J. Phys. Chem. 1995,99, 17269-17272
17269
Unique Photochemistry of Surface Nitrate R. Vogtt and B. J. Finlayson-Pitts* Department of Chemistry, University of Califomia, Irvine, Irvine, Califomia 9271 7-2025 Received: July 12, 1995; In Final Form: September 12, 1995@
Unique inorganic surface nitrate species are known to be formed by the reactions of alkali halides such as solid NaCl with gaseous NO2, HNO3, and N205. We report here that these surface nitrate species do not give nitrite ions upon UV photolysis, unlike stable crystalline inorganic nitrates such as NaNO3. No infrared active products are detected in the salt while the surface nitrate photodecomposes, demonstrating that the surface nitrate species has a unique photochemistry that is distinct from that of crystalline NaNO3. On the other hand, if the surface nitrate is transformed into microcrystallites of NaNO3 through a water-induced surface reorganization, the formation of nitrite is observed upon photolysis, as expected for the stable crystalline salt. A possible mechanism for the decomposition of the surface nitrate involves production of N02: N03-su,.f hv NO2 O-surf(8) rather than N02- 0 or ONOO- as observed in earlier studies. The atmospheric implications of these observations are discussed.
+
-
+
+
as gaseous chlorine-containing compounds:
Introduction Numerous studies of the irradiation of solid alkali nitrates have been reported over many decades.' A characteristic decomposition product is the nitrite ion (NOz-), which has been observed in all such studies reported in the literature to date. For example, Narayanswamy2fist reported the detection of the nitrite ion (NO23 from the UV photolysis of a series of solid inorganic nitrates using a mercury arc lamp. Since then, a variety of studies3-I2have confirmed this production of nitrite although there has been considerable controversy about the mechanism of its production. Recently, Plumb and Edwards12 used diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) to detect photolysis products after irradiation of KNO3 powder at 254 nrp. They observed the formation of bands at 721, 815, and 943 cm-', which they assigned to peroxonitrite (ONOO-). They further suggested that this intermediate, which had also been observed in earlier studies,6~8~'0~11 was a major source of nitrite via reaction 1 followed by reaction 2:
+ hv - ONOOONOO- + hv NO2- + 0 NO3-
-
+ hv -NO3NO3- + hv - NO2- + 0 ONOO-
(1)
(2) (3)
(4)
The peroxonitriteion photoisomerizes back to Nos-, reaction 3, under photolysis at 300 nm.I2 At 254 nm, the photoisomerization of the peroxonitrite is slower and, in addition, the decomposition (2) to nitrite ion occurs.12 While the photochemical studies described above have utilized stable inorganic crystalline nitrate salts, nitrate can also be formed by chemical reactions of solid alkali halides with various oxides of nitrogen. For example, NaCl and NaBr reactI3 with gaseous NOz, HNO3, and N205 to form nitrate ions as well
* Author to whom correspondence should be addressed. Phone, (714) 824-7670; fax, (714) 824-3168; e-mail,
[email protected]. Now at Max-Planck-Institut fur Chemie, Abt. Luftchemie, D-55020 Mainz, Germany. @Abstractpublished in Advance ACS Abstracts, November 1, 1995. +
NaCl,,,
+ 2N02(,, -ClNO(,, + NaNO,(,,
(5)
Recent DRIFTS studiesI4 of these reactions in our laboratory have shown that the nitrate formed in the earliest stages of these reactions has an infrared spectrum different from that of bulk crystallineNaNO3 powders. Rather than a broad absorption due to the v3 asymmetric stretch in the 1300-1500 cm-' region, which is characteristic of the crystalline powder, in the earliest stages of the reaction two sharp bands at 1333 and 1460 cm-' are formed, indicative of NO3- ions in different environments on the NaCl surface. At larger extents of reaction, additional overlapping bands due to surface nitrate grow in, generating a broader absorption feature in this region. Upon exposure to water vapor below the deliquescence points of NaCl and NaN03, followed removal of adsorbed water by pumping and heating, these surface nitrate species coalesce into microcrystallites of NaN03 attached to the NaCl c r y ~ t a l . ' ~ , ' ~ We report here studies of the photolysis of the nitrate ions formed by reaction 5 of NaCl with NO2, as well as of the microcrystallites formed subsequently by the surface reorganization induced by exposure to water vapor. For com$arison, photolysis of a mixture of NaN03 in NaCl was also studied. Nitrite was observed as a product of the photolysis of crystalline powders of NaNO3 as expected. In addition, nitrite was observed from the photolysis of the nitrate microcrystallites formed by reaction 5 followed by the water-induced recrystallization. However, when the surface nitrate ions formed by the reaction of NaCl with NO2 without subsequent water exposure were photolyzed, nitrite was not formed and no new infrared bands attributable to products were observed, despite the loss of nitrate. This very surprising result suggests an effect of the matrix surrounding the nitrate that has not before been reported.
