J. Phys. Chem. 1995, 99, 4620-4625
4620
Effect of Sodium on the Adsorption of SO2 on A1203 and on Its Reaction with H2S A. B. Mohammed SaadJ 0. Saw$ Y. Wang,* C. P. Tripp? B. A. Morrow,**$and J. C. Lavalley*?+ Laboratoire Catalyse et Spectrochimie, URA CNRS 414 ISMRA,Universite, 6 Boulevard du Markcha1 Juin, 14050 Caen Cedex, France, and Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K I N 6N5 Received: July 1, 1994; In Final Form: September 30, 1994@
The adsorption of SO2 on A1203 and on 3% sodium on A1203 preactivated at 350 and 600 "C has been studied by gravimetry, thermodesorption, infrared spectroscopy, and Raman spectroscopy. The quantity of SOz adsorbed is greater on NdA1203. A comparison of the results showed that physical adsorption occurs on weak basic 0,- or OH- sites. Chemisorption mainly arises from adsorption on basic sites through the formation of sulfite species (AlOS02) which are characterized by an infrared band near 1060 cm-'. Other minor species such as disulfite or hydrogen sulfite were postulated in order to account for (1) a perturbation of surface O H groups and (2) the constant quantity of chemisorbed species which form as a function of the temperature of activation. The sulfite species were more stable on NdAlzO3 than on Al2O3, and they were less reactive toward HzS. This is attributed to the greater basicity of the 3% sodium-doped catalyst and accounts for the lower reactivity of NdAl2O3 relative to pure A1203. On either A1203or NdAlzO3, no sulfur was produced if gaseous SO2 was added to preadsorbed H2S. However, in the reverse sequence, 2HzS(g) S02(ads) (3/n)S, 2H20, sulfur was detected via Raman spectroscopy.
-
+
+
Introduction Alumina is the most commonly used catalyst for the modified Claus reaction. The active sites for this reaction are basic,' and 2HzS
+ SO, - (3/n)S, + 2 H 2 0
(1)
although it is well-known that the addition of sodium increases the basicity of alumina,2prior studies have shown that a catalyst which contained 3.9 mass % sodium on A1203 gave rise to a very basic catalyst which was only moderatively active for the modified Claus rea~tion.~ Two explanations have been proposed for this behavior: (i) the greatly augmented basicity makes the catalyst less active for this reaction (i.e., there is an optimum basicity, and the presence of sodium goes beyond the optimum, making the catalyst too basic) or (ii) the reaction needs basic and acidic sites, the latter being poisoned by the presence of excess sodium. The purpose of the present work was to study the effect of sodium on the adsorption of SO2 on A1203, and on sodiumimpregnated A1203, and to compare the reactivity of the adsorbed SO?;species with gaseous H2S on these catalysts. The catalysts are A1203 and y A1203 impregnated with 3 mass % sodium. The techniques used are gravimetry, infrared (IR) and Raman spectroscopies, and temperature-programmeddesorption (TPD). To our knowledge, this is the first time that Raman spectroscopy has been used to study the adsorption and reactivity of oxo-sulfur species on oxide catalysts or in the modified Claus reaction.
cm3/g after vacuum activation at 350 or 600 "C. Sodium impregnation was carried out from a solution of sodium acetate so as to give 3% by mass after drying. This sample was dried at 120 "C, was calcined in air overnight at 450 "C, and had a BET surface area of 235 m21g and a pore volume of 0.45 cm3I g. For Raman studies, a Degussa alumina having a BET surface area of 100 m2/g was used. Some of the infrared work was also carried out using this alumina, and no significant spectral differences were noted. Prior to use, samples were activated by heating under vacuum for about 2 h at a specific temperature, during which time the base pressure dropped to less than lop4 Torr. For convenience, the various samples will be denoted below as A1203(C) or Na/A1203(C) where C is the activation temperature in degrees Celsius, i.e., Na/A1203(600)is a sample of 3% by mass sodium on alumina which has been activated in vacuum at 600 "C. Infrared studies were carried out using Nicolet MX1 FTIR, Bomem DA3-02, or Bomem Michelson MB FTIR spectrometers at a resolution of 4 cm-'. Raman spectra were recorded using a Jobin-Yvon HG2 intrument (8 cm-' resolution), and sample irradiation was achieved using an argon ion laser (488 nm) with a laser power at the sample ranging from 50 to 500 mW. For TPD studies, about 5 @a of SO2 was contacted with the activated samples at room temperature (22 rt 1 "C), and after 20 min, the samples were evacuated at room temperature for 1 h. They were then heated (5 "Clmin) under a helium flow (20 cm31min),and the desorbed gases were analyzed quantitatively and qualitatively by gas chromatography. Gravimetric measurements were done using a McBain thermobalance.
