J. Phys. Chem. 1986, 90, 981-982 the peak near 2570 cm-'. On this evidence we must conclude that the peaks in this frequency region which were observed by Datta and Cavell do not arise from the adsorption of pure HzS. We have always found that the ratio of the intensities of the 1570/ 1340-cm-' bands is constant (as was found by Datta and Cavell for HzS adsorption on an alumina activated at 700 "C) and that both bands shift to lower wavenumber when HzS is coadsorbed with 10% I3Cl6O2 (1528/1314 cm-I) or lzC'sO (1548/1301 cm-I) but not a t all when using D2S l2CI6O2. [Using 10% COz the 1570/1340-cm-I bands were much more intense than those which arise from the adsorption of unpurified H2S. We have not investigated the minimum level of COz which would give rise to these spectral observations since our objective was to determine the isotopic shifts using the same conditions which were used previo~sly.~] Further, the adsorption of pure COS on to a similarly activated alumina gives rise to the identical 1570/1340-cm-I pair. Thus, these bands can definitively be assigned to a type of surface thiocarbonate species. From the spectroscopic point of view there is no doubt that alumina does not have "intrinsic" bands at 1560 and 1480 cm-' and that a vibration due to A1=0 at 1578 cm-' does not exist, regardless of the temperature of activation. Furthermore, the deformation mode of gaseousi0 or solid" HzS is near 1182 cm-' whereas that for HzS in CC14 is reported12 to be at 1172 cm-I, far below the 1334-cm-' band assigned by Datta and Cavell to the deformation mode of coordinated HIS (coordination is not expected to alter the frequency significantly, as has been found for coordinated wateri3). We agree that H2S adsorbed on actb 1:ed Alz03 gives rise to an increased absorption in the 3000-3' t 1 . k m - l spectral region. We have also found that this is true for the adsorption of CH3SH and that the OH intensity increase per mole of adsorbed CH3SH or HzS is identical. That is, both molecules dissociatively adsorb to yield AlOH and AlSCH3 (from CH,SH) or AlSH (from HzS). We deduce, therefore, that HzS adsorption at room temperature does not yield the sulfide via the reaction
+
2A10
+ H2S
-
2A10H
+ Sz-
To conclude, our picture of HzS aasorption is unchanged from that reported previo~sly.~-~ We believe that the 1570/1334-cm-I bands observed by Datta and Cavell must be due to thiocarbonate formation. We have found that similar spectral features always appear unless extreme precautions are taken to remove traces of COz from commercial HzS (freeze-pump-thaw cycles are not sufficient). We recognize that our alumina samples are different (Datta and Cave!l do not specify the crsytalline form, nor the Kaiser description) but we note that Amenomiya et al. have r e p ~ r t e dlittle ~ , ~ difference for the adsorption of C 0 2 and y- and 7-alumina. Acknowledgment. We are grateful to C N R S (France) and to Imperial Oil Ltd. (Canada) for financial support and we thank NATO for the award of a Collaboration Research Grant (B. A. Morrow and J. C. Lavalley). Registry No. H,S, 7783-06-4; A1,03, 1344-28-1 ( I O ) Collins, T. U.; Johnson, W. H.; Nier, A . 0. Phys. Rev. 1954,94, 398. (11) Lohman, J. B.; Reding, F. P.;Homing, D. F. J . Chem. Phys. 1951, 19, 252. (12) Saumagne, P.; Dorval, P.C. R. Acad. Sci., Ser. E 1966, 263, 963. (13) Falk, M. Spectrochim. Acta, I'art A 1984, 40, 43.
