Ind. Eng. Chem. Res. 1995,34, 699-702
699
RESEARCH NOTES FTIR Characterization of the Interaction of Oxygen with Zinc Sulfide Ranjani V. Siriwardane* and Steven Woodruff Morgantown Energy Technology Center, US.Department of Energy, P.O. Box 880, Morgantown, West Virginia 26505
Interaction of ZnS with oxygen at 823, 873, and 923 K was studied using Fourier transform infrared spectroscopy (FTIR). The reaction of ZnS with oxygen was found to be pressure Torr dependent. At 823 K the reaction products during oxygen exposures of 5 x 10-3-4x Torr were adsorbed sulfur dioxide and sulfite, while at oxygen pressures greater than 4 x sulfate was the most prominent species. At 873 K, adsorbed SO2 and sulfite were the only species Torr of oxygen but sulfate started to form at 4 x Torr. However, at formed below 4 x 873 K adsorbed SO2 was the most prominent species observed for all oxygen pressures. The reaction profile at 923 K was similar to that at 873 K, but there were more types of adsorbed S02. On the basis of the experimental results, a mechanism for the reaction of ZnS with oxygen is proposed.
Introduction Several zinc-based sorbents (Grindley and Stienfeld, 1984; Lew et al., 1989; Woods et al., 1989) have been shown to be promising regenerable high-temperature sorbents for hydrogen sulfide removal from fuel gas in 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. Thus, the reaction of metal sulfide with oxygen to form sulfur dioxide is a critical step in regeneration. In addition to SO2 formation, sulfate is also formed during the regeneration reaction, which has negative effects on sorbent durability (Siriwardane and Poston, 1994). The kinetics of ZnS oxidation have been studied by several researchers (Ong et al., 1956; Prabhu, 1984). Thermogravimetric analysis (TGA)has also been performed by previous researchers t o understand the mechanism of sulfate formation (Flytzani-Stephanopouoloset al., 1987). However, data are not reported on the spectroscopic identification of reaction intermediates on the solid surface to understand the reaction mechanism of sulfate formation. It is important to understand whether sulfate is directly formed by the reaction of oxygen with zinc sulfide or whether SO2 is formed first and subsequently reacts with zinc oxide to form sulfate in the presence of oxygen. In this study, research focused toward understanding of the mechanisms of zinc sulfide oxidation was performed using FTIR (diffuse reflectance) spectroscopy.
Experimental Section Diffuse reflectance spectra were recorded using a transfer optics device (Model DRAH2MO1) and a hightemperature diffuse reflectance accessory (Model HVC DR2), both made by Harrick Scientific Corporation. FTIR spectra were obtained using a Matteson Cygnus 100 spectrometer. Single beam spectra were acquired at 4 cm-l resolution with 512 scans a t 823 K and 1024
* To whom all inquiries should be addressed.
scans at 873 and 923 K. ZnS (Alfa) was heated in the diffuse reflectance cell Torr) at the desired temperature for 2 days before exposure to gas. The single beam spectrum at the desired temperature obtained immediately before exposure t o gas was taken as the reference. In order to observe the reaction progress, each spectrum was also ratioed to the spectrum obtained immediately before the desired spectrum. The spectrum of the ZnS04 standard (Alfa) was also obtained for comparison.
Results and Discussion (a) Oxygen Exposures at 823 K. FTIR has been utilized by other researchers to identify different sulfur species in solid materials. The IR bands assigned to various species are shown in Table 1. The spectra, after exposure at 5 x Torr of oxygen and a t 823 K (ratioed to the spectrum obtained before the exposure) are shown in Figure 1 (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. A broad band in the range of 1160-1650 cm-l appeared after 2 min of oxygen exposure. In the spectrum after 78 min of oxygen exposure there was a small additional band at 950 cm-l which may be assigned to sulfite based on the literature values in Table 1. After evacuation, the peaks at 1560,1150, and 1315 cm-l within the broad band decreased in intensity indicating they corresponded to weakly adsorbed species which also agree with the literature values in Table 1. The broadness of the spectrum at 1160-1650 cm-l indicated that these adsorbed species were in random orientation. The spectra after the exposure a t 1 x 10-2 The Torr of oxygen were similar to those a t 5 x reaction reached a steady state for this pressure of 1 x Torr in less than 44 min, while at 5 x lop3 Torr the reaction continued beyond 1h. Thus in the pressure Torr of oxygen, only the range 5 x 10-3-1 x various types of adsorbed sulfur dioxide species (1160-
This article not subject to U S . Copyright. Published 1995 by the American Chemical Society
Ind. Eng. Chem Res., Vol. 34, No. 2, 1995 I
I 1
,
I
,
1700
12'00'
'
'
700
Wavenumbers (cm-1)
Figure 1. Ratio of IR spectra of ZnS after oxygen exposures of 5 x Torr a t 823 K to spectrum of unexposed ZnS a t 823 K (1) 2 min; (2) 78 min.
