Ind. Eng. Chem. Res. 1997, 36, 5277-5281
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In Situ Fourier Transform Infrared Characterization of Sulfur Species Resulting from the Reaction of Water Vapor and Oxygen with Zinc Sulfide Ranjani V. Siriwardane* and Steven Woodruff U.S. Department of Energy, Federal Energy Technology Center, P.O. Box 880, Morgantown, West Virginia 26505
Steam is utilized as a diluent in the regeneration of zinc-based hot gas desulfurization sorbents used in fuel gas cleanup. Therefore, the effect of water vapor on the interaction of zinc sulfide (ZnS) with oxygen at 823, 873, and 923 K was studied using Fourier transform infrared (FTIR) spectroscopy. When ZnS at 823 K was exposed to oxygen, sulfate (SO42-) was the most prominent species observed at oxygen pressures greater than 5 × 10-2 Torr while sulfite (SO32-) was the major species observed during the water vapor exposures. When ZnS at 823 K was exposed to an equimolar mixture of water vapor and oxygen, sulfite and sulfate were the most prominent products but the intensity of the sulfite peak was more than that of the sulfate. The sulfur species formed during the water vapor and oxygen exposures at both 873 and 923K were similar to those at 823 K, but the relative intensities of the peaks corresponding to these sulfur species were different. At all three temperatures, the presence of water vapor enhanced the formation of sulfite, which can be decomposed more readily than sulfate. Thus, it is hypothesized that it would be desirable to have water vapor present during the regeneration of ZnS to form ZnO. Introduction Several zinc-based sorbents (Siriwardane, 1996; Lew et al., 1989; Woods et al., 1989; Siriwardane et al., 1994a, 1996) have been shown to be promising regenerable high-temperature sorbents for hydrogen sulfide removal from fuel gas during coal gasification. The performance of these sorbents depends on both their sulfidation ability and their regenerability. Regeneration of sulfided sorbents is usually performed utilizing oxygen. Steam and nitrogen are used as diluents during regeneration. Major sulfur species formed during the regeneration are sulfur dioxide and sulfate (Siriwardane et al., 1994a,b). However, the formation of sulfate is undesirable during the regeneration of sulfided sorbents. Steam is primarily used as a diluent since the reaction of oxygen with ZnS is highly exothermic. However, the effect of steam on the formation of undesirable sulfur species during regeneration is not known. Thus, the reaction of a metal sulfide with oxygen and steam is a critical step in regeneration (Siriwardane et al., 1994a,b). The reaction mechanism of the interaction of oxygen with ZnS has been studied previously (Ong et al., 1956; Prabhu et al., 1984; Siriwardane and Woodruff, 1995; Flytzani-Stephanopouolos et al., 1987). However, studies on the effect of water vapor on the reaction mechanism have not been reported. In this study, research focused on understanding the effect of water vapor on the reaction mechanism of zinc sulfide oxidation was performed by using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Experimental Section Diffuse reflectance spectra were recorded using a diffuse reflectance attachment (Model DRAH2MO1) and a high-temperature cell (Model HVC DR2) with ZnSe windows, both made by Harrick Scientific Corp. FTIR spectra were obtained using a Matteson Cygnus 100 * To whom all correspondence should be addressed. S0888-5885(97)00343-6
