J . Phys. Chem. 1985,89, 443-449
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Claus Catalysis. 1. Adsorption of SO2 on the Alumina Catalyst Studied by FTIR and EPR Spectroscopy' Arunabha Datta, Ronald G. Cavell,* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 Robert W. Tower, and Zacheria M. George Alberta Research Council,z Edmonton, Alberta, Canada T6G 2C2 (Received: April 4, 1984) The adsorption of SO2on y-alumina activated at 400 and 700 "C has been studied by FTIR and EPR spectroscopy. Four different types of adsorbed species have been identified on alumina activated at 400 "C, and an additional fifth species has been detected on the sample activated at 700 "C. The most strongly held species appears to be a surface sulfite bonded in a unidentate fashion to sites involving metal ions. The other four remaining species probably involve S02-likestructures, the strengths of adsorption of which are dependent on the nature of the adsorption sites. Two of the more strongly held species can be described as SO2bonded to metal atoms through the sulfur in either pyramidal or planar form. Of the remaining two, the species which is more strongly adsorbed appears to be SO2 attached to an anionic oxygen site, whereas the weakly held species are adsorbed on surface hydroxyls. The EPR studies demonstrate the formation of SO, radicals upon adsorption of SO2on alumina at 25 "C. The decrease in the concentration of radicals upon heating to 200 "C parallels the disappearance, at this temperature, of the hitherto unobserved bands at 1255 and 1189 cm-'. However, the increase in the radical signal upon further heating to 400 "C did not correlate with the behavior of any of the infrared bands.
Introduction The adsorption of SOzon alumina has been studied previously by infrared spectro~copy.~" Bands due to both physically adsorbed as well as chemisorbed species were observed. In recent ~ t u d i e sin ~ .which ~ compensation for the background absorbance of the alumina was applied, an additional band around 1080-1060 cm-' was observed. In this case a reference sample was placed in the reference beam of a double-beam spectrophotometer. Chang5assigned this band to the formation of a sulfite-like species whereas Karge et a1.6 assigned it to SOT radicals. It has also been suggested6 that the species responsible for this band was an important intermediate in the Claus reaction: 2H2S
+ SO2
-
2 H z 0 + (3/n)S,
The adsorption behavior of SO2 on alumina has also been investigated by electron paramagnetic resonance (EPR) spectroscopy.'** The formation of SO2- radicals has been demonstrated, and conditions of their formation and their thermal stability have been demonstrated. However, although Karge et aL6 observed some similarities in the behavior of the 1080-cm-' infrared band and the EPR signal due to SO2- radicals, recent investigationsg done concurrently with our study have indicated that there is no correlation between the intensities of the major infrared bands of adsorbed SO2and the EPR signals. The EPR signal apparently arises from a very small proportion of SO2radicals (ca. 1% of adsorbed SO2),so the species may not be detectable by infrared measurements. In the present work in connection with our studies on the nature of the Claus reaction, we have reinvestigated the adsorption of SO2 on alumina using a Fourier transform infrared (FTIR) spectrometer. With this technology the infrared bands, particularly in the low-wavenumber region close to the strong adsorption of alumina, can be observed and monitored much more easily than with dispersive instrumentation. This is because a FT instrument provides advantages of (a) much higher sensitivity, precision, and (1) Presented in part at the 66th Canadian Chemical Conference, Calgary, Alberta, June, 1983. (2) Alberta Research Council contribution no. 1252. (3) Deo,A. V.;Dalla Lana, I. G.; Habgood, H. W. J. Catal. 1971,21, 270. (4) Fiedorow, R.; Dalla Lana, I. G.; Wanke, S. E. J. Phys. Chem. 1978, 82, 2414. ( 5 ) Chang, C. C. J . Caral. 1978, 53, 374. (6) Karge, H. G.; Tower, R. W.; Dudzik, 2.;George, Z . M. Stud. Surf. Sci. Caral. 1981, 7, 643. (7) Ono, Y.;Takagiwa, H.; Fukuzumi, S. J . Caral. 1977,50, 181. ( 8 ) Khulbe, K. S.; Mann, R. S. J. Caral. 1978, 51, 364. (9) Karge, H. G.; Trwizan de Suarez, S.; Dalla Lana, I. G. J. Phys. Chem. 1984, 88, 1782.
