450
J. Phys. Chem. 1985,89, 450-454
Claus Catalysis. 2. An FTIR Study of the Adsorption of H,S on the Alumlna Catalyst Arunabha Datta and Ronald G. Cavell* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 (Received: June 25, 1984)
The adsorption of H2S on alumina activated at 400 and 700 "C has been studied by Fourier transform infrared (FTIR) spectroscopy. Contrary to earlier reports, it appears that H2S is adsorbed on alumina in not one but two different forms. Both these forms involve adsorption on an aluminum ion site. In one case, the adsorption is dissociative through interaction with an adjacent hydroxyl and oxide ion and leads to the formation of A1-S species. In the other case, the H2S is adsorbed in an undissociated form. The latter form is found to desorb on heating to 100 "C whereas the AI-S species are inferred to be still present at temperatures above 300 "C. Dissociative adsorption takes place to a much lesser extent on the 700 "C activated sample, but the extent of updissociative adsorption is similar in the samples activated at 400 and 700 "C.
Introduction The adsorption of H2Son y-alumina has been previously probed by means of infrared spectroscopy. Dalla Lana et al.' observed bands at 2560 and 1335 cm-I which they assigned to the Stretching and bending modes of an H2S molecule adsorbed on a surface hydroxyl. These bands did not disappear even after evacuation of the sample at room temperature for 1 h, indicative of chemisorption of H 3 on alumina. Later Slager and Amberg2 observed an additional strong band at 1568 cm-' which they assigned to an A1=0 vibration, the double-bond character being imparted to the aluminum-oxygen band by the adsorbed HIS. Subsequent worker^^,^ however observed that when H2S was adsorbed on alumina, only a band at 2585 cm-' was observed, and it was suggested that the 1340- and 1570-cm-' bands observed by others were probably due to the coadsorption of COz (present as an impurity in the H2S sample) on alumina. In the course of a study of the adsorption of SO2 on a l ~ m i n a , ~ we observed that additional and much more detailed information could be obtained from the system using a modern FTIR instrument in contrast to the dispersion instruments used by the previous investigators. Consequently, we have reinvestigated the adsorption behavior of HzSon alumina activated at both 400 and 700 "C in order to garner further insight into the nature of the Claus reaction. The two different activation temperatures were chosen because we had evidence5 that, in accordance with predictions based on the models6 proposed for the surface structure of catalytic aluminas, the nature of the adsorption sites on alumina is different when activated at 400 and 700 "C. Experimental Section The experimental procedure used has been described previ~usly.~ As described therein, the Kaiser Alumina wafers (approximate "thickness" of 25.5 mg/cm2) were activated by heating them in the cell' under vacuum for 16 h at the temperatures indicated. The BET surface area of a typical wafer was 296 m2/g, and the average mass of a wafer was approximately 98 mg. The infrared spectra were recorded on a Nicolet 7199 Fourier transform infrared spectrometer using a Globar source, a germanium-coated KBr beam splitter, and a mercury-cadmium telluride detector operating at 77 K. The mirror velocity was 0.8 cm-'. Usually 1000 consecutive scans were summed to obtain a spectrum with a resolution of 4 cm-I over the spectral range. A (1) Deo, A. V.; Dalla Lana, I. G. J. Catal. 1971, 21, 270. (2) Slager, T. L.; Amberg, C. H. Can. J . Chem. 1972, 50, 3416. ( 3 ) Lavalley, J . C . ;Travert, J.; Laroche, D.; Saur, 0. C. R . Hebd. Seances Acad. Sci., Ser. A 1977, 285, 385; Chem. Abstr. 1978, 88, 553206. (4) Saur, 0.; Cherreau, T.; Lamotte, J.; Travert, J.; Lavalley, J. C. J . Chem. Soc., Faraday Trans. 1 1981, 77, 427. ( 5 ) Datta, A.; Cavell, R. G.; Tower, R. W.; George, Z. M. J . Phys. Chem., preceding article in this issue. ( 6 ) Knozinger, H.; Ratnasamy, P. Catal. Rev.-Sci. Eng. 1978, 17, 31. (7) The Kieslev type cell used herein is described in detail by: Karge, H. G. Z. Phys. Chem. (Wiesbaden) 1971, 76, 133.
