J . Phys. Chem. 1993,97, 11464-1 1473
11464
Temperature-Dependent Rate Constants and Product Branching Ratios for the Gas-Phase Reaction between CH302 and C10 Frank Helleis, John N. Crowley,. and Geert K. Moortgat Max- Planck- Institut fur Chemie. Division of Atmospheric Chemistry, Postfach 3060, 55020 Mainz, Germany Received: April 29, 1993; In Final Form: August 5, 1993'
Kinetic and product branching data have been measured for the reaction of CH3O2 radicals with C10 in the temperature range 225-355 K using the discharge-flow/mass spectrometry technique. The pressure-independent overall reaction rate constant is described by k(12)(225-355 K) = (3.25 f 0.50) X 10-12 exp((-114 & 38)/T) cm3 molecule-' s-l. Two reaction channels were identified, leading to C H 3 0 ClOO (12a) and CHJOCl 0 2 (12b), respectively. The branching ratios, alza = klza/k12 and (Y12b = kl~b/k12,are also independent of pressure and are described by a12a = (1.51 f 0.56) exp((-218 f 93)/7') and (Y12b = (0.080 f 0.059) exp((377 f 178)/T). These expressions yield roughly equal rate constants of klza and k12bof ca. 1 X cm3 molecule-' s-1 at the low temperatures prevalent in the polar winter and early springtime stratosphere. We thus identify CH3OCl as a potentially important species in ozone hole chemistry.
+
+
1. Introduction
The Severe in Ozone first Observed during the to be the Of Antarctic spring in 19851are now catalytic reaction sequences involving the chlorine monoxide radical (C10). Simultaneous observations of Ozone depletion, and high clo concentrations (see refs and for review) are largely explained in terms of a reaction mechanism including formation of the C10 dimer (ClOOCl) and subsequent photolysis to regenerate C1 atoms and C10.4
-
+ C10 + M ClOOCl + M ClOOCl + hv C1+ ClOO ClOO + M C1+ 0, + M 2(C1+ 0,) 2(C10 + 0,)
C10
-
+ -
(1) (2)
(3) (4)
net: 20, 30, (5) These catalytic sequences are made possible by heterogeneous reactions that transform NO, into HN03s96and thus prevent the removal of ClO via reaction with NO,: C10
+ NO,
M
ClONO,
+M
(6) Polar stratospheric cloud (PSC) particles, consisting mainly of nitric acid trihydrate,'J provide reactive surfaces to convert stable C1-containing reservoir species into the active chlorine species which initiate catalytic destruction of ozone under sunlit conditions. Heterogeneousreactions (het) of the principal chlorine reservoir HCI with, for example, chlorine nitrate,9JO and with HOCl,llJ2 generate photolabile C12. ClONO,
+ HCl
hct
C1,
+ HNO,
(7)
hct
+ HOCl C1, + H,O C1, + hv + C 1 + C1
HCl
(8)
(9)
Modeling studies have recently shown that reaction 8 is an extremely important route for conversion of HCl into Cl2 under conditions in which nitrogen-containingreservoir species such as Correspondence to this author. Abstract published in Advance ACS Absrracrs, September 15, 1993.