Experimental Section The y alumina sample used for all of the gravimetric, temperature-programmeddesorption (TPD), and infrared studies reported herein was provided by RhBne-Poulenc, France, and had a BET surface area of 244 m2/g and a pore volume of 0.52 URA CNRS 414 ISMRA.
* University of Ottawa.
@Abstractpublished in Advance ACS Abstracts, March 1, 1995.
0022-365419512099-4620$09.0010
Results (a) Gravimetry. A series of gravimetric measurements were made on each sample by contacting the samples with 40 Torr of SO2 at room temperature. The total quantity adsorbed and the quantities physically or chemically (irreversibly) adsorbed are shown in Table 1. The total quantity adsorbed represents both physically and chemically adsorbed species. The amount 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 13, 1995 4621
Effect of Sodium on the Adsorption of SO2
TABLE 1: Gravimetric Measure of the Quantity of SO2 Adsorbed (molecule/nm*) as a Function of the Temperature of Activation
c N
A1203(350) Na/.4lz03(350) Al203(600) Na/A1203(600)
so2 total SO2 chemisorbed
SO2 physisorbed
3.1 1.8 1.3
4.3 2.8 1.5
3.0 2.0 1.0
4.4 2.8 1.6
chemically adsorbed was determined to be that which remained following evacuation for 2 h at room temperature,the difference being the quantity physically adsorbed. These results show that the quantity adsorbed is greater for the Na/Al;?03samples but that there is not a large difference related to the temperature of activation. We also measured the mass change of each sample after the evacuation at room temperature followed by heating at 2 "C/min up to 600 "C and found that the curves were quasi-parallel (Figure lA), indicating that the quantity of chemisorbed SO2 was always greater on the NdAl203 sample. Further, all SO;?was desorbed from pure A1203 after heating to 600 "C, whereas about 0.4 molecule/ nm2 remained on the Na/A1203. (b) TPD Experiments. Figure 1B shows the TPD profiles following adsorption on the 600 "C activated samples. For both samples, there was a relatively sharp desorption near 100-120 "C. Further, there was a tapering off of the desorption curve to high temperature, with a poorly defined peak near 290 "C for A1203(600) and a more distinct peak near 380 "C for Na/ A1203(600). In agreement with the gravimetric results, all SO;? desorbed from pure A1203 by 600 "C, but not from NdAlzO3. (c) Infrared and Raman Experiments of Adsorbed S02. Gaseous SO2 has absorption bands at 1361 (strong) and 1151 cm-I (medium), respectively, due to the antisymmetric and symmetric SO;?stretching modes4 Following the introduction of 40 Torr of SO2 on A1203 and on Na/Al203, IR bands near these frequencies were observed. After spectral subtraction of the contribution from the gas phase, symmetrical bands at 1337 and 1148 cm-' for A1203(600) or at 1340 and 1151 cm-' for Na/A1203(600)(Figure 2) were observed. Similar bands were also observed following addition of SO2 to both catalysts when activated at 350 "C. These bands could be removed by brief evacuation at room temperature, showing that they arose from physisorbed SO;?. The bands were more intense in the case of Na/A1203 than for pure A1203 and Table 2 shows relative integrated intensities (band areas) for all four samples. In agreement with the gravimetric results, we found that the activation temperature did not greatly influence the band areas, but they were greater for the sodium-containing sample. The spectra of the species which were irreversibly adsorbed (chemisorbed) at