repeatability on the frequency scale, (b) increased speed of analysis, and (c) higher throughput and larger S/N ratio compared to conventional dispersive spectrometers. Moreover, the necessary computer and laser calibration system allow facile and reliable coaddition of spectra. In addition, it is possible to subtract a previously acquired reference spectrum of the solid from that of the same solid after treatment with gas, a procedure which provides an additional advantage over the conventional procedure of spectral compensation by means of a blank sample in the reference beam because the computerized subtraction procedure on the same sample before and after treatment yields a more true difference spectrum. Modern dispersive spectrometers with attached computational facilities provide features similar to, but not identical with, the FT instrumental capability. In addition to the use of vibrational spectra, the adsorption of SOz on alumina has also been studied by EPR using herein the same conditions as were employed for the infrared studies, in order to investigate whether specific infrared bands could be correlated with the presence of the SO2- radical. Similar investigations were carried out independently e l ~ e w h e r e . ~ Finally, because current models for the surface condition of catalytic aluminaslo suggest that the nature of the active sites on the catalyst surface varies according to the temperature of activation of the sample (because of different hydroxyl environments developed during the dehydroxylation process at different temperatures), we have studied the dehydroxylation process. This temperature dependence is also evident from the observation6 that the rate of the Claus reaction increases with increasing activation temperature from 400 to 700 "C and maximizes at 700 "C although the surface area of the sample remains unchanged through this temperature range. Herein we report studies of vibrational spectra of alumina dehydroxylated at different temperatures, the adsorption of SOz on alumina activated at 400 and 700 OC, and the EPR behavior of SO2 adsorbates prepared on alumina activated at these temperatures. Experimental Section
The alumina was manufactured by Kaiser Alumina. Wafers made from this alumina ("thickness" of 25.5 mg/cm2) were activated under vacuum torr) for 16 h at temperatures of 400-700 "C (heating rate was 5 "C/min) in a Kiselev type cell designed by Karge." The BET surface area of a typical wafer (10) Knozinger, H.; Ratnasamy, P. Catal. Rea-Sci. Eng. 1978, 17, 31. (11) Karge, H. G. Z . Phys. Chem. (Wiesbaden) 1971, 76, 133.
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was 296 m2/g, and the average weight of the wafer was approximately 98 mg. The SO, (Matheson) used was purified by several freeze-thaw cycles under vacuum, chilling with liquid nitrogen. Only the middle fractions from each cycle were retained. For the infrared studies the SOzwas added in small increments to the alumina wafer. In each case the gas was allowed to adsorb at room temperature for 30 min before the spectrum was recorded. On addition of the first few doses of SO, (0.09 and 0.06 mmol, respectively, for the 700 and 400 OC activated samples) the pressure within the cell rapidly fell to zero. On subsequent increments, there was a minor residual gas pressure after the 30-min adsorption time but no infrared bands due to gaseous SO, were detected. The infrared spectra were recorded on a Nicolet 7 199 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-I which optimizes the signal-to-noise ratio for this detector in the wavenumber range of interest to this work (4000-1000 cm-I). Usually 1000 consecutive scans were summed to obtain a spectrum with a resolution of 4 cm-I over the spectral range. A 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 SO, was obtained by a 1:l subtraction of a stored spectrum of the activated alumina sample from that of the alumina sample with SO, adsorbed on it. 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 nor derived from the plotted curves shown in the figures. Prior to plotting the spectrum, the raw data was in general subject to an 11-point smoothing treatment. For the EPR studies 125 mg of alumina (20/30 mesh) was placed in a quartz tube and was activated in the same manner as described for the IR experiments. The same sequence of adding incremental amounts of SO, with an adsorption time of 30 min after each addition was employed, but the EPR spectrum of the SO2adsorbed on alumina was recorded only after the total amount of SO, had been added. The EPR spectra were recorded with a Varian Model E-102 spectrometer at a frequency of -9.5 GHz, with a power level sufficiently low to prevent saturation and a modulation amplitude appropriate for the desired resolution. The frequency was measured by a Hewlett-Packard 5245M electron counter connected to a 5255A frequency converter. The spectra were measured in quartz sample tubes at ambient temperature. Spin concentrations were calculated by double integration of the first derivative with a DEC PDP-11/24 computer. The calculations were based upon a comparison with a standard cellulose char sample calibrated against a known amount of diphenylpicrylhydrazyl, a CuS04. 5 H 2 0 single crystal, and a Varian pitch standard. The g factors were measured against a cellulose char standard with a g factor of 2.002 76 calibrated against 1,4-benzosemiquinone free radical (g = 2.00468).
