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J. Phys. Chem. 1986, 90, 296-300
Chemisorption and Decomposition of H,S on Rh( 100) R. I. Hegde and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: June 10, 1985)
The chemisorption and decomposition of H2S adsorbed on Rh( 100) at 100 K have been studied by thermal desorption and Auger electron spectroscopy. Hydrogen sulfide exposure to clean Rh( 100) leads to both molecular desorption and dissociative processes upon heating. On sulfur-covered rhodium, there is less decomposition of H2S. The adsorption of HIS at 300 K is completely dissociative and hydrogen desorbs during exposure. Preadsorbed D(a) does not alter the desorption energetics of H2S. Coadsorption of D and H2S leads to no detectable deuterium incorporation in desorbing hydrogen sulfide but the isotopic forms of molecular hydrogen all desorb in relatively large amounts. Preadsorbed D(a) inhibits the decomposition of H2S. Postdosing H2Sdoes not displace D(a) from the surface at 100 K but does shift the deuterium desorption to significantly lower temperatures.
Introduction In spite of its use as a precursor to surface sulfur in a large number of surface science studies, the adsorption, desorption, and decomposition of H2S has only recently begun to be actively studied.'-9 These studies identify important molecular and dissociative decomposition channels that vary with coverage. The vibrational spectra of surface SH has been identified'-4 and the core and valence levels have been s t ~ d i e d . ~ Fisher studied the coverage dependence of H2S adsorption on R u ( l l 0 ) at 80 K.' At low HIS exposure complete dissociation occurred. The presence of S H was observed at intermediate coverages and at higher coverages molecular H2S dominated. Battacharaya and co-workers4 found a small amount of SH on W( 100) at room temperature. Adsorption of H2S on a Ni film at 77 K was mainly m o l e c ~ l a r .Baca ~ and c o - ~ o r k e r sstudied ,~ H2S on Ni(100) at 110 K and used EELS to identify both molecular and atomic species on the surface. Their coverage-dependent EEL spectra showed a trend of decreasing dissociative adsorption as the coverage was increased. At higher exposures of H2S two distinct molecular species were observed. The coveragedepqdent EELS showed the dissociation of H2Son Ni( 100) at 170 K . In another study on polycrystalline Pt,6 molecular and dissociative processes were identified and there was evidence for the formation of islands of molecular H2S at 100 K. Sulfur poisoning has been extensively studied on both unsupported and supported metal catalysts but the mechanisms are still uncertain. Some authors1"-l3 propose that the dominant effect of sulfur-containing species is the blocking of adsorption sites. Other investigator^'^^^^ have concluded that longer-range electronic interactions between sulfur and metal play a major role. Recently Hardegree et al.? have studied the adsorption of H2Son Ni( 100) a t 300 K and have concentrated on the effects of submonolayer
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quantities of sulfur on the adsorption-desorption behavior of C O on Ni(100). They conclude that the major effect is on the nearest neighbors, i.e. quite short ranged but more than geometric site blocking. Earlier studies of H2Sadsorption on metals include Ni,I6 W,17 Ag,I7 Cu,'* and Pb.I9 As part of our continuing investigation of H,S on a variety of transition metal^,^,^ we report in this paper an investigation of the Rh( 100) surface. The experiments involve temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and hydrogen-deuterium isotope exchange.
Experimental Section The experiments were carried out in a turbopumped stainless-steel UHV system which has been described elsewhere.20s21 The base pressure was in the low torr range. This system includes a single-pass cylindrical mirror analyzer and an electron gun for AES and a line-of-sight mass spectrometer interfaced to a computer for TPD. The temperature ramp for TPD was 9.5 K s-l. The Rh(100) single crystal was mounted on a liquid nitrogen cooled and resistively heated sample holder capable of cycling between 100 and 1400 K. A chromel-alumel thermocouple was spot-welded to the back of the crystal. Cleaning before each run was accomplished by heating at 1300 K in 5 X lo-? torr of O2 for 15 min, followed by annealing in vacuo for 5 min at 1400 K. This treatment gave a clean surface as judged by AES. Hydrogen sulfide and deuterium were adsorbed at 100 K by backfilling the chamber to p < 2 X low7torr. Pressure readings were corrected for ion gauge sensitivity (relative to nitrogen) by the factorsZZof 2.2 for H2S and 0.35 for D,.
