J. Phys. Chem. 1995,99, 10433-10437
10433
ARTICLES Photochemistry of Chlorine Dioxide in Ice Christopher J. Pursell,” Jay Conyers, Philip Alapat, and Ruksana Paweed Department of Chemistry, Trinity University, 715 Stadium Drive, San Antonio, Texas 78212 Received: January 30, 1995; In Final Fom: April 28, 1 9 9 9
The photochemistry of chlorine dioxide, OClO, in amorphous ice has been investigated using FTIR and UV-vis spectroscopy. Exposure to ultraviolet light (A = 360 nm) quantitatively converted the OClO to the chlorine peroxy radical, C100. The asymmetric stretch of OClO at 1100 cm-l and the 0-0 stretch of ClOO at 1440 cm-I, along with the characteristic UV absorptions at 360 and 260 nm, respectively, were used for monitoring the photochemical process. Under the dilute conditions used, the only photoproduct appears to be C100, in contrast with the gas-phase photolysis which yields predominantly CIO. These findings have implications for stratospheric ozone loss. Namely, the photochemical conversion of OClO to ClOO in stratospheric ice particles may represent a new ozone-depleting mechanism.
Introduction
(jo
Our laboratory has begun to study photoinduced chemical reactions in amorphous and polycrystalline solids with special emphasis on solids which simulate atmospheric aerosols. The reactions of interest are those that may lead to stratospheric ozone destruction. This paper reports our initial study of the photolysis of chlorine dioxide, OCIO, in amorphous ice. The photochemistry of OClO has recently attracted attention due to its possible role in stratospheric ozone loss. The dominant gas-phase photolysis pathway is OClO
+ hv -c10 + 0
(1)
which does not lead to net ozone loss. However, other possible pathways have been suggested. One involves the isomerization to the chlorine peroxy radical, CIOO, followed by dissociation to 0 2 and C1, Is2
+ hv - ClOO ClOO - 0, + c1
OClO
while another is the direct photodissociation to OClO
+ hv
0,
+c1
(2) 0 2
and C1. (3)
The photochemical pathways are shown in Figure 1. The ClOO isomer is thermodynamically more stable than OClO but is kinetically very unstable and will quickly dissociate. The production of ozone-destroying C1 atoms by either of these last two pathways would represent a potentially new mechanism for ozone loss. Unfortunately, the importance of the pathways leading to C1 atom formation and the actual quantum yield for the production of C1 atoms is a matter of debate.”-“) There are also questions about the wavelength dependency of the photolysis and whether an isomerization state is even involved?-i4
* To whom correspondence should be addressed.
’National Science Foundation-REU
Student, Summer 1994 (Incarnate Word College, San Antonio, TX). Abstract published in Advance ACS Absrructs. June 1, 1995. @
0022-3654/95/2099- 10433$09.00/0
kd
0
0
f
\
+
W
g + 0 Figure 1. General reaction scheme for the photolysis of chlorine dioxide, OCIO. In the gas phase the photochemistry occurs predominantly (cf ’90%) by the dissociation pathway ( k d ) that produces CIO 0, while the isomerization pathway (ki) that produces the chlorine peroxy radical, CIOO,and the direct dissociation pathway (kdd) that produces 0 2 CI are minor channels. (The ClOO isomer is kinetically unstable and will quickly dissociate to 0 2 CI.) In amorphous ice at 80 K the only photoproduct is CIOO, which occurs directly by the isomerization pathway or indirectly by one of the dissociation pathways followed by “cage effect” recombination.