0022-3654/85/2089-0450$01 S O / O
spectrum of the empty infrared cell was used as the instrument background, and all single-beam spectra of the samples were ratioed to this background. The spectrum of the adsorbed H2S was obtained by a 1:l subtraction of a stored spectrum of the activated alumina sample from that of the alumina sample with adsorbed H2S. All spectral subtractions and subsequent plotting of the spectra were carried out by the Nicolet 1180 computer which controls the instrument. The peak positions noted on the graphs were located on the primary Fourier transformed interferogram and not derived from the curves shown in the figures which have been in general subject to an 11-point smoothing treatment before plotting. Hydrogen sulfide (Matheson) was degassed via a freeze-thaw cycle under vacuum. Treatment pressures ranged from 0.03 to 5.0 torr (0.5 mmol). At low pressures, up to 0.6 torr (0.06 mmol), the total amount of gas introduced was adsorbed after an absorption time of 30 min. At higher pressures increasingly smaller fractions were adsorbed. Results Adsorption and Desorption of H2S on Alumina Activated at 400 "C. At low doses, the adsorption of HzS on alumina (Figure 1) gives rise to a band at 2578 cm-' which increases in intensity and shifts to lower wavenumber values on further addition of H2S. As suggested earlier,' this band can be assigned to the S-H stretching mode of adsorbed H2S, and the width and asymmetry of the band probably suggest that it contains contributions from the other stretching mode (vl) of the free molecule and probably also the first overtone of the bending vibration. The band observed at 1334 cm-I can be ascribed to the bending mode of adsorbed H2S. The band at 1620 cm-' corresponds to the bending vibration of water, and the intensity of this band increases with increased addition of H2S. This indicates that H2Sadsorbs dissociatively on alumina, leading to the formation of water. The band at 1556 cm-l and the much weaker one at 1420 cm-' are similar to those observed by Slager and Amberg2 and will be discussed later. The behavior of the hydroxyl region on adsorption of H2S (Figure 2) is in sharp contrast to that observed during the ad~ observe that the hydroxyl bands sorption of SOzon a l ~ m i n a .We are affected by the first dose of 0.05 mmol of H2S. The 3750and 3780-cm-l hydroyxls interact first as is evident from the decrease in the intensities of these bands upon the addition of H2S. Next, the hydroxyl type indicated by the band around 3732 cm-', which is initially masked by the stronger 3750-cm-' band and becomes visible only when the 3750-cm-' band has decreased considerably in intensity due to H2S adsorption, is affected. Finally, the 3680-cm-' hydroxyl begins to disappear, but this band is still present even after the addition of 0.5 mmol of H2S. In addition, the adsorption of H2S is accompanied by strong absorption throughout the 3600-3200-~m-~range which is indicative of the reformation of hydroxyl groups on the alumina surface. This arises from the water which is formed by the dissociative adsorption of HIS. 0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 451
Adsorption of H2S on Alumina Catalyst 0.40~
1 w 0.30. 0 2
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8
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,
,
,
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,
,
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1600
Figure 1. FTIR spectra (SH stretch region) of the adsorption of HIS on alumina which has been activated at 400 "C. The curves show the resultant spectra after exposure to ( 1 ) 0.03, (2) 0.06, (3) 0.09, (4) 0.20, and (5) 0.50 mmol of H2S.
3 ,v,,
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3600
3400 ,
0.00 2700
WAVENUMBERS (cm-1)
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,
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WAVENUMBERS (cm-1) Figure 2. FTIR spectra (OHstretch region) of the adsorption of H2S on alumina which has been activated at 400 O C : (1) before exposure to H2S. Subsequent spectra were obtained after exposure to (2) 0.03, (3) 0.06, (4)0.09, (5) 0.20, and (6) 0.50 mmol of H2S.
