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Ind. Eng. Chem. Res. 1998, 37, 1196-1202
Deactivating Effects of Lead on the Selective Catalytic Reduction of Nitric Oxide with Ammonia over a V2O5/WO3/TiO2 Catalyst for Waste Incineration Applications Raziyeh Khodayari* and C. U. Ingemar Odenbrand† Department of Chemical Engineering II, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
Lead is one of the poisonous elements for de-NOx catalysts especially in the case of municipal waste incineration plants. Samples of a commercial selective catalytic reduction catalyst were impregnated with different amounts of Pb(NO3)2, and their catalytic activities were studied. The XPS and SEM analysis showed higher lead concentration on the outer surface of the monolith than on the inside of the wall for all samples. Pb covers the surface of inactive TiO2 sites, likewise active V2O5/WO3 sites, in a thin, noncrystalline layer. Both activity and NH3 chemisorbed decreased in the same manner with increasing poison coverage. This suggests that the area available for reaction was the same or at least proportional to that for the adsorption of NH3 and deactivation of the catalyst may be due to competitive chemisorption of the poison on the acid sites instead of by pore blocking. 1. Introduction Waste incineration is a route increasingly used to get rid of waste, mainly due to lack of space for landfill. According to Energy from Waste (EfW), 10 million tons of waste (of the total amount of about 80 million tons) are annually burned in nine member countries and contributed to 22 TWh energy consumption (Svensson, 1996). The NOx emission from waste incineration plants is in the same range as for coal-fired power plants (Herrlander, 1990). A commonly applied technique to remove NOx from the flue gas is selective catalytic reduction (SCR). The cost of replacing catalyst is a major part of the maintenance of the SCR system; hence, it is desirable to prolong catalyst life. Catalyst deactivation has been associated with the removal of active sites via the strong chemisorption of impurities on the surface, thus blocking access of the reactants (Chen and Yang, 1990; Morimune and Hirayama, 1988). Contaminants from the flue gas can deactivate the catalysts not only by covering sites but also by blocking pore entrances, thus decreasing the accessibility to unpoisoned sites. Consequently, resistance to poisonous elements is the most important subject to overcome. Municipal waste incineration produces more heavy metals per usable energy unit than other solid-fuel-fired plants. A normal waste incineration plant produces 20-30% (by weight) ash of the original waste. This ash contains most of the heavy metals, e.g., Zn, Pb, Cu, and Ni. The concentration of Pb in dust is between 6 and 30 mg/g for two Swedish waste incinerators and about 6-40 mg/m3 in the flue gas before dust precipitation for some Swedish waste incineration plants (Carlsson, 1986). The concentration after the dust precipitator will be much lower (0.5-1.4 and 13 mg/m3 for two different power plants). Brunner and Mo¨nch (1986) determined material balances for several metals in different sub* Author to whom correspondence is addressed. Telephone: +46 46 222 8309. Fax: +46 46 149156. E-mail:
[email protected]. † Telephone: +46 46 222 8284. E-mail:
[email protected].
units of a municipal waste incinerator. The flue gas contains about 5% of the total amount of lead, while 58% of the lead remains in the slag and 37% is adhered to the dust. Taylor Eighmy et al. (1995) analyzed ESP ash from municipal solid waste incineration and reported 27 mg of Pb/g of ash. The transfer of metals to the dust and the flue gas is strongly affected by the waste composition. The formation of lead compounds in the waste is influenced by elements such as sulfur, carbon, chloride, and fluoride, especially O2, CO2, HCl, SO2, and HF (Brunner and Mo¨nch, 1986). Winter et al. (1994) injected aqueous metal salts of arsenic, cadmium, chromium, and lead into a laboratory post flame reactor. The observed Pb species were PbO and Pb2O3. The reactor temperature was varied from 600 to 1100 °C. Morselli et al. (1993) investigated the amounts and types of heavy metals in both input waste and each incineration effluent and reported that the flue gas contained 3.6% of the total input of lead. Stuart and Kosson (1994) investigated waste incinerators and reported that lead is preferentially deposited onto the fly ash, by either volatilization or entrainment, and is carried out of the combustion chamber with the flue gas. According to Lin and Biswas (1994), three species of lead are most predominant in waste incinerator exhausts and they are elemental lead, lead oxide, and lead chloride. Chen et al. (1990) reported that the deactivating effect of PbO was between Na2O and K2O, which are known as quite strong poisons. Tokarz et al. (1991) investigated SCR catalysts used in a tail-end municipal waste incinerator. After an operating time of 1840 h, they measured 90 ppm lead on the catalyst. We have investigated an SCR catalyst (TiO2, V2O5, WO3) used in a high-dust municipal waste incinerator. The lead concentration increased irregularly during the time and was 3350 ppm after an operating time of 1908 h (Janner, 1995). The aim of the present research was to study the effects of lead on the SCR catalysts and especially if it is possible to correlate, as one of many possible poisonous compounds, the activity in the SCR reactions to the
S0888-5885(97)00606-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/20/1998
Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1197
Figure 1. Experimental setup of the activity test: 1, 2, 3, 4, gas cylinders for He, NO, NH3, and O2; 5, rotameters; 6, mass flow regulators; 7, gas mixing; 8, pressure transducers; 9, preheating; 10, oven; 11, reactor; 12, 3-way magnetic valve; 13, mass spectrometer; 14, mass spectrometer electronics.