1650 cm-') and sulfite are formed. Sulfite may be formed by SO2 further reacting with the surface as shown by O\
S
/O
Zn -0-
When the sample was exposed to 5 x lo-, Torr of oxygen, a broad band at 1360-1575 cm-l, corresponding to various types of adsorbed SO,, was observed initially, similar t o the observations after the oxygen exposures at 1 x Torr. However, after 14 min of oxygen exposure, distinct peaks at 1600, 1330, and 1180 cm-l were observed as shown in Figure 2. The distinct nature of these bands indicates that the adsorbed species at this pressure are more structured than those formed at lower pressures (5 x 10-3-1 x Torr). The strong peaks 1120 and 1180 cm-l observed after 90 min of oxygen exposure a t 5 x lo-, Torr, as shown in Figure 2, were not observed at low (5 x 10-3-1 x lop2Torr) oxygen pressure as shown in Figure 1. The peaks at 1120 and 1180 cm-l correspond to sulfate peaks as shown in Table 1. The IR spectrum of 1% ZnS04 in ZnS obtained at 773 K also showed peaks at 1120 and 1180 cm-'. The spectra ratioed to the spectrum corresponding to the previous oxygen exposure indicated that after 14 min of oxygen exposure, only the peak centered around 1120 cm-l (sulfate) mainly continued to grow, as shown in Figure 3. When the oxygen was evacuated, the peaks corresponding to some adsorbed SO2 (1578, 1330,1180 cm-l) and sulfate (1120 cm-l) continued to decrease in intensity, as also shown in Figure 3 (the peaks above the baseline correspond to the species leaving the solid). The spectra after exposure at 8 x lo-, Torr of oxygen Torr of oxygen. Thus, were similar to those a t 5 x Torr of oxygen pressure is the reaction above 5 x different from that at lower (5 x 10-3-1 x Torr) pressure indicating the pressure dependency of the reaction. However, a t all oxygen pressures (5 x 8 x Torr) the initially formed species correspond
to different types of adsorbed SO2 species. Thus, it is reasonable to conclude that the initially formed species are SO2 from the reaction of ZnS and oxygen. In addition to S02, sulfite (950 cm-l) was also formed initially at all the oxygen exposures. When the ZnS was exposed to oxygen at 823 K the sulfate formation only started to take place at high oxygen pressures ( 2 5 x lop2Torr). The peaks corresponding to adsorbed SO2 did not change after a certain time while the sulfate continued to grow. Thus, it may be concluded that as the concentration of the initially formed SO2 increases, it reacts to form sulfate in the presence of oxygen. Decomposition of sulfate was observed when the sample was evacuated as shown in Figure 3. The oxygen exposures at 823 K were also conducted continuously for 4 h while the oxygen pressure was increased (1 x 10-2-4 x lo-' Torr). The pressure was kept constant for about 1h and was increased aRer each hour. The spectra at the end of some pressure periods are shown in Figure 4. The broad band a t 1160-1650 cm-l (adsorbed SO21 and the band at 960 cm-l (sulfite) Torr of oxygen exposure were observed up to 4 x similar to the observations after oxygen exposures at separate pressures. The IR bands became sharper and new peaks corresponding to sulfate appeared at 1120 and 1180 cm-l at oxygen exposures above 4 x lo-, Torr, and a typical spectrum is shown in Figure 4. The spectra at the beginning of each pressure period ratioed to the last spectrum at the previous oxygen pressure period are shown in Figure 5. The major changes occurring during 1 x lop2 Torr and 4 x lo-, Torr pressure change were the growth of the peaks a t 1575, 1360, and 1175 cm-l which correspond to various types of