spectrometer with a mercury cadmium telluride (MCT) detector. Single-beam spectra were acquired at 4 cm-1 resolution with 512 scans at 823 K and 1024 scans at 873 and 923 K. ZnS (Alfa, 99.999%) utilized in the experiments had a BET nitrogen surface area of 20 m2/ g. The ZnS sample was heated in the high-temperature cell (10-5 Torr) at the desired temperature for 2 days before exposure to gas. The single-beam spectrum at the desired temperature obtained immediately before exposure to gas was taken as the reference. The gas exposures were performed continuously, and spectra were obtained in situ during the gas exposures. The peaks in the region 450-750 cm-1 could not be easily identified since the noise level of this region was high after normalization of the spectra. Results and Discussion (a) Oxygen and Water Vapor Exposures at 823 K. Infrared peak assignments for the various sulfur species are based on the information listed in Table 1. According to the data in Table 1, “sulfite” peaks are in the region 925-984 cm-1. Various types of SO2 are reported to be in the region 1140-1685 cm-1, but the majority of SO2 peaks are reported around the region 1350 cm-1. The peaks around 1120 and 1180 cm-1 corresponding to “sulfate” obtained by the authors (Table 1) for the ZnSO4 standard were utilized to identify sulfate peaks in this study. When ZnS was exposed to pure oxygen, IR peaks corresponding to sulfur dioxide (1160-1760 cm-1) and sulfite (950 cm-1) were observed below oxygen pressures of 5 × 10-2 Torr, and those of sulfate (1120 and 1180 cm-1) and sulfur dioxide (1600 and 1330 cm-1) were observed above oxygen pressures of 5 × 10-2 Torr (Siriwardane and Woodruff, 1995) as shown in parts a and b of Figure 1, respectively. Spectra were collected every 10-15 min during oxygen exposures, but not all of these are shown. The peaks below the baseline indicate the growth of the corresponding species. The FTIR spectrum of ZnS after exposures to 1 × 10-2 Torr of water vapor at 823 K is shown in Figure 2a.
This article not subject to U.S. Copyright.
Published 1997 by the American Chemical Society
5278 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 Table 1. Literature Values of IR Bands in Sulfur Containing Compounds species -2)
(1) sulfite (SO3
(2) physisorbed SO2 weakly chemisorbed SO2 adsorbed SO2 gaseous SO2 physisorbed SO2 weakly bound SO2 strongly bound SO2 weakly bound SO2 perturbed adsorbed SO2 at high P perturbed chemisorbed SO2 (high P, high T) adsorbed SO2 at high T (3) sulfate (SO42-) general ZnSO4(D16 2h) SO42- (Td) chelating bidentate SO42-/γ-alumina CaSO4 ZnSO4 (4) hydrogen sulfide H2S/Al2O3 H2S/Al2O3 gaseous H2S
IR bands (cm-1)
ref
950 950
Martin et al. (1987) Newman and Powel (1963) Nyberg and Larsson (1973)
925, 984 1330, 1151 1320, 1140 1250, 1200 1350, 1330 1276, 1022, 813 1370, 1375, 1147 1326, 1070 1330, 1140 1685, 1240 1570, 1440 1505, 1434
Berben et al. (1988) Thompson and Palmer (1988) Chang (1978) Deo and Dalla Lana (1971) Low et al. (1971)
1104, 981, 613, 451 1085, 1190, 1150, 1085, 1075, 1005, 695, 605, 451 1163, 1118, 676 1205, 1155, 1114, 997, 675, 613, 594 1400, 1100 1185, 1160, 1120, 1020, 675 1120, 1180 2560, 1330 2560, 1335 3789, 2684, 2611, 2422, 1290
Figure 1. IR spectra of ZnS at 823 K after oxygen exposures of (a) 1 × 10-2 Torr for 76 min and (b) 8 × 10-2 Torr for 74 min and (c) nitrogen exposures of 1 × 10-2 Torr for 75 min.
Distinct peaks corresponding to adsorbed water (1570 cm-1), sulfate (1180 cm-1), adsorbed SO2 (1330 cm-1), and sulfite (strong broad peak in the 700-990 cm-1 region and centered around 900 cm-1) were observed. In addition, a broad band in the 2600-3650 cm-1 region that corresponds to adsorbed water was also observed. The peak centered at 2450 cm-1 was assigned to adsorbed H2S. When ZnS was exposed to water vapor, the peaks obtained above 5 × 10-2 Torr (Figure 2b) were similar to those observed at the lower pressure (Figure 2a). Thus, the species formed during water exposures are not pressure-dependent, which is different from the observations during the exposures to oxygen. When the ZnS that had been exposed to water vapor was evacuated to 10-5 Torr, adsorbed water (2930 cm-1), hydrogen sulfide (2450 cm-1), and sulfur dioxide (1330 cm-1) desorbed from the sample and sulfate (1180 cm-1) and sulfite (930 cm-1) decomposed.