room temperature following evacuation are shown in Figure 3. (These are difference spectra, after background subtraction, A1203 being totally absorbing below about 1000 cm-'.) For NdAk;?O3,there was a broad asymmetric band having a maximum near 1050 cm-' and a shoulder near 1150 cm-'. With pure A1203, the band maximum was shifted to higher wavenumber for a higher temperature of activation, being at 1070 cm-' for 600 "C and near 1050 cm-I for 350 "C activation; in each case, there was a shoulder near 1190 cm-'. Using Raman spectroscopy, the bands near 1150/1340 cm-' due to physically adsorbed SO;?were observed for A1203 (parts a and b of Figure 4) or Na/A1203;they disappeared following evacuation at room temperature. There was additionally a very weak band near 1060 cm-' for both catalysts (see Figure 4b for A1203)which was not removed following evacuation. In the AlOH spectral region, the IR spectrum of pure alumina after activation at 600 "C showed three distinct bands at 3779, 3731, and 3690 cm-' due to AlOH vibrations, whereas for Na/
I
I
i5
1s ;
2&5
3i5
5b5 6bO "c
T E M P E RATU RE Figure 1. (A) Effect of temperature of the mass of chemisorbed SO2 on A1203(600) (a) and Na/Al203(600) (b). (B)TPD spectra of SO2 adsorbed on A1203(600) (a) and Na/Al~O3(600)(b).
N
0
1
300 WAVENUMBER Figure 2. Infrared spectra of physically adsorbed SO2 on A1203(600) (a) and NdAl204600) (b).
AlzO3(600), there was a single band centered near 3750 cm-I (Figure 5). Following adsorption of SO;?,the sharper bands in the 3800-3650 cm-' spectral region decreased in intensity and were replaced by a very broad absorption extending from about 3600 to 2800 cm-'. This effect is shown more clearly by the
4622 J. Phys. Chem., Vol. 99, No. 13, 1995
Mohammed Saad et al.
TABLE 2: Relative Band Areas (arbitrary units) for Physically Adsorbed SO2 A1203(600) NdAl203(600) &03(350) NdAlzOs(350) 1150 cm-l 3 9 4 6 1340 cm-I 22 41 26 44 difference spectra in Figure 6 where the negative peaks in the OH spectral region correspond to the decrease in intensity of the positive peaks in Figure 5 as a result of the adsorption of S02. The corresponding difference spectra for A1203(350) and Na/&03(350) are shown in parts c and d of Figure 6, respectively. For any temperature of activation, this broad band was much more intense for NdAl203 than for pure A1203. However, in all cases, there was a broad weak absorption near 2350 cm-'. (d) Comparison between SO2 and COZ. In a previous study, we showed that the preadsorption of SO2 on A1203 blocked the subsequent adsorption of COz (SO2 is a stronger acid than COz), indicating that both of the probes SO2 and COP adsorbed on common sites.5 In particular, the hydrogen carbonate species formed by adsorption of C02 on the OH groups did not form if the alumina has been pretreated by S02, suggesting that hydrogen sulfite species might be formed by reaction with the same OH sites. Therefore, in order to further characterize the adsorption sites, we have studied the effect of addition of C02 on A1203(350) and on Na/A1203(350). The adsorption of C02 on basic oxides gives rise to IR bands in the spectral region between 1200 and 1700 cm-' due to carbonate or hydrogen carbonate speciesS6 On A1203, C02 adsorption mainly gives rise to hydrogen carbonates characterized by bands at 3610 (YOH), 1650, 1480 and 1450 (YOCO), and 1235 cm-' (60H). Such species are also formed on Na/ A1203 (corresponding bands at 3620, 1650, 1435, and 1235 cm-') with additional