adsorbed SO2 species. Thus, up to 4 x lo-, Torr pressure of oxygen, ZnS reacts with oxygen t o form SO2 and s0s2-,but sulfate formation does not take place. When the pressure is changed from 4 x lov2to 7 x Torr, the major growth was observed in the sulfate peaks at 1120 and 1150 cm-' as shown in Figure 5. This sulfate peak continued to grow up to 4 x 10-1 Torr of oxygen. When this exposed surface was evacuated, the peaks corresponding to adsorbed SO2 desorbed from the surface and some sulfate also started to decompose. The reaction of ZnS with oxygen has been postulated in the past (Flytzani-Stephanopouolos et al., 1987) to proceed via two alternative pathways. Free energy values reported by Flytzani-Stephanopodas et al. (1987) for reactions 1,2, 3, and 4 at 1000 K are -87.7, -18.7, -106.4, and 18.7 kcal/mol (1 kcal = 4.2 x lo3 kJ), respectively. path 1 ZnS ZnO path 2
+ (3/2)0,- ZnO + SO,
+ SO, + (1/2)0, - ZnSO,
+ 20, - ZnSO, ZnSO, - ZnO + SO, + (1/2)0, ZnS
(1) (2)
(3) (4)
In path 1, SO2 is formed first and later reacts to form sulfate while in path 2 sulfate is formed first and later decomposes to form , 5 0 2 . The reaction mechanism of ZnS with oxygen at 823 K can be suggested on the basis of the results of the present work. The data in the present study agree with reaction path 1. A detailed mechanism conforming to present data is given in eqs
Ind. Eng. Chem. Res., Vol. 34, NO. 2, 1995 701 Table 1. Literature Values of IR Bands in Sulfur-ContainingCompounds species
IR bands (cm-l)
1
sulfite (SO&)
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 adsorbed SO2 a t high T sulfate (s04'-) general ZnSOd ( D : ~ )
3
T)
s0.i2-( T d )
chelating bidentate S042-/y-alumina Cas04
I I I I
I I
I I I I
1 1 I I
I
I
I
I
I
I
I
I
I
bhl.lL 1
I
I
reference
950 950 925,984 1330,1151 1320,1140 1250,1200 1350,1330 1276,1022,813 1370,1375,1147 1326,1070 1330,1140 1685,1240 1570,1440 1505,1434
Martin et al. (1987) Newman and Powel (1963) Nyberg (1973) Berben e t al. (1988)
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
Steger (1964)
Thompson and Palmer (1988) Chang (1978) Deo and Dalla Lana (1971) Low et al. (1971)
I I I I I I
I 1700
1200
700
Wavenumbers (cm-1)
Figure 2. Ratio of IR spectra of ZnS after oxygen exposures of 5
Martin et al. (1987) Chang et al. (1978) Low et al. (1971)
I I I I I I
1
I ,
1 ,"I
I
1700 1200 Wovenum bers (cm - 1 )
,
I
700
Figure 3. Ratio of IR spectra of ZnS after oxygen exposures of 5
x
Torr a t 823 K to spectrum of unexposed ZnS at 823 K (1) 14 min; (2)98 min.
x Torr at 823 K (1)ratio of 36 min exposure to 14 min exposure; (2)ratio of after 12 min evacuation to the last oxygen exposure.
5-8. This mechanism agrees with the TGA work reported by Flytzani-Stephanopoulas et al., 1987.
oxygen pressure. Thus, the SO2 formed during the reaction readily reacts with the surface to form sulfate. After evacuation at 873 K, adsorbed SO2 desorbed from the surface and sulfate started to decompose. The reaction profile at 923 K was similar to that a t 873 K, but there were more different types of adsorbed SO2 a t 923 K compared to those at 873 K. However, at 923 K and at 7 x Torr of oxygen, the intensity of the adsorbed SO2 was comparable to that of the sulfate. This is different from what was observed a t 873 K, in which the intensity of the sulfate peak was much higher than the adsorbed SO2 peak at 7 x Torr. Thus, a t all three temperatures the first products of ZnS oxidation were adsorbed SO2 and sulfite while sulfate formation takes place only a t higher oxygen pressures after longer exposures according to the reaction sequence 1-4.
