J. Phys. Chem. 1994,98, 3141-3155
3747
A Diffuse Reflectance Infrared Fourier Transform Spectroscopic (DRIFTS) Study of the Surface Reaction of NaCl with Gaseous NO2 and HN03 Rainer Vogt and Barbara J. Finlayson-Pitts’ Department of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, Calvornia 92634 Received: October 12, 1993; In Final Form: January 3, 1994’
The heterogeneous reactions of gaseous NO2 and HNO3 [(2-29) X 1014molecules ~ m - with ~ ] particles of NaCl in the 1-5” size range at 298 K have been followed in real time using diffuse reflectance infrared Fourier transform spectrometry (DRIFTS)to obtain kinetic and mechanistic data. Both the NO2 and H N 0 3 reactions gave an identical sequence of absorption bands attributed to nitrate ions, the expected product of these reactions: (1) HNO3(,) NaCl,,) NaNO3(,) HCl(,) and (2) 2N02(,) NaCl(,) NaN03(,) ClNOb). The reaction of D N 0 3 or NO2 with NaCl pretreated with D2O showed no new infrared absorptions, indicating that none of the bands were due to adsorbed HN03. Bands observed in the earliest stages of the reaction at 1333 and 1460 cm-l in the nitrate v3 antisymmetric stretch region, and at 1042 cm-l in the 11 symmetric stretch region, are attributed to isolated nitrate ions in different localized environments on the NaCl surface. When this surface is exposed to gaseous water at pressures below the deliquescence points of NaCl and NaNO3 and then heated and pumped, bands due to the isolated NO3- disappear, and broad absorptions characteristic of bulk N a N 0 3 are formed. This suggests that the water exposure leads to the formation of a quasi-liquid layer and drying to recrystallization into separate regions of N a N 0 3 and NaC1. Finally, the kinetics of the NO2-NaCl reaction are shown to be, within experimental error, second order in NO2. Assuming that N204 is the reactant, the gas-solid reaction probability for the N204-NaC1 reaction was found to be (6 f 2) X (la). Under typical tropospheric conditions, the NOz/N204 reaction will be too slow to compete with others, for example the NzOs-NaCl reaction.
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Introduction While diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) has been used successfully for the analysis of solids,’ it has not been applied extensively to studying the kinetics and mechanisms of gas-solid reactions. This technique holds promise for following in real time the reactions of alkali halides which are transparent in the infrared, since the formation of infrared absorbing intermediates and products can be followed with ease. Despite the potential of this technique, to date only one DRIFTS study has been carried out in which the products of the NaCl reactions with NO2 and N2O5 in the presence of water were studied qualitatively.2 The studies reported here were carried out to test the utility of DRIFTS in obtaining detailed kinetic and mechanisticinsights into the reactions of solid NaCl with gaseous NO2 and HN03: HN03(,) 2N02,,)
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+ NaCl,,,
+ NaCl,,,
+ HCl,,)
(1)
+ ClNO,,,
(2)
NaNO,,,)
NaNO,,,,
These reactions, as well as those with N2O5 and ClONO2,
C10N02,,,
+ NaCl,)
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NaNO,,,,
+ Cl,,,,
(4)
are potentially important in the atmosphere. Thus, the major component of sea salt particles generated by wave action is NaCl, and HN03, NO2, and N205 are known to be present in the atmosphere as well.’ Such reactions may also be important in unique situations such as in the plumes from oil well burning in Kuwait where the particles contain significant concentrations of NaC1.6’ In addition, the eruption of alkaline volcanoes such as El Chichon has been observed to inject significant quantities of
* To whom correspondence should be. addressed.
Abstract published in Advance ACS Absfracts. March 1, 1994.