Laboratoire de Spectrochimie U.A. 41 4, UniversitP de Caen 14032 Caen Cedex, France
Jean-Claude Lavalley A. B. Mohammed Saad
Carl P. Tripp B. A. Morrow*
Department of Chemistry University of Ottawa Ottawa, Canada, K I N 9B4 Received: July 8, 1985
0022-3654/86/2090-098 1$01.50/0
98 1
On the Adsorption of H,S on Activated Alumina: A Reassessment Sir: Recent detailed evidence provided by Lavalley et a1.I and a ieevaluation of our own experiments indicates that the interpretation of our previous studyZrequires revision particuiarly with respect to the assignment of the infrared hands obtained in the 1800-1200-~m-~ region when alumina was treated with hydrogen sulfide. The evidence presented' suggests strongly that a carbonaceous species ( C 0 2 or another carbon source) is responsible for the bands that we observed at 1570 and 1334 cm-' and possibly also at 1640 cm-'. The observations of Lavalley et al.' ind'cate that the 1334-cm-' band is not the bending motion of H 2 and now appears to be more probably a thiocarbonate stretch ana that both the 1570- and 1334-cm-' bands are due to this source. This being the case it follows that there is no infrared evidence for a second adsorbed form of H 2 S 2 Our conclusion2 that there are two forms of adsorbed HzS is therefore not substantiated by infrared measurements nor is the statement that a nondissociative adsorption occurs. We and Lavalley et al.' agree that H2S does adsorb dissociatively as indicated by the appearance of a band at 2560-2570 cm-' and the rehydroxylation of the alumina in the 3600-3880-~m-~region. If the features in question at 1570 and 1334 cm-' derive from specifically introduced COz impurity then the implication is that our HzS source was grossly contaminated, which we are certain was not the case, or the system exhibits very marked sensitivity to C 0 2 adsorption. The strong features reported previously by Lavalley et aL3 were obtained at levels of 10% C 0 2 in H2S, a contamination level we can rule out. The actual analysis provided for our HzS source4 indicated nondetectable levels of COz (and likewise ethane, methane, and ethylene). A "trace" of Nzwas found. Even with very generous estimates for detection limits of these impurities, there could not have been more than 250 ppm of COz in our input HzS gas. The analyses reveal, however, that we do have a source of gaseous carbonaceous material in our H2S, namely propylene, which is a significant contaminant (0.18%), and propane (0.05%). We have now also found, via electron ~pectroscopy,~ that carbon is present on the alumina svrface before any treatments are carried out. This carbon signal (the'binding energy of which is indicative of reduced [e.g. hydrocarbonj carbon) increases during the activation heating (when the bands in the 1800-1200-~m-~ region are decreasing). There is no significant contribution to the C I S signal from species with binding energies indicative of oxidized (e.g., carbonate or bicarbonate) carbon according to electron spectroscopy but, if these particular kinds of species are strong IR absorbers, they could appear prominently in the infrared spectrum even if present in small amounts. We also note that the C I Ssignal, relatively strong after activation of the alumina at 400' C,5 does not change appreciably after treatment of the sample with either SO2 or HzS; these two gases did not therefore appear to introduce significant carbonaceous material. At the same time, however, several substantial S2, signals arose from adsorbed species. Slager and Amberg6 noted that bands at 1568 and 1460 cm-' grew as their alumina was exposed to C-H containing species (but they did not mention a similar enhancement of the 1334-cm-' band). It is clear then that the 1800-1200-~m-~ region is fraught with difficulties. Various species are formed either from C 0 2 or other contaminants carried in with H2S or form carbonaceous contamination on the alumina itself. The (1) Lavalley, J. C.; Mohammed Saad, A. B.; Tripp, C. P.; Morrow, B. A. J. Phys. Chem., prLceding comment in this issue. (2) Datta, A,; Cavell, R. G. J . Phys. Chem. 1985, 89, 450. (3) Lavalley, J. C.; Travert, J.; Chevrean, T.; Lamotte, J.; S a w , 0. J . Chem. Soc., Chem. Commun. 1979, 146. (4) Analyses reported by Matheson Co., East Rutherford, NJ for our specific sample of H2S. (5) Pua, C.; Tower, R.; Vandentop, G . ; Li, Xizhong; Cavell, R. G.unpublished data. (6) Slager, T. L.; Amberg, C. H. Can. J. Chem. 1971, 50, 3416.
