(13) Spurny, K., “Assessment of Airborne Particles”, T. T. Mercer et al.. Eds., P 60, Thomas. 1972. (14) Spurny, K:,Havlova, J., Lodge, J. P., Sheesley, D. C., Wilder, B., Staub-Reinhalt Luft, 3 5 , 7 7 (1975). (15) Fan. K. C.. Lee. J.. Gentrv. J. W.. “The Effect of Gas Comoosition on the Collection Efficiency of Model Grid and Nuclipore Filters”, 50th Int. Conf. on Colloid and Surface Sci., San Juan, Puerto Rico, 1976. (16) Fan, K. C., Lee, J., Gentry, J. W., in “Colloid and Surface Science 11”,M. Kerker, Ed., Academic Press, New York, N.Y., 1976. (17) Chapman, S., Cowling T . G., “The Mathematical Theory of Non-Uniform Gases”, pp 151-67, Cambridge, 1960. (18) Fan, K. C., PhD dissertation, University of Maryland, College Park, Md., 1977. (19) Mercer, T. T., “Collection of Ti02 Particles Using Nuclepore Filters, 10th Aerosol Technology Meeting, Albuquerque, N.M., 1977. (20) Zebel, G., J . Aerosol Sci., 5,2473-82 (1974). (21) Leaseburge, C., Hyun, Y., Shen, P., Gentry, J., unpublished data, 1977. (22) Gentry, J.,Spurny, K., ibid (23) Dautrebande, L., “Microaerosols”, pp 1-22, Academic Press,
New York, N.Y., 1962. (24) Hyun, Y., Gentry, J., “A Variable Frequency Electrostatic Mobility Analyzer”, 13th IRCHA Colloquium, Paris, France, 1978. (25) Spurny, K., Madelaine, G., Collect. Czech. Chem. Commun., 36, 2749 (1971). (26) Pich, J., ibid., 29, 2223-7 (1964). (27) Gormley, P. G., Kennedy, M., Proc. Royal Irish Acad., 52A, 163 (1949). (28) Twomay, S., Bull. Obs. Pug de Dome, p 173 (1962). (29) Smith, T., Phillips, C., Environ. Sei. Technol., 9, 564-8 (1975). (30) Smutek, M., Pich, J., Aerosol Sei., 5, 17-24 (1974). (31) Parker, R., PhD dissertation, Duke University, Durham, N.C., 1975. (32) Fan, K. C., Leaseburge, C., Hyun, Y., Gentry, J., Atmos. Enuiron., submitted for publication (1978).
Receiued for reuieu: September 6,1977. Accepted June 12,1978. Work supported by N S F u n d e r Grant No. 76-09381. K.C.F. receioed support f r o m the Minta Martin Foundation of the University of Maryland.
Effects of Nitrogen Dioxide and Water Vapor on Oxidation of Sulfur Dioxide over V205 Particles Brigitte Barbaray, Jean-Pierre Contour, and Gerard Mouvier* Laboratoire de Physico-Chimie Instrumentale, Universite Paris VII, 2, place Jussieu, 75221 Paris Cedex 05, France 8 The effect of nitrogen dioxide and water on the adsorption and oxidation of sulfur dioxide over V2O5 is studied by x-ray photoelectron spectroscopy (XPS). SO2 does not chemisorb onto V2O5 below 150 “C, and it oxidizes a t 200 “C into adsorbed sulfate and sulfur trioxide. But if nitrogen dioxide or water is added in the adsorption cell, SO2 is chemisorbed and oxidized into sulfate from 25 “C. These results indicate that in the absence of nitrogen oxide or water, the VzO5 aerosols contribute little to the production of atmospheric sulfuric acid and that the nitrogen oxide enhances the , 5 0 2 oxidation as in the homogeneous reactions.