OO22-3654/94f 2098-3741$04.50f 0
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salt into the stratosphere: which may have impacted the chemistry immediately after the eruption via reactions such as (2)-(4).9 Reactions 2-4 could be particularly important in the troposphere since they form gaseous photochemically labile products which will photolyze to form chlorine atoms. Atomic chlorine may play an important role in the formation and fate of troposphericozone. It can either react with O3lo0r,alternatively, react rapidly with organics10.11 in a manner similar to OH. In the presence of NO,, the Cl-organic reaction may result in the formation3 of 03,rather than its destruction. However, while the overall stoichiometry and products of reactions 1-4are well-known,12-22therearenodataon thekinetics of the HNOs and CION02 reactions and few on the r e a c t i o n ~ ~ ~ J ~ of . 2 3NO2 - ~ ~and N2O5. Perhaps more important, there is little mechanistic understanding of these reactions on a molecular level. We show here that DRIFTS can provide mechanistic details not available through other methods and that kinetic data can also be obtained. Specifically, we present the first evidence for different sites on the alkali halide surface which give unique NO3absorption bands in the initial stages of the H N 0 3 and NO2 reactions with NaCl. Evidence is also obtained indicating that a quasi-liquid layer forms on the salt surface upon exposure to water vapor at concentrations below the deliquescence point of the salt. Finally, the kinetics of the NO2 reaction are obtained, and the atmospheric implications of these kinetic and mechanistic data are discussed. Experimental Section The reactions of NaCl were studied in a newly constructed continuous slow-flow apparatus. The system consisted of a Harrick Scientific diffuse reflectance attachment (Model DRA2CS) with vacuum chamber (Model HVC-DR2) located in the sampling compartment of a Mattson RS Fourier transform infrared spectrometer equipped with a Mattson high-speed transfer board. Data from 64 scans, 1-cm-’ resolution, taken over 1.1 min were averaged for one spectrum. 0 1994 American Chemical Society
3748 The Journal of Physical Chemistry, Vol. 98, No. 14, I994
Vogt and Finlayson-Pitts
w Pnrun-z.
*. URlOI
Figure 1. Schematic of the DRIFTS apparatus and flow system.
Weighed amounts of large crystals of NaCl were placed in a Wig-L-Bug hall mill (Crescent Dental Manufacturing Co.) and typically ground for 5 min. To investigate the effects of the salt anion and cation on the positions of the infrared hands, several runs were also carried out with KCI, NaBr, and KBr prepared in the same manner. Particle sizes of the salt (typically 1-5 pm) were measured using scanning electron microscopy (SEM) on parts of the pellet mounted on adhesive tape and coated with a 200-A gold/palladium layer. Photographs of the salt particles were recorded by a scanning electron microscope (JOEL Model 35CF) applying acceleration voltages of I s 2 0 kV. To obtain reproduciblepacking of the DRIFTS sampling cup, the powder (250mg) waspressed into thecup(1 I-"diameter, 2-mm depth) using a packing device similar to the one described by TeVrucht and Griffiths.26 The sample could be heated and the temperature of the sample cup measured by a thermocouple located directly underneath. The outer walls of the reaction chamber were maintained at room temperature by circulating cooled water through a jacket surrounding the cell. The reaction chamber was connected to a vacuum system shown in Figure 1. A measured flow of carrier gas was passed into the chamber and was pumped through an outlet at the bottom ofthe sample cup. In most runs, helium (UHP, >99.999%, Spectra Gases Inc.) was used in order to detect any infrared absorption bandsdue to intermediates whichcould beoxidized by02. Some runs with NO2 were carried out in air (Ultrazero grade, THC 5 X 10-8 at Torr concentrations of NO2, assuming that NO2 was the reactant in a single-step process.I7 Sverdrup and Kuhlman23reported a mass accommodation coefficient for loss of NO2 on an artificial sea salt surface in the range l W - l t 7 , with the higher values at higher relative humidities of -88%. However, the concentrations of NO2 used were not reported, and these values represent the total uptake of NO2 rather than the reaction probability. Atmospheric Implications. Reactions 2-4 produce photochemically labile species ClNO, ClN02, and C12, respectively. Given their absorption cross sections in the actinic region10.42.43 they will photolyze rapidly, generating highly reactive chlorine atoms which may be important in the formation and fate of tropospheric ozone. Our studies establish that these reactions will not be limited to the surface of NaCl under atmosphericconditions. Thus, even at water vapor concentrations below the deliquescence point of NaCl, the isolated surface nitrate ions initially formed appear to become mobilized in a "quasi-liquid layer". On drying of the particle, coalescence to form microcrystallites of NaNOp on the surface of the NaCl occurs, regenerating fresh NaCl surfaces for reaction. Under atmospheric conditions where the relative humidity and temperature cycle repeatedly, a similar phenomenon is expected. Hence, the amount of available chloride for release
