J. Phys. Chem. 1994,98, 126-131
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Temperature Dependence of the Reaction between O(3P) and OClO at L o w Pressure James F. Gleason,'*f Fred L. Nesbitt,# and Louis J. Stief Laboratory for Extraterrestrial Physics, NASAIGoddard Space Flight Center, Greenbelt, Maryland 20771 Received: February 26, 1993; In Final Form: August 27, 1993'
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The rate coefficient for the reaction O(3P) OClO has been measured at low pressure (1-5 Torr) over the temperature range 200-400 K. The experiments were performed in a discharge flow system with atomic resonance fluorescence detection of OpP). The ratio of [OClO]/[O] was kept very high (usually 103-104) in order to avoid secondary reactions. At 298 K, k = (1 .O f 0.3) X 10-13 molecules-1 cm3 s-1, which is consistent with recent results obtained at high pressure (8-800 Torr argon) and extrapolated to zero pressure. In the present low-pressure experiments, the rate coefficient increases with increasing temperature from 243 to 400 K while from 200 to 225 K the rate coefficient decreases with increasing temperature. The Arrhenius expression for the 243-400 K region is k = (2.4 f 0.8) X 10-12 exp[ (-960 f 120)/ T] molecule-' cm3 s-l. The trend observed here for T = 243-400 K is opposite to that inferred from the extrapolation of high-pressure results ( 2 0 4 0 0 Torr argon) at T = 248-312 K. Our high-temperature results (243-400 K) are consistent with the bimolecular abstraction channel, O(3P) OClO C10 0 2 .
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Introduction Recent observations of OClO in both the Antar~ticl-~ and Arctice polar stratospherehave renewed interest in the chemistry of this molecule. The observations indicate enhanced levels of stratospheric chlorine, which are associated with stratospheric ozone destruction. The primary source of stratospheric OClO is the reaction7
BrO
+ C10
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OClO Br (1) OClO is a link between the C1 and Br cycles in the stratosphere, and the analysis of the OClO diurnal variation has been used to calculate the relative contributions of different proposed ozone destruction mechanism^.^-* The use of a molecule, like OC10, as a tracer species requires that all of its sources and sinks be well characterized. Although photolysis is expected to be the major atmospheric loss process for OC10, potential chemical loss processes have not been well studied. At the time we initiated this work, rate coefficients had been measured for the reaction ofOC10withCl,90(3P),9H,9N0,9Br,ioOH,1i S0,iZand03.i3J4 Although the temperature dependence of the rate coefficients have been determined for the C1, OH, and O3 reactions, only room-temperaturedata were available for the O(3P),H, Br, NO, and SO reactions. Of these, the most interesting for atmospheric science is probably the reaction
o(3~) +o c i o
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CIO+ 0,
(2) The reaction is slow at room temperat~re:~ kz (298 K) = (5:;). X 10-13 molecule-' cm3 s-l. With a reasonable estimated positive temperature dependence for an abstraction reaction,'s the reaction would be unimportantunder stratospheric conditions. However, the recent demonstration by Colussii6that the reaction shows a very significant pressure effect at 298 K emphasizes the need to determine the temperature dependence of k2. The temperaturedependence of the low-pressure rate coefficientwould provide insight into whether the reaction has two independent channels (bimolecular and termolecular pathways) or proceeds solely via a collision-complex mechanism. The reaction between O(3P) and OClO may also be significant in complex laboratory studies designed to reveal the spectra, NASA/NRC Resident Research Associate. Present address: Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Mail Code 916, Greenbelt, MD 20771. 8 Also at: Departmentof Natural Science,Coppin State College,Baltimore, MD. Abstract published in Advance ACS Absrracts, December 1, 1993.
0022-3654f 94f 2098-0126$04.50/0
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photochemistry, and chemical reactivity of C1 oxides such as the C10 dimer (C10)2 and the molecule Cl203, especially those that use OClO as a starting material.i7J* In these experiments we have undertaken a study of the reaction O(3P)+ OClOat low pressure (1-5 T o K H ~in) order todetermine the temperature dependence of the absolute rate coefficient from 200 to 400 K.
Experimental Section The experiments were performed in a discharge flow reactor using resonance fluorescence and mass spectrometricdetection. The basic apparatus has been described previously,19 and only the modifications made for this experiment will be described. A four-way cross was added to the end of the flow tube for the resonance fluorescence detection. Oxygen atom resonance radiation at A = 130 nm was produced by passing He with trace amounts of 0 2 through a 2450-MHz microwave discharge. The light from the resonance lamp, with a MgF2 window, was uncollimated and used a series of 6-mm baffles to reduce the amount of scattered light entering the flow tube. The resonantly scattered photons were detected by a baffled solar blind PMT with a 1-mm CaF2 cutoff filter (A > 120 nm) at right angles to the resonance lamp. The oxygen atoms were produced by passing low concentrationsof 02 in He through a 2450-MHz microwave discharge. Typical O(3P) atom concentrationswere in the range 5-8 X 1010 molecules cm-3 and were determined by measuring mass spectrometricallythe change in the 0 2 concentration with the microwave discharge on and off, Le. [O]= 2 X A[O2]. The OClO was prepared by a standard method and purified as described by Toohey.20 In summary, C12 was passed through a U-tube containing sodium chlorite (NaC102) supported on glass beads. The OClO was collected in a cold finger at 223 K and purified by the following bulb to bulb distillation procedure. The residual Clz was removed by pumping on the OClO held at 223 K,and the liquid OClO was them warmed to 243 K and distilled into a second cold finger immersed in liquid nitrogen. This procedure removed any traces of HzO or higher C1 oxides. A waxy orange solid and some white ice frost was usually left in the first cold finger. The second cold finger was warmed to 243 K, and the middle fraction of this second distillation was collected in the first cold finger, immersed in liquid nitrogen. The OClO from this three-step procedure was stored at 193 K when not in use. The use of unpurified OClO directly from the NaC102 0 1994 American Chemical Society
Temperature Dependence of O(3P) lowo,
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The Journal of Physical Chemistry, Vol. 98, No. 1 , 1994 127 .