Experimental Section A schematic of the DRIFTS apparatus (Harrick Model DRA2CS with Model HVC-DR2 vacuum chamber) is shown in Figure 1. The cell has ZnSe windows for the entrance and exit
0022-365419512099-17269$09.0010 0 1995 American Chemical Society
Vogt and Finlayson-Pitts
17270 J. Phys. Chem., Vol. 99, No. 47, 1995
n a) Wore photolysis (x 0.4)
A b
O.l{
S
Ellipsoid
1600
1400
Zases
1200
1000
800
Wavenumbers
Figure 2. DRIFTS spectra: (a) a dilute (0.058% w/w) mixture of
I
FTIR Spectrometer
OLt
Figure 1. Schematic diagram of DRIFTS photolysis apparatus.
of the IR beams, as well as a quartz window to allow in situ photolysis. NaCl was prepared by grinding single crystals of NaCl (Harshaw) in a Wig-L-Bug (Crescent Dental Manufacturing Co.) for 5 min. This has been shown in previous studiesi4 to give particles with typical diameters of ~ 1 - 5pm, which are ideal for DRFI'S studies.16 To remove adsorbed water and traces of organic impurities that are often observed, the salt was typically heated to 450 K for 40-50 min, cooled to room temperature in a flow of dry He, and then exposed to the beam from the xenon lamp for 30-50 min to further desorb traces of organics. The salt was then reacted to form the surface nitrate by passing a mixture of NO2 in He over the salt. Simultaneously, infrared spectra were recorded at 1 cm-l resolution by averaging 64 scans taken over 1.1 min using a Mattson R/S series FTIR spectrometer with a cooled mercury cadmium telluride detector. In some studies, the salt was then exposed to water vapor at concentrations below the NaCl and NaNO3 deliquescence points. This was followed by pumping to remove the water, which induces reorganization of the surface into microcrystallites of NaNO3 attached to the NaC1.I4-I5 This was confirmed by following the changes in the v3 region of two bands at 1333 and 1460 cm-', assigned to the isolated nitrate ions, into a broad band in the 1300-1500 cm-' region, similar to that of crystalline NaNO3 powders. Dilute mixtures of crystalline NaNO3 powders with NaCl were also photolyzed for comparison. NaNO3 (Aldrich, A.C.S. Reagent Grade) was ground with NaCl in the Wig-L-Bug and further diluted with ground NaCl to give mixtures of approximately 0.1 % (w/w). Photolysis using a high-pressure Xe lamp (Cermax, 300 W, Model LX300W operated at 150 W) was carried out while He (ultrahigh purity, >99.999%, Spectra Gases) flowed over the salt from the top of the pellet to the bottom. The lamp output is continuous, rising from iz 200 nm to a plateau at -260 nm. The infrared radiation from the lamp was absorbed using a 10 cm water filter placed between the lamp and the
-
NaNOflaCI before photolysis, where the spectrum has been reduced to 40% of the original for clarity and shifted upward for clarity; (b) difference spectra after photolysis with an unfiltered high-pressure Xe lamp for the times shown; (c) a dilute (0.24% w/w) NaNOdNaCI mixture, where the spectrum has been reduced to 40% of original and shifted downward for comparison purposes.
DRFI'S cell. In some studies, a 1 mm Pyrex glass filter was located in the beam to absorb W; the 50% and 10% transmission points were at 325 and 307 nm, respectively. At measured times, the photolysis lamp was turned off and infrared spectra were recorded. During the photolysis, the temperature at the surface of the sample rose approximately 5 "C as measured in a separate experiment by a thermocouple placed directly into the top layer of the salt pellet.