amount of ammonia chemisorbed on the catalyst. Future work will be published dealing with other compounds as well. 2. Experimental Section 2.1. Catalyst. The catalyst used for this investigation was a commercial monolith SCR catalyst produced by BASF. The active phase consisted of 5% WO3 and 2% V2O5 based on a TiO2 support (anatase). The wall thickness of the one-channel monolith was 1 mm, and the pitch was 7.3 mm. 2.2. Preparation of Artificially Poisoned Catalysts. It is not possible to impregnate catalyst samples directly with PbO, PbF2, or PbSO4 due to their low solubilities in water (97.5% for all samples except crushed samples poisoned with 2.31 and 2.88 wt % which showed >95.0% selectivity). 3.3. BET Surface Area and Pore Volume. The BET surface areas were measured for two fresh samples after the activity test, and they were 70 and 72 m2/g, respectively. The BET surface area for the poisoned catalysts was about 67-70 m2/g, showing a very low dependence on the lead coverage. As mentioned before, it is assumed that a lead coverage of about 33 wt % is needed for covering a monolayer. Thus, only 8.7% of a monolayer can be covered with the sample containing 2.88 wt % lead. Furthermore, the poison precursors tend to be small molecules. This suggests deactivation due to specific either physical or chemical adsorption of lead on the acid sites of the catalysts. The pore volumes of the fresh catalyst before and after the activity test were 0.296 and 0.295 cm3/g, respectively. For the poisoned catalysts, the total pore volume was about 0.274-0.291 cm3/g, still showing no dependence on the lead coverage. The catalyst has quite a low micropore volume in comparison to the total pore volume. As Table 3 shows, the micropore volume for the poisoned catalysts fluctuates and is quite different for two fresh samples (0.0021 and 0.0011 cm3/g). This can be explained by the low accuracy of the method of
1200 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 Table 3. BET Surface Area, Total Pore Volume, and Micropore Volume of Different Monolithic Samples samplea
BET surface area (m2/g)
total pore volume (cm3/g)
micropore volume (cm3/g)
fresh fresh* 0.05 wt % Pb 0.19 wt % Pb 0.63 wt % Pb 1.16 wt % Pb 1.85 wt % Pb 2.31 wt % Pb 2.88 wt % Pb
72 70 68 69 69 67 69 70 68
0.295 0.296 0.291 0.292 0.275 0.278 0.274 0.283 0.281
0.0021 0.0011 0.0010 0.0001 0.0012 0.0007 0.0020 0.0020 0.0017
a Asterisk indicates a different piece of the same catalyst sample.