Reaction of NaCl with NO2 and H N 0 3 into the gas phase is not only that on the surface of the particle, but essentially all of the chloride within the bulk of the particle. This recrystallization may also explain why sea salt particles essentiallytotally depleted of chloride are often observed in marine atmospheres. The NO2 reaction is sufficientlyslow that the DRIlTS studies could not be extended down into the ppb range of NOz concentrations directly relevant to the atmosphere. However, given that the reaction is second order in N02, its rate will fall off rapidly with concentration. The dependence on water concentration has not been investigated, although based on the work of Sverdrup and K ~ b l m a n ?it~may be somewhat higher in the presence of significant concentrations of water as found in the atmosphere. Using the published equilibrium constant for reaction 9, the concentration of N2O4 in equilibrium with 0.1 ppm NO2 in the atmosphere will be only 0.07 ppt. Competing with the N2O4NaCl reaction will be others such as that with NzO5, reaction 3 above. The reaction probability25 for the NzOs reaction with dry NaCl is >2.5 X 10-3. The sticking coefficient for NzO5 on wet NaCl aerosols has been reported by Zetzsch and co-~orkers19,20 to be -0.03, with the yield of ClNOz continuing to be significant (-30%) at relative humidities above 90%. While there have been no direct measurements of N2O5 in the troposphere, its equilibrium concentrations have been calculated to be as high as 10-1 5 ppb based on the measured NO3 and NO2 concentrationsaU From eq IV, the relative rates of formation of nitrate in the reactions of N 2 0 4 and NzO5 with NaCl can be obtained by comparing @[G](l/M)'/z for each reaction, where [GI is the reactant gas concentration and M is its molecular weight. Assuming an NO2 concentration of 0.1 ppm, an NzO4 in equilibrium with it of 0.07 ppt, and an NzO5 concentration of 1 ppb, the NzOs reaction will be at least 5 orders of magnitude faster than the N2O4 reaction. Hence, the NO2/N204 reaction appears unlikely to be important under typical tropospheric polluted conditionscompared to the N2O5 reaction. Further work is underway in this laboratory to investigate this reaction. Conclusions DRIFTS is a very useful tool for elucidating both the kinetics and mechanisms of gas-solid reactions. Thus, the formation of isolated nitrate in different molecular environments has been observed in the initial stages of the reactions of NO2 and HNOs with NaCl, while at longer reaction times the spectrum more closely resembles that of crystallineNaN03. A surfacecontaining the isolated nitrate species can be converted to one similar to NaNO3/NaCl mixtures by exposure to water vapor followed by drying, suggesting that a quasi-liquid layer is formed even at water vapor pressures below the salt deliquescence point. DRIlTS has also been shown to beuseful for obtaining reaction orders and reaction probabilities. The N02-NaCl reaction was shown to be approximatelysecond order in NO2, and the reaction probabilities were found to be sufficiently low that the NO2 reaction is not likely to be significant compared to the N2O5 reaction under typical tropospheric conditions. Work is underway to extend this approach to other reactions of NaC1, including the N 2 0 5reaction. Acknowledgment. We are grateful to the National Science Foundation (Grant ATM-9005321) for support of this work and to Professor James N. Pitts Jr. for helpful discussions and review of the manuscript. Helpful discussions with Professor Peter R. Griffiths and Professor Kazuo Nakamoto are appreciated. We also thank Professor Robert Koch and Mr. Steve Karl for the SEM pictures and Mr. Dan Klevisha of Mattson Instruments,
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3755 Inc., for technical assistance. R.V. acknowledges partial postdoctoral support by the Deutsche Forschungsgemeinschaft. References and Notes (1) Griffiths, P. R.; Fuller, M. P. In Advances in Infrared and Raman Spectroscopy;Clark, R. J. H., Hater, R. E., Eds.; Heyden and Sons: London, 1982; Vol. 9. (2) Junkermann, W.; Ibusuki, T. Atmos. Enuiron. 1992, 26A, 3099. (3) Finlayson-Pitts, B. J.; Pitts, Jr., J. N. Atmospheric Chemistry: Fundamentals and Experimental Techniques; Wiley: New York, 1986. (4) Sheridan,P. J.;Schnell,R.C.;Hofmann,D. J.;Harris,J. M.;Deschler, T. Geophys. Res. Lett. 1992, 19, 389. (5) Cahill, T. A.; Wilkinson, K.; Schnell, R. J. Geophys. Res. 1992,97, 14513. (6) Parungo, F.; Kopwicz, B.; Nagamoto, C.; Schnell, R.; Sheridan, P.; Zhu, C.; Harris, J. J. Geophys. Res. 1992, 97, 15867. (7) Lowenthal, D. H.; Borys, R. D.; Rogers, C. F.; Chow, J. C.; Stevens, R. K.; Pinto, J. P.; Ondov, J. M. Geophys. Res. Lett. 1993, 20, 691. (8) Woods, D. C.; Chuan, R. L.; Rose, W. I. Science 1985, 230, 170. (9) Michelangeli, D. V.; Allen, M.; Yung, Y. L. Geophys. Res. Lett. 1991, 18, 673. (10) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.;
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