Results and Discussion Figure 2a shows the DRIFTS spectrum in the 800-1600 cm-l region of a 0.058% (w/w) mixture of NaNO3 in NaCl. The strong v3 asymmetric stretch in the 1300-1500 cm-I region is observed, as are the weaker vi symmetric stretch at approximately 1050 cm-l and the v2 out-of-plane bend at about 830 cm-I. Figure 2b shows the difference spectra upon photolysis of the mixture whose spectrum is shown in Figure 2a. All three bands attributable to nitrate decrease, and a band at -1260 cm-l is formed. In some runs, a weak band at 828 cm-I was also observed. By comparison to the spectrum of a dilute NaN02lNaCl mixture (Figure 2c), these bands are assigned to NOz-. In this experiment, the bands at 721, 815, and 943 cm-I reported by Plumb and Edwards and attributed to peroxonitrite (ONOO-) were not observed; this is not surprising in our studies, however, since the NaNO3 was present as a very dilute mixture, whereas pure KNO3 and much longer photolysis times were used in their studies. In addition, our light source was unfiltered so that rapid secondary photoisomerization of the peroxonitrite back to NO3- by radiation around 300 nm as reported earlier by Plumb and Edwards'*is expected. However, in separate experiments with a 5.1% KN03/KCl mixture photolyzed for 188 min using a low-pressure mercury lamp, where the most intense line is at 254 nm, a weak band at 943 cm-' was observed. Figure 3a shows the DRIFTS spectrum in the 800-1600 cm-' region after NaCl has reacted with NO2 and after the reacted surface has then undergone water-induced surface reorganization into microcrystallites of NaNO3 attached to the NaC1.I4.l5 Figure 3b shows the difference spectra upon photolysis, first with a 1
J. Phys. Chem., Vol. 99, No. 47,1995 17271
Unique Photochemistry of Surface Nitrate
1
0.044 a) Before phdolyrir
b
s
0.02
0
b) After p4mtalyris
1600
1400
1200
1000
800
Wavenumbers Figure 3. DRIFTS spectra: (a) after the reaction of NaCl with NO2 (1.3 x lOI5 molecules cm-') for 1 1 min followed by exposure to water vapor (4.2 x 10'' molecules cm-)) for 5 min, then pumping, heating to 443 K for 5 min, and cooling to 298 K (spectrum has been reduced to 20% and shifted upward for clarity); (b) difference spectra after photolysis with and without a Pyrex filter for varying times.
mm Pyrex plate to filter out the W and then without this plate. Essentially, no change occurs during the 10 min photolysis with the Pyrex filter in place. However, upon removal of the filter, the bands due to nitrate decrease rapidly and a strong band at approximately 1260 cm-' grows in. As in the photolysis of the NaN03NaCl mixture, this is attributed to the formation of nitrite. In short, photolysis of the microcrystallitesof NaNO3 formed by the water-induced surface reorganization gives the same results as those from an authentic sample of crystalline NaNO3 mixed with NaCl. Figure 4a shows the DRIFTS spectrum after NaCl reacted with NO2 to a sufficient extent so that a relatively broad and strong absorption in the v3 region from overlapping bands due to surface nitrate is observed. Figure 4b shows the difference spectra obtained upon photolysis, first with the Pyrex filter in place and then with it removed. Although a small amount of photolysis occurs with the W filter in place after 20-40 min, the photolysis is much faster when the filter is removed. The bands due to nitrate decrease, yet there is no corresponding increase in infrared absorption bands due to nitrite. Indeed, no additional bands in the infrared attributable to other solid products were observed under these conditions. DRIFTS interrogatesportions of the pellet to varying degrees depending on the depth. In earlier studies,I4 we showed that while the top portion produces most of the signal, absorptions from depths of 0.2-0.5 mm from the top were still detectable but with reduced intensity. To test whether the decrease in the intensity of the observed infrared absorption of nitrate in Figure 4 was due to a shift in the distributionof nitrate to lower regions of the sample, in several experiments the pellets were sliced into two pieces horizontally and analyzed for nitrate using the wet chemical method of Greenberg et al." A pellet, reacted with NO2 under conditions known to replace about 19% of the total available surface chloride (averaged over the whole pellet), was irradiated and analyzed in this manner. The top half of the pellet lost about 14% of the nitrate compared to a similarly reacted but unirradiated pellet, while the bottom part showed an approximately 13% increase in its nitrate concencentration. In a second photolysis experiment, 50% of the nitrate was lost from the top portion and an increase of about 44% was measured in the bottom portion compared to a pellet reacted under the same conditions but not photolyzed. These results suggest that a gaseous product formed in the upper layers of the pellet by photolysis was carried by the