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Further suggestions have been made concerning the role that heterogeneous chemistry may play in the photoproduction of C1 atoms. The presence of atmospheric aerosols, i.e., polar stratospheric clouds (PSCs), may influence the photolysis and possibly enhance C1 production. Recent studies of OClO photolysis in water have determined that C1 formation occurs 10-20% of the time (cf. gas-phase yields are Il%).i0*15-17 Additionally, the photochemistry of OClO in inert matrices has been extensively studied. 8-21 In argon and neon, photolysis quantitatively converts OClO to CIOO. It appears that this “apparent” photoisomerization is due to the matrix cage effect; i.e., the matrix restricts the mobility of the photoproducts, and they recombine to form potentially new chemical species. To address the possibility that OClO photochemistry on atmospheric aerosols could ultimately lead to ozone loss, we have initiated the present laboratory studies. This first report is concerned with the photolysis of OClO at 360 nm (the peak of the UV absorption) in amorphous ice at 80 K. Using FTIR 0 1995 American Chemical Society
10434 J. Phys. Chem., Vol. 99, No. 26, I995 spectroscopy along with UV-vis spectroscopy, we have discovered that OClO is quantitatively converted to C100, similar to the inert matrix results. There is no evidence that C10 is formed under our experimental conditions. These results suggest that heterogeneous OClO photolysis in the stratosphere would produce C100. Once released into the gas phase, the ClOO would dissociate to 0 2 and C1 atoms, which would ultimately lead to ozone loss.
Purse11 et al.
9 c,
Experimental Section The experimental setup was similar to a standard matrix isolation setup. An optically transparent window (KBr for both IR and UV light) was attached to the copper block of a liquid nitrogen-cooled dewar (International Cryogenics). An indium ring between the window and copper block ensured good thermal contact. The temperature was measured using a silicon diode embedded in the copper block and was controlled by adjusting the flow of liquid nitrogen and by adjusting the current through a heating wire attached to the block (Lakeshore 321 Temperature Unit). The dewar, which had outer KBr windows attached to the vacuum jacket, could be transferred between the sample compartments of both an FTIR (Nicolet Magna 550) and UV-vis (IBM 9420) spectrometer. A standard glass vacuum line with a water-cooled diffusion pump was used for handling the gases. The chlorine dioxide was synthesized from oxalic acid and potassium chlorate (with a few drops of water) at 363 K according to the following reaction,
2KClO,(s)
+ (COOH),(s)2OClO(g)
+ 2CO,(g) + 2KOH(s)
The yellow OClO gas was dissolved in water as it was produced for easier handling. Since chlorine dioxide is much more soluble than carbon dioxide in water,*? the resulting solution was rich in OC10. After three pump-thaw cycles the chlorine dioxide solution was held at -263 K. [In order to determine the relative amount of water vapor to OClO above this solution, the vapors were deposited onto the cooled KBr window, and IR spectra were taken. The results indicate that the vapors above the OClO in water solution are nearly pure OClO.] Pure water vapor was mixed with the chlorine dioxide gas in ratios of approximately 1OO:l. After allowing for mixing, the gas mixture was slowly sprayed onto the cold KBr window ( T = 80 K) using a stainless steel injector tube. The gas flow was controlled using a needle valve such that the deposition rate was 15 pmollh, and the total deposition time was -30 min. The ice growth was monitored using FTIR spectroscopy. In particular, the characteristic ice feature at 840 cm-’ was used to determine the ice thickness. Tolbert ef al.23 have measured the absorbance of the ice peak at 840 cm-’ as a function of thickness and have thereby determined a “Beer’s law” conversion factor of 0.163 Abs/pm. Using this conversion factor, we typically produced -5 p m thick ice films. Under our experimental conditions (Le., T = 80 K) the ice would be considered After growing the ice films (with a dilute concentration of OClO), they were characterized using FTIR and UV-vis spectroscopy. The films were then exposed to low power ultraviolet light (1= 360 nm, P 100 p W ) at 5 s intervals for the first 100 s and then for longer intervals as the photolysis slowed. The UV light source was a 75 W Xe arc lamp with a high-throughput tunable monochromator (F’TI LPS-220/A1010). After each photolysis interval an IR spectrum (8 cm-’ resolution and 120 scans) was collected. The asymmetric stretch of OClO
-
8 3000
2000
1000
Wavenumber (cm-’) Figure 2. Infrared spectrum of amorphous ice at 80 K with a small concentration (1OO:l) of OC10. The asymmetric stretch of OClO at 1100 cm-I was used for monitoring the photochemistry and is displayed in the inset. (This ice absorbs essentially all the infrared radiation at 3200 cm-I which is the reason for the noise in that spectral region.)