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Bmo
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E
,o 0
o"'"""""""""~'""~'""''"~~~' 0.0
0
10
15
M
25
30
Reaction Time I ms Figure 1. 0 atom signal vs reaction time at pressure = 1.1 Torr and T = 298 K [OCIO]units are molecules ~ m - ~ .
generator gave nonreproducible results. The purified material can bequite unstableandshould be handled with extremecaution. We had two spontaneous decompositionsof OCIO, both resulting in equipment damage and personal injury. The cause of the explosions has not been identified, so the amount of purified material should be kept to a minimum to reduce this hazard. The flow of the reagent OClO was determined by two methods. High concentrations(2040% OClO/He) were used in the initial experiments. The total flow of the mixture was monitored using a calibrated mass flow controller, and the OClO concentration was determined by optical absorption at X = 405 nm. These mixtures showed no loss of OClO over a 6-h period. The optical setup consisted of a low-pressure Hg lamp, absorption cell, and 0.2-nm monochromator coupled to a photomultipliertube. The OClO cross section was taken from Wahner et al,,zo u (405nm) = 2.19 X cm-2. The flow controller readings were unstable at higher concentrations(>40%) of OC10. In the second method we used the vapor from the top of pure OClO liquid in a thermostated bath. The flow of pure OC10, from a calibrated volume, was determined for each experiment by measuring AP/ ATwhile OClO was continuously monitored by optical absorption. The flow of pure OClO was pushed into the flow reactor by a small additional flow of He gas. The experiments were performed by monitoring the decay of the 0 atom resonance signal as a function of the position of the movable injector. The concentration of OClO is changed, and the process is repeated (see Figure 1). The decay of the atomic oxygen is represented by ln[O], = -kohl + ln[O]o, where the pseudo-first-order rate coefficient kob is given by kob= k2 [OClO] kwsll. The slope of each decay line plotted as a function of the OClO concentration yields the second-order rate coefficient for reaction 2, at that temperature and pressure. The gases used were UHP grade and used without additional purification.
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The ratecoefficient for reaction 2 was measured under pseudofirst-order conditions with [OClO] >> [Ole. As a check on the modified apparatus and on the quantitative handling of OC10, we first measured the rate coefficient for the reaction
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c1+OClO c10 + c10 (3) Formation of the C10 product from reaction 3 was observed via collision-free sampling mass spectrometry using low-energy electron impact ionization. In addition, we observed C1203as a product of subsequent reaction of the C10 product with the OClO reactant. For the study of reaction 3, we monitored the decay
2.0
1.0
4.0 5.0 6.0 7.0 [OCIO] / 10 l4 molecule cm .3
8.0
3.0
9.0
10.0
Figure 2. kob vs [OCIO]for OCP) + OClO at T = 298 K. The open circles represent data taken at 1 Torr. The open squares are data taken at 5 Torr. The line is the least-squares fit to the 1 Torr data.
10
o
0.0
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1.0
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2.0
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3.0
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4.0
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5.0
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6.0
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7.0
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8.0
9.0
10.0
[mol / 10 "molecule cm 4
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Figure 3. kob vs [OCIO]for OQP) OClO at T = 200 K. Symbols represent the data, and the line is the least-squares fit.
of C1 using resonance fluorescence under pseudo-first-order conditions. We performed six experimentswith [Cl] = 1 X 10*1 molecule cm-3 and [OClO] = 3-8 X 1012 molecule cm-3. Our result, k3 (298 K) = (6.6 f 1.2) X lo-" molecule-' cm3 s-1, is in excellent agreement with the only published determination of the rate coefficient, k3 = (5.9 f 0.9) X 10-11 molecule-1 cm3 s-1.9 The rate coefficient for reaction 2 was measured over a wide temperature range, 200-400 K. The bulk of the experiments were conducted at about 1 Torr total pressure. Additional experiments were conducted at a higher pressure (5 Torr, 298 K). The experimental first-order decay constant, kob, was obtained from the slope of the plot of the ln(0 atom signal) vs reaction time. Typical decays are shown in Figure 1. Corrections to kob to allow for axial diffusion of O(3P) atoms in the He carrier gas proved to be quite small (