Results and Discussion A . Dehydroxylation of Alumina. It is knownI2 that the hydrated surface of y A 1 2 0 3retains 13 molecules of water per 100
A2 of surface after evacuation at 25 OC for 100 h. Even after
drying at 120 OC it appears to retain as many as 8.25 molecules per 100 A,. On heating to temperatures above 100 "C, however, some of the adsorbed water is desorbed as molecular water while some reacts to form hydroxyl groups. At still higher temperatures both these and preexisting hydroxyl groups gradually condense to eliminate water. In crystalline 7-A1203,at least five different types of hydroxyls have been observed and it has been suggested'O that the stretching vibrational frequencies of these hydroxyls depend on their net (12) Peri, J. B. J . Phys. Chem. 1965, 69, 211.
Datta et al.
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Figure 1. FTIR spectrum (OH region) showing progressive dehydroxylation of alumina at (1) 100, (2) 200, (3) 300, (4) 400, (5) 500, ( 6 ) 600, and (7) 700 "C under vacuum. Spectral presentation conditions are described in the Experimental Section.
charge. These frequencies are within or above the range expected for the stretching vibrations of hydroxyls that are not hydrogen bonded', and can be referred to as "isolated" hydroxyls. The actual occurrence and relative concentration of these hydroxyls will depend upon the relative contributions of the different crystal faces (that is the (1 lo), (loo), or (1 1 1) faces) to the total surface layer. Thus, the relative intensity of the surface hydroxyl bands for different alumina samples varies according to the proportion of each of these planes which is preferentially exposed on the surface. This is dependent upon the detailed preparation conditions of each alumina sample. Since it is also highly likely that the catalytically active sites on alumina are created by dehydroxylation of the surface hydroxyls and since there are bands indicative of five different hydroxyl environments, it can be expected that the nature of the active sites created on the surface of the catalyst during dehydroxylation will be related to the type of surface hydroxyls originally present on the sample. The identification of the types of hydroxyls present on the surface by infrared analysis should therefore give an idea of the nature of the active sites present on the activated sample. From the dehydroxylation pattern (Figure 1) of the alumina sample used in the present study, it can be seen that initially, even on heating the sample at 100 OC for 12 h, the hydroxyl tail extends to about 3200 cm-I. Although at this stage there is no evidence for the presence of isolated hydroxyls, the lack of an infrared band at approximately 1630 cm-' (which corresponds to the bending mode of water) suggests that the strong absorption throughout the hydroxyl region cannot be due to adsorbed water and must consequently be due to hydrogen-bonded hydroxyl groups. At higher temperatures (200 and 300 "C) bands corresponding to isolated hydroxyls begin to appear at 3675 and 3750 cm-' but the tail still extends to around 3200 cm-I, indicating that there is still a considerable proportion of hydrogen-bonded hydroxyl groups remaining on the surface. On heating to 400 O C , the spectrum of the isolated hydroxyl region becomes better defined with distinct bands at 3675 and 3750 cm-' and a shoulder at 3785 cm-'. This is accompanied by a pronounced reduction of the tail intensity in the low-frequency region. It is interesting to note that alumina is known to have very little catalytic activity until it is activated to temperatures of about 400 O C . It is evident from the present study that at this temperature there has been a significant reduction in the number of hydrogen-bonded hydroxyls
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 445
Adsorption of SO2 on Alumina Catalyst
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Figure 2. FTIR spectra (S-O region) of the adsorption of SO2on alumina which has been activated at 400 OC. The curves represent addition of (1) 0.03, (2) 0.06, (3) 0.09, (4) 0.12, ( 5 ) 0.15, and ( 6 ) 1.0 mmol of SO2,respectively. Spectral presentation conditionsare described in the
Experimental Section. and distinct structural hydroxyls have been generated. On heating to higher temperatures of 500, 600, and 700 OC, the 3680-an-' band gradually disappears while the 3750-an-' band becomes more prominent. The band a t 3780 cm-' which appears as a weak shoulder at 400 O C also persists as the dehydration temperature is increased to 700 O C . Following the model proposed by Knozinger and Ratnasamy,lo it can be suggested that the 3680-cm-' band corresponds to the stretching frequency of a hydroxyl bonded to three aluminum atoms in octahedral coordination. The gradual disappearance of this band on heating above 400 OC suggests that at 700 O C there should be sites consisting of clusters of aluminum ions along with coordinatively unsaturated oxygen atoms created by the dehydroxylation of the hydroxyl groups (which are characterized by the 3680-cm-' band) by combining amongst themselves. A result of this process of self-condensation may be the creation of strained and therefore active sites. The persistence of the 3790-cm-' band at high temperatures was unexpected because the model suggests that this band is due to a hydroxyl attached to an aluminum in octahedral (or tetrahedral) coordination and should therefore be dehydroxylated easily (i.e., at lower temperatures). The presence of this band at high temperatures is probably due to the reformation of these hydroxyls as a result of the increased mobility of surface hydrogen and possibly, although to a much lesser extent, of aluminum ions, at temperatures above 400 OC. B. Adsorption and Desorption of SOzon Alumina Activated at 400 OC. ( i ) Adsorption. Figure 2 shows the spectra resulting from different loadings of SOz adsorbed on alumina. On exposure to 0.03 mmol of SOz a broad band at 1050 cm-' with shoulders at 1134, 1189, and 1322 cm-' can be detected. On further addition of SOz (up to 0.09 mmol) there is a pronounced increase in the intensity of the band at 1050 an-'. With still higher doses of SOz the bands at 1322 and 1334 cm-' preferentially increase in intensity and shift to higher frequency until they appear at around 1334 and 1148 cm-I, respectively, when about 1.0 mmol of SOz has been added. In the hydroxyl region (Figure 3), addition of 0.03 mmol of SO2provides very little change in the hydroxyl bands, suggesting that the adsorbed species do not interact with the hydroxyls.
