J. Phys. Chem. 1984,88, 3638-3643
3638
41-
NdC13, 3.9, is considerably less than the values, from 5.3 to 6.2, listed for the other solutions. Thus, it is an interesting question as to whether measurements with more dilute solutions of NdC13, 1.O m, will give values of n = 6. Such experiments are currently in progress. We finally note that the data contain no apparent evidence for direct ion pairing, or contact ion pairs, a result that has also been ~ found for NiC12 solutions of comparable ionic ~ t r e n g t h .Considerations based on the ionic radii of Nd3+ and C1- and on the crystal structure* of solid NdC13-6H20suggest that if ion pairin exists in the solution it will be characterized by a peak near 3 in both GNd(r)and Gcl(r). No such peak occurs in GNd(r) (shown in Figure 6), and the weak feature at -2.9 A in Gcl(r)is almost certainly a truncation artifact. However, as we pointed out earlier, a definitive statement about ion pairing will come from a knowledge of gNdCl(r) which requires one further experiment on NdC13 in which the isotopic states of both Nd and C1 have been changed. We expect that the nature of the Nd-Cl arrangement will explain why the hydration number of the C1- ion is so low.
I
NdC13 2.85m
-
3
x
-I 0 ~
2
6
6
10
[AI
Acknowledgment. We thank Mrs. Sue Langron for preparing the isotopically substituted samples. The Bristol group (S.B., J.E.E.) also acknowledge the support of the Science and Engineering Research Council, U.K., who provided travel grants. This research was sponsored by the Divisions of Materials Sciences and Chemical Sciences, U S . Department of Energy, under contract W-7405-eng-26 with the Union Carbide Corp. Registry No. C1-, 16887-00-6; NdC13, 10024-93-8.
Figure 6. Radial distribution function for Nd interactions, G N d ( r ) as , given in ref 1.
at different concentrations of the same salt, that there is a small tendency for the coordination number, n, to decrease as R and R'decrease. Thus, for LiCl, n decreases by -lo%, from 5.9 to 5.3, as R and R'decrease by a factor -3. For NiCl,, n stays approximately constant, within experimental error, as R and R' decrease by a factor of -,I3. Note also that the values of R and R' for NdC13 are comparable to those of several of the other concentrated solutions in Table V; yet the value of n found for
Kinetics of the Reaction 0
+ CIO
-
CI
(8) A. Habenschuss and F. H. Spedding, Cryst. Struct. Commun., 9, 71 (1980).
+ O2
James J. Margitan Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91 109 (Received: December 8, 1983)
-
The kinetics of the reaction 0 + C10 C1 + O2 ( k l )were studied by using resonance fluorescence to monitor the decay of 0 atoms which had been generated from the laser photolysis of C10. A discharge flow system was used to generate the C10 and its concentration was measured directly by multipass absorption spectrometry. The results are k1(298) = (4.2 0.8) X lo-" cm3 s-' (including systematic errors) with E/R determined to lie in the range h200 K. The literature data are reviewed and a composite value of k,(g = 6.0 X lo-" exp[-(lOO/T)] is recommended for stratosphericmodeling calculations. The atmospheric implications of the revised rate constant are discussed briefly.
Introduction The reaction of oxygen atoms with chlorine monoxide radicals Q + c10 c1+0 2 (1) plays a uniquely important role in stratospheric chemistry since it is the rate-determining step in the chlorine-catalyzed destruction of ozone, completing the cycle initiated by C1+ O3 C10 O2 (2) The potential depletion of the stratospheric ozone column, caused by anthropogenic sources of chlorine, has an approximately linear dependence on kl (see Discussion).' Uncertainties in kl are thus directly reflected in model calculations of ozone depletion. In addition, reactions 1 and 3 control the Cl/ClO ratio throughout
-
+
(1) R. J. Cicerone, S. Walters, and S. C. Liu, J. Geophys. Res., 88, 3647-61 (1983).