carbonate species characterized by bands near 1600, 1345, 1090, and 1060 cm-'. Progressive heating under vacuum in the temperature range from 100 to 350 "C resulted in the preferential removal of the bands due to hydrogen carbonate species. Higher temperatures are necessary to eliminate all of the species from Na/A1203 than to eliminate them from pure A1203 (350 vs 250 "C, respectively), demonstrating the greater basicity of the former catalyst. The increased basicity of NdAl203 was also demonstrated in a sequence of experiments whereby SO2 was preadsorbed at room temperature, followed by evacuation of SO2 at a given temperature, and finally, COz was added at room temperature. We found that it was necessary to evacuate the SO2-treated A1203 at 150 "C before the subsequent addition of CO2 produced a spectrum of hydrogen carbonates. However, using NdAlzO3, evacuation at 250 "C was necessary before carbonates were observed. This again demosntrates that SO2 is more tenaciously held on the more basic Na/A1203 than on pure A1203. The significance of these results will be discussed in more detail later in connection with the HzS/S02 reaction on these catalysts. (e) Reaction between Chemisorbed SO2 and HzS. Details of the adsorption of H2S alone on A1203 and on Na/A1203are not of concern in this paper insofar as we are concerned mainly with the adsorption of SO2 and its subsequent interaction with H2S. This aspect will appear in a subsequent publication, but the salient features to note are, in agreement with other work, that HzS physically adsorbed on the AlOH groups of alumina produced an IR or Raman band at 2570 cm-I and that it additionally dissociated to give AlOH and AlSH species. No water was p r ~ d u c e d . ~ On NdA1203, more complex spectra were observed. NaSH species were produced, as shown by IR spectroscopy (2685
I
1150 ld00 WAVENUMBER
1300
lis0
ld00
Figure 3. Infrared spectra (after subtraction of the background spectrum of A1203 or Na/A1203) of chemisorbed SO2 on A1203(600) (a), N d Al203(600) (b), Al203(350) (c), and Na/kd203(350) (d).
1
1500
I
I
500
1000
100
c m-' Figure 4. Background Raman spectrum of AhO@OO) (a), after the addition of SO2 (P, = 2 Torr) to the substance producing the spectral line a (b), and after evacuation of the substance producing the spectral line b for 5 min, followed by the addition of H2S (P, = 1 Torr) (c). cm-'), as well as H20 on NdA1203 (1640 cm-' HOH deformation mode), and there was evidence for the presence of surface sulfides8 Finally, about 50% more H2S was adsorbed (either physically or irreversibly) on NdA1203 than on pure A1203.8 In order to study the reaction between chemisorbed SO2 and gaseous H2S, SO2 was chemisorbed on A1203(350) (Figure 7a). The addition of H2S (P,= 3 Torr) to this sample resulted in the disappearance of the shoulder at 1190 cm-', a decrease in the intensity of the 1060 cm-' band, and the appearance of a band at 1640 cm-l, the latter being indicative of the formation of water (Figure 7b). The addition of H2S did not result in the appearance of a SH band as was previously observed for adsorption of H2S alone on A1203.739Finally, heating caused a further decrease in the intensity of the 1060 cm-' band (Figure 7c-e). Qualitatively similar results were found for A1203(600).
Effect of Sodium on the Adsorption of SO2
J. Phys. Chem., Vol. 99, No. 13, 1995 1623
b
a
u)
-3aoo
3650 3500 WAVENUMBER
Figure 5. Infrared spectra of the hydroxyl groups on A1203(600) (a) and Na/Al203(600) (b).