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Figure 3. FTIR spectrum (OH region) of the adsorption of SO2 on alumina which has been activated at 400 "C. Curves represent addition of (1) 0.0, (2) 0.03, (3) 0.06, (4) 0.09, ( 5 ) 0.12, ( 6 ) 0.15, and (7) 1.0 mmol of SO2,respectively. Spectral presentation conditions are described in the Experimental Section.
Further addition of SOz,however, causes the 3750-cm-' band to disappear progressively while a new band appears at around 3735 cm-I. This suggests that the adsorbed SOz species perturbs the 3750-cm-' hydroxy1 even though the SOz species does not interact directly with these hydroxyls. After the addition of 0.09 mmol of SOz the 3750-cm-' band has disappeared completely and the 3680-cm-' band begins to decrease in intensity. This is accompanied by the appearance of a broad band around 3550 cm-I, indicating the interaction of SOz molecules directly with the 3680-cm-' hydroxyls. (ii)Desorption. From the desorption cycle (Figure 4), it is seen that evacuation at room temperature for 1 h results in the disappearance of the bands at 1334 and 1148 cm-' with the intensity of the remaining bands remaining more or less unaffected even after room-temperature evacuation for 50 h. On evacuation a t 100 O C , however, the band at 1322 cm-l disappears and the band at 1138 shifts to 1131 cm-'. Further evacuation at 200 O C causes the band at 1189 cm-' to disappear completely, but those at 1050 and 1129 cm-l are retained but with reduced intensity. After evacuation at 400 OC the intensity of the same two bands is further reduced but they are still observable. These bands disappear completely only upon evacuation at 600 OC. In the hydroxyl region (Figure 5 ) , evacuation at room temperature for 1 and 50 h causes the bands at 3728 and 3660 cm-' to reappear. However, the broad band (-3500 cm-') is still present and disappears only after evacuation at 100 O C . At 200 O C , the 3750-cm-' band begins to emerge. On evacuation at 400 O C the distinct hydroxyl bands are completely restored so that, at 400 and 600 OC, the hydroxyl region of the spectrum corresponds well with that of fresh alumina which has been dehydroxylated at these temperatures. C. Assignment of the infrared Bands. The set of bands at 1334 and 1148 cm-I is the last to appear in the adsorption cycle and the first to disappear on room-temperature evacuation. Hence, these can be ascribed to a weakly bound physically adsorbed species. These two bands can be assigned to the antisymmetric and symmetric S-0 stretching vibrations of physically adsorbed
sop
The bands at 1322 and 1 140 cm-' disappear only after evacuation at 100 "C. These bands therefore are due to more strongly
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The Journal of Physical Chemistry, Vol. 89, No. 3, 1985
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Figure 4. FTIR spectrum (SO region) showing the desorption of SO2 which has been adsorbed on alumina activated at 400 O C . Curves display the result of (1) evacuation at room temperature for 1 h, (2) evacuation at room temperature for 48 h, (3) evacuation at 100 "C for 0.5 h, (4) evacuation at 200 O C for 0.5 h, ( 5 ) evacuation at 400 "C for 0.5 h, and (6) evacuation at 600 O C for 0.5 h. Spectral presentation conditions are
described in the Experimental Section. bound species and can be attributed to the antisymmetric and symmetric S-0 stretching vibrations of chemisorbed SO,. On evacuation at 200 O C , the band at 1189 cm-I disappears, accompanied by the appearance of an absorption around 1 130 cm-' which may have been masked by the band at 1140 cm-I. The bands at 1050 and 1130 cm-' show a similar behavior on evacuation. Their intensities decrease on evacuation at 400 O C , and they disappear completely on heating to 600 O C . Hence, these two bands can be ascribed to the same or similar species whereas the band at 1189 cm-I is due to a different species. In total there appear to be, according to the IR band patterns, a minimum of four different types of adsorbed SO2 species: type 1 with bands at 1334 and 1148 cm-I, type 2 with bands at 1322 and 1140 cm-', type 3 