0 1986 American Chemical Society
982
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986
surface carbon may undergo facile reaction with HzS to yield CS-containing species which appear to account for the prominent bands in this region. The source of the band at 1620-1624 cm-' is also not clear. Parkyns' in a study of COz adsorption on alumina suggested that a band at 1640 cm-' was due to bicarbonate and gave isotopic shift evidence in support. In that same work a band was observed at 1480 cm-' which was comparable in intensity to this 1640-cm-' band and these two bands maintained a direct proportionate relationship. We observe on the 400 "C activated alumina a prominent band at 1620 cm-' with no corresponding 1480-cm-' peak. The 1620-cm-' band grows in prominently late in the HzS absorption cycle and decreases early (heating to ca. 100 to 200 "C) in the desorption cycle. In contrast the same source of H2S adsorbed on the 700 OC activated alumina gave only a very weak feature (at similar levels of H2S (2565 cm-') signal) at 1620 cm-' (also with no companion peak at 1480 cm-I) but in this case the 1566- and 1334-cm-I bands are very prominent. The variable behavior of all the bands between 1200 and 1800 cm-' cannot be wholly rationalized on the basis of contaminant CO, introduced with the HzS. Involvement of propylene with highly activated alumina, already contaminated with carbon species, might account for the formation of this 1620-cm-' band. Another possibility is the growth of "bicarbonate" concentration as protons, released by the dissociative adsorption of H2S, react with carbon-oxygen species already present on the surface; these species may be indicated by peaks at 1565 and 1479 cm-' in, for example, the case of the sample activated at 400 OC.* These peaks vanish after activating the alumina at 600 OC? thus the species which account for these features are of much lesser importance following activation a t 700 "C and this difference may resolve the lesser contribution of the 1620-cm-' feature of HzS adsorption on alumina activated at 700 OC (see curve 5, Figure 9 ) . The revision of interpretation of part 2 has an impact on part 39 in that arguments based on two sulfur species from HzS cannot (7) Parkyns, N. D. J . Chem. SOC.A 1969, 410. (8) Datta, A,; Cavell, R. G. unpublished IR spectral data.
Comments be substantiated and the origin of the 1640-cm-I band is not clear. It is important to note that the allegedly C 0 2 related species (at 1570 and 1333 cm-') do not appear in the reaction cycle of HzS reacting with SOz adsorbed on alumina (Figure 1 part 39) originally activated at 400 OC. There is a small feature growing in at 1455 cm-' and perhaps also at 1550 cm-' (Figure 2 part 39) as the 1630-cm-' band increases. The difference of band position and the prominence of this peak at 1633 cm-' suggests that it is not of the same origin as that observedZwith H2S adsorption on alumina alone but we are now not certain of the source. The 1620-cm-' band is also prominent at the start of the SO, reaction with adsorbed H2S (Figure 5 part 39) but the band shape in this region is not as well defined as that in curve 1 of Figure 3 part 2,2 which shows comparable HzS signal and represents ostensibly the same state, illustrating that there is possibly some difference between our alumina samples. This 1620-cm-' band grows as SOz is added to adsorbed HzS and is indicative of a species being formed in the reaction. A shoulder appears at about 1550 cm-' which is more prominent than a similar feature which grows at this position in the reaction of H2S with adsorbed SO, described above. The contiguous spectra (Figure 4 part 39) for this reaction show a small 1333-cm-I peak which is replaced as reaction proceeds by the characteristic 1326-cm-' peak arising from adsorbed SO2. Again we are not certain of the origin of the 1620-cm-' band (or the 1550-cm-' shoulder), but we think that bicarbonate is unlikely in this latter case. We are presently completing our study of the system with electron spectroscopy in order to elucidate more completely the species involved and we shall be reporting our results as soon as possible. Registry No. H2S, 7783-06-4; A1,0,, 1344-28-1. (9) Datta, A.; Cavell, R. G. J . Phys. Chem. 1985, 89, 454
Department of Chemistry University of Alberta Edmonton, Alberta, Canada T6G 2G2 Received: October 16, 1985
R. G . Cavell