Steger and Schmidt (1964) Martin et al. (1987) Chang (1978) Low et al. (1971) Siriwardane and Woodruff (1995) Desyatov et al. (1990) Deo and Dalla Lana (1971)
Figure 2. IR spectra (region 700-1800 cm-1) of ZnS at 823 K after water exposures of (a) 10-2 Torr for 44 min and (b) 7 × 10-2 Torr for 54 min.
Adsorbed H2S may be formed after the water exposure by the following reaction.
ZnS + H2O f ZnO + H2S
(1)
H2S may be readsorbed on the surface. After exposing the ZnS sample to 1 × 10-2 Torr of an equimolar mixture of water and oxygen, a broad band at 1350-1600 cm-1 corresponding to various types of adsorbed SO2 and a peak centered around 930 cm-1 corresponding to sulfite were observed, as shown in Figure 3a. This was similar to the observations after the oxygen exposures at 1 × 10-2 Torr, as shown in Figure 1a. In addition, peaks corresponding to adsorbed water vapor (2400-3300 cm-1) were observed, but a peak corresponding to adsorbed hydrogen sulfide (∼2460 cm-1) was not observed. Formation of H2S was observed
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when ZnS was exposed to pure water vapor. Thus, the presence of oxygen inhibited the formation of H2S. When the pressure of the equimolar mixture of oxygen and water was increased to 1.2 × 10-1 Torr, the peak corresponding to sulfate (∼1180 cm-2) appeared, while the peaks within the broad band (1350-1600 cm-1) corresponding to adsorbed SO2 became more distinct. This indicates the formation of a more structured form of adsorbed SO2 than at the lower pressures. The sulfite peak (∼930 cm-1) was still the strongest peak. This is different from the observations made during the exposures to oxygen above 5 × 10-2 Torr, as shown in Figure 1b, in which sulfate was the strongest peak. Thus, the presence of water promotes the formation of sulfite at the expense of the formation of sulfate. Both sulfite and sulfate peaks continued to grow when the exposure of the ZnS sample to an equimolar mixture of water and oxygen was increased up to 1.7 × 10-1 Torr. The intensities (obtained by fitting Voigt bands to spectra transformed by the Kubelka-Munk method) of sulfite to sulfate during both water-oxygen and oxygen exposures are shown in Figure 4, and the intensity of the sulfite peak is clearly higher than that of sulfate in the presence of water. Zinc sulfite is reported to be unstable above 473 K (Lide et al., 1995; Schwitzgebel and Lowell, 1969). However, sulfite type structures which are unstable at elevated temperatures have been observed as intermediates by previous researchers during the reaction of CaO with SO2 and O2 (Siriwardane, 1989; Martin et al., 1987). When ZnS was evacuated at 10-5 Torr after the exposures to the water-oxygen mixture, the intensities of the peaks corresponding to water vapor (1574 cm-1), adsorbed SO2 (1330 cm-1), sulfate (1180 cm-1), and sulfite (∼930 cm-1) decreased, indicating the removal of these species from the solid during evacuation. Sulfite seems to be the most prominent species being removed from the ZnS solid during this evacuation. When ZnS was exposed to oxygen, formation of adsorbed SO2, sulfite, and sulfate may take place via the following reactions:
ZnS + 3/2O2 f ZnO + SO2
(2)
ZnO + SO2 f ZnSO3
(3)
ZnO + SO2 + 1/2O2 f ZnSO4
(4)
(b) Oxygen and Water Vapor Exposures at 873 K. When ZnS was exposed to oxygen at 873 K, adsorbed SO2 (1150-1750 cm-1) was the major species observed at both low (1 × 10-2 Torr) and high (7 × 10-2 Torr) pressures. At higher oxygen pressures (7 × 10-2 Torr), an additional band in the 800-1150 cm-1 region was observed within the broad spectral region, as shown in Figure 5a. However, it is not possible to clearly observe the growth of individual peaks within the broad spectrum. In order to see the growth of the peaks when the pressure was changed, the spectrum at 7 × 10-2 Torr was ratioed to that at 1 × 10-2 Torr and it was possible to see the growth of the sulfate peaks (1120 and 1180 cm-1); this may have contributed to the growth of the additional band in the 800-1150 cm-1 region at higher oxygen exposures. The type of species formed during the water vapor exposures at 873 K were not pressure-dependent. The spectrum after the water vapor exposures at 7 × 10-2 Torr is shown in Figure 5b. Even at 873 K, the sulfite
Figure 3. IR spectra (region 700-1700 cm-1) of ZnS at 823 K after 1:1 oxygen-water vapor exposures of (a) 1 × 10-2 Torr for 62 min, (b) 1 × 10-1 Torr for 1 min, and (c) 1.2 × 10-1 Torr for 64 min.