with bands at 1189 cm-I, and type 4 with bands at 1130 and 1050 cm-'. Although the assignment of the bands due to species 1 and 2 is fairly unambiguous, it is rather more difficult to identify species 3 and 4 since their bands are close to the broad alumina absorption. Moreover, these species may have vibrational modes below the cutoff region of alumina which are not observed in the present experiments. In addition, although it is possible to eliminate the absorption due to alumina through spectral subtraction, it is known13 that considerable errors can occur if digital subtraction is done for spectra with absorbances greater than 2. In most cases an upper limit of 1.O is imposed for reliable treatment of such cases. Consequently, although the presence of a broad band around 1050 or 1065 cm-I is unequivocal, it is not possible to unambiguously determine whether there is more than one band in this region. In this context, it is fruitful to compare the present results with studies that have been done on the adsorption of SO, on Mg014,15 since this substrate absorbs strongly only below -750 cm-I. In that case the behavior of SO2 appears to be very similar to that of SO2 adsorbed on alumina. In addition to bands in the 1300cm-' region due to physically adsorbed and chemisorbed SO,, there (13) Koenig, J. L. ACC.Chem. Res. 1981, 14, 171. (14) Gwdsel, A. J.; Low, M. J. D. Takezawa, N. Environ. Sci. Technol. 1972, 6,268. (15) Schoonheydt, R. A.f Lunsford, J. H.J . Card. 1972, 26, 261
0
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Figure 5. FTIR spectrum (OH region) illustrating the desorption of SO2 which has been adsorbed on alumina activated at 400 OC. The numbered curves represent (1) evacuation at room temperature for 1 h, (2) evacuation at room temperature for 48 h, (3) evacuation at 100 O C for 0.5 h, (4) evacuation at 200 O C for 0.5 h, ( 5 ) evacuation at 400 O C for 0.5 h, and ( 6 ) evacuation at 600 "C for 0.5 h. Spectral presentation conditions are described in the Experimental Section.
are bands in the 1150-950-cm-' region which have been assigned to sulfite-like species bonded to the surface in both a monodentate and a bidentate fashion. For sulfite-cobalt complexes bands due to unidentate sulfite occurI6 at 11 17, 1075, 984, and 625 cm-'. Therefore, in the present case, the two bands observed at 1135 and 1050 cm-l can be assigned to a similar unidentate sulfite-like species. The fact that the frequencies are higher than those observed for a free sulfite ion (1010 and 961 cmW1)l7 suggests that the adsorbed species is bound to the surface through the sulfur atom. The other two vibrational modes of the sulfite occur below 1000 cm-I and are not observed. We have ascribed the band at 1189 cm-' to v, of SO, bonded to surface metal (Al) atoms (or ions) through the sulfur. Given that A13+,with no "d" electrons, is unlikely to provide extensive a back-donation interactions, we can consider the surface complex to arise principally by a Lewis acid-base interaction involving donation of the lone pair of SO, (Le., the 4a, orbital) to the A13+ ion forming a &SO2 with either pyramidal or planar structure. It has been suggested that SO2 bonding to metals in a pyramidal S u donation into the form generally results from strong M 2b, (LUMO) orbital with rehybridization of the sulfur atom to sp3. In contrast, the planar configuration of SO, is favored if the metal fragment is capable of accepting an electron pair from the 4a1 orbital and a back-bonding into the 2bl orbital of sulfur. Since the A13+ ion is potentially a very strong Lewis acid, T bonding is unlikely and the main interaction with SO, is expected to arise from the strong polarization of the sulfur lone-pair orbital to form a relatively strong S M u bond. This mode of bonding should therefore favor a planar configuration for Strong u bonding would have the effect of lowering the S-0 stretching frequencies due to the weakening of the S - 0 bond, and
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(16) Baldwin, M. E. J . Chem. SOC.1961, 3123. (17) Ryan, R. R.; Kubas, G. J.; Moody, D. C.; Eller, P.G. Srruct. Bonding (Berlin) 1981, 46, 47.