0022-3654/84/2088-3638%01.50/0
*
the stratosphere, with reaction 1 dominating above about 38 km, and reaction 3 below that altitude.2 NO
+ C10
-
C1 + NO2
(3)
Early measurements of k , were in good agreement: direct measurements of k , using discharge-flow/resonance flourescence by Bemand et al.3 (5.3 X lo-" cm3 s-l) and Clyne and Nip4 (5.2 X lo-" cm3 s-l) agreed well with the relative measurement of the k 1 / k 2ratio by Zahniser and Kaufmans ( k , = 4.2 X lo-" cm3 s-l, (2) WMO Report No. 11, "The Stratosphere 1981, Theory and Measurement", NASA/GSFC, Greenbelt, MD, 1982. (3) P. P. Bemand, M. A. A. Clyne, and R. T. Watson, J . Chem. SOC., Faraday Trans. 1,69, 1356-74 (1973). (4) M. A. A. Clyne and W. S.Nip, J . Chem. Soc., Faraday Trans. I , 12, 2211-7 (1976). (5) M. S.Zahniser and F. Kaufman, J . Chem. Phys., 66,3673-81 (1977).
0 1984 American Chemical Society
Kinetics of the Reaction 0
+ C10
-
C1
+ O2
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3639 n
using a consensus value for k2). A less precise measure of k , was also obtained by Bemand et al.3 by using mass spectrometric observation of C10 disappearance in excess 0 to yield kl = (5.7 f 2.3) X lo-" cm3 s-I. These early studies were averaged to obtain 5.0 X lo-" cm3 sd as the recommended value6,' for modeling calculations, with a recommended E / R (130) intermediate between those of Clyne and Nip (224) and Zahniser and Kaufman (11). Recently, Leus has reexamined k l , once again using the DF/RF technique, and has obtained a somewhat lower value of 3.6 X lo-" cm3 s-l, with a T dependence near the average ( E / R = 96). Although the spread in k l measured among these studies is not dissimilar to that normally encountered, the extreme sensitivity of stratospheric models to kl dictates that another investigation be conducted to determine if kl lies closer to the old value (5.0 X lo-" cm3 s-l) or nearer the more recent, and extensive, study of 3.6 X lo-" cm3 s-'. The 30% decrease in kl represented by these two numbers leads to a 40-50% decrease in the calculated depletion of the ozone column.8 In this study, reaction 1 was studied by a combined discharge-flow/flash-photolysis/resonance-fluorescence technique. A flow system was used to generate C10 via the reaction of C l 2 0 with excess C1
c1 + c120
-
c10 + Cl2
(4)
A laser at 266 nm was used to photolyze a small ( 1%) fraction of the C10 and generate 0 atoms, whose decay was followed by resonance fluorescence. Unlike prior investigations, which determined [ClO] from a knowledge of the concentration of added precursor (03, C120, OCIO) and an assumed unit conversion efficiency in processes such as reaction 4, the [ClO] was measured directly in this work with a multipass absorption cell.
&
'c'
Ar
CIjHe
METER
m
c CI2/He
9 METER
Ill Ill
puqpz HeiO2
INSULATION
L
l
h
-
CaF2 FILTER
I
E 3
(6) W. B. DeMore, R. T. Watson, D. M. Golden, R. F. Hampson, M. Kurylo, C. J. Howard, M. J. Molina, and A. R. Ravishankara, JPL Publication no. 82-57, Pasadena, CA, 1982. (7) D. L. Baulch, R. A. Cox, R. F. Hampson, Jr., J. A. Kerr, J. Troe, and R. T. Watson, J . Phys. Chem. Ref. Data, 9, 295-471 (1980). (8) M.-T. Leu. J . Phvs. Chem.. 88. 1394 (1984). (9j (a) J. J. Margitai, J . Phys. Chek., 87,674-9 (1982); (b) J. Geophys. Res., 88, 5416-20 (1982). (10) M. A. A. Clyne and J. A. Coxon, Trans. Faraday SOC.,62, 1175-89 (1966); Proc. R . SOC.London, Ser. A, 303, 207-231 (1968). (11) R. T. Watson, J . Phys. Chem. Ref. Data, 6 , 871-917 (1977).