peak (∼930 cm-1) was the most significant peak during the water vapor exposures. In addition, peaks corresponding to adsorbed water (2400, 3300, and 1570 cm-1) and sulfur dioxide (1350 cm-1) were also observed; this was similar to the observations at 823 K. The intensity of the peak corresponding to sulfite (∼930 cm-1) was higher at 873 K than that at 823 K. When ZnS was exposed to an equimolar mixture of water and oxygen at 873 K, a peak corresponding to sulfite (∼930 cm-1) was observed at both low (2 × 10-2 Torr) and high (1.5 × 10-1 Torr) pressure, as shown in Figure 6. This was different from the spectra observed with oxygen exposures at 873 K. However, the other peaks were similar to those observed with the oxygen exposures. This again indicated that the water vapor enhanced the formation of sulfite even in the presence of oxygen at 873 K. (c) Oxygen and Water Vapor Exposures at 923 K. As shown in Figure 7b, when the sample at 923 K was exposed to oxygen, a very broad peak in the region of 1150-1725 cm-1 (various types of adsorbed sulfur dioxide) and a band around 950 cm-1 corresponding to sulfite were observed at both low (1 × 10-2 Torr) and high (7 × 10-2 Torr) pressures. At 7 × 10-2 Torr of oxygen, an additional peak corresponding to sulfate (∼1120 cm-1) was observed, also shown in Figure 7b. Sulfite (930 cm-1) was the major species formed during water vapor exposures at 923 K, as shown in Figure 7a. Thus, the reaction profile at 923 K is similar to that at 873 K. When the ZnS sample was exposed to an equimolar mixture of water vapor and oxygen, sulfite (∼930 cm-1) was the strongest peak, as shown in Figure 7c. This again indicates that, even at 923 K, the formation of sulfite was enhanced by the presence of water vapor. Since the presence of water vapor decreases the sulfate formation, it may be desirable to have steam present during the regeneration of zinc sulfide utilizing oxygen. The decomposition temperature of zinc sulfite is 473 K, while the decomposition temperature of zinc sulfate is 873 K (Lide et al., 1995; Schwitzgebel and Lowell, 1969). Therefore, it is easier to decompose the sulfite type intermediate, which forms in the presence
5280 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997
Figure 4. Intensities of sulfite and sulfate peaks as a function of time during oxygen and oxygen-water vapor exposures.
Figure 5. IR spectra (region 700-1700 cm-1) of ZnS at 873 K after exposures of (a) oxygen at 7 × 10-2 Torr for 20 min and (b) water vapor at 7 × 10-2 Torr for 20 min.