Adsorption of SO2 on Alumina Catalyst
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 441 2.50
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Figure 6. FTIR (S-0 region) spectrum of SOz (0.03 mmol) adsorbed on alumina activated at (1) 400 and (2) 700 O C . The absorbance scale is in arbitrary units. Spectral presentation conditions are described in
Figure 7. FTIR (SO region) spectra of SO2adsorbed on alumina which has been activated at 700 OC. The numbered curves illustrate increasing doses of SOz: ( I ) 0.03, (2) 0.06, (3) 0.09, (4) 0.12, (5) 0.15, and ( 6 ) 1.0 mmol. Spectral presentation conditions are described in the Experimental
Section.
the Experimental Section.
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the extent of lowering would be dependent on the strength of the S M band. It has been observed" that the vibrational frequencies of &SO2 in ( MS02}complexes is dependent primarily on the ancillary ligand of the metal fragment rather than on the geometry of SO2. Accordingly, the range of v, and u, values reported is relatively large (1045-1 140 and 1190-1 300 cm-I) for a planar configuration. The 1189-cm-I band has therefore been tentatively assigned to v, of SO2 bonded to an aluminum ion in planar form. The us bond would be expected in the 1050-cm-' region and may well contribute to the broad absorption in that region. D. Possible Adsorption Sites. We can draw several reasonable inferences from the behavior of the hydroxyl region during adsorption and desorption of SO2. (a) Since, on desorption at room temperature, the 3730-cm-l band reappears first followed by the 3680-cm-' hydroxyl and since the broad band at -3550 cm-' disappears only after evacuation at 100 OC,it can be assumed that both species 1 and 2 are bonded directly to hydroxyls. Out of these, type 1 are probably associated with the 3730-cm-I hydroxyl whereas type 2 are associated with the 3680-cm-' hydroxyls. (b) Since the disappearance of the 1189-cm-' band on evacuation at 200 OC coincides with the reappearance of the 3750-cm-I hydroxyl band, we suggest that the type 3 species are adsorbed on sites which do not directly affect the 3750-cm-' hydroxyls but that a slight perturbation of the environment causes a 20-cm-' shift in the stretching frequency. (c) Type 4 species do not affect any of the hydroxyls and must therefore be adsorbed on the coordinatively unsaturated aluminum or oxygen ions or the aluminum ion clusters, created during the dehydroxylation process. Thus, there appear to be at least three different types of adsorption sites: (i) the hydroxyls themselves (3730 and 3680 cm-' hydroxyls), (ii) a site near a 3750-cm-' hydroxyl, and (iii) coordinatively unsaturated aluminum and oxygen ions and aluminum ion clusters. E . Adsorption and Desorption of SO2on Alumina Activated at 700 O C . The general pattern of the adsorption and desorption
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Figure 8. FTIR (OH region) of SO2 adsorbed on alumina activated at 700 OC. The numbered curves represent the addition of (1) 0.00, (2) 0.03, (3) 0.06, (4) 0.09, ( 5 ) 0.12, ( 6 ) 0.15, and ( 7 ) 1.0 mmol of SO2, respectively. Spectral presentation conditions are described in the Ex-
perimental Section. behavior of SO2 on alumina activated at 700 "C (Figures 6-10) is similar to that on alumina activated at 400 OC. However, there are the following differences: (i) The broad band in the low-wavenumber region occurs at around 1065 cm-' instead of 1050 cm-I. (ii) On adsorption, an additional band at 1255 cm-' appears which disappears upon evacuation at 200 OC. Although the
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The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 2.50
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Figure 9. FTIR spectrum (SO) region illustrating the desorption of SO2 which has been adsorbed on alumina activated at 700 OC. The numbered curves represent (1) evacuation at room temperature for 1 h, (2) evacuation at room temperature for 48 h, (3) evacuation at 100 "C for 0.5 h, (4) evacuation at 200 OC for 0.5 h, ( 5 ) evacuation at 400 O C for 0.5 h, and (6) evacuation at 600 "C for 0.5 h. Spectral presentation conditions are described in the Experimental Section.
