2499
J. Phys. Chem. 1980, 84, 2499-2503
ARTICLES Rate of Reaction of OH with CSJ P. H. Wine,* R. C. Shah, and A. R. Ravishankara" Molecular Sciences Group, Engineering Experiment Station, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received: March 3 1, 1980)
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The flash photolysis-resonance fluorescencetechnique has been employed to study the kinetics of the reaction OH CS2 products (12') over the temperature range 251-363 K. Complicatingsecondary reactions involving CS2photofragments were eliminated only when the photoflash was filtered by 10 torr cm of CS2 and SF6 was used as the buffer gas. The rate constant was found to be much slower than previous measurements indicated. Based on our experiments, upper limits for k1 (in units of cm3molecule-' s-') are 9.9 at 251 K, 1.5 at 297 K, and 1.6 at 363 K. Our results suggest that the title reaction is unimportant as a source for COS in the atmosphere.
+
Recent observations of large COS mixing ratios in both the troposphere' and the stratotsphere2 have led to the postulate3 that COS is a significant contributor to the stratospheric sulfate layer. To better define the role of COS in atmospheric chemistry, accurate kinetic data for possible source and sink reactions is needed. One COS production mechanism which warrants consideration is the oxidation of CS2, a process which could be initiated by hydroxyl radical attack (reaction 1):
OH
+ CS2
ki
products
(1)
Reaction 1has been studied in several laboratories. The two earliest experiments, by Kurylo4 and Atkinson et a1.,5 both employed the flash photolysis-resonance fluorescence technique. :Kurylo's experiments appear to be free of secondary chemistry complications, so his value-kl = 1.85 X cm3 molecule-' s-' at 296 K and 10 torr of Ar buffer-has been recommended for atmospheric modeling! While the experiments reported here were in progress, reaction 1was also being investigated in three other labReported values for hl at room temperature now range from > [OH] (pseudo-first-order conditions), simple first-order kinetics will be obeyed: In ([OH],/[OH],] = (kl[CS2] k3)t h’t (4) The bimolecular rate constant, kl, is determined from the slope of a h’ vs. [CS,] plot. Observation of OH temporal profiles which are exponential (Le., obey eq 4), a linear dependence of k’ on [CS,], and invariance of hl to variations in experimental parameters such as the flash intensity and the water concentration serve as proof that reactions 1-3 are, indeed, the only processes which significantly affect the OH time history, and, therefore, validate the measurement of kl. Initial attempts to study reaction 1 involved flash photolysis of H20,CS2, Ar mixtures (100-250 mtorr of H20, 35 torr of Ar) with the filter cell empty. Under these conditions, the OH decays were nonexponential with the initial fast component becoming faster with increasing flash energy. At all flash energies (30-120 J),the rate of the fast component increased linearly with added CS2 at low concentrations but became invariant to addition of CS2 at high concentrations. When higher flash energies were employed, the initial rate of OH decay in the high CS2limit was much faster than when lower flash energies were employed. The results are shown in Figure 1. The above observations indicated that C S z photofragments were important in the post-flash OH chemistry.
b cn 0
P
-18-
I r J
i
+
-2045
55
50
60
:( l o 3 crn-l) Figure 2. Absorption spectra for CS, and H20. The CS, spectrum is taken from ref 19 and the H,O spectrum is taken from ref 20.
Absorption spectra for CS2 and H 2 0 in the wavelength region of interest are shown in Figure 2. Convolution of these spectra with the spectral distribution of the photoflash (nearly wavelength independent) and the Suprasil transmission curve indicates that the fraction of CS2 photolyzed by an unfiltered flash is about two orders of magnitude greater than the fraction of H20 photolyzed. Actinometry experiments on our system have shown that photolysis of 150 mtorr of HzO at a flash energy of 60 J produces -5 X 1O1O OH per crn3.l5 Therefore, in most of the experiments described above 1012-1013CS2 photofragments per cm3were produced. Callear has studied the flash photolysis of CSZlsand deduced that the primary photochemical step in the 1900-2100 A region is
+
S(3P) cs* (5) where CSwis vibrationally excited (ground electronic state)
cs2
Rate of Reaction of OH with CS,
.
20
:t *s
I-
0 0
0
0
0 0
0
I
I
1
o
l o
oLL0
10
20 m o l e c u ~ e / c m3 )
LCS~I
Flgure 3. k’ - k, vs. [CS,] data obtained with the flash filtered by 10 torr cm CS, and with Ar as the buffer gas. Experimentalconditions: T = 297 K; PHp = 250 mtorr; P = 35 torr; flash energy 35 J (0), 110 J (0).