In a separate experiment, the procedure used to generate the spectrum shown in Figure 7b was followed, the gaseous H2S was evacuated at room temperature, and then C02 (P, = 1.2 Torr) was introduced. This resulted in the appearance of the spectrum of hydrogen carbonate species. Recall that under similar conditions but without introduction of HpS, CO2 did not adsorb on A1203 pretreated with SOz. Therefore, OH sites capable of interacting with CO2 must have been created during the SOp/H2S reaction. These basic hydroxyls could have been formed either from the reaction of H2S with the species responsible for the 1060 or 1190 cm-' bands or as a result of the formation of water. However, when Hp0 alone was added to chemisorbed SO2 on A1202(350) (or A1203(600)),the 1190 cm-' band disappeared and there was practically no effect on the 1060 cm-' band. Therefore, we conclude that the H20 generated from the SO2/H2S reaction was not responsible for the decrease in the intensity of the 1060 cm-' band, but rather, this resulted from a reaction with HpS. We can make no conclusion regarding the reactivity of the species responsible for the 1190 cm-' band with H2S. The same series of experiments were carried out using SO2pretreated Na/A1203(350) which has a band at 1050 cm-' and a shoulder at 1150 cm-'. The addition of HpS caused the preferential disappearance of the 1150 band and a decrease in the intensity of the 1050 band, but less so than on pure A1203. Water was also formed (1640 cm-'), but less so than for pure A1203. After evacuation at room temperature, the addition of C02 gave no bands due to carbonates or hydrogen carbonates, confirming that adsorbed SO2 species are less reactive on Na/ A1203 than on Al2O3. Finally, for either Alp03 or NalA1203, the appearance of a yellowish color after the introduction of HzS suggests formation of sulfur via the modified Claus reaction. Raman spectroscopy confirmed this conclusion. Following addition of H2S to preadsorbed SO2 on either catalyst, intense Raman bands at 151, 220, and 480 cm-' characteristic of s8 were observed (Figure 4c). (f) Reaction between Chemisorbed H2S and SO2. After having chemisorbed H2S at room temperature on A1203(350 or 600), SO2 was added. This caused the disappearance of the YSH band but did not hinder the chemisorption of S02, giving
0
z
a m a 0
tn
m
a
N
'$boo
3250
2500
1 k O
1600
WAVENUMBER Figure 6. Difference infrared spectra after the addition of SO2 to &os(600) (a), Na/A1201(600) (b), A1203(350) (c), and Na/Al203(350) (d).
rise to a broad band near 3550 cm-l and appearance of a band near 1060 cm-'. The band resulting from the formation of water was hardly visible. We conclude that there is some interaction but that the conditions are not favorable for the modified Claw reaction to take place. The same conclusions have been reached using Raman spectroscopy. F'readsorbed H2S did not react with SO2 to produce Raman bands characteristic of SS. Conversely, if H2S was re-added after the above addition of SOz, then s8 was again detected. Similar results were observed using Nal A1203.
Discussion (a) Nature of the Catalysts. The IR spectra of our y A1203 in the OH stretching region are similar to those reported in the
4624 J. Phys. Chem., Vol. 99, No. 13, 1995
Mohammed Saad et al.
11
\Id &
2ooo
I
1600
1200
cm-1
WAVENUMBER
Figure 7. Infrared spectra of A1203(600) (background subtracted) after the addition of excess SOz, followed by evacuation (a),after the addition of H2S to the substance producing spectral line a (P,= 3 Torr) at room
temperature (b), after heating the substance producing spectral line b at 100 "C (c). after heating the substance producing the spectral line c at 150 "C (d), and after heating the substance producing the spectral line d at 200 "C (e). literature.1°-13 However, given that the surface structure of y A1203 from the surface science point of view is largely unknown,14 it would be inappropriate to attempt a detailed description of the Na/Al203 surface. Impregnation with sodium results in the removal of most of the fine structure in the OH stretching region (e.g., Figure 5 ) . In agreement with other work relative to Na/A1203, the disappearance of the high- and lowwavenumber OH bands at 3779 and 3690 cm-' shows that Al0-Na groups are formed, and this suggests that the sodium is well-dispersed on the ~ u r f a c e . ~ However, .'