ZnS
+ (3/2)0,ZnO + SO,
ZnO
(adsorbed in various forms) ( 5 )
+ SO, - ZnSO,
a t P > 0.04 Torr
+ +
-
(6)
(7)
ZnS 0, SO, ZnSO, (8) (b)Oxygen Exposures at 873 and 923 K. At 873 K, adsorbed SO2 was the major product a t lower pressures ( < 4 x lop2Torr) and a continued growth of the peaks corresponding t o SO2 was observed at these pressures. In addition, sulfite was also observed. At 873 K, the growth of sulfate band was observed a t 4 x lo-, Torr which was different from the reaction at 823 K. Once sulfate begins to form ( 2 4 x Torr), the formation of adsorbed SO2 rapidly reached a steady state while the sulfate continued to grow at a given constant
Summary 1. Different types of adsorbed ,902 and sulfite were Torr of observed at 823 K up to a pressure of 4 x
702 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
4. The reaction profile as a function of pressure at 923 K was similar to that at 873 K. However, more different types of adsorbed SO2 species were observed at 923 K compared to those at 873 K. 5. At all three temperatures, the reaction mechanism can be summarized as
+ 0, - ZnO-SO,(ads) ZnO + SO, - Zn-SO, ZnO + SO, + (1/2)02- ZnSO, ZnS
Literature Cited
1700 1200 Wavenumbers (cm-1)
700
Figure 4. Ratio of IR spectra of ZnS after continuous oxygen 7x lo-', and 4 x lo-' Torr) a t exposures 4 x Torr, 823 K to spectrum of unexposed ZnS at 823 K (1)4 x 45 min; (2) 4 x lo-' Torr, 15 min.
I
1700
I I
I/ I I1 1
I
II
I
1
1200
700
Wavenumbers (cm-1)
Figure 6. IR spectra of ZnS after continuous oxygen exposures 4 x ratio of 4 x Torr to 4 x
7x lo-', and 4 x lo-' Torr) at 823 K (1) Torr, 42 min; (ii) ratio of 7 x Torr to Torr, 45 min.
oxygen. The adsorbed SO2 was randomly oriented as indicated by the broadness of the IR band. 2. When the oxygen pressure was greater than 4 x Torr at 823 K, sulfate began t o form and the relative intensity of the IR sulfate peak was greater than that of adsorbed S02. The adsorbed SO2 became more structured at higher pressures. 3. When the reaction was performed at 873 K, adsorbed SO2 and sulfite were the only products obTorr of oxygen. Sulfate began to served up to 4 x form at 4 x Torr of oxygen and continued to form at higher pressures. In contrast to 823 K, the strongest peak at 873 K was due to adsorbed SO2 at all pressures. However, the rate of the growth of the sulfate peak was more than that of the adsorbed SO2 peaks at oxygen pressure > 4 x lo-, Torr.
Berben, P. H.; Kappers, M. J.; Gevs, J. W. An FTIR Study of Adsorption of Sulfur Dioxide on Alpha and Gamma Alumina. MikroChim. Acta m i e n ] 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. 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, October 1987. Grindley, T.; Stienfeld, G. Desulfurization of Hot Coal Gas by Zinc Ferrite. In Acid and Sour Gas Treating Processes; Newman, S . A,, Ed.; Gulf Publishing: New York, 1984; Chapter 16. Lew, S.; Jothimurugesan, K.; Flytzani-Stephanopoulos, M. High Temperature Regenerative H2S Removal from Fuel Gases by Regenerable Zinc Oxide-Titanium Dioxide Sorbents. Znd. Eng. Chem. Res. 1989,28, 535-541. 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 SulfurOxygen Species Resulting From The Reaction Of SO2 With CaO and CaC03. Appl. Spectrosc. 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 Spalerite. J . Met. 1966, Feb, 257-263. Prabhu, G. M.; Ulrichson, D. L.; Pulsifer, A. H. Kinetics of Oxidation of Zinc Sulfide. Znd. Eng. Chem. Fundam. 1984,23, 271-273. Siriwardane, R. V.; Poston, J. A.; Evans, G., Jr. Spectroscopic Characterization of Molybdenum-Containing Zinc Titanate Desulfurization Sorbents. Znd. Eng. Chem. Res. 1994, 33, 2810-2818. 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. Insitu Fourier Transform Infrared Diffuse Reflectance and Photoaccoustic Spectroscopy Characterization of Sulfur-Oxygen Species Resulting from the Reaction of SO2 with CaC03. Appl. Spectrosc. 1988, 42, 945-951. Woods, M. C.; Leese, K. E.; Gangwal, S. K.; Harrison, D. P.; Jothimurugesan, K.Reaction Kinetics and Simulation Models for Novel High-Temperature Desulfurization Sorbents; Final Report, DE-AC21-87MC24160, DOEMETC, February 1989. Received for review November 8, 1994 Accepted December 2, 1994@ IE930602+ Abstract published in Advance ACS Abstracts, January 1, 1995. @