Table 4. Concentration of Pb and Other Elements on the Surface of the Monolith Measured by XPS (atom %) lead content (wt %) element fresh Si O V Ti C W Al S Pb
0.05
0.19
0.63
1.16
1.85
2.31
2.88
3.70 3.57 3.89 4.85 4.24 4.40 4.91 5.50 61.19 62.57 57.33 58.57 54.91 58.93 59.57 62.45 0.62 0.73 0.55 0.45 0.45 0.48 0.64 0.54 15.12 14.55 12.34 14.00 13.14 13.62 13.98 14.93 14.82 14.00 20.61 17.70 23.35 18.98 16.52 11.77 2.14 2.07 1.52 1.92 1.79 1.78 1.81 1.87 1.39 1.00 1.69 1.30 1.55 0.97 1.29 2.09 1.04 1.49 1.70 0.84 0.38 0.23 0.45 0.30 0.00 0.01 0.37 0.37 0.19 0.61 0.83 0.56
Table 5. Concentrations of Pb and Other Elements inside the Monolith Wall Measured by XPS (atom %) lead content (wt %) element fresh Si O V Ti C W Al S Pb
Figure 4. Pb concentrations in the profile of the catalyst wall for the sample poisoned with 1.85 wt % lead.
determination of the micropore volume. The low values show that the catalyst is mesoporous and the effect of the micropores is insignificant. 3.4. SEM and XRD Analysis. To determine the location of lead and the other elements on the catalyst before and after poisoning and to analyze these elements semiquantitatively, the catalyst surface and the catalyst wall were analyzed for some samples by SEM. It must be observed that the SEM analysis was made after the activity tests and maybe the reactor environment affected the distribution of different elements in the monolithic catalysts. If the poison was adsorbed slowly and was highly mobile, it would distribute itself uniformly throughout the catalyst. The fresh and the poisoned catalysts had almost identical distributions of all other catalytic elements over the full depth of the monolith. Hence, the poison had no effect on the distribution of these elements. Figure 4 shows lead concentration in the profile wall of the sample poisoned with 1.85 wt % lead. The poison deposits were concentrated more near the outer surface than inside the wall for the sample poisoned with 0.19 and 1.85 wt % lead. To determine whether crystalline phases of PbO or any other compounds were present on the poisoned samples, XRD analysis was carried out on the catalyst with the 2.88 wt % lead. No crystalline phases of any lead compounds were detectable, confirming that the lead oxide is distributed over the whole catalyst surface and penetrates into the wall. Furthermore, no XRD diffraction lines due to the active material were found. 3.5. XPS Analysis. The locations of lead and other elements on the catalyst were also determined by XPS. Both the catalyst surface and the inside of the catalyst wall were analyzed. Compared to fresh catalyst, two additional peaks were considered for the poisoned samples. These peaks were attributed to lead oxide. Tables 4 and 5 show concentrations of lead and other
0.05
0.19
0.63
1.16
1.85
2.31
2.88
5.09 5.09 3.97 5.88 6.34 4.00 5.01 3.79 43.70 43.70 48.76 51.62 44.53 44.81 44.57 42.95 0.37 0.37 0.67 0.75 0.29 0.18 0.48 0.24 9.99 9.99 9.09 11.53 9.62 7.99 9.23 10.79 36.90 36.90 35.42 26.29 34.72 39.47 36.20 38.42 1.65 1.65 1.35 1.90 1.68 1.58 1.71 1.62 1.95 1.95 0.57 1.43 2.52 1.68 2.43 1.65 0.33 0.33 0.00 0.54 0.16 0.00 0.00 0.20 0.02 0.02 0.16 0.06 0.15 0.29 0.36 0.34
elements on the surface and inside the wall for different samples. The lead amount increased with higher lead coverage. The concentration of lead over the full depth of the monolith was almost the same for the poisoned catalysts with higher than 1.16 wt % lead coverage. The analysis showed higher lead concentration on the outer surface of the monolith than inside the wall for all samples. This agrees with the result from the SEM analysis. Lead is being concentrated on the outer surface and in a zone less than 50 µm deep. The catalysts with lower lead coverage than 1.16 wt % showed lower lead concentration over the full depth of the monolith, still lower than the concentration on the outer surface. However, the most important feature of the XPS analysis was the determination of the chemical structure of the lead components. The lead doublet was found to have a constant energy separation of 4.80 ( 0.13 eV over the entire range of samples. By referencing the observed lead 4f7/2 peaks to the C1s peak, any charging in the sample would also appear in the binding energy of the C1s. Since the C1s electron binding energy was assumed to be 285.0 eV, any shifts in the lead values can be approximately corrected. The