E x p e r i m e n t a l Conditions
We used very high-purity V2O5 (Merck), treated before adsorption by heating a t 200 “ C and to 10-9 torr in the preparation chamber of the spectrometer for 14 h. The SO2 (Merck) was used without any further purification (SO2 = 99.95%, HzO = 0.02%). The spectra were recorded on an AEI ES 200 B spectrometer operating with FAT 130 scanning mode and equipped with a Mg anticathode (Mg K a = 1253.65 eV). The binding energies were determined by using the Is peak of carbon contamination as internal reference. The binding energy of these electrons is set a t 285 eV, relative to the Fermi level (14).
Numerous studies on the homogeneous oxidation of SO:, in the presence of other gaseous pollutants (NO2, hydrocarbons, H 2 0 , 0 3 , OH, HOP,R 0 2 . . .) show that nitrogen oxides and hydroxyl radicals play a very important role in the oxidation of SO2 and formation of fine aerosol particles that constitute “acid smog” (1-3). However, two other processes are involved in the oxidation of atmospheric SO2: oxidation in the aqueous phase ( 4 , 5 ) ,and heterogeneous oxidation in the presence of solid particles (6-9). These two mechanisms have not been as widely published. In particular, the catalytic action of certain solid particles has never been clearly demonstrated (9, 10). We have therefore studied the adsorption and the oxidation of SO2 upon contact with Vz05 particles in the presence of preadsorbed nitrogen dioxide and water vapor. Recent reports have revealed the utility of X-ray photoelectron spectroscopy (XPS) in the determination of the chemical state of adsorbed pollutants (6, 8, 1 1 ) . I t seemed interesting to us to apply this technique to the study of the adsorptions and the surface reactions occurring in the S02NOz-H:,O-V:,05 system. The chemical reactions between SO2 and V205 have long been studied (12,13),but photoelectron spectroscopy, which is a technique for surface analysis, makes it possible to investigate more precisely the initial phase of these reactions. 1294
Environmental Science & Technology
The preparation of the sample and the adsorption are performed in the spectrometer preparation chamber. The adsorption pressure is set a t IO-* torr, and the temperature can be varied from 25 to 450 O C . After adsorption the SO:, is pumped out at the adsorption temperature, and the chamber is evacuated until a pressure of lop8 torr is reached. This pressure is maintained in the spectrometer while the spectra are recorded. These experiments were carried out in the absence of light to avoid any photochemical contribution to the SO2 oxidation ( 1 , I O ) . When the effect of NO2 and H20 on SO2 oxidation was studied, the following experimental procedure was adopted: NO2 adsorption at 10-2 torr or HzO adsorption a t 1torr during 15 min, followed by evacuation of NO:, until a pressure of lops torr is reached; after pumping down to torr during 15 min, introduction of SO2 and adsorption as described above. These procedures were chosen in preference to the introduction of a mixture of NO:, and SOz, since it makes it possible to study the NO2- or HiO-SO:, interactions in the adsorbed state by eliminating the possibility of homogeneous gas-phase reactions (10). R e s u l t s a n d Discussion
Adsorption of SO2. The spectra were recorded before and after SO2 adsorption (Table I). Under these conditions,
0013-936X/78/0912-1294$01.00/0 @ 1978 American Chemical Society
-.
I 100
1 I'CI
Figure 2. Chemisorption of SOn and NOz on V2O5 Comparison of ratios of S 2p (0)and 0 Is shoulder ( 0 )to V 2p intensities plotted vs. temperature
Figure 1. SOz chemisorption isobars (lo-' torr) on V2O5 Effect of NO2 and H20: ratios of N Is and S 2p to V 2p intensitiesare plotted vs. temperature. N 1s: ( 0 )NO2 alone, (0) NO2 t SOP.S 2p: (0)SO2 alone, (A)SO2 NOz, (A)SO2 t H20
A
+
Table 1. Binding Energies of S 2p and N 1s Electrons from SO2 and NO2 Adsorbed on V205 (Relative to C 1s = 285 eV) T(OK)
329 373 423 523 573 623
ss%
(h0.3)
...