I-
/ v
- 4 -
_I'
N
Experimental Section The apparatus is shown in Figure 1. The laser photolysis/ resonance fluorescence system has been described previo~sly.~0 atoms are detected at 130 nm by using a microwave discharge of a trace of O2in He as a resonance lamp, whose emission passes through a CaF2 window to remove Lyman-a, and an 0.5-cm cell containing a small amount of O2 in N2 to prevent emission > 135 nm from entering the cell and exciting C12molecular fluorescence, which would contribute to the background. Fluorescent emission at 130 nm was detected without wavelength discrimination by an EMR 541G phototube and was recorded on an N D 600 MCA, usually with s/channel. The 0 atoms were generated by photolysis of C10 with a quadrupled Nd:YAG laser at 266 nm (Quanta Ray DCR l ) , usually at 15 mJ/pulse, 10 Hz, with an expanded beam of -2 cm diameter. The C10 was generated by adding C1,O to an excess of C1, which had been generated in a microwave discharge of a C12/Ar mixture. The C1,O was generated by passing a 2% C12/Ar mixture through a 40-cm column of yellow mercuric oxide. The [C120] was determined in some experiments by its absorption at 253.7 nm in a 10-cm cell in the inlet line prior to addition to the main system. The [ClO] for the kinetic studies was determined in a six-pass White cell (122 cm total pathlength) via absorption at 277.5 nm with a deuterium lamp light source and a McPherson 218 monochromator (Ah -0.3-0.5 nm). Due to the diffuse nature of the spectrum, the effective cross section is not very sensitive cm2),10,11 to the resolution; the literature value (u = 7.26 X
COLUMN
FLUORESCENCE SIGNAL I
PUMP
HigiClg FLUID l l
WHITE CELLFOR [ C I O I
Figure 1. Schematic diagram of the apparatus.
however, had been determined under similar conditions. The absorption cell was located in a room temperature section of the apparatus downstream of the reaction cell so that the temperature dependence of u was not needed. Measured [ClO] were converted to [ClO] in the reaction zone by the temperature factor 298/T. The analyzing light phototube output was fed to a Keithley 417 picoammeter operated in the current suppression mode. Approximately 90% of the current was suppressed; the remainder was amplified to full scale and displayed on a chart recorder with a 10-s time constant to remove short-term noise. Longer term instability was circumvented by alternately turning the C10 on and off to repeatedly measure I and Io. C10 cycling was achieved by having the Cl20 flow bypass the cell and be added downstream of the point where C10 absorption was measured. Measurement of absorption using the suppression system does not result in the self-cancellation of inaccuracy in the I and Io measurements. Therefore, the absorption as measured by suppression was calibrated from known concentrations of C12and Cl20,with the known concentrations being determined from either (a) absorption in the White cell measured without suppression, (b) C l 2 0 absorption in the inlet system, or (c) pressure measurements. Cross calibrations among these methods showed that (1) the effective absorption pathlength is the same as that measured physically (1 22 cm) and (2) the absorption as measured by suppression is -8% lower than the true absorption. This deviation is within the -5-10% accuracy with which these calibrations can be done, but the consistent tendency of suppression measurements to lie 4-1 5% low indicates that the small (-8%) calibration factor is needed. Most experiments were done in Ar at 10 torr pressure. Flow, pressure, and temperature measurement and control have been described previo~sly.~ Results Figure 2 is a typical plot of the semilogarthmic decay of 0 atoms, generated from C10 photolysis, due to reaction 1. The
Margitan
3640 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 2500 3.3
29 JUN 83 RUN 10
3.1 2.9
KOBS = 915.6 sec-1 [CaOl = 1 . 6 X t O 1 3 [~01'=1.78E+013
2.7
T - 2 % K lOTORR 15 MJ EXPANDED BEAM
; 2
1500
2.5
2.3
-
2.1
-
-I
2-
% YI
1
v)
,O 1.9
3
1000
1. 7
1.5 1.3
500
1.1
0.9
0 0
TIME (msecl
Figure 2. Typcial semilogarithmicdecay plot. [CIO] is the concentration as measured in the absorption cell; [ClO]' is the concentration in the reaction zone and is obtained from [CIO] corrected for the loss of C10 due to the C10 C10 reaction.