Jl.
c,
Results Adsorption of H2S. H2S was adsorbed on clean Rh( 100) at 100 K and examined by AES. The results (Figure 1) indicate very rapid initial adsorption followed by a very slow approach to saturation (exposure of at least 50 langmuirs is required). The inset to Figure 1 shows an expanded version of the AES ratio over the range 0-1 langmuir. Over the first 0.8 langmuir of H2S exposure the AES ratio increases linearly. Since this represents a major fraction of the total sulfur accumulated, we conclude that H2S adsorption proceeds through a mobile precursor state. Although the saturation sulfur coverage on Rh( 100) is not known,
(6) Thrush, K. A . Ph.D. Disseration, University of Texas, 1985. (7) Hardegree, E. L.; Ho, Pin; White, J. M. Surf. Sci., submitted for publication. (8) Kelemen, S. R.; Fischer, T. E. Surf. Sci. 1979, 87, 53. (9) Salmeron, M.; Somorjai, G. A.; Wold, A.; Chianelli, R.; Liang, K. S. Chem. Phys. Lett. 1982, 90, 105. (10) Madix, R. J.; Thornburg, M.; Lee, S. B. Surf. Sci. 1983, 133, L447. (1 1) Madix, R. J.; Lee, S. B.; Thornburg, M. J . Vac. Sci. Technol. A 1983, 1 . 1254. (12) Erley, W.; Wagner, H. J. Caral. 1978, 53, 287. (13) Rhcdin, T. N.; Brucker, C. F. Solid Stare Commun. 1977, 23, 275. (14) Kiskinova, M.; Goodman, D. W. Surf. Surf. Sci. 1981, 108, 64. (15) Goodman, D. W.; Kiskinova, M. Surf. Sci. 1981, 105, L265.
(16) Bechtold, E.; Weisberg, L.; Block, J. H. Z . Phys. Chem. (Frankfurt am Main) 1975, 97, 97. (17) Saleh, M.; Kemball, C.; Roberts, M. W. Trans. Faraduy SOC.1961, 57, 1771. (18) Vy, P. S.; Bardolle, J.; Bujor, M. Surf. Sci. 1983, 134, 713. (19) Saleh, G. B.; Wells, B. R.; Roberts, M. R. Trans. Faraday Sot. 1964, 60, 1865. (20) Hegde, R. I.; Tobin, J.; White, J. M . J . Vac. Sci. Techno/.A 1985, 3, 339. (21) Hegde, R. I.; White, J. M. Surf. Sci. 1985, 157, 17. (22) Summers, R. L. NASA Technical Note TND-5285, NASA, Washington, DC, June 1969.
(1) Fisher, G. B. Surf. Sci. 1979, 87, 215. (2) Koestner, R. J.; Salmeron, M.; Kollin, E. B.; Gland, J. L. to be submitted for publication. (3) Baca, A . G.; Schulz, M. A.; Shirley, D. A. J . Chem. Phys. 1984, 81, 6304. ..... (4) Battacharaya, A . K.; Clarke, L. J.; Morales de la Garza, L. J . Chem. SOC.,Faraday Trans. 1 1981, 77, 2223. (5) Brundle, C. R.; Carley, A. F.Faraday Discuss. Chem. Sot. 1975, 60,
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0022-3654/86/2090-0296$01.50/0
0 1986 American Chemical Society
H2S on Rh( 100) I
The Journal of Physical Chemistry, Vol. 90, No. 2, 1986 297 I
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Figure 1. S(152 eV)/Rh(302 eV) Auger peak-to-peak intensity ratio as a function of H2S exposure. ( 0 )Dose and measure AES at 100 K. (A) Dose at 100 K, flash to 600 K, cool to 100 K, and measure AES. (D) 40 langmuirs of D2followed by x langmuir of H2S at 100 K, flash to 600
K, cool to 100 K, and measure AES. Inset shows an expansion of the low-exposure region for HIS exposure and AES measurement at 100 K.
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Figure 4. H2 TPD spectra for various indicated exposures of H2S on
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Figure 2. S(152 eV)/Rh(302 eV) Auger peak-to-peakintensity ratio as a function of H2S exposure at 300 K. ( 0 )Dose and measure AES at 300 K. (A)Dose at 300 K, flash to 600 K, cool to 300 K, and measure AES.