... 168.9 169.2 169.2 169.5
NO2
N 1s
SO2
s 2P
+ NO2
N 1s
SO2
+ H20
s 2P
(h0.3)
(f0.3)
(f0.3)
(h0.3)
406.1 406.0 407.0
169.4 169.2 169.0 169.5 169.4 169.5
406.0 406.0 406.9
168.7 169.4 169.7 169.1 169.1 169.3
...
... ...
... ... ...
0
v. ,.,,,I,, '11,,",
535
chemisorption of SO2 cannot be detected a t room temperature. The sulfur 2p peak is only observed above 200 "C. The binding energy of the 2p sulfur electrons is close to that of sulfate ions ( E 169 eV) and increases little with the temperature (Tables I and 11).However, contrary to Craig et al. ( 6 ) , we are unable to detect several S 2p peaks after SO2 adsorption. As the FAT 130 scanning mode has a lower energy resolution than the FRR mode used by Craig e t al., we suppose that the S 2p lines of sulfate and adsorbed SO2 or sulfite are no longer resolved. The binding energies of electrons arising from substrate atoms do not change after adsorption. However, at 400 "C the shoulder observed a t about 532 eV on the oxygen Is peak is greater than a t low temperature. The intensity of the sulfur 2p peak compared to that of vanadium 2p goes through a maximum a t about 300 "C (Figure 1). Effect of Preadsorbed Nitrogen Dioxide and Water. NO2 A d s o r p t i o n . We first studied the temperature dependence of the adsorption of NO2 alone on V2Oj. The binding energy of the nitrogen Is electrons is close to that of nitrate ions (406.5 eV) and increases slightly with the temperature (Table I). The intensity of this peak is greatest a t room temperature and decreases regularly as far as 250 "C, above which the signal corresponding to nitrogen disappears (Figure 1). The adsorption isobar of NO2 is therefore similar to that observed for the adsorption of NO2 on FesO4 (14). A d s o r p t i o n a n d O x i d a t i o n of SO2 in Presence of NO2. Chemisorption of SO2 in the form of sulfate ions is observed from ambient temperature upward (Tables I and 11). The quantity of sulfur adsorbed is very much greater than that detected in the absence of NO2. The chemisorption isobar shows a minimum toward 100 "C and a maximum near 250 "C
7-1
--.,
,mT' , r.-'i,/ > , , ,, ~TT,-~F, I
I,,
530
, I
tnlrql., Bindiw
Figure 3. Deconvolution of 0 1s peak recorded after H 2 0 and SOn chemisorption on V 2 0 5at 323 O K (1) total peak, (2) chemisorbedoxygen peak, (3) lattice oxygen peak, (4) background. All binding energies are relative to C Is = 285 eV
(Figure 1).The energy of Is electrons of chemisorbed nitrogen is still close to that of nitrate ions (406.5 eV), but the quantity of NO2 remaining chemisorbed after reaction with SO2 is much less than that found when the NO2 is adsorbed alone. Moreover, the temperature dependence of the relative intensity of the nitrogen Is peak is the opposite of that of sulfur 2P. The 1s peak of oxygen consists of several signals. The main peak appears a t 530.5 eV and has a shoulder a t 532 eV whose intensity varies with the temperature in the same way as the relative intensity of the sulfur 2p peak (Figure 2). The binding energies of the nitrogen Is and sulfur 2p electrons depend little on the temperature, as has been observed for the adsorption of SO2 and NO2 alone. A d s o r p t i o n a n d O x i d a t i o n of SO2 i n Presence of W a t e r . Under our adsorption conditions, SO2 is found to be adsorbed in the form of sulfate from