electron binding energies for carbon and lead were 288.79 and 142.6 eV for 0.19 wt % lead coverage. So, a chemical shift of about 1 eV was observed. The same chemical shift was observed for other lead coverages and a chemical shift of about 4 eV was observed for both Ti and Si. The binding energies for different samples are summarized in Table 6. According to Kim et al. (1973), the Pb4f electrons of PbO2 have lower binding energies than those of PbO, and Morgan et al. (1973) have reported that the Pb4f electrons of PbO2 have lower binding energies than those of lead and PbO. These results are not predicted by the normal correlation of binding energies to the oxidation state of the atom, but similar observations of binding energy reversal have been reported by other authors. Consequently, the chemical shift in our samples suggests PbO or Pb3O4. Kim et al. (1973) have also reported that PbO contrib-
Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1201 Table 6. Experimental Binding Energies for Fresh and Poisoned Samples Measured by XPS (eV) lead content (wt %) element fresh
0.05
0.19
Ti 2p3/2 Si 2p1/2 Al 2s1/2 V 2p3/2 S 2p1/2 C 1s1/2 Pb 4f7/2 W 4d5/2 O 1s1/2
462.8 107.5 123.7 519.8 173.0 288.7
0.63
1.16
1.85
2.31
2.88
463.1 107.8 123.9 520.2 173.4 288.6 142.6 251.2 251.1 251.6 533.7 534.0 534.3
463.1 107.7 123.9 520.0 171.0 288.6 142.7 251.4 534.1
463.1 107.7 124.0 519.9 173.1 288.7 143.0 251.5 534.1
462.6 107.3 124.1 519.7 172.8 288.4 142.5 251.2 533.7
462.7 107.8 123.9 520.0 172.7 288.8 142.7 251.3 534.1
462.7 107.5 124.2 519.7 173.2 288.7 142.7 251.2 533.9
462.6 107.3 124.0 519.4 172.7 288.5
Figure 6. Effect of lead content on chemisorbed amount of ammonia at 200 oC and different pressures: 0.48 Torr (3), 0.97 Torr (b), 1.95 Torr (0).
Figure 5. Isotherms of chemisorption of NH3 on the active sites of the poisoned catalyst samples: fresh (9), fresh* (4), 0.05 wt % lead (O), 0.19 wt % lead (0), 0.63 wt % lead (f), 1.16 wt % lead (3), 1.85 wt % lead (b), 2.31 wt % lead ([), 2.88 wt % lead (×). * ) repeated on a different piece.
utes to two O1s peaks, while PbO2 and Pb3O4 contribute to only one O1s peak. In the lead-oxygen system the O1s signal in XPS could distinguish between different oxides and different crystal structures of PbO. Nevertheless, it must be observed that in our case many other oxides such as WO3, TiO2, V2O5, Al2O3, and SiO2 are present in the catalyst and contribute to the change of the O1s peaks and it is very difficult to determine how each oxide contributes to the measured O1s peak. 3.6. NH3 Adsorption. To verify the lead oxide effect on the catalyst, ammonia adsorption measurements were performed on the fresh and poisoned samples. Figure 5 illustrates the isotherms of chemisorbed ammonia. These isotherms represent the amount of NH3 being chemisorbed by the acid-base reaction between NH3 and the acidic sites. In all, the amount of ammonia bonded to active sites was decreased with increasing lead coverage. Thus, the chemisorption of the poison on the surface covers active sites in a uniform manner, such that the net activity of the surface is a direct function of poison chemisorbed. Comparison of isotherms showed that the amounts of NH3 chemisorbed on the samples with 0.05 and 0.19 lead coverage were somewhat higher than that for the fresh catalyst over the whole pressure range. Repetition of the adsorption measurement on the fresh catalyst led to almost the same results. Figure 6 illustrates the effect of lead content on chemisorbed amounts of ammonia at different pressures and 200 °C. A gradual decrease in the amount of chemisorbed NH3 was observed with increasing lead coverage up to 1.85 wt %. A further increase in lead coverage up to 2.88 wt % led to no remarkable changes in the amount of chemisorbed NH3. The same amount of NH3 chemisorbed on these three catalysts indicates