+
slopes of such plots are the pseudo-first-order rate constants, k', and consist of the sum of k , [ClO] kk where kdlis largely due to diffusion loss of the 0. The bimolecular rate constant k l can be determined as the slope of a plot of the measured k"s vs. [CIO]. The [ClO], however, is not that measured in the absorption cell since, under these experimental conditions, loss of C10 in the 0.2 s between the resonance fluorescence cell and the absorption cell due to reactions 5 and 6 is not negligible. Cox et al.I2 and Basco
+
-
+ C10 C10 + C10 + M C10
products -+
products
(5) (6)
and HuntL3have recently determined k5 and k6 and their values are in good agreement. The combined average value for the cm3 s-'. For C10 conditions here of 10 torr of Ar is 1.5 X ~ , corconcentrations in the reaction zone of 1 4 X lOI3 ~ m - the rection factor for this loss was 520%. At higher [ClO], the factor was correspondingly larger. Only data where the factor was 520% were used in the determination of k l . At 298 K, a plot of k' vs. [ClO] (corrected) has a slope of k l = (4.2 f 0.1) X lo-" cm3 s-' (21 experiments, uncertainty is lu, precision only). Experiments in which the [ClO] corrections were >30% did not deviate significantly from extrapolation of the data with smaller correction factors but were not used in the determination of kl. A plot of k' vs. [ClO] (uncorrected) also appears linear with a slope of 4.9 X lo-" cm3 s-I, linearity obviously being not sufficient to demonstrate the absence of complications. Empirical verification of the magnitude of the C10 loss correction was obtained by comparing the [ClO] measured in the absorption cell, [Cl0lab, to the initial [Cl20], which is identical with [CIO]o. For the second-order loss process given by reactions 5 and 6 [C10],,,/[C1,0]
= 1 - 2kt[ClO],b,
A plot of [C10],b,/[C120] vs. [CIO],b, was linear and had a slope indicating k = 1.5 X cm3 s-', in agreement with the literature value. The actual intercept of 0.94 is acceptably close to the expected value of 1, and the difference can be explained as either (12) R. A. Cox, R. G.Derwent, A. E. J. Eggleton, and H. J. Reid, J . Chem. SOC.,Faraday Trans. 1 , 1 5 , 1648-66 (1919). (13) N. Basco and J. E. Hunt, Int. J . Chem. Kinet., 11, 649-64 (1979).
1
2
3
4
5
(d3
[C~OI cm-3) Figure 3. k' vs. [CIO] plot for 10-torr data at 298, 268, and 241 K. The line is a least-squares fit to all the data.