at 1100 cm-I and the 0-0 stretch of ClOO at 1440 cm-’ were used for monitoring the photochemical process. Similarly, the characteristic UV absorptions for OClO and ClOO at 360 and 260 nm, respectively, were monitored for some of the experimental runs.
Results Infrared Spectroscopic Studies. A typical IR spectrum of an -5 p m thick ice film is shown in Figure 2, and is characteristic of amorphous i ~ e . This ~ ~ ice . ~ film ~ absorbs essentially all the infrared radiation at 3200 cm-’ which is the reason for the noise in that spectral region. The inset is an enlargement of the 1100 cm-’ region where OClO has a strong absorption due to the ~3 asymmetric stretch. The shoulder to the red of the main peak is due to the 37Clisotopomer (note the correct 3:l ratio). The positions of these peaks are in good agreement with matrix isolation The weaker V I symmetric stretch at 940 cm-’ is unobservable due to the very strong ice absorption centered at 840 cm-I, while the v z bend at 450 cm-’ is hidden by the KBr window cutoff at -500 cm-I. The v3 band at 1100 cm-’ has therefore been used for following the photochemical process. Before exposure to the UV light, a background spectrum of the ice film (with dissolved OC10) was collected and stored. A spectrum taken after an interval of photolysis using this background would then represent a “difference” spectrum. Figure 3 is an example of one such difference spectrum after a total of 6 min of UV exposure. Since the OClO is being destroyed, the peak at 1100 cm-’ is negative going. The positive going spectral feature at 1440 cm-’ is due to the 0-0 stretching band of C100, the chlorine peroxy radical. This characteristic peak is in excellent agreement with previous matrix studies.’*-*’ The very small peaks around the 1440 cm-’ peak are due to atmospheric water vapor interferences, resulting from the removal and then replacement of the dewar in the sample compartment of the FTIR after UV photolysis. The ClOO is known to also have IR absorptions at 414 and 201 cm- I ,20.21 which are beyond the cutoff of our KBr windows and were not observed. As discussed above, the gas phase photolysis of OClO yields predominantly C10, which should have an absorption at 970 cm-I. Using our “difference” spectrum technique, we are able
Photochemistry of Chlorine Dioxide in Ice
J. Phys. Chem., Vol. 99, No. 26, 1995 10435
5 0
5 0
Q
0
8 0
UP 1400
1 I00
1200
1300
Wavenumbers (cm-I) Figure 3. “Difference” spectrum (see text for details) after 6 min of photolysis at 360 nm. The OClO peak at 1100 cm-’ has decreased (and is therefore negative going), whereas a new peak at 1440 cm-I (positive going) has appeared and is assigned to the 0-0 stretch of C100. (The very small peaks around the 1440 cm-I peak are due to atmospheric water vapor interferences.)
200
250
300
400
350
&-----A 00
20
40
60
EO
100
120
0
Time (sec) Figure 4. Absorbance of OClO at 1100 cm-I (left axis, filled circles) and ClOO at 1440 cm-I (right axis, filled squares) as a function of photolysis time. The lines through the data points represent exponential fits. The average first-order rate constants are 0.0167 f 0.0042 and 0.0158 f 0.0044 s-I for the loss of OClO and the production of C100, respectively (the uncertainty is one standard deviation). to greatly reduce the “apparent” intensity of the 840 cm-’ peak of ice and thereby increase the sensitivity in that spectral region. The results of our studies indicate that there is no C10 being produced, at least not in any appreciable amount with our present sensitivity. These results are consistent with previous matrix isolation studies which have no spectral interferences near 970 cm- 1.18-21 The absorbance of the OClO peak at 1100 cm-’ and the ClOO peak at 1440 cm-’ was measured as a function of UV exposure time, and a typical plot is shown in Figure 4. The photolysis follows first-order kinetics, with average rate constants of 0.0167 z t 0.0042 and 0.0158 & 0.0044 s-I for loss of OClO and production of C100, respectively (the uncertainty is one standard deviation). UV-vis Spectroscopic Studies. W - v i s spectroscopy was also used for studying the photochemistry. For most experimental runs a spectrum was taken before and after photolysis.