no further covering of active sites with increasing lead coverage. Hence, a certain number of active sites remained unaffected on these catalysts. This result in combination with SEM results may be evidence that Pb covers the surface of the catalyst nonselectively in a thin, noncrystalline layer. It is assumed that only uncovered active sites are capable of chemisorbing NH3. Consequently, a lower adsorbed amount of NH3 for highly poisoned samples indicates that lead oxide covers active sites on the catalyst. Inomata et al. (1980) has reported that in the SCR reaction the NH3 first chemisorbs on the Bro¨nsted acid sites, followed by the bonding of nitrogen monoxide on the chemisorbed NH3 and subsequent decomposition of the complex to N2 and H2O. The rate of chemical adsorption of reactants or desorption of products may be rate-limiting. Catalyst poisoning is a special case of parallel competitive chemisorption. The strength of a poison is directly related to the strength of the chemisorptive bond. Hence, high adsorptivity of the catalyst for a compound means inhibition of a reaction by this compound. Further, the presence of a poison on a surface can affect the strength of bonding of other molecules on unpoisoned sites. The chemisorption bonding strength for reactant molecules decreases as the surface coverage of poison increases. Thus, a direct correlation exists between the amount of chemisorbed ammonia and the activity of the poisoned catalysts. According to Chen and Yang (1990), the Bro¨nsted acid sites of the V-OH groups are the active sites for the reaction. The poison is able to be preferentially chemisorbed on the catalytic surface in the presence of reactants and products, and chemisorption may be reversible or irreversible. Since both the activity and the amounts of NH3 chemisorbed decreased in the same manner with increasing poison coverage, this was taken as evidence that the area available for reaction was the same or at least proportional to that for the adsorption of NH3. 4. Conclusions The effect of lead poisoning on a commercial SCR catalyst produced by BASF was studied. It was shown that lead is a serious poison for SCR catalysts and causes deactivation by chemically reacting with the active sites or by physically introducing a barrier between them and the gas phase. (a) Lead deactivates the catalysts to a very high extent as shown by the results from the crushed samples. In this case, a lead coverage of about 0.19 wt
1202 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998
% is sufficient to reduce the conversion by almost 12% at 340 °C, while the conversion decreased by only 1% at the same conditions for the monolith catalyst. (b) Both cases showed no dependence of lead coverage on the selectivity toward N2 production. (c) Poisoning of the catalyst resulted in a concentrating of the major part of the lead on the outer surface and in a zone less than 50 µm deep. The levels of lead inside the walls were much lower. XPS suggested PbO or Pb3O4 as possible lead compounds. (d) NH3 adsorption showed that activity and NH3 chemisorbed decreased in the same manner with increasing poison coverage. This suggests that the area available for reaction was the same or at least proportional to that for the adsorption of NH3. Furthermore, deactivation of the catalyst may be due to competitive chemisorption of the poison on the acid sites instead of by pore blocking. Acknowledgment Financial support from NUTEK, the Swedish National Board for Industrial and Technical Development, is gratefully acknowledged. We also thank Mrs. Birgitta Svensson for help with XRD and BET analyses and Mr. Christer Jo¨nsson for help with SEM analyses. Furthermore, we thank Anders Widelo¨v for all the interesting discussions throughout the XPS analyses. Finally, special thanks goes to Carsten Bartels for his master thesis on the subject. Literature Cited Beeckman, J. W.; Hegedus, L. L. Design of Monolith Catalysts for Power Plant NOx Emission Control. Ind. Eng. Chem. Res. 1991, 30, 969. Brandin, J. Investigation of SCR Catalysts, Main and Side Reaction, Characterisation and Kinetics. Ph.D. Dissertation, University of Lund, Sweden, 1995. Brunner, P. H.; Mo¨nch, H. The Flux of Metals through Municipal Solid Waste Incinerators. Waste Manage. Res. 1986, 4, 105. Carlsson, K. Heavy Metals from “Energy from Waste” Plants: Comparison of Gas Cleaning Systems. Waste Manage. Res. 1986, 4, 15. Chen, J. P.; Yang, R. T. Mechanism of Poisoning of the V2O5/TiO2 Catalyst for the Reduction of NO by NH3. J. Catal. 1990, 125, 411. Chen, J. P.; Buzanowski, M. A.; Yang, R. T. Deactivation of the Vanadia Catalyst in the Selective Catalytic Reduction Process. J. Air Waste Manage. Assoc. 1990, 40, 1403. Gmelins Handbuch der Anorganischen Chemie; Blei, 8th ed.; Verlag Chemie: Weinheim, Germany, 1969; Part C. Herrlander, B. SCR DeNOx at the Munich South Waste Incinerator. 83rd Annual Meeting & Exhibition, Pittsburgh, PA, June 24-29, 1990.
