J . Phys. Chem. 1984, 88, 1538-1543
1538
Infrared Studies of SO, Adsorption on a Claus Catalyst by Selective Poisoning of Sites Hellmut G. Karge* Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6,1000 Berlin 33 (Dahlem), Federal Republic of Germany
and Ivo G . Dalla Lana Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 (Received: April 18, 1983)
Adsorption of SOzas well as interaction of SO2 with H2Son alumina (ALON) are investigated by IR and volumetric adsorption measurements. Strongly held SO2 is indicated by a low-frequency band around 1065 cm-' (LFB), while weakly adsorbed SO2gives rise to the bands at 1330 and 1150 an-'. Site-blocking experiments with bases and acids show that strong chemisorption of SO2 occurs on basic sites, whereas acidic sites seem to be responsible for weak adsorption. Chemisorbed SOzwas detected on the alumina surface during Claus reaction and observed to be reactive toward H2S.
Introduction Even though the catalytic reaction step in the modified Claus process 2HzS + SO, s 3/2S2+ 2 H 2 0 (1)
n = 3, ..., 8 (2) (n/2)Sz @ S, plays an important role in industrial sulfur recovery, the mechanism of this reaction remains a mystery. Several attempts have been made to elucidate this mechanism via IR spectroscopic investigations. Deo, Dalla Lana, and Habgood' were the first to study both HIS and SO, adsorption on aluminas and zeolites. Also, they carried out the Claus reaction in a static IR experiment involving the contacting of HIS with SO, preadsorbed on those catalysts (or vice versa). The most important bands that Deo et al. obsetved after H2S adsorption on alumina were around 2560 and 1335 cm-I. They attributed the high-frequency band to contributions from the SH stretching vibrations vl and v3 (261 1 and 2684 cm-' for the free molecule) and the first overtone of the bending vibration (219, 2422 cm-I for the free molecule). Accordingly, the 1335-cm-' band of the adsorbate was ascribed to the scissorlike bending mode v2 of the SH bonds. Similarly, Slager and Amberg2 found bands of HzS adsorbed on alumina at 2568 and 1341 cm-I. At variance with Deo et al., they interpreted their 2568-cm-' band to be solely due to the asymmetric stretching mode v3 of the HzS molecule. Deo et al.' reported that upon adsorption of SO, on alumina at room temperature and low pressure (1.3 kPa) the main bands occur at 1330 cm-' (vg, asym stretch) and 1140 cm-' (vI, sym stretch). Weaker bands at 2470 and 2340 cm-' were assigned to the combination modes vl v3 and 219, respectively. Only at higher pressures did another pair of (very weak) bands appear a t 1410 and 1090 cm-l, and these were attributed to perturbed adsorbed SOzspecies. These latter bands were readily eliminated merely by pumping. In a study of hydrogen sulfide adsorption on Na-A and NaCa-A zeolites, Forster and Schuldt3were able to provide some evidence for dissociative adsorption of small amounts of H2S on Na-A. A band close to 2500 cm-' was attributed3q4 to the SH stretching vibration v3, in agreement with ref 2. Dissociative adsorption of H2Swas studied in more detail by Karge and Rask$ using as adsorbents faujasite-type zeolites with a systematically varied Si/A1 ratio. These authors demonstrated that the dissociation of H2S occurs at poorly shielded cations of the sodium faujasite structure since a close correlation emerged between the
+
(1) A. V. Deo, I. G. Dalla Lana, and H. W. Habgood, J . Catal., 21,270 (1971). (2) T. L. Slager and C. H. Amberg, Can. J . Chem., 50, 3416 (1972). (3) H. Fbrster and M. Schuldt, J . Colloid Interface Sci., 52, 380 (1975). (4) H. Forster and M. Schuldt, Spectrochim. Acta, Part A, 31A, 685 (1975). ( 5 ) H. G. Karge and J. Rask6, J . Colloid Interface Sci., 64, 522 (1978).
0022-3654/84/2088-1538$01.50/0
population of low-shielded S(II1) sites and the intensity of O H and SH bands developing on H2Sadsorption. Dissociative HzS adsorption on zeolites was confirmed in a subsequent study by George et aL6 Employing a series of Me-X and Me-Y zeolites (Me = Li through Cs), Karge and Ladebeck' showed that not only the number of cations but also their nature affects both the adsorption of HzS and of SO, as well as the oxidation of hydrogen sulfide. The oxidation reaction was monitored by IR spectroscopy and GC. In those of the cited studies that included interaction of SO, with preadsorbed H2S (or vice versa) on the surface of aluminas or zeolite^,'^^^^-^ it was observed that the HzS bands (around 2500 cm-') or the SOzbands (around 1330 and 1150 cm-I) were readily eliminated from the spectra. Accordingly, upon adsorption of a stoichiometric mixture of H2S and SOz, i.e. during the Claus reaction, none of these bands did appear (see, for example, ref 7). Moreover, inspection by IR analysis revealed that the above bands of HzS and SO2 disappear upon subsequent H 2 0admission, indicating that the respective H2S and SOz species are easily replaced by water.8 This is surprising since one would expect that at least one of the reactants (HzS, SOz) would adsorb on the working catalyst and interact with the IR beam. More recently, both Changg and Karge et al.1° observed that adsorption of SO2 alone on alumina gives rise to a hitherto unknown low-frequency band (LFB) at 1060-1080 cm-'. This band was difficult to detect because of the low transmittance of alumina in that particular spectral region. Current IR investigations are much facilitated by computer-supported spectrometers, even with very low transmitting adsorbents. Accordingly, the long-wavelength region of A1203/S02systems should particularly be reinvestigated for possible adsorbed SOz species. Furthermore, by using the method of preadsorption of suitable probe molecules,* it might be possible to characterize the nature of SO2 adsorption sites on alumina surfaces. Therefore, the goals of the present study were to investigate (i) whether or not the SO2 species indicated by the low-frequency band (LFB, 1060-1080 cm-') are reactive toward HzS and are present during the Claus reaction and (ii) whether the LFB species adsorb on basic or acidic sites of A1203. (6) Z . M. George, R. W. Tower, and H. G. Karge, Preprints of the 6th Canadian Symposium on Catalysis, Aug 19-21, 1979, Ottawa, Ontario, 1979, p 87. (7) H. G. Karge and J. Ladebeck, Proceedings of the 5th International Conference on Zeolites, Napoli, Italy, June 2-6, 1980, R. Sersale, C. Colella, and R. Aiello, Eds., p 180; Recent Progress Reports and Discussion, Napoli, 1981. (8) Ch.-L. Liu. Ph.-D. Thesis, The University of Alberta, Department of Chemical Engineering, Edmonton, Alberta, Canada, 1978. (9) C. C. Chang, J . Catal., 53, 374 (1978). (IO) H. G. Karge, R. W. Tower, Z. Dudzik, and Z. M. George, Proceedings of the 7th International Congress on Catalysis, June 30-July 4, 1980, Tokyo, Japan, T. Seiyama and K. Tanabe, Eds., Tokyo, 1981, p 643.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 8,1984 1539
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