desorption behavior of this band is similar to that of the 1189-cm-' band, it must be associated with a different species since the 1255-cm-' band does not appear during adsorption on alumina activated at 400 O C . However, the strength of adsorption of the two species would appear to be similar. In essence, therefore, it seems that there is an additional type of species adsorbed on alumina activated at 700 O C . (iii) From the hydroxyl region of the spectrum it can be seen that the hydroxyls are hardly affected even when 0.06 mmol of SO2 has been added, indicating that there are now more sites involving coordinatively unsaturated aluminum and oxygen ions. This is, of course, to be expected since the alumina activated at 700 O C has been dehydroxylated to a much greater extent. Since on desorption the 1255-cm-' band shows a behavior similar to that observed for the band at 1189 cm-', it can also be assigned to v, of a SO2molecule bonded to surface aluminum. However, the higher S-0 stretching frequency of 1255 cm-I suggests a weaker S M interaction. It is expected that dehydroxylation of the 3680-cm-' hydroxyls at 700 "C (which are believed to be attached to three aluminum ions in octahedral coordination) would produce [OAl,] clusters. An SCF-MO calculation2' of such clusters has shown that the oxygen acquires increased electron density from the Al, in the cluster formation process. An [OAl]species yields atomic charges for AI and 0 of 0.74+ and 0.74-, respectively, whereas the corresponding values for the [OAl,] cluster are 0.35+ and 1 . 4 s . It is possible therefore that the species responsible for the 1255cm-I band is SO2 bonded to the aluminum of a cluster. The decreased charge on such an aluminum ion would produce a weaker S M u bond and hence a higher S-0 stretching frequency. Whether the decreased acidity of the aluminum ion, with the consequent increase in the possibility of M S bonding, would favor a pyramidal configuration for SO2 is not obvious since the stretching frequency of 1255 cm-I falls within the range of vibrational frequencies observed for SO2 bonded in either planar or pyramidal form." Using the assignments and structures in Chart I, we can suggest a mechanism for the adsorption process. In the first step the SO2 is attached to either the acidic (positively charged A1 ions) or basic
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Figure 10. FTIR spectra (OH region) illustrating the desorption of SO2 which has been adsorbed on alumina activated at 700 O C . The treatments are (1) evacuation at room temperature for 1 h, (2) evacuation at room temperature for 45 h, (3) evacuation at 100 OC for 0.5 h, (4) evacuation at 200 OC for 0.5 h, ( 5 ) evacuation at 400 "C for 0.5 h, and (6) evacuation at 600 OC for 0.5 h. Spectral presentation conditions are described in the Experimental Section.
Chart I. Possible Nature of Adsorbed Species (i) type 1, physically adsorbed
SO, with bands at 1334 and
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1148 c m - l
OH
I (ii) type 2, chemisorbed SO, with bands at 1322 and 1140 cm-'
O\/
I A I/O\*
I
I1
(iii) type 5 with band a t 1255 cm-'
planar or pyramidal (see text) on AI cluster
0\s/o
I
(AI),
I11 (iv) type 3 with band at 1189 cm-l
planar (see text) on single AI ion
0\s/o
I
AI
IV (v) type 4 with bands at 1135 and 1065 (1050) cm-'
0
\g/.0
0
I
AI
V
sites (negatively charged oxygen ions). The adsorption on acidic sites gives rise to structures 111 and IV which are relatively stable. The adsorption on basic sites giving rise to structure I1 can lead
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 449
Adsorption of SOz on Alumina Catalyst
Activation Temp
1
:
.E& 50 20
0m 30
.-V
U m E 20
INlT ADSN
'b 25'
100'
200'
400'
600'
700'
Temperature of Evacuation ("C) Figure 11. Concentration dependence of radical species arising from the adsorption of SO2on alumina activated at different temperatures (400 and 700 "C) and the growth and decay of the radical species upon desorption of the adsorbed SO2 at different temperatures. to the removal of an oxygen atom from the surface producing a sulfite-like species. This species can then attach to metal atoms in a unidentate fashion to give rise to structure V. At higher doses of SO2, when all the acidic and basic sites have been occupied, adsorption takes place directly on the hydroxyls giving rise to the weakly held physically adsorbed species (structure I). The bands observed at 1189 and 1255 cm-I and assigned to structures 111 and IV have not been previously reported. Only a band at 1060 cm-I has been reported.'