~ XPS or zero-point of charge (zpc) measurements on similar materials have shown that, at sodium coverages similar to ours, there could also be islands of Na20 or sodium aluminate.16 In our case, we do not believe that the formation of these species is excessive because the surface area and the pore volume were not significantly changed by the addition of sodium. Further speculation concerning the nature of the surface is unwarranted. (b) Physisorbed S02. The two stretching modes of SO2 in the gas phase are at 1361 (va,antisymmetric SO2 stretch) and 1151 cm-' (vS,symmetric SO2 stretch). The complexation of SO2 with some acids and bases has been studied." With strong electron acceptors (A1 or Ga halides), an A1:OSO or Ga:OSO interaction arises and Y, and Y, shift to near 1450 and 1090 cm-', respectively. With strong electron donors such as pyridine or triethylamine, a N:SO2 interaction arises and these modes shift to near 1275 and 1125 cm-l, respectively. In the present study, the IR or Raman bands of physisorbed SO2 on A1203 are very close to those of the free molecule, showing that the interaction with the surface is weak. We assume that the interaction is mainly via the AlOH groups. This is supported by the IR study of the OH region which shows that some of the SO2 is in interaction with the hydroxyls that give rise to the band at 3730 cm-', an OH type which does not have a strong acidity or basicity.I0 We also note (Tables 1 and
2) that there is more physisorbed SO2 on A1203(350) than on A1203(600), the former having a higher OH c~ntent.~-ll The gravimetric (Table 1) and IR (Table 2) studies show that more SO2 is physisorbed on Na/A1203 than on A1203 alone. In a previous study, we showed that the quantity of physisorbed SO2 increased as the sodium loading was increased in 0.05% increments from 0.0 to 3%.18 The IR bands were observed at 1340 and 1151 cm-', very close to those reported for SO2 complexation with very weak bases such as ethylene, ethanol dioxane, and water. l7 Therefore, we assume that adsorption occurs on basic 0 2 2 - or OH- on NdA1203. On A1203, Karge and Dalla Lanalg studied the adsorption of SO2 after poisoning the catalyst with different acid or base probes. Because NH3 and pyridine partly hindered the observation of the 1334 cm-' band, and BF3, HCl, or CH3COOH did not, they concluded that physisorbed species mainly reside on acidic sites. The present results show that physisorption also occurs on basic sites. For both A1203and NdAl203, the TPD results show that the physisorbed species are desorbed at a relatively low temperature, 100-120 "C. (c) Chemisorbed S02. These species can be attributed to IR bands below 1200 cm-l which remain after evacuation. Both the IR and gravimetric results show that the number of these species is greater for sodium-loaded alumina. The spectra with or without Na are characterized by a strong broad band near 1060 cm-', accompanied by a shoulder to higher wavenumber, near 1150 cm-' for NdA1203 and near 1190 cm-' for A1203. The TPD results show that the chemisorbed species desorbed in a wide temperature range, having a maxima from 290 to 350 "C, thus demonstrating their greater thermal stability relative to the physisorbed species. The main peak near 1060 cm-' on A1203(600) has been universally attributed to the formation of a surface sulfite on a l ~ m i n a . l ~We - ~ ~have studied the shift of this peak for l80 substitution. The peak shifted by 10 cm-' to lower wavenumber for adsorption of Sl802 on normal A1203, but it shifted by about 30 cm-l to lower wavenumber for adsorption of Sl6O2 on l80exchanged Al2O3. Although these shifts cannot be used for any quantitative structural determination (note that l80exchange of the surface of A1203 is not expected to be complete), the results suggest that SO2 is interacting with a surface 02-, probably resulting in the formation of an A1-O-SO:, type species. The adsorption of SO2 on NdA1203 has only been the subject of a few studies. Zotin and Faro3 showed that the presence of Na on alumina increased the quantity of SO2 irreversibly adsorbed at 100 "C. It went from 0.49 mmoVg on pure y A1203 to 0.76 on 3.9% Na/A1203, which corresponds respectively to 1.5 and 2.6 molecules/nm2. These values are close to those found in the present study (Table l), confirming the greater adsorption capacity of Na/Al2O3 relative to A1203. We also found that the species formed were more thermally stable on NdA1203, a result which will be discussed further below in connection with the H2S/S02 reaction. The increased basicity of Na/Al2O3 undoubtedly is responsible for both the increased number of sulfite species and their thermal stability. The 1050 cm-l band observed in the case of NdA1203(600) is assigned to sulfite species on alumina whose basicity is increased by the presence of sodium. It is not possible to show that bulk Na2SO3 is formed because its characteristic IR band4 at 970 cm-' is below the limit of transmission (about 1000 cm-') of A1203. However, the number of chemisorbed species does not depend significantly on the activation temperature of either catalyst (Table 1). This is not what one would expect if the interaction was only between SO2 and 02-,the latter species being expected