CS. Callear further deduced that S(3P)is a very efficient vibrational relaxer for CS. Our data, in conjunction with Callear’s results, suggest that, in the forementioned experiments, the postflash OH chemistry was controlled by the following reaction scheme:
S(3P)+ OH -%SO + H AH = -22.5 kcal/mol
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S(3P)+ CS,
k7
CS
+ S,
AH = -9.0 kcal/mol (7)
S(3P) I- cs*22 S(3P) + cs
OH + C9
k0
H
+ COS
(6)
(8)
AH = -45.4 kcal/mol (9)
(Heats of reaction are calculated from thermochemical data given by Benson.)17 The initial fast component in the OH temporal profile is probably due to reaction 6. Eventually, Sf‘P) is depleted by reaction 7 to the point where reaction 9 becomes important. k6 is unknown but is probably near gas kinetic, while k9 is probably faster than, but of the same order of magnitude as, the rate constant for reaction 10,
OH + co k
l o - ~+ co2
(10)
cm3 molecule-l s-l.18 which is known to be 1.4 X To reduce the concentration of CS2photofragments, the photolysis flash was filtered with 10 torr cm of CS2. Calculations based on known absorption cross section^^^^^^ (Figure 2) indicated that this would reduce CS, photolysis by about a factor of 200 while reducing HzO photolysis by a factor of 3. A factor of 3 reduction in resonance fluorescence signal was, in fact, observed. Filtering the flash resulted in a considerable reduction in the measured OH disappearance rates. The temporal profiles were also much closer to single exponentials although some deviation from the predictions of eq 4 was still observed. Furthermore, the same nonlinear dependence of decay rate on CS, concentration which was observed when the flash was not filtered (Figure 1) was still observed (Figure 3). The k’ vs. [CS,] dependence shown in Figure 3 was found to be independent of the total argon pressure (35-100 torr) and the water partial pressure (0.1-1.0 torr) but dependent upon flash energy. At low [CS,] the slope of the k’ vs. [CSJ plot was larger when high flash energies were em-
ployed than when low flash energies were employed. Typical OH temporal profiles measured in experiments with argon buffer gas are shown in Figure 4. Experimentally, we found that the chemistry which led to nonlinear k’ vs. [CS,] behavior could only be eliminated when SFGwas substituted for Ar as the buffer gas and the photoflash was filtered. (Experiments with SF6buffer gas and the filter cell empty gave essentially identical results with those obtained under similar conditions with Ar buffer.) This strongly suggests the importance of CS* in the postflash OH chemistry under conditions where the flash was filtered but Ar was used as the buffer gas. Apparently, only S(’P) and SF6 are capable of relaxing CS* rapidly enough to make it unimportant in the OH chemistry, and, with the flash filtered, S(3P)was not present in sufficient quantity to cause rapid relaxation. With SF6diluent ( P = 50 torr) and the flash filtered, the OH decays were very nearly exponential (Figure 5) and, within experimental error, increased linearly as a function of added CS2. Furthermore, kl was found to be invariant to a factor of 3 variation in flash intensity. Thus, we conclude that, under these experimental conditions, only reactions 1-3 contributed to the OH temporal behavior, and, therefore, the variation of k’ with [CS,] represents a measurement of kl. k’ vs. [CS,] data was obtained under the desired experimental conditions at three temperatures. The results are shown in Figure 6. At 297 K, additian of 0.3 torr of CS, to the reaction mixture resulted in an increase in OH decay rate of only 10 &. Experiments with CS2concentrations greater than 0.3 torr, while desirable, were not possible because the already weak signal was further reduced because of fluorescence quenching by CS, (the initial OH concentration in most experiments was 2-3 X 1O1O molecule/cm3). Because large quantities of CS2were required to observe any OH removal, the possibility exists
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2502
The Journal of Physical Chemistry, Vol. 84, No. 20, 1980
r
'1
I
I
Wine et al.
TABLE I: Comparison of Our Results with Those Obtained by Other Investigators. Our Reported Upper Limits Represent the Rate Constant Obtained from a Least-SquaresAnalysis of the k'vs. [ CS,] Data Plus Two Standard Deviations exptl technique" FP-RF FP-RF
cc cc DF-MS FP-RF
T,K 296 298 425 297 298 298
l O l 5 k , , cm3 molecule-'^-^
ref
185.t 34
4