continuous flow of He into the bottom layers, where it reacted, at least in part, to regenerate nitrate. Unfortunately, it is not possible in this apparatus to reverse the gas flow in order to release the products into the gas phase for infrared analysis since this dislodges the pellet from the sampling cup. In short, crystalline NaNO3, either as a bulk powder or as microcrystallites formed by reaction of NaCl with NO2 and subsequent treatment with water, photolyzes to form NO2- , consistent with the findings of earlier studies.'-'2 In contrast, nitrate ions formed on the surface of an NaCl matrix by reaction with oxides of nitrogen also photolyze but do not form nitrite. This appears to be the first such observation of nitrate photolysis without the concomitant production of nitrite, indicating that the photochemistry of the surface nitrate species cannot be described by reactions 1-4 as is the case for crystallineKNO3.l2 The nature of the gaseous photolysis product responsible for the increase in nitrate in the lower portion of the pellet, which accompanies the NO3- loss in the upper layers, is not known. One possibility is photolysis to N02, NO3-
+ hv - NO2 + 0-
(8)
followed by reaction 5 of NO2 with NaCl as it is carried by the continuous He flow through the lower portions of the pellet. However, if this is the case, it is somewhat surprising that significantincreases in nitrate are observed in the lower portions of the pellet, given that the reaction of NO2 with NaCl is relatively slowl4 and the concentrations of NO2 generated by photolysis will be small. Whether or not adsorbed water also plays a role is not clear. Small amounts of adsorbed water still reside on the surface even after heating and pumpingI4and may assist in the photochemical decomposition. For example, in aqueous solution,'* photolysis of NO3- proceeds in part via reaction 8 followed by reaction 9 of 0- with water:
0-
+ H 2 0- OH + OH-
(9)
This suggests the following reaction sequence:
m03(ads)
HN03(,)
-.
(11)
m03(g)
-
+N a q S )
Hq,)
+ NaNO,(,)
(12)
The reaction of HN03 with NaCl is f a ~ t e r ' ~ -than ~ l that of NOz, which is more consistent with trapping of the gaseous photolysis product in the lower layers of the NaCl pellet than with the production and subsequent reaction of NO2 itself. The formation of nitrite as a product of the photolysis of bulk crystallineNaNO3, in contrast to the photolysis of surface nitrate species, may be due to several factors. First, the 0-N-0 bond angles22in NO2- and NO3- are 115" and 120°, respectively, so there is not a large change in geometry when NOz- is formed from NO3-. With Na+ ions located above and below the nitrogen atom in nitrite and nitrate:2 stabilization of the NO2in the three-dimensional lattice can occur via interactions of the oxygen with sodium ions in adjacent levels. At the surface, however, this three-dimensional stabilization cannot occur so that other channels such as (8) above may become more favorable. Second, reactions such as (8)-(12) are much less
11212 J. Phys. Chem., Val. 99, No. 47,1995
Vogt and Finlayson-Pitts 1
I
a) B&o
A 0.06-
b
photolyaia
x 0.2
A
0.04-
40 min wtth pyrex nlter 6 min
no pyrex fllter b) After photolysis
1600
1400
1200
1000
800
Wavenumbers
Figure 4. DRIFTS spectra: (a) after the reaction of NaCl with NO2 (3.5 x lOI5 molecules cm-3) for 5 min, where the spectrum has been reduced to 20%of original and shifted upward for clarity; (b) difference spectra after photolysis with and without a Pyrex filter for the times shown.
likely for the crystalline NaNO3 since the nitrate ions are not adjacent to the surface-adsorbed water. In the atmosphere, alkali halides are the major components of sea salt particles generated in marine areas by wave action.13 In addition, significant concentrations of alkali halides were measured in the plumes from oil well burning in and in the stratosphere immediately following the eruption of El C h i ~ h o n . ~ Reactions ~,~* of dry salt aerosols with gaseous oxides of nitrogen found in the a t m o ~ p h e r eincluding ,~~ NOz, HNO3, and N2O5, will form the surface nitrate species initially. It is generally assumed that once alkali nitrates are formed, they do not react to regenerate gas phase oxides of nitrogen. However, if reaction 8 represents the major path for photochemical decomposition of the surface nitrate, this may not be the case, and regeneration of HNO3 or other oxides of nitrogen may occur. Clearly, further studies of the wavelength dependence and quantum yields, as well as the mechanisms involved, are needed using a variety of techniques.