2300
1900
1500
WAVENUMBERS (cm-1)
Figure 3. FTIR spectra (SH stretch region) of the desorption of adsorbed H2S from alumina activated at 400 "C. The numbered curves display the resultant spectra after evacuation at (1) room temperature for 50 h, (2) 100 "C for 0.5 h, (3) 200 "C for 0.5 h, (4) 400 "C for 0.5 h, and (5) 600 "C for 0.5 h.
The desorption study (Figure 3) indicated that the adsorbed H2S was retained on the alumina surface even after evacuating the sample for 100 h at room temperature. Thus, H2S is not physically adsorbed. Heating to 100 OC for '/' h was however sufficient to remove the H2S bands, so we conclude that H2S is much more weakly bound to alumina in comparison with SO2. The 1624-cm-l band normally ascribed to the bending mode of water is retained even after the sample was heated to 100 "C, and the band disappears only on subsequent heating to 200 O C . This is in accordance with the normal behavior of a hydroxylated alumina s ~ r f a c e .The ~ bands at 1578 and 1447 cm-' decrease in intensity upon heating but disappear completely only after heating at 600 OC. The band at 1578 cm-' has been assigned by Slager and Amberg' to an A 1 4 vibration with the adsorbed molecule imparting the double-bond character to the aluminum-oxygen bond. In contrast, Lavalley3s4et al. believe this band and the one at 1447 cm-I to be due to the coadsorption of C 0 2on the alumina surface. It is known* however that alumina has intrinsic bands at 1560 and 1480 cm-' which can be removed completely and irreversibly only by heating alumina in vacuo at 600 "C. These bands were also observed during the dehydroxylation studies of alumina5 wherein it was found that the position of these bands varied with the extent of dehydroxylation and disappeared upon heating to 600 OC. The variation in the band positions was in agreement with the results of Scholten et al.' At the same time it is also knownI0 that adsorbed C 0 2 has three major bands at 1636, 1484, and 1235 cm-I, and all of these bands can be completely removed by evacuation at 200 OC. Considering the above facts and the observations made from the desorption studies that the bands due to adsorbed H2S disappear on heating to 100 O C whereas those at 1578 and 1447 cm-' persist up to 600 OC, it is apparent that the 1578- and 1447-cm-' bands are not due to either adsorbed H2S or the coadsorption of C 0 2 and must be associated with alumina itself. (8) Parkyns, N. D. J . Chem. SOC.A 1969, 410. ( 9 ) Scholten, J. J. F.; Mars, P.; Menon, P. G.; Hardevald, R. V. Proc. Int. Congr. Catal., 3rd, 1964 1965, 881. (10) Amenomiya, Y.; Morkawa, Y.; Pleizier, G. J . Catal. 1977,46, 431.
452 The Journal of Physical Chemistry, Vol. 89, No. 3, 1985
Datta and Cave11
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1600
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Figure 5. FTIR spectra (SH stretch and water bend region) showing the adsorption of H2S on alumina which has been activated at 700 OC after exposure to (1) 0.03, (2) 0.05, (3) 0.09, (4)0.2, and ( 5 ) 0.5 mmol of H2S.
4 00
3800
3600
3400
3200
WAVENUMBERS (cm-1)
Figure 4. FTIR spectra (OH stretch region) of the desorption of adsorbed H2S from alumina activated at 400 OC. The curves display the resultant spectra after evacuation at (1) room temperature for 50 h, (2) 100 "C for 0.5 h, (3) 200 "C for 0.5 h, (4)400 OC for 0.5 h, and ( 5 ) 600 OC for 0.5 h.