500
Since the KBr windows in our dewar cutoff at 200 nm and produce a sharp rise in the absorbance as 200 nm is approached, we have had to subtract from each “raw” spectrum a KBr “background” spectrum. Figure 5 displays representative spectra from before and after photolysis using KBr background subtraction. The ringing-like noise near 200 nm is due to the subtraction procedure. The broad, structured spectrum centered ~ of OClO at 360 nm is the characteristic 2A2 2 B transition before photolysis and resembles both the g a s - p h a ~ e and ~~.~~ matrix isolation spectra.20-21The sharper, unstructured spectrum centered at 260 nm appears after photolysis and is the characteristic absorption band of C100. This band also resembles the gas p h a ~ e ~ and ~ J ’matrix isolation spectra20-21 with only a small (5-10 nm) red shift of the peak. If significant amounts of C10 were being formed during the photolysis, a unique absorption band centered at -275 nm should have been observable as a shoulder to the 260 nm peak of C100. Its absence indicates that the only photoproduct is C100, which further supports the IR results. Further evidence for the quantitative conversion of OClO to C100, with no production of C10, comes from the peak intensities of the UV-vis spectra. The absorption cross sections for OClO and ClOO have been reported in the gas phase at comparable temperatures (ca. 204 and 191 K, r e ~ p e c t i v e l y ) . ~ ~ ~ ~ ~ The ratio of absorption cross sections for the peak maximum is (T (OClO)/o(ClOO) = 0.5 1. This is in excellent agreement with our experimental ratio for maximum peak intensities which is typically Z(OClO)/Z(ClOO) = 0.5. (We have assumed that the ratio of absorption cross sections does not change significantly from -200 to 80 K.) This being so, the experimental intensities confirm that the OClO is converted quantitatively to ClOO with no C10 being produced. W - v i s spectra were also collected as a function of photolysis exposure time for some experimental runs, similar to the IR time studies. The results agree with the IR data in that the photolysis follows first-order kinetics with an average rate constant of 0.0158 s-I. Finally, we have estimated the quantum yield for OClO photoconversion to C100. Using the gas-phase W absorption cross section for OC10, our experimental absorbance value, and rate constant, along with the ultraviolet light power, the quantum
-
0
450
Wavelength (nm) Figure 5. UV-vis spectrum of amorphous ice at 80 K with a small concentration (100:1) of OCIO. The spectrum before photolysis, with a peak maximum at 360 nm, is characteristic of OCIO. The spectrum taken after -10 min of 360 nm photolysis has a peak at 260 nm and is characteristic of C100. Using the peak absorbances and literature absorption coefficients, the UV-vis spectra confirm the quantitative conversion of OClO to C100. (The ringing-like noise near 200 nm is due to the subtraction of the KBr window’s absorption profile.)
10436 J. Phys. Chem., Vol. 99, No. 26, 1995
Purse11 et al.
yield is approximately 0.7 f 0.4 (where the uncertainty is a conservative estimate).
restrictive ice cage, the highly compressed OClO molecule passes through the transition state to form the ClOO isomer.