ClONO2 and N205 have been converted into inactiveHN03.13.14 In addition, Crutzen et al.14 have suggested that reaction 8, in conjunction with a gas phase reaction between 1 ('0 and CH302, may help to account for the almost complete conversion ofHC1 into C10, species and hence more rapid ozone depletion. Central to the hypothesisis a relatively rapid reaction (12a) (kl% = 10-12 cm3 molecule-l s-l)which generates the methoxy radical, CH30. Subsequent reactions convert C H 3 0 to HO2, supplementing the generation of HO, via known reactions involving o3 and OH. H02 may then react with C10 to make HOCl which is eventually converted to Cl2 via reaction 8 on a PSC particle: C1+ CH4
- + - + - + -+ + - + - + - +
+ 0,
C H 3 0 2 HCl
(10)
0,
(11)
C1+ 0, C10
+
C H 3 0 2 C10
CH30
ClOO
+ M Cl 0, M C H 3 0 + 0, HO, H C H O H C H O + hv (+20,) CO 2H0, HO, + C10 HOCl 0, ClOO
(12a) (13) (14) (15) (16)
To date, the reaction between C10 and CH3O2 has been the subject of two experimental studies. The first measurement's of a rate constant for reaction 12 was made using the molecular modulation technique, generating both CH3O2 and C10 in the broad band photolysis of a C12-CH4-C120-02 mixture at 300 K and at a pressure of 230-240 Torr. A rate constant of (3.1 1.7) X 10-12 cm3 molecule-' s-* was derived by least-squares fitting of measured C10 concentration profiles to an assumed reaction scheme in order to separate primary and secondary chemistry and deconvolute composition absorptions. Product measurements using both FTIR (for detection of HCHO and HCOOH) and UV absorption spectroscopy (for detection of OC10) enabled the authors to conclude that the major reaction channel ( 8 5 f 15%) leads to C H 3 0 + ClOO formation (reaction 12a)
*
+
C H 3 0 2 C10
-
CH30
+ ClOO
(12a) CH30C1, OC10, and O3 were not detected, and the authors concluded that channels 12b, 12c, and 12e are unimportant. The generation of HCHO in this study was thought to be via the
0022-3654/93/2097- 11464%04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 44, I993 11465
CH3O2 and C10 Gas-Phase Reaction methoxy radical (reaction 14), though a contribution from a channel directly forming HCHO + HCl + 0 2 (12d) could not be ruled out.
CH30, + C10
-
-
CH,OCl
+ 0,
MW Dischargr F2-He
( 12b)
CH30 + OClO
(1 2c)
HCHO + HC1+ 0,
(12d)
--
+iuy NO
Sliding
+
CH,C1 0, (12e) DeMorel6 undertook a low temperature study (197-21 7 K) of this reaction, in this case employing photolysis of Cl2-CH4-03O2-N2 mixtures to generate C10 and CH3O2. Ozone removal rates and product measurements provided kinetic and mechanistic information, respectively. Unable to detect deviations in ozone decay rates from that expected in the absence of a coupling of the C10 and CH3O2 chemistry, DeMore set a conservative upper limit of 4 X 10-12 cm3 molecule-' s-1 a t 200 K for a reaction channel that may regenerate C1 atoms from C10, i.e. (12a), although the experimental results were compatible with a rate constant of k(1h) = 1 X 10-12cm3molecule-' s-1. Product analysis enabled Demore to set upper limits on channels k(l2b) (based on CH30Cl measurements) and k(12c,(based on OClO measurements) of 1 X 10-12and 1 X le15cm3molecule-1 s-1, respectively, at ca. 200 K. As discussed by DeMore, the upper limit for a CH30Cl forming channel is likely to be inaccurate as infrared absorption cross sections are not known for this molecule. Both studies are thus fairly indirect, and the 200 K results of DeMore are presently preferred over the 300 K data of Simon et al. for the purpose of low temperature stratospheric m0de1ing.l~ It is clear from the above discussion that, given the potential importance of a reaction between CH3O2 and C10 for ozone hole chemistry, a direct measurement of both temperature-dependent kinetic parameters and product branching ratios is highly desirable. To this end we have undertaken a study of this reaction using the discharge-flow technique coupled with mass spectrometric detection of both radicals and products.