Adsorption and Desorption on Alumina Activated at 700 O C . The essential adsorption behavior of HIS on alumina activated at 700 O C (Figure 5) is similar to that observed on the 400 O C activated sample. The stretching mode of adsorbed HIS is observed a t 2576 cm-I and shifts to 2562 cm-' on addition of 0.5 mmol of H2S. The corresponding bending mode occurs at 1334 cm-', and the band at 1620 cm-' corresponds to the bending mode of water. However, the 1620-cm-' band is clearly quite weak and has significantly lower intensity than the corresponding band observed in the case of the 400 O C activated sample. This indicates that the dissociative adsorption of H2S, leading to the formation of water, takes place to a much lesser extent. This is also evident from the spectrum of the hydroxyl region (Figure 6) where it can be seen that although a broad H-bonding band appears at around 3400 cm-', it is not as extensive as in the case of the 400 "C activated sample where there is strong absorption throughout the region 3600-3200 cm-'. The desorption behavior (Figure 7) is similar to that observed with the sample activated at 400 "C. However, in the hydroxyl region (Figure 8) the structural hydroxyls show up more clearly and distinctly at lower desorption temperatures since the alumina, in this case, is not extensively hydroxylated. Discussion The adsorption and desorption behavior in this system seems H2S is not very strongly to suggest that, in contrast to adsorbed on the alumina surface. Also, the adsorption takes place dissociatively with the liberation of water and hence adsorption must take place on a site close to a hydroxyl group. If H2S adsorption were exclusively dissociative on the alumina surface, we would then expect the extent of adsorption (as evident
from the intensity of the stretching mode of the adsorbed H2S) to be much higher in the 400 OC activated sample compared to the samples activated at 700 O C owing to the much larger concentration of hydroxyls on the former samples. For instance, it is known that the extent of dehydroxylation of an alumina surface is about 50% on activation at 400 O C and around 90% on heating to 700 O C 6 In the present case however the extent of adsorption is comparable in both the 400 and 700 OC activated samples. Moreover, since dissociative adsorption will probably lead to the formation of M-S or possibly M-SH bonds, we would expect neither the stretching nor bending modes of adsorbed H2S to be observed in the former case, whereas in the latter case only the S-H stretching vibration should be observed. In the present case both the stretching and bending modes of adsorbed H2S are observed, indicating that some nondissociative adsorption also occurs. The above facts suggest, contrary to what has been previously concluded, that H2Sis adsorbed on alumina is nor one bur two different forms. One of these involves dissociative adsorption whereas the other results from nondissociative adsorption. Dissociative adsorption, as mentioned earlier, must necessarily involve a structural hydroxyl on the alumina surface, and consequently a likely adsorption site could be schematically depicted as 0-
OH
I
I
The adsorption process can then be represented as follows: H~=H\S/H*..O-
I
AI
l
AI
l
AI
-
s
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in which H2S adsorbs on the Al' site and then transfers protons to adjacent hydroxyls and oxygen atoms forming respectively adsorbed H 2 0 and bound OH units. The formation of an A1-S bond is the most likely product of dissociative adsorption by analogy with the known tendency of H2S for dissociative adsorption on many metals over a wide range of temperatures and coverages with resultant formation of metal sulfides." Moreover, (1 1) Bartholomew, C. H.; Agrawal, P.K.; Katzer, J. R.Adu. Curd. 1982, 31, 135.
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 453
Adsorption of HzS on Alumina Catalyst
"I w
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1 1900
0 1500
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Figure 7. FTIR spectra (SHstretch and water bend region) showing the desorption of adsorbed H2S from alumina activated at 700 OC. The curves display the resultant spectra after evacuation at (1) room temperature for 50 h, (2) 100 "C for 0.5 h, (3) 200 OC for 0.5 h, (4) 400 OC for 0.5 h, and (5) 600 OC for 0.5 h.
W
0
z
4000
3800
3600
3400
3200
WAVENUMBERS (cm-1)
Figure 6. FTIR spectra (OH stretch region) showing the adsorption of H2S on alumina which has been activated at 700 O C (1) before exposure to H2S and after exposure to (2) 0.03, (3) 0.06, (4) 0.09, (5) 0.20, and (6) 0.50 mmol of H2S.