room temperature upward; from 200 to 300 "C, the chemisorption isobar is little different from that observed in the presence of SO2 alone (Figure 1);above 300 "C, the amount adsorbed continues to increase with the temperature. The binding energy of the sulfur 2p electrons is close to 169 eV and depends little on the temperature (Table I). The main peak of the oxygen Is electrons appears a t 530.5 eV with a shoulder a t 532 eV which disappears above 150 "C (Figure 3). Discussion of Results. In the absence of nitrogen dioxide, the chemisorption and oxidation of SO2 can be explained in terms of the mechanisms proposed for the oxidation of SO2 Volume 12,Number 12,November 1978
1295
Table II. SO2 Adsorption on V205: Binding Energies Relative to C 1s = 285 eV samples
V 2p 312
517.2
v205
f 0.2
f 0.2 532 530.1 f 0.2 532 530.3 f 0.2 532 530.6 f 0.2 532.7 f 0.2 530.2 f 0.2 532.2 f 0.2 530.2
+ so2 + NO2 (RT) V2O5 + NO2 + SO2
517.1 f 0.2
(537 K)
v205
517.6
f 0.2
517.7
f 0.2
517.3
f 0.2
(RT)
+ H20 + SO2
(RT) ("4)2S04 NO2SO3 NH4N03
169.2
f 0.3
169.4 f 0.3
406.1
f 0.3
406.0
f 0.3
168.7 f 0.3 168.5 eV 167.0 eV 406.0 eV 402.1 eV
NO,
NH:
on certain industrial catalysts containing V205 (15-1 7). The oxidation of SO2 occurs by a mechanism of the EleyRideal type, between the gaseous SO2 and the surface oxides:
+ 0 2 - (a) + SO,'- (a) SO2 (g) + 0- (a) + SO, (a) SO,'- (a) + V5+ + V3+ + SO3 (g) SO3 (9) + 0 2 - (a) + SO,'- (a) SO, (a) + 0- (a) + SO,'- (a) SO2 (g)
(1)
(2) (3)
(4) (5)
Reactions 4 and 5 show how the sulfate ions are formed when the SO:! is chemisorbed a t high temperature. These results are in good agreement with those of Sabroux who observes that V 2 0 5 aerosols have no catalytic effect on the formation of sulfuric acid in industrial smoke between 25 and 150 "C (18). In the presence of nitrogen dioxide, the mechanism of adsorption and oxidation is more complex. Nitrogen dioxide is adsorbed from room temperature; the nitrogen atom is then involved in an entity similar to the nitrate ion. This entity can be formed from oxygen ions, 0- and 02-,present a t the V205 surface (chemisorbed oxygen and lattice oxygen). The nitrogen dioxide then takes part in the oxidation of the SO2, but it is not a catalytic effect since the NO2 is consumed in the cycle of oxidation reactions. The intensity of the N Is peak changes conversely to that of the S 2p peak and is always less than that measured in the absence of SO2. Above 250 "C, when NO2 is no longer chemisorbed, the amount of sulfate formed decreases rapidly to the value observed in the absence of NO2. The formation of nitrosyl sulfuric acid a t the surface seems unlikely, since NOHS04 decomposes a t 73.5 "C; moreover, no N 1s peak corresponding to this compound appears in the spectra. However, it can be assumed that the SO2 molecule is adsorbed on the nitrate ions in the form of an unstable intermediate complex (S04NO)- which decomposes to regenerate V5+ ions and to release nitrogen monoxide according to the following reaction scheme:
NO2 (g)
+ 02-(a) +NO,
(a)
+ e-
(6)
V5+ + e- e V4+
(7)
SO2 (g) + NO, (a) + (SO4NO)- (a)
(8)
(S04NO)- (a)
+ V4+
-
SO:- (a)
+ V5+ + NO (9)
(9)
The presence of preadsorbed water also favors the chemisorption and oxidation of SO2 to sulfate even a t room tem1296
N 1s
N
v205
V205
s 2P
0 1s
Environmental Science 8 Technology