experimental uncertainty or a first-order C10 loss of -0.2 s-I. For experiments at 298 K and 30 torr total pressure, ( k , 4- k6[M]) = 2.3 X cm3 s-l, and the maximum [ClO] in the reaction zone, consistent with the -20% correction factor limit, was -2 X 1013~ m - ~ A .k' vs. [ClO] plot of ten experiments under those conditions gave kl = (4.4 f 0.2) X lo-'' cm3s-I. Due to the wider range of conditions available at 10 torr, the k , derived from the 10-torr data is the preferred 298 K value. At temperatures other than 298 K, the C10 loss cannot be estimated from the two literature studies as they did not measure the T dependence of k5 and k6. Since k5 is a slow bimolecular reaction, it probably has a positive T dependence, whereas k6,being termolecular, would have a negative T dependence. Thus, the direction of the temperature dependence is unknown. It was observed here, however, that, for a given [C120]o,the [ClO] measured in the absorption cell was virtually independent of the temperature, indicating that the integrated C10 loss was the same at all temperatures. Since [ClO] in the reaction cell is higher at the lower temperatures, and the reaction time is longer (due to a slower velocity), the apparent C10 disappearance rate constant ( k , + k,[M]) must decrease with decreasing temperature. Values of (k5 + k6[M]) at the lower temperatures were therefore decreased so as to maintain the same correction factors as at room temperature. Analyzed individually, the data at 268 and 241 K gave k1(268) = (4.13 f 0.14) X lo-'' cm3 s-' and k1(241) = (4.08 & 0.26) X lo-" cm3 s-l. That these differences are not significant can be seen in Figure 3 which is a plot of the 10-torr data for all three temperatures. The composite slope of kl = (4.17 f 0.07) X lo-" cm3 s-' has a slightly smaller uncertainty and a slightly better correlation coefficient (R = 0.997) than the 298 K data alone, indicating that the inference of E / R = 40 K from the k l (T ) data is unwarranted. In view of the experimental uncertainties (especially the C10 + C10 loss correction as a function of temperature), it can only be stated that E / R lies in the range f200 K. An E / R at either limit would have resulted in only a 15% change in k from 298 to 241 K. An independent determination of kl was made under conditions which established a firm lower bound to k , . C1,O was added through an injector -0.1-supstream of the detection region in order to minimize loss of C10 via self-reaction. The [C1,0] was determined from 254-nm absorption measurements in the inlet
Kinetics of the Reaction 0 + C10
-
C1 + O2
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3641
line and a knowledge of flow rates. The assumption that there was no C10 loss, [ClO] = [C120]o,slightly overestimates [ClO] and, thus, the value of k l derived from a k' vs. [ClzO] plot underestimates kl. The value of kl = 4.0 X lo-" cm3 s-l obtained from the slope is very close to the 4.2 X lo-" cm3 s-l obtained in the other experiments with corrected [ClO]. These experiments did show substantial curvature in the k1 vs. [C120] plot, indicative of a second-order C10 loss prociss. It is unlikly that there are any complicating reactions other than C10 C10 in the experiments. The rapidity of the 0 C10 reaction minimizes the potential for interference from any other reactions of 0 atoms. The presence of excess C1 atoms (verified by C1 resonance fluorescence and, flow and microwave discharge power variation) ensures compiete reaction of the ClzO and eliminates any potential interference from C1 formed as a product or from the photolysis. The presence of Clz does not interfere since 0 Cl, is relatively slow. The [ClO] was determined directly by absorption, so that no assumptions regarding the stoichiometry of the C120 C10 conversion were necessary. Other reactive atomic species present in the disdharge (H and 0) may remove some C10 prior to the reaction region, but this process must be small as evidenced by the good agreement between the empirical observation of C10 loss in the [C10]/[C120] ratio measurements and that calculated based on the literature value of the C10 + C10 rate constant. The products of the C10 C10 reaction have not been firmly established, but the major products of the bimolecular channel appear to be either (Cl, 0,) or (C1+ ClOO) which yields (2C1 Oz). The termolecular association forms a dimer C1202which is postulated to react rapidly with the excess C1 atoms.l2 The reaction of C1 with a dimer of the structure ClOOCl would yield (Cl, C 1 0 0 ) which would give (Cl, C1 Oz). If the dimer structure were ClOClO,%hefinal products would be ClZ 2C10 following a subsequent reaction of C1 with either a Cl2O or OClO product. If the dimer were the ClOClO form, then the termolecular C10 C10 reaction might not lead to loss of C10. At 10 torr, the termolecular channel represents only about 25-30% of the total C10 removal rate, so that even if a substantial fraction of the dimer is recycled to C10, the effect on the C10 loss correction factor is minimal. Since the data analysis was limited to those experiments with corrections of