Figure 6. IR spectra (region 700-1800 cm-1) of ZnS at 873 K after 1:1 water-oxygen exposures of (a) 1 × 10-2 Torr for 54 min and (b) 1.5 × 10-1 Torr for 56 min.
of water vapor, so complete regeneration of zinc sulfide to zinc oxide may be obtained. Inhibition of the formation of sulfate in the presence of steam has been observed during regeneration studies of zinc ferrite utilizing a thermobalance reactor (Focht et al., 1989).
2. When ZnS at 823 K was exposed to an equimolar mixture of water vapor and oxygen at low pressures (1 × 10-2 Torr), adsorbed sulfur dioxide and sulfite were observed. At high exposures (1.2 × 10-1 Torr), both sulfite and sulfate peaks were observed but sulfite was the strongest peak. This was different from the observations made during the pure oxygen exposures, in which sulfate was the strongest peak. 3. The presence of water vapor promoted the formation of sulfite at the expense of the formation of sulfate at 823 K. The sulfite peak was readily removed from the sample during evacuation of the water vapor. 4. At 873 K, sulfite was the strongest peak observed during both water vapor and water-oxygen exposures.
Summary 1. The type of chemical species formed during the reaction of ZnS with water vapor at 823 K was not pressure-dependent, and this was different from the observations made during oxygen exposures. The major species formed during the water vapor exposure was sulfite. In addition, adsorbed water, hydrogen sulfide, and adsorbed sulfur oxides were also observed.
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Figure 7. IR spectra (region 700-1800 cm-1) of ZnS at 923 K after exposures of (a) water vapor at 7 × 10-2 Torr for 35 min, (b) oxygen at 7 × 10-2 Torr for 35 min, and (c) water-oxygen at 8 × 10-2 Torr for 30 min.
In contrast to this, when ZnS was exposed to pure oxygen at 873 K, the major species formed were adsorbed sulfur dioxide and sulfate. This again indicated that the presence of water vapor enhanced the formation of sulfite. 5. The reaction profile at 923 K was similar to that at 873 K. 6. It may be desirable to have steam present during the regeneration of ZnS to ZnO utilizing oxygen since the presence of water decreases the formation of undesirable sulfate and promotes the formation of a sulfite type intermediate which can be decomposed easily. Literature Cited Berben, P. H.; Kappers, M. J.; Gevs, J. W. An FTIR Study of Adsorption of Sulfur Dioxide on Alpha and Gamma Alumina. Mikrochim. Acta 1988, 2, 15-18. Chang, C. C. Infrared Studies of Sulfur Dioxide on GammaAluminas. J. Catal. 1978, 53, 374-385. Deo, A. V.; DallaLana, I. G. Infrared Studies of the Adsorption and Surface Reactions of Hydrogen Sulfide and Sulfur Dioxide on Some Aluminas and Zeolites. J. Catal. 1971, 21, 270-281. Desyatov, I. V.; Paukshtis, E. A.; Mashkina, A. V. Infrared Spectroscopic Studies of H2S Adsorption on Al2O3. React. Kinet. Catal. Lett. 1990, 41, 85-88. Flytzani-Stephanopouolos, M.; Gavalas, G. P.; Jothimurugesan, K.; Lew, S.; Sharma, P. K.; Bagajewicz, M. J.; Patrick, V. Detailed Studies of Novel Regenerable Sorbents for High Temperature Coal Gas Desulfurization. Final Report DOE/MC 22193-2582; U.S. DOE Office of Science and Technical Information: Oak Ridge, TN, Oct 1987. Focht, G. D.; Ranade, P. V.; Harrison, D. P. High Temperature Desulfurization Using Zinc Ferrite: Regeneration Kinetics and Multicycle Testing. Chem. Eng. Sci. 1989, 44 (No. 12), 29192929.