perature. This reaction can be attributed to surface hydroxyl groups taking part in a process analogous to that which occurs in homogeneous phase and at the surface of carbon particles. But water vapor can also favor a reduction of V5+ ions to V4+ ions by the S02. In the presence of water, the V2O5 is reduced by the SO2 to V02; this reaction is very slow when the temperature is below 400 O C ; but photoelectron spectroscopy, which analyzes only the first 50 A of the surface, is able to detect the initial phase of this reaction: V205
+ SO2 + H20 + 2 VO2 + H2S04
(10)
This reaction is faster than that which, in the absence of water, leads to the formation of VOS04. In both cases, these surface reductions increase the concentration of V4+ ions that are responsible for the mechanisms of adsorption and oxidation of S02. Conclusions
X-ray photoelectron spectroscopy appears to be an excellent tool for studying the adsorption of pollutants and their transformation in the adsorbed state. These results are in good agreement with those of Madelaine et al. (18,19) who showed that V205aerosols contribute little to the oxidation of SO2 to sulfate and sulfuric acid a t low temperature. In our experimental conditions, that is the ones of the sulfate formation in the flues of oil-fired boilers or in the plumes immediately exiting the stack, the adsorption and oxidation of SOz over V z 0 5 only occur above 150 "C, but the presence of nitrogen dioxide or water induces the reaction'even at room temperature. Acknowledgment
The authors are grateful to A. Salesse (Groupement RBgional de Mesures Physiques) for technical assistance in photoelectron spectroscopy. L i t e r a t u r e Cited (1) Sander, S . P., Seinfeld, J. M., Enuiron. Sci. Technol., 10, 1114 (1976). (2) Smith, J. P., Urone, P., ibid., 8,743 (1974). (3) Finlayson, B. J., Pitts, J. N., Science, 192,111 (1976). (4) Matteson, M. J., Stober, N., Lutmer, H., Ind. Eng. Chem. Fundam. 8,677 (1974). (5) Saint-Yreix. A,. Sibut-Pinote. R.. Courtecuisse. M.. Dmst 7172-765-Fr., 1973. (6) Craig, N. L., Harker, A. B., Novakov T., Atmos Enuiron , 8,15 I
(7) Conte, C., Devito-Francesco, G., Petronio, B., Staub. Reinhalt. Luft. 35,51 (1975). (8) Novakov, T., Chang, S. G., Harker, A. B., Science, 186, 259 (1974).
(9) Judeikis, H. S., Siege],S., Atmos. Enuiron., 7,619 (1973). (10) Boulaud, D., thesis (Fr.),Paris, 1977. (11) Barbaray, B., Contour, J. P., Mouvier, G., Atmos. Enuiron., 11, 351 (1977).
(12) Theobald, F., thesis (Fr.),Besancon, 1975. (13) Glemser, O., Angew. Chem., 73,785 (1961). (14) Contour, J. P., Mouvier, G., J . Catal., 40,342 (1975). (15) Ashmure, P. G., “Catalysis and Inhibition of Chemical Reactions”, p 235, Butterworths, London, England, 1963. (16) Michaud, M., Leroy, M. C., Livage, J., Mat. Res. Bull., 11, 1425 (1976).
(17) Paranavo, G., Symp. on Electronic Phenomena in Chemisorption and Catalvsison Semiconductors (Moscow.1968)D 111.Walter De Gruyter, Berlin, 1969. (18) Sabroux,J. C., thesis (Fr.),Paris, 1974. (19) Boulaud, D., Madelaine, G., Vigla, D., 3rd Int. Clean Air Congress (1973),C113-VDI Verlag Gmbh, Dusseldorf, Germany.
Received for review March 27, 1978. Accepted J u n e 12,1978. Work supported in part by the minist6re de la qualit6 de la uie. One o f t h e authors (B.B.) thanks the Electricit6 de France for its financial support.