Acknowledgment. We are grateful to the National Science Foundation (Grant No. ATM-9302475), the Department of Energy (Grant No. DE-FG03-94ER61899), and the Joan Irvine Smith and Athalie R. Clarke Foundation for support of this work. We also thank Dr. Ming Taun Leu for permission to cite work prior to publication and Dr. Vladimir Bondybey and Dr. James N. Pitts, Jr. for helpful discussions and comments on the manuscript.
References and Notes (1) Johnson, E. R. The Radiation-Induced Decomposition of Inorganic Molecular Ions; Gordon and Breach: New York, 1970. (2) Narayanswamy, L. K. Trans. Faraday SOC. 1935, 31, 1411. (3) Doigan, P.; Davis, T. W. J. Phys. Chem. 1952, 56, 764. (4) Hochanadel, C. J. Radiat. Res. 1962, 16, 286. (5) Kalecinski, J. Bull. Acad. Pol. Sci. 1972, X X , 279. (6) Yuramazova, T. A.; Koval, L. N.; Serikov, L. V. Khim. Vys. Energ. 1983, 17, 151. (7) Dolganov, V. S.: Borisova, I. A. Khim. Vys. Energ. 1987,21, 557. (8) Nevostruev, V. A.; Miklin, M. B. Khim. Vys. Energ. 1987,21, 154. (9) Khisamov, B. A,; Khaliullin, R. Sh. Khim. Vys. Energ. 1989, 23, 371. (10) Anan'ev, V. A,; Miklin, M. B.; Nevostruev, V. A. Khim. Vys. Energ. 1990, 24, 146. (11) Miklin, M. B.; Kriger, L. D.; Anan'ev, V. A,; Nevostruev, V. A. Khim. Vsy. Energ. 1989, 23, 506. (12) Plumb, R. C.; Edwards, J. 0. J . Phys. Chem. 1992, 96, 3245. (13) Finlayson-Pitts, B. J. Res. Chem. Intermed. 1993, 19, 235. (14) Vogt, R.; Finlayson-Pitts, B. J. J. Phys. Chem. 1994, 98, 3747. Vogt, R.; Finlayson-Pitts, B. J. J. Phys. Chem. 1995, 99, 13052. (15) (a) Vogt, R.; Elliott, C.; Allen, H. C.; Laux, J. M.; Hemminger, J. C.; Finlayson-Pitts, B. J. Atmos. Environ., in press. (b) Allen, H. C.; et al. In preparation. (16) Griffiths, P. R.; Fuller, M. P. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden and Sons: London, 1982; Vol. 9. (17) Greenberg, A. F., Trussell, R. R., Clesceri, L. S., Franson, M. A. H., Eds. Standard Methods f o r the Examination of Water and Waste Water; American Public Health Association, American Water Works Association and Water Pollution Control Federation; American Public Health Association: Washington, DC, 1985. (1 8) Brezonik, P. L. Chemical Kinetics and Process Dynamics in Aquatic Systems; Lewis: Boca Raton, FL, 1994. (19) Laux, J. M.; Hemminger, J. C.; Finlayson-Pitts, B. J. Geophys. Res. Lett. 1994, 21, 1623. (20) Fenter, F. F.; Caloz, F.; Rossi, M. J. J. Phys. Chem. 1994, 98, 9801. (21) Leu, M.-T.; Timonen, R. S.; Keyser, L. F.; Yung, Y. L. J. Phys. Chem. 1995, 99, 13203. (22) Megaw, H. D. Crystal Structures: A Working Approach; Saunders: Philadelphia, PA, 1973. (23) Cahill, T. A.; Wilkinson, K.;Schnell, R. J. Geophys. Res. 1992, 97, 14, 513. (24) Sheridan, P. J.; Schnell, R. J.; Hohann, D. J.; Harris, J. M.; Deshler, T. Geophys. Res. Lett. 1992, 19, 389. (25) Parungo, F.; Kopcewicz, B.; Nagamoto, C.; Schnell, R.; Sheridan, P.; Zhu, C.; Harris, J. J. Geophys. Res. 1992, 97, 15, 867. (26) Lowenthal, D. H.; BOTS, R. D.; Rogers, C. F.; Chow, J. C.; Stevens, R. K.; Pinto, J. P.; Ondov, J. M. Geophys. Res. Lett. 1993, 20, 691. (27) Woods, D. C.; Chuan, R. L. Geophys. Res. Lett. 1983, 10, 1041. (28) Woods, D. C.; Chuan, R. L.; Rose, W. I. Science 1985, 230, 170. (29) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques; Wiley: New York, 1986.
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