Lew, S.; Jothimurugesan, K.; Flytzani-Stephanopoulos, M. High Temperature regenerative H2S Removal from Fuel Gases by Regenerable Zinc Oxide-Titanium Dioxide Sorbents. Ind. Eng. Chem. Res. 1989, 28, 535-541. Lide, D. R., Frederikse, H. P. R., Eds. CRC Handbook of Chemistry and Physics, 75th ed.; CRC Press, Inc.: Boca Raton, FL, 1995; pp 4-113. Low, M. L. D.; Goodsel, A. J.; Takezawa, N. Reactions of Gaseous Pollutants with Solids (1) Infrared Study of the Sorption of SO2 on CaO. Curr. Res. 1971, 5, 1191-1195. Martin, M. A.; Childers, J. W.; Palmer, R. A. Fourier Transform Infrared Photoaccoustic Spectroscopy Characterization Of Sulfur-Oxygen Species Resulting From The Reaction Of SO2 With CaO and CaCO3. Appl. Specrosc. 1987, 4, 120-126. Newman, G.; Powel, D. B. The Infrared Spectra and Structures of Metal-Sulfite Compounds. Spectrochim. Acta 1963, 19, 213224. Nyberg, B.; Larsson, R. Infrared Absorption Spectra of Solid Metal Sulfites. Acta Chem. Scand. 1973, 27, 63-70. Ong, J. N., Jr.; Wadsworth, M. E.; Fasell, W. M., Jr. Kinetic Study of the Oxidation of Sphalerite. J. Met. 1956, Feb, 257-263. Prabhu, G. M.; Ulrichson, D. L.; Pulsifer, A. H. Kinetics of Oxidation of Zinc Sulfide. Ind. Eng. Chem. Fundam. 1984, 23, 271-273. Schwitzgebel, K.; Lowell, P. Applicability of Metal Oxides to the Development of New Processes for Removing SO2 from Flue Gas. Final Report Contract PH 86-68-68; National Air Pollution Control Administration: Washington, DC, 1969. Siriwardane, R. V. Interaction of SO2 and O2 Mixtures with CaO(100) and Sodium Deposited on CaO(100). J. Colloid Interface Sci. 1989, 132, 200-209. Siriwardane, R. V.; Grimm, U.; Poston, J.; Monaco, S. J. Fixed Bed Testing of Durable Steam Resistant Zinc Oxide Containing Sorbents. AIChE Annual Meeting, San Francisco, CA, 1994a; Symposium on Gas Purification, Paper No. 247g. Siriwardane, R. V.; Poston, J. A.; Evans, G., Jr. Spectroscopic Characterization of Molybdenum Containing Zinctitanate Desulfurization Sorbents. Ind. Eng. Chem. Res. 1994b, 33, 28102818. Siriwardane, R. V. and Woodruff, S. Ind. Eng. Chem. Res. 1995, 34, 699-702. Siriwardane, R. Fixed Bed Testing of Durable Steam Tolerant Zinc Containing Sorbents. Proceedings of the Thirteenth Annual International Pittsburgh Coal Conference: “Coal-Energy and the Environment”, Pittsburgh, 1996; Chiang, S. H., Ed.; University of Pittsburgh: Pittsburgh, PA, 1996; Vol. 1, pp 590-595. Steger, V. E.; Schmidt, W. Infrarotspektren Von Sulfaten Und Phosphaten fur Physikalishe Chemie. Ber. Bunsen-Ges. Phys. Chem. 1964, 68, 102-109. Thompson, M. M.; Palmer, R. A. In Situ Fourier Transform Infrared Diffuse Reflectance and Photoaccoustic Spectroscopy Characterization of Sulfur-Oxygen Species Resulting from the Reaction of SO2 with CaCO3. Appl. Spectrosc. 1988, 42, 945951. Woods, M. C.; Leese, K. E.; Gangwal, S. K.; Harrison, D. P.; Jothimurugesan, K. Reaction between H2S and zinc oxidetitanium oxide sorbents. 1. Single-pellet kinetic studies. Ind. Eng. Chem. Res. 1990, 29, 1160-1167.
Received for review May 12, 1997 Revised manuscript received September 22, 1997 Accepted October 1, 1997X IE970343E
X Abstract published in Advance ACS Abstracts, November 1, 1997.