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DeRosset et al.lz report that the heat of adsorption of H2S on a dehydrated y-alumina surface is -38 kcal/mol, a value consistent with the formation of AI-S bonds. The observation of both the stretching and bending modes of adsorbed HzS indicates that H2S can also be adsorbed in an undissociated form. The adsorption site in this case is most probably an isolated aluminum ion or an aluminum cluster, and the adsorbed species can be represented as H\/H
I
AI IR features due to this species disappear on heating to 100 OC, suggesting that undissociatively adsorbed HzS is relatively easily desorbed. SOz molecules which appear5 to be similarly bonded to aluminum ions or clusters through sulfur desorb a t 200 O C . The difference in the apparent strength of interaction between S and Al, leading to the higher required desorption temperature for SOz,is due to the fact that SOzis more acidic (less basic) than (12)
DeRosset, A. J.; Finstrom, C. G.; Adams, C. J. J. Coral. 1962, 1,235.
37506P3732
4000
3600
3800
3400
3200
WAVENUMBERS (cm-l) Figure 8. FTIR spectra (OH stretch region) showing the desorption of adsorbed H2S from alumina activated at 700 OC. The curves display the resultant spectra after evacuation at (1) room temperature for 50 h, (2) 100 OC for 0.5 h, (3) 200 OC for 0.5 h, (4) 400 OC for 0.5 h, and (5) 600 O C for 0.5 h.
-
H2S, leading to a weaker S M u bond in the latter. A strongly bonded AI-S species formed by dissociative adsorption would be expected to remain on the surface well beyond temperatures of 100 OC because M-S bonds are generally strong." It is however not possible to monitor by infrared spectroscopy the presence, and
5
454
J. Phys. Chem. 1985,89,454-451
perhaps the subsequent disappearance on desorption at high temperatures, of such A1-S species owing to the strong absorption of alumina below 1000 cm-'. A technique such as X-ray photoelectron spectroscopy could perhaps be fruitfully used for this purpose. Further evidence for the presence of two different types of adsorbed H2S species is provided by the work of Lunsford et al.I3 on the EPR spectrum of N O adsorbed on y-alumina wherein it was found that H2S, if preadsorbed on the alumina, effectively blocked the sites which yielded the characteristic 27Alhyperfine structure in the nitric oxide spectrum. It was concluded that this was due to adsorption of H2S on the A1 ion sites which thereby prevented the NO from using these same sites. Lunsford et al. also found that, after degassing a sample poisoned with H2S for 1 h at 300 "C, only about 50% of the N O spectrum could be (13) Lunsford, J. H.; Zingery, L. W.; Rosynek, M. P. J. Catal. 1975,38,
179.
developed, suggesting that half of the aluminum ion sites were still blocked by the H2S, and even heating the sample at 300 "C did not remove the H2S and liberate the N O adsorption sites. These observations are consistent with our interpretation. The restoration of 50% of the nitric oxide spectrum is attributed to the unblocking of some of the Al' sites through desorption of the undissociatively adsorbed form of H2Sat 100 OC. The Al+ sites which are still blocked after degassing at 300 OC are probably those on which HIS had dissociatively adsorbed, leading to the formation of A1-S species, which, as discussed earlier, being very stable, is not expected to desorb or break up to temperatures below 300 OC.
Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for support of our research. We also thank R. W. Tower of the Alberta Research Council, Edmonton for preparation of the sample pellets and the A.R.C. for the loan of the infrared cell. Registry No. HIS, 7783-06-4;alumina, 1344-28-1.
Claus Catalysis. 3. An FTIR Study of the Sequential Adsorption of SO, and H,S on the Alumina Catalyst Arunabha Datta and Ronald G. Cavell* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 (Received: June 25, 1984)
The sequential adsorption of H2S and SO2on y-alumina catalyst activated at 400 OC has been studied by Fourier transform infrared (FTIR) spectroscopy. When incremental amounts of H2S were added to a sample of alumina upon which SO2had been preadsorbed, the Claus reaction took place but the species responsible for the band at 1055 cm-I (characteristic of SO, adsorbed on alumina) showed very low reactivity toward H2S. No infrared bands due to adsorbed H2S were observed, but this cannot be taken as conclusive evidence for the absence of adsorbed H 2 0 . On addition of SO2 to an alumina sample on which HzS had been preadsorbed, the Claus reaction also proceeded but to a much lesser extent probably because of the dissociative adsorption of part of the preadsorbed H2S. Also, in this case bands due to adsorbed SO, were observed throughout the reaction. The dependence of the rate of the Claus reaction on the activation temperature of the catalyst, the nature and mechanism of catalyst poisoning, and an alternative approach for carrying out the Claus reaction are also discussed.