Discussion
Conclusion
The photochemistry of chlorine dioxide in amorphous ice quantitatively converts the OClO to the chlorine peroxy radical, C100. As one would expect, the photoisomerization follows first-order kinetics. There is no evidence, from either the IR spectra or UV-vis spectra, that the C10 radical is formed. All these results are consistent with previous matrix isolation studies of OClO photochemistryla-*I and are in contrast to gas-phase photolysis which yields predominantly C10.2-10 There are at least two possible explanations for the apparent change in the photochemical pathway. One possible explanation may simply be the “cage effect”. Similar to an inert matrix, the amorphous ice at 80 K can greatly restrict the mobility of the OClO and its photoproducts. If in fact the photoproducts are the same as in the gas phase (cf.C10 and an 0 atom), the ice would prevent these species from escaping their cage. After losing the excess energy from the photodissociation process to the cage walls, the reactive species would recombine. Most probably they would recombine to the most thermodynamically stable product, which happens to be C100. This also assumes that the reaction C10 0 ClOO is chemically feasible. In other words, the predominant photochemical pathway in Figure 1 has not changed. The rate for photodissociation (kd) is still larger than the rate for photoisomerization (k,) and the rate for direct photodissociation ( k d d ) to 0 2 c1. However, unlike the gas phase where the photoproducts are able to separate, the ice physically restricts the C10 and 0 atom mobility, and they recombine to the thermodynamically more stable isomer, C100. There is speculation, however, that the photolysis process would produce enough energy to melt the cage and allow the photoproducts to escape.28 If this is true, then the cage effect alone cannot account for our present results. Another possible explanation, and a more interesting one, is that the chlorine dioxide’s electronic states are actually perturbed by the amorphous ice. It may be that the ice enhances the coupling of the excited *A2 state of OClO to the potential energy surface that leads to isomerization. This would suggest that the predominant photochemical pathway in Figure 1 is actually changed when OClO is dissolved in ice. The rate for photoisomerization (k,) is now greater than the rate for photodissociation ( k d ) . Furthermore, the ice would stabilize the ClOO and prevent it from dissociating to 0 2 C1. As mentioned in the Introduction, the existence of an isomerization pathway is even questionable. Davis and Lee* have studied the gas-phase photolysis of OClO and have measured the translational energy of the photofragments. Their results indicate a concerted decomposition, as opposed to a twostep process involving isomerization followed by dissociation, i.e., the direct dissociation pathway (kdd) that produces 0 2 C1. They suggest that the production of 0 2 C1 is the result of vibronic coupling of the excited *A2 state to the strongly bent *B2 state and that the transition state would most likely be an OClO molecule with a severely compressed bond angle. If the mechanism for the photoproduction of 0 2 C1 from OClO is as Davis and Lee8 suggest, then our results might be understood in either of the following two ways. ( 1 ) The predominant pathway is the photodissociation to C10 0; the cage effect does not allow the separation of products, and they recombine to form ClOO (ie., same as above). ( 2 ) The ice perturbs the OClO excited electronic states and enhances the coupling of the ’A2 state to the *B2 state. Because of the
We have examined the photochemistry of chlorine dioxide in amorphous ice using both FTIR and UV-vis spectroscopy. Exposure of dilute mixtures of OClO dissolved in ice to ultraviolet light quantitatively converted the OClO to the chlorine peroxy radical, C100. In contrast to the gas phase, there is no evidence for C10 radical formation. Depending on the extent of OClO uptake by ice particles in the stratosphere, our results suggest that heterogeneous OClO photolysis in the atmosphere would produce C100. Once released into the gas phase, the ClOO would dissociate to 0 2 and C1 atoms. If OClO were continually taken up by ice particles during the early spring in the Antarctic and converted to C100, this would represent a new heterogeneous catalytic cycle leading to ozone loss. We are presently investigating the mechanism for the photochemistry of OClO in ice at stratospheric temperatures (i.e., 185-195 K) in order to further determine the importance for atmospheric ozone loss.