Synchronous Spectroscopy for Analysis of Polynuclear Aromatic Compounds Tuan Vo-Dinh”, Richard B. Gammage, Alan R. Hawthorne, and John H. Thorngate Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
The technique of synchronous excitation is used to enhance the selectivity of luminescence spectrometry. The applicability of this simple methodology to the monitoring of trace organic pollutants such as the polynuclear aromatic compounds (PNA) originating from coal conversion processes is emphasized. Two specific examples are presented: the characterization of naphthalene derivatives in Synthane gasifier wastewater by synchronous fluorimetry, and multicomponent analysis of several PNA by synchronous room-temperature phosphorescence.
rather common occurrence of broad emission bands. Secondly, the methodology, as discussed briefly here, might provide a practical tool for the rapid analysis of samples filtered or spotted on paper adsorbents. In 1972 Schulman and Walling (6) observed intense phosphorescence a t room temperature from a number of salts of organic compounds absorbed on a variety of supports such as silica, alumina, filter paper, asbestos, and cellulose. Winefordner and his coworkers (7-10) have developed the procedures based on this effect into a powerful analytical tool. This simple and novel method of analysis has opened up a host of possibilities for monitoring organic pollutants deposited on surfaces or collected on filter membrane systems.
In recent years interest in monitoring polynuclear aromatic compounds (PNA) has increased because of their potential for carcinogenic action (as initiators, cocarcinogens, or tumor promoters) and their frequent occurrence in the environment in such various forms as processed shale oils, coal tars, waste waters, or contaminated urban air ( I ) . Luminescence spectroscopy has provided an important analytical tool to detect these organic pollutants. Despite the choice it affords of selecting both the excitation and the emission wavelengths, standard luminescence spectroscopy methods have limited applicability to the analysis of complex mixtures. While being a good research technique, it is often not suitable for use in industrial hygiene or routine environmental analyses. A recently developed methodology (2) is based on the idea of synchronous excitation suggested by Lloyd (3, 4 ) . This technique has been used previously by forensic researchers and oil spill analysts only in an empirical manner to provide fingerprints of some complex samples, such as crude oils ( 5 ) . This work reports preliminary attempts to apply and extend this underexploited analytical technique. Emphasis is placed on the suitability of this method for monitoring organic pollutants, such as PNA compounds, collected on filter paper or found in wastewater or by-products from coal conversion processes. Two specific examples will illustrate the applicability of the synchronous technique to luminescence spectroscopy: fluorescence analysis of by-product water of the Synthane gasifier, and room-temperature phosphorescence (RTP) multicomponent analysis of a mixture of several PNA. The first example involves the fluorimetric characterization of naphthalene derivatives in solution, whereas the second example presents the R T P assay of organic compounds adsorbed on filter paper. The R T P method was investigated for various reasons. First, it would seem to be a good area of application because of the 0013-936X/78/0912-1297$01.00/0
@ 1978 American Chemical Society
Experimental Apparatus and Procedure. A Perkin-Elmer spectrofluorimeter (Model 43A, Perkin-Elmer, Norwalk, Conn.) was used for all spectroscopic measurements (fluorescence and phosphorescence). A 150-W xenon arc lamp was used as the excitation light source. The detector was a R508 photomultiplier (Hamamatsu Co., Middlesex, N.J.) with a photocathode spectral response from 200 to 750 nm. The monochromator gratings, having 1200 lines per mm, can provide a maximum resolution of 0.2 nm. The measured spectra were not corrected for instrumental response and were recorded on a strip-chart recorder (Perkin-Elmer, Model 023). For synchronous luminescence measurements, both excitation and emission monochromators were locked together and scanned simultaneously. Fluorescence measurements were performed using standard 1 X 1 cm quartz cells. Most of the fluorescence spectra were recorded with 1 nm spectral resolution, whereas a 2-nm bandpass in both monochromators was used for the determination of the limit of optical detection of 2 methylnaphthalene. For RTP measurements, a cylindric phosphoroscope was used. A 10-nm spectral bandpass was used for R T P analysis. Since RTP spectra usually show broad emission bands, higher resolution was not necessary. A special sample holder with interchangeable heads for phosphorimetric measurements has been constructed in the laboratory. I t consists of a modified version of the finger-type sample holder described in previous work (9).The R T P method is characterized by the simplicity of its experimental procedures compared with the complexity of traditional phosphorimetric techniques which normally require the incorporation of the analyte in polymer matrices, or the time-consuming process of degassing solutions, or the Volume 12, Number 12, November 1978 1297