Inomata, M.; Miyamoto, A.; Murakami, Y. Mechanism of the Reaction of NO and NH3 on Vanadium Oxide Catalyst in the Presence of Oxygen under the Dilute Gas Condition. J. Catal. 1980, 62, 140. Inomata, M.; Miyamoto, A.; Toshiaki, U.; Kobayashi, K.; Murakami, Y. Activities of V2O5/TiO2 and V2O5/Al2O3 Catalysts for the Reaction of NO and NH3 in the Presence of O2. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 424. Janner, J. Deactivation of an SCR-Catalyst in a Municipal Waste Incinerator. Department of Chemical Engineering II, University of Lund, Sweden, 1995. Kim, K. S.; O’Leary, T. J.; Winogard, N. X-Ray Photoelectron Spectra of Lead Oxides. Anal. Chem. 1973, 45 (13), 2214. Lefers, J. B.; Lodder, P.; Enoch, G. D. Modelling of Selective Catalytic DeNox Reactors: Strategy for Replacing Deactivated Catalyst Elements. Chem. Eng. Technol. 1991, 14, 192. Lin, W. Y.; Biswas, P. Metallic Particle Formation and Growth Dynamics During Incineration. Combust. Sci. Technol. 1994, 101, 29. Morgan, W. E.; Van Wazer, J. R. Bonding Energy Shifts in the X-Ray Photoelectron Spectra of a Series of Related Group IV-a Compounds. J. Phys. Chem. 1973, 77 (7), 964. Morimune, T.; Hirayama, N. Study of Catalytic Reduction of NOx in Flue Gas from a Municipal Refuse Incinerator. JSME Int. J. 1988, 31, No. 1, 135. Morselli, L.; Zappoli, S.; Militerno, S. The Presence and Distribution of Heavy Metals in Municipal Solid Waste Incinerators. Toxicol. Environ. Chem. 1993, 37, 139. Odenbrand, C. U. I.; Bahamonde, A.; Avila, P.; Blanco, J. Kinetic study of the selective reduction of nitric oxide over vanadiatungsta-titania/sepiolite catalyst. Appl. Catal. B 1994, 5, 117. Ramis, G.; Busca, G.; Lorenzelli, V.; Forzatti, P. Fourier Transform Infrared Study of the Adsorption and Coadsorption of Nitric Oxide, Nitrogen Dioxide and Ammonia on TiO2 Anatase. Appl. Catal. 1990, 64, 243. Stuart, B. J.; Kosson, D. S. Characterization of Municipal Waste Combustion Air Pollution Control Residues as a Function of Particle Size. Combust. Sci. Technol. 1994, 101, 527. Svachula, J.; Alemany, L. J.; Ferlazzo, N.; Forzatti, P.; Tronconi, E.; Bregani, F. Oxidation of SO2 to SO3 over Honeycomb DeNoxing Catalysts. Ind. Eng. Chem. Res. 1993, 32, 826. Svensson, M. Energi Ka¨ llan, NUTEK (Sweden) 1996, 5, 11. Taylor Eighmy, T.; Eusden, J.; Kranozowski, J. Comprehensive Approach toward Understanding Element Speciation and Leaching Behavior in Municipal Solid Waste Incineration Electrostatic Precipitator Ash. Environ. Sci. Technol. 1995, 29, 629. Tokarz, M.; Ja¨rås, S.; Persson, B. Poisoning of DE-NOx SCR Catalyst by Flue Gases From a Waste Incineration Plant. Stud. Surf. Sci. Catal. 1991, 68, 523. Winter, R. M.; Mallepalli, R. R.; Hellem, P. Determination of As, Cd, Cr, and Pb Species Formed in a Combustion Environment. Combust. Sci. Technol. 1994, 101, 45.
Received for review August 28, 1997 Revised manuscript received December 23, 1997 Accepted January 7, 1998 IE9706065