Introduction In spite of the commercial importance of the modified Claw reaction for sulfur recovery
2H2S + SO2
+3s + 2H20 alumina
the mechanism of this reaction is still not established. Attempts have been made by various authors to understand the mechanism through infrared spectroscopic studies of the adsorption of SO, and HIS on alumina, both individually and in combination. Earlier work led to the conclusion that SO2was adsorbed on alumina in two or possibly three different one of which was very strongly adsorbed. In contrast, H2S was believed to be weakly adsorbed in only one form.'sbs Accordingly, Karge et al.4 suggested that the Claus reaction proceeded between adsorbed SO2 and gas-phase H,S. It has also been proposed4 that SO, radicals which form when SO2 is adsorbed on alumina were responsible (1) Deo, A. V.;Dalla Lana, I. G.;Habgood, H. W. J . Catal. 1971,21,270. (2) Fiedorow, R.; Dalla Lana, I. G.;Wanke, S. E. J . Phys. Chem. 1978, 82, 2474. (3) Chang, C. C. J . Catal. 1978, 53, 374. (4) Karge, H. G.;Tower, R. W.; Dudzik, Z.; George, Z. M. Stud. Surf. Sei. Caral. 1981, 7 , 643. (5) Karge, H. G.;Dalla Lana, I. G. J. Phys. Chem. 1984, 88, 1538. (6) Sager, T. L.; Amberg, C. H. Can. J . Chem. 1972, 50, 3416. (7) Lavalley, J. C.; Travert, J.; Laroche, D.; Saw, 0. C. R. Hebd. Sceances Acad. Sei., Ser. A 1977, 285, 85; Chem. Abstr. 1978, 88, 55320b. (8) Saur, 0.; Cherreau, T.; Lamote, J.; Travert, J.; Lavalley, J. C. J . Chem. SOC.,Faraday Trans. 1 1981, 77, 427.
for Claus conversion and that the species responsible for the band a t 1080 cm-' in the spectrum of adsorbed SO, might be an important intermediateS in the Claus reaction. Our recent studyg of the adsorption of SO, on alumina using FTIR spectroscopy has shown however that SO, may be adsorbed in as many as five different forms with varying strengths of adsorption. We also foundlo that, in contrast to earlier reports, HIS is strongly adsorbed on alumina in not one but two different forms. Herein we report the next logical step in the study of Claus catalysis, the study of the sequential adsorption and reactions of SO, and H2Son alumina undertaken with the hope of identifying which of the adsorbed SO2 and H,S species are reactive in the Claus reaction. We have studied both approaches: the reaction of SO2with preadsorbed H2Son alumina and the reaction of H2S with preadsorbed SO2. Experimental Section The experimental procedures used have been described prev i o ~ s l y .As ~ described therein, the Kaiser Alumina wafers (approximate "thickness" of 25.5 mg/cm2) were activated by heating them in the cell" under vacuum torr) for 16 h at the temperatures indicated. The BET surface area of a typical wafer (9) Datta, A.; Cavell, R. G.; Tower, R. W.; George, Z. M. J . Phys. Chem., accompanying article in this issue. (10) Datta, A.; Cavell, R. G.J. Phys. Chem., preceding article in this issue. (1 1) The Kieselev type. cell used herein is described in detail by: Karge. H. G. 2. Phys. Chem. (Wiesbnden) 1971, 76, 133.
0022-365418512089-0454$01 SO10 0 1985 American Chemical Society