2. Experimental Section A schematic diagram of the discharge-flow/mass spectrometer system used in these experiments is shown in Figure 1. The main reactor is a 80 cm long by 3 cm i.d. Pyrex flow tube, the temperature of which could be controlled by passing a thermostated fluid through an outer jacket. The inner wall of the reactor was coated with halocarbon wax. Linear flow velocities of between 5 and 10m s-l were established within the reactor by variation of the bulk flow rate, consisting predominantly of He, and by throttling a mechanical pump. Calibrated flow rates were maintained either by Tylan mass flow controllers (type 260) or by home-built constant pressure flow controllers which could be calibrated online using the dp/dt technique. The sample region was pumped by a 7-in. oil diffusion pump, the ionization region by a turbo molecular pump. Flow tube pressures were measured at the center of the reaction zone by a differential pressure gauge (Baratron, 0-10 Torr). The maximum pressure in the flow tube, constrained by the chosen linear velocities and the maximum allowable pressure in the mass spectrometer, was ca. 3 Torr. Radical and stable species, continuously sampled at the downstream end of the flow tube via a two-stage molecular beam interface, were detected by mass spectrometer. Beam modulation at cu. 200 Hz was provided by a tuning fork type chopper. Ions, generated by electron bombardment (1 5 eV, 0.05-1 mA) in an open cross-beam ion source, were selected by quadrupole mass filter (Riber/Balzers) and detected by a channeltron electron multiplier. The spectrometer was run in selective ion monitoring mode with mass dependent variation of the ion current and signal
I
No
Thermostatted fluid
-7
+Pumps
Chopper --/\
Mass
-b
Spectrometer
Figure 1. Schematic representation of the discharge-flow mass spectrometer set-up showing inlet ports for the CH3O2 and ClO radical precursors and NO.
integration times. The detection limit (SIN = 1) for CH302was 5 X 109 molecule cm-3 for an integration time of 20 s. Two 2450-MHz microwave discharges were used, one to dissociate F2 and one Cl2. Ceramic (A1203)insets were used to reduce F atom reaction at the glass surface where the microwave cavity was attached to glass tubing and low microwave power (18 W) was employed to reduce dissociation of impurities in the F2/He mixture. For the Cl2 discharge a glass tube that had been cleaned with 10% HF, washed with H20, and rinsed in H3P04 was used. The discharge was also run at 18 W. 2.1. Generation of CHJO~and C10. CH3O2 radicals were generated by the termolecular reaction between CH3 radicals and 02 in a movable double injector concentric to the main flow tube. F atoms, generated in a microwave discharge through a flow of highly diluted F2 in He, passed through the outer injector chamber, a CH4/02/He mixture through the inner, and were mixed in the last 5-8 cm. This flow arrangement was chosen to ensure that conversion of CH3 to CH3O2 could compete with the recombination to form CzHs. The inner wall of the larger injector was coated with halocarbon wax to reduce losses of F atoms and CH3 radicals.
F
+ CH4
4
CH,
-
+ 0, + M CH, + CH, + M CH,
+ HF
+M C,H6 + M
CH,O,
(17) (18)
(19) The end of the injector was constricted to increase the pressure to cu. twice that in the flow tube and help drive the termolecular reaction between CHs and 0 2 toward completion. In order to determine the most favorable conditions for the production of CH3O2 in the injector, experiments were carried out in which the m/e 47 signal was measured as a function of added CH4 and 0 2 for a given F2 concentration. Plateaus in the increasing CH3O2 signal and the decreasing C2Ha signal with added O2 were taken as criteria for optimized production. In order to assess the role of possible secondary chemistry occurring in the injector, numerical simulations's were carried out using a
Helleis et al.
11466 The Journal of Physical Chemistry, Vol. 97, No. 44, 1993
reaction scheme that is essentially identical to that of Pilling and Smithlg adapted for 3 Torr He. The simulations showed that with typical starting concentrations (in the injector) of [F] = 2.5 X 1012, [CH4] = 1.5 X 1014, and [02] = 2 X 10l6molecules cm-3, ca. 75%of the F atoms were converted to CH3O2. The 25% loss is due to reactionsof the CH3 radical other than with 0 2 , mainly with CH3O2 (k20= 4.5 X 10-" cm3 molecule-' s-1 20) to form C H 3 0 and in a self-reaction to make C2H6 (k19= 4 X 10-11 cm3 molecule-I s-l 21).
-
CH3O2 and C10, and which might contribute to CH3O2 removal
C1+ CH,O, products (24) k(2.4)= 2 X 10-10 cm3 molecule-' s-1.23 Generally, [ClzO]o was kept equal to ca. 0.5 [ClO]o. This value represents a compromise between keeping the concentration of Cl20 sufficiently high to rapidly scavenge C1 atoms but at the same time reducing the size of the Cl20 ion fragment at m / e = 51 (see below). [03]0 and [OClOj0were usually 50-60% greater than [ C l o ] ~ .Under some experimental conditions where [Cl0]0 was low (