* It has been suggested by Karge et a1.18 that the species responsible for this band is adsorbed on basic sites. However, this appears to contradict the o b s e r ~ a t i o n that ' ~ the number of acidic sites increases with increasing activation temperature up to 550 OC whereas the number of basic sites is a maximum at 140 O C . It is also known that the intensity of the 1060-cm-' band increases with increasing activation temperatures. The mechanism proposed above explains these two divergent observations since the formation of the species responsible for the 1060-cm-' band would require both basic and acidic sites. This is because the formation of this species involves first an adsorption at an unsaturated oxygen (basic site) followed by the incorporation of the oxygen from the surface leading to the formation of a sulfite which is then bonded to a positive metal atom (acid site) through the sulfur atom. This would also explain the observations of Chang5 that on allowing the AlZO3/SO2system to stand at room temperature for 2 h there was a reduction in the intensity of the band at 1326 cm-l and an increase in the intensity of the 1060-cm-' band. F. EPR Studies. Exposure of alumina activated at either 400 or 700 OC to SOzyielded an EPR signal with axially symmetric gvalues of g,,= 2.0081 (f3) and g , = 2.0041 (f3). The gvalues agree with those previously for the SO, radical anion on alumina. (18) Karge, H. G.; Dalla Lana, I. G. J . Phys. Chem. 1984, 88, 1538. (19) Vit, Z.; Vala, J.f Malek, J. Appl. Cutul. 1983, 7, 159. (20) Nakamoto, K. "Infrared and Raman Spectra of Inorganic and Coordination Compounds", 3rd ed. Wiley: New York, 1978; p 338. (21) Schwartz, M. E.; Quinn, C . M. Surf. Sci. 1981, 106, 258.
The variation of the spin concentration of these radicals upon initial adsorption of SO2and subsequent desorption (Figure 11) is similar for both the samples activated at 400 OC and those activated at 700 OC. In both cases the concentration of SOT rises to a maximum at a desorption temperature of 400 OC and then decreases upon heating to higher temperatures (700 "C). This behavior is in partial agreement with the results of Khulbe and Mann,8 who reported that a noticeable signal due to SOz- could be detected only when the S02/A1203species had been heated to 450 OC. In the case of the 700 "C activated sample, however, the concentration of SOT radicals following the initial adsorption of SOz is higher and there is a noticeable decrease of the radical signal on desorbing at 200 OC. This might suggest that a particular alumina site is responsible and this site is more plentiful following activation at 700 O C . There appears to be no obvious correlation between the infrared bands and the EPR signal since the intensities of all the infrared bands decrease on desorption at elevated temperatures whereas the EPR signal becomes initially more pronounced on heating. It would appear therefore that none of the observed infrared bands can be assigned to the SOT radical. A similar suggestion has also been made recently by Karge et aL9 It is, however, interesting to note that the initial higher concentration of SO, radicals formed when SO2 is adsorbed at room temperature on a sample activated at 700 OC and the subsequent decrease of the resultant EPR signal upon heating to 200 O C closely parallel the behavior exhibited by the infrared spectra. It was also observed that in contrast to the sample activated at 400 OC,which shows only a band at 1189 cm-I due to adsorbed SOz, the 700 "C activated sample also shows an additional band at 1255 cm-l which we have ascribed to a different adsorbed species. Furthermore, both these bands disappear on desorbing the SOz at 200 OC. However, the increases in the EPR signal upon further heating to 400 O C cannot be correlated with the behavior of any of the infrared bands, and especially in view of the inherently greater sensitivity of EPR as opposed to IR spectroscopy, we think that no correlation can be substantiated. It is not clear how the radical anion species are generated since in this case activation is provided only by thermal conditioning of the surface. In contrast, UV activation of MgO was used to generate SO2- radical anions on that solid.22 The growth of the SOz- radical signals on alumina with heating suggests that radical-forming sites (possibly partially reduced metal centers) are thermally formed on the alumina either prior to adsorption of SO2 or with SO2present on the surface. The initial decrease in concentration arises because desorption occurs without formation of additional active centers. This suggests that temperatures of e200 "C are insufficient to create radical formation sites. At higher temperatures more radical formation sites are created and adsorbed SOz available on the surface can be used to form radical species. At the highest temperatures all the SO2desorbs and the signals decay. The fact that the g values of the radical species are the same at all temperatures suggests that the same (sulfur) species is involved in all cases.
Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for the support of the research of R.G.C. (via Strategic and Operating grants) at the University of Alberta. We would also like to thank Dr. H. G. Karge et al. for preprints of papers in press. Registry No. SO2, 7446-09-5; AI2O3, 1344-28-1. (22) Schoonheydt, R. A,; Lunsford, J. H. J. Phys. Chem. 1972, 76, 323.