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Acknowledgment. We thank Professor V. Vaida at the University of Colorado for helpful discussions and encouragement. We also appreciate some constructive criticism by the referees. This work has been made possible by the generous financial support of Trinity University, the Dreyfus Foundation (Camille and Henry Dreyfus Start-up Grant Program for Undergraduate Institutions), the Research Corporation (Cottrell College Science Award), and the donors of the Petroleum Research Fund (administered by the ACS). Miss Parveen thanks the National Science Foundation for her summer support (NSFREU Program). Finally, we thank Mr. Rex Leonard of International Cryogenics for construction of the liquid nitrogen dewar and invaluable assistance during the course of this research. References and Notes (1) Vaida, V.; Solomon, S.; Richard, E. C.: Riihl, E.: Jefferson. A. Nature 1989, 342, 405. (2) Vaida, V.; Simon, J. D. Science, in press. (3) Lawrence, W. G.; Clemitshaw, K. C.; Apkarian. V. A. J. Geophys. Res. 1990, 95 (Dll), 18591. (4) Ruhl, E.; Jefferson, A.; Vaida, V. J. Phys. Chem. 1990, 94. 2990. (5) Colussi, A. J. J. Phys. Chem. 1990, 94, 8922. (6) Bishenden, E.; Haddock. J.; Donaldson, D. J. J. Phys. Chem. 1991. 95, 2113. ( 7 ) Bishenden. E.; Donaldson. D. J. J. Chem. Phys. 1993, 99. 3129. ( 8 ) Davis, H. F.: Lee, Y. T. J. Phys. Chem. 1992, 96. 5681. (9) Vaida, V.; Richard, E. C.; Jefferson, A,; Cooper, L. A,; Flesch, R.: Riihl, E. Ber. Bunsen-Ges. Phys. Chem. 1992. 96. 391. (10) Vaida, V.; Goudjil, K.: Simon, J. D.: Flanders. B. N. J. Mol. Liq. 1994, 61, 133. (11) Gole, J. L. J. Phys. Chem. 1980, 84. 1333. (12) Jafri, J. A.; Lengsfield, 111, B. H.; Bauschlicher, Jr.. C. W.; Phillips. D. H. J. Chem. Phys. 1985, 83, 1693. (13) Craven, W.; Knowles, D. B.; Murrell, J. N.; Vincent, M. A,: Watts. J. D. Chem. Phys. Lett. 1985, 116. 119. (14) Peterson, K. A.; Werner, H.-J. J. Chem. Phys. 1992, 96, 8948. (15) Dunn, R. C.; Richard, E. C.; Vaida. V.: Simon, J. D. J . Phys. Chem. 1991, 95, 6060. (16) Dunn, R. C.; Simon, J. D. J. Am. Chem. SOC. 1992, 114, 4856. (17) Dum, R. C.; Flanders, B. N.; Vaida, V.: Simon, J. D. Spectrochim. Acta 1992, 48A, 1293. (18) Arkell, A.; Schwager, I. J. Am. Chem. Soc. 1967. 89, 5999. (19) Rochkind. M. M.; Pimentel. G. C. J. Chem. Phys. 1967, 46, 4481. (20) Johnsson, K.: Engdahl, A,: Nelander. B. J. Phjr. Chem. 1993. 97, 9603. (21) Muller, H. S. P.; Willner. H. J. Phys. Chem. 1993. 97, 10589.
Photochemistry of Chlorine Dioxide in Ice (22) Masschelein, W. J. Chlorine Dioxide: Chemistry and Environmental Impact of Oxychlorine Compounds: Ann Arbor Science: Ann Arbor, MI, 1979. (23) Tolbert, M. A.; Middlebrook, A. M. J. Geophys. Res. 1990, 95 (D13), 22423. (24) Bertie, J. E.: Whalley, E. J. Chem. Phys. 1964, 40, 1637. (25) Johnston, H. S.; Morris, Jr., E. D.; Van den Bogaerde, J. J. Am. Chem. SOC. 1969, 91, 7712.
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