Effect of Sodium on the Adsorption of SO2 to be more abundant when the activation temperature leading to dehydroxylation is higher. Therefore, we assume that SO2 can react with equal facility with 02-or with OH- to form sulfite or even to form a hydrogen sulfite species, HS03- or HOSOz-, or disulfite, &Os2-. The formation of HOS02- could account for the very broad band in the 3500-3300 cm-' spectral region when SO2 chemisorbs on NdA1203(350). Note that the formation of &Os2- has been observed when introducing SO2 to a NdSi02 catalyst.23 This species is characterized by a band at 657 cm-'. However, solid Na2S205 has a strong band24in the IR at 1180 cm-' which might correspond to the weak shoulder near 1150 cm-' for NdA1203. (d) Reaction between SO2 and HzS. The purpose of this work has been to compare the adsorption of SO2 on A1203 and on NdA1203and to compare the reactivity of chemisorbed SO2 with H2S on both catalysts. For pure Al2O3, the decrease in the intensity of the sulfite band near 1060 cm-' with the concomitant formation of water when H2S was added has been reported by other^.'^^^^^^^ However, we have shown that this species, although present to a greater extent on Na/A1203than on A1203 (gravimetric, IR, and Raman results), is less reactive to H2S on NdA1203,an effect which can be attributed to the greater basicity of the NdA1203 catalyst. That is, the sulfite is more strongly chemisorbed on NdA1203,and its reactivity with H2S is accordingly reduced. The TPD results c o n f m this trend; higher temperatures are required to desorb sulfite from N d A1203. The Raman results have shown that the generation of sulfur requires the presence of chemisorbed SO2 insofar as s8 was only detected if H2S was added to preudsorbed SO2 and not if SO2 was added to preadsorbed H2S. Moreover, in the latter instance, sulfur was generated following the re-introduction of H2S after addition of SOz. Noting that SO2, being a stronger acid than H2S, can displace adsorbed H2S (the SH band disappears), we are tempted to conclude that the reaction to produce sulfur at room temperature on either catalyst involves the reaction of gaseous H2S with chemisorbed SOz. Finally, this is the first time that s8 has been positively identified during the modified Claus reaction at room temperature. Further speculation concerning the mechanism of the reaction to generate sulfur is unwarranted in the absence of additional experimental evidence. For example, we have shown that the shoulders near 1150 or 1190 cm-' disappear after contacting chemisorbed SO2 with either H2S or H20, the latter being a product of the reaction. Quite apart from not knowing the structure of these species, we can conclude nothing vis-&vis their reactivity with HzS.
Conclusions At room temperature, sulfur dioxide reversibly adsorbs on A1203 or on NdAI2O3, and we have concluded that weak basic sites are implicated in the physisorption process. In addition, at least two strongly chemisorbed species are produced. The major species has a strong IR band near 1060 cm-' and has been attributed to an adsorbed sulfite type species, probably involving an AlOS02 bond. This species is somewhat reactive with gaseous H2S and leads to the formation of water and sulfur, two products of the modified Claus reaction. More sulfite is formed on NdAl2O3 than on A1203. However, the sulfite on
J. Phys. Chem., Vol. 99, No. 13, 1995 4625 the more basic NdA1203catalyst is more strongly chemisorbed than it is on pure Al2O3, and it is less reactive with gaseous H2S. Sulfur is formed when gaseous H2S is contacted with chemisorbed SO2, but not when gaseous SO2 is contacted with chemisorbed H2S. The decrease of reactivity after impregnation with a large quantity of sodium can be attributed to the greater stability of the sulfite species adsorbed on the more basic N d A1203 catalyst3 than the stability on pure A1203, these sulfite species being considered intermediates in the modified Claus r e a ~ t i o n Zotin . ~ ~ and ~ ~Faro3 ~ ~ ~proposed ~ that the lower activity of a high sodium-loaded alumina catalyst relative to pure alumina was due to either (1) the catalyst being too basic with excessive sodium or (2) the need to have both acid and base sites and sodium poisons the acid sites. We conclude that hypothesis (1) is more plausible; a greater basicity if not necessarily a favorable factor for the modified Claus reaction.
