room temperature in a conventional grease-free glass vacuum system. The gases were mixed in blackened glass storage vessels with connecting glass tubing painted black to prevent photochemical reactions in the system. Each storage vessel has a volume of 5 L which is sufficient to store enough gas for 2-5 h of operation. Magnetic stirrers are used to spin mixing vanes in the bulbs. Although no significant variation in the gas composition could be detected with the mass spectrometer after 40 min of stirring, gases were mixed for 2 h to ensure adequate mixing for the experiments and the gases were stored overnight before use. The NO, used in the experiment was supplied by Matheson Gas Products and had a minimum purity of 99.5%. The CFCI, used was also supplied by Matheson Gas Products and had a minimum purityof 99.9%. The O2 used in the experiment was provided by Northern Cryogenics with a minimum purity of 99.97%. CF2ClBr was obtained from SCM Specialty Chemicals at a purity of 99.9%. After introduction into the system the gases (except 0,) were degassed by repetitive freeze-thaw cycles with liquid nitrogen until there was no detectable residual pressure. Both the NO2 and 0, were dried by passing them over CaS04. To make accurate rate constant measurements for the reaction of NO, with peroxy radicals, it is necessary to exclude NO from the reaction mixture since NO reacts with peroxy radicals via reaction 4. The equilibrium ratio of NOINO, is insignificantI6 in the storage bulbs as the few Torr of NO2 present is mixed with 5C-IO0 Torr of 02.Even for the much lower 0, partial pressures in the reactor, the calculated N O / N 0 2 equilibrium ratio is less for all the experiments. This small amount of NO than 5 X would only change the measured rate constant by about 0.1% at most. However, N O can be produced photochemically in the ;~~~'~ visible or ~ltraviolet.'~The reverse reaction is S ~ O Wtherefore, as mentioned above, light was excluded as much as possible from the vacuum system. Also, the equilibrium between NO2and N204 is important; even at a few Torr of pressure pure NO, contains a few percent of N 2 0 4 . The measured partial pressure of NO2 (5-10 Torr) used in preparing a reaction mixture was assumed to include an equilibrium mixture of NO2 and N2O4. The partial pressure of NO, in the reactor was calculated by assuming the N204was completely dissociated at the NO2 partial pressure in the reactor (