Acknowledgment. We are grateful to the CNRS (France), RhBne-Poulenc, the Natural Sciences and Research Council of Canada, and Esso Petroleum Canada for financial support. B.A.M. and J.C.L. are grateful to NATO for an International Collaborative Research Grant. References and Notes (1) George, Z. M. In Sulfur Removal and Recovery from industrial processes; Pfeiffer, J. B., Ed.; Advances in Chemistry Series 139; American Chemical Society: Washington, DC 1975; p 75. (2) Scokart, R. 0.; Amin, A,; Defosse, C.; Rouxhet, P. G. J. Phys. Chem. 1981, 85, 1406. (3) Zotin, J. L.; Faro, A. C. Catal. Today 1989, 5, 423. (4) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination compounds, 4th ed.; John Wiley & Sons: New York, 1986. (5) Lavallev. J. C.: Janin.. A.:. Preud'homme. J. React. Kinet. Catal. Lett.'1981, 18, i 5 . (6) Morterra. C.; Zecchina. A.; Coluccia, S.; Chiorino. A. J. Chem. Soc., Faraday Trans. I 1977, 73, 1544. (7) Saur, 0.;Chevreau, T.; Lamotte, I.; Travert, J.; Lavalley, J. C. J. Chem. SOC.,Faraday Trans. 1 1981, 77,427. (8) Mohammed Saad, A. B.; Saur, 0.; Lavalley, J. C., to be published. (9) Lavalley, J. C.; Mohammed Saad, A. B.; Tripp, C. P.; Morrow, B. A. J. Phys. Chem. 1986, 90, 908. (10) Knozinger, H.; Ratnasamy, P. Catal. Rev.-Sci. Eng. 1978, 17, 31. (11) Morrow, B. A. Stud. Surf.Sei. Catal. 1990, 57A, A161. (12) Zecchina, A.; Coluccia, S.; Morterra, C. Appl. Spectrosc. Rev. 1985, 21, 259. (13) Mohammed Saad, A. B.; Ivanov, V. A,; Lavalley, J. C.; Nortier, P.; Luch, F. Appl. Catal. 1993, 94, 71. (14) Henrich, V. E.; Cox, P. A. The Sulface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1994. (15) Vordonis, L.; Koutsoukos, P. G.; Lycourghiotis, A. J. Catal. 1986, 98, 296. (16) Deo, D. V.; Dalla Lana, I. E.; Habgood, H. W. J . Catal. 1971,21, 270 (17) Belopukhov, S. L.; Dmitrevsky, L. L.; Knyazev, D. A,; Konchits, V. A. Koord. Khim. 1987, 13, 1047. (18) Waquif, M.; Mohammed Saad, A. B.; Bensitel, M.; Bachelier, J.; Saur, 0.; Lavalley, J. C. J. Chem. SOC.,Faraday Trans. 1992, 88, 2931. (19) Karge, H. G.; Dalla Lana, I. G. J . Phys. Chem. 1984, 88, 1538. (20) Chang, C. C. J. Catal. 1978, 53, 374. (21) Karge, H. G.; Dalla Lana, I. G.; Trevisan De Suarez, S.; Zhang, Y. Proceedings of the 8th International Congress on Catalysis, Berlin, 1984; VOl. m,453. (22) Datta, A.; Cavell, R. G.; Tower, R. W.; George, 2. M. J . Phys. Chem. 1985, 89, 443. (23) Wang, Y.; Mohammed Saad, A. B.; Saur, 0.; Lavalley, J. C.; Morrow, B. A. To be published. (24) Sato, S.; Higuchi, S.; Tanaka, S. Appl. Spectrosc. 1985, 39, 822. (25) Datta, A.; Cavell, R. G. J. Phys. Chem. 1985, 89, 454. JP94 1675P