on Ni(100) sulfur coverages saturate at 0.5 m ~ n o l a y e r .Based ~ on AES sensitivities, a similar coverage is calculated for Rh(100) when the S(152)/Rh(302) AES ratio reaches 0.66. With this in mind, we estimate that the initial sticking coefficient of H2S on Rh( 100) is greater than 0.5. Figure 1 also shows that the S/Rh AES ratio increases when the sample is heated in vacuo to 600 K and cooled back to 100 K. This is taken as evidence of stimulated desorption of H2S within the area irradiated by the electron beam. The increase of the S/Rh ratio upon heating is attributed to migration and decomposition of H2S from surrounding areas. Exposure to HIS at 300 K (Figure 2) gives a sulfur uptake curve which is identical, within experimental error, with that measured at 100 K. Heating this surface to 600 K caused no change in the S/Rh ratio. This is consistent with the above electron-stimulated desorption model since no molecular HIS remains after adsorption at 300 K. Thermal desorption after exposure at 300 K gave no desorption products. Following adsorption at 100 K, thermal desorption was a strong function of H2S exposure (Figure 3). At low exposures only hydrogen was detected. Its desorption peak grew rapidly and saturated at about 2 langmuir exposure. Comparing Figure 3 with Figure 2 indicates that H2 desorption saturated in the same way as the S / R h ratio. Above about 0.25 langmuir, molecular H2S
desorption began and grew monotonically with exposure. Hydrogen was the only other desorbing product. Comparing Figures 1 and 3 shows that the S/Rh ratio saturates at much lower exposures than the H2S TPD peak area. This is consistent with electron-stimulated desorption and decomposition (by the AES beam) of molecular H2S. Measuring the exposure required for saturation coverage of H2S was difficult since the mass spectrometer sensitivity often dropped dramatically after heavy exposure to H2S. Our best estimate is that saturation is reached between 5 and 10 langmuirs. The detailed hydrogen thermal desorption spectra for several H2Sexposures at 100 K are shown in Figure 4. At low exposures, the desorption peaked at 290 K just as it does for low exposures of H2 on clean Rh(100). At higher exposures, there was a steady grljwth of a low-temperature peak near 200 K. At intermediate exposures the spectrum is complicated, indicating a mixture of desorption from regions of clean rhodium and regions perturbed by H2S and/or S. This is consistent with the formation of islands of sulfur on Rh(100). Desorption of H2S (Figure 5 ) shows two peaks (130 K and between 220 and 300 K). Both the low- and high-temperature peaks grow steadily with exposure, at least up to 5.0 langmuir. The low-temperature peak increases more slowly than the hightemperature peak. We propose that the high-temperature peak represents chemisorbed molecular H2S adsorbed within sulfided regions of Rh(100) and that the low-temperature peak represents physisorbed H,S boundwithin islands of chemisorbed hydrogen sulfide. H2S on SIRh(100). Large exposures of HIS at 100 K with heating to 600 K produced a sulfided rhodium surface with a S(152)/Rh(302) ratio of 0.9 0.05 (Figure 1). Based on the
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The Journal of Physical Chemistry, Vol. 90, No. 2, 1986
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Figure 8. Isotopic molecular hydrogen sulfide desorption spectra for 40
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Figure 9. (a) D2 TPD after a 40-langmuir D, exposure at 100 K. (b-d)
D2, HD, and H2 TPD after a 40-langmuir D2 and 0.8-langmuir H2S exposure on Rh(100) at 100 K. I
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Figure 7. HIS TPD spectra for 0.8 langmuir of H2S (lower) and 40 langmuirs of D, followed by 0.8 langmuir of H,S (upper) on Rh(100) at 100 K.
estimate given above, this corresponds to a sulfur coverage of 0.73 monolayers. The hydrogen desorption from this sulfided rhodium (after H2S exposure) is shown in Figure 6. The major effect is a strong attenuation of the low-temperature (220 K) hydrogen desorption peak. The intensity around 300 K that appears in the
lower curve is also diminished but not eliminated in the presence of sulfur. Slightly more H2Sdesorbs (not shown) for a given dose on a sulfided surface than on a clean surface, but the peak positions are not altered by preadsorbed sulfur. H2S on DIRh(100). Exposure of clean Rh(100) to 40 langmuirs of D2 at 100 K is more than sufficient to saturate the surface.2’ Subsequent exposure to 0.8 langmuir of H2S (Figure 7) leads to H2STPD spectra which are not distinguishable from those observed in the absence of D(a). D2S and HDS desorption were also followed by monitoring m / e 36 and 35 amu, respectively. Their intensities (Figure 8) are approximately 100 times less than the intensity of H2S.
The Journal of Physical Chemistry, Vol. 90, No. 2, 1986 299
H2S on Rh(100) Deuterium desorption from clean rhodium and from rhodium exposed to both D2 and H2Sis shown in Figure 9. In the absence of H2Sthere are two major desorption peaks, in agreement with previous work.21 In the presence of H2S, deuterium desorption at high temperatures is strongly attenuated and there is an increase in the intensity near 170 K. In this same temperature regime, significant quantities of H D and H2 also desorb.
Discussion H2S on Rh(100). Adsorption of H2S on Rh(100) at 100 K proceeds via a mobile precursor state. The coverage of sulfur grows rapidly up to 1 langmuir H2S exposure but saturation is reached only slowly. On the basis of our AES results, we estimate that at 100 K (with no heating to 600 K) the saturation S coverage on Rh( 100) corresponds to 0.54 f 0.04 monolayers (i.e., S(152)/Rh(302) = 0.66 in Figures 1 and 2). This conclusion is supported by the thermal desorption results as follows. Since D(a) is not displaced by HIS adsorption and since the saturation D(a) coverage is 1 monolayer,26we can use the isotopic hydrogen desorption peak areas (like those of Figure 9) to estimate the amount of H desorbing and, by stoichiometry, the amount of dissociated H2S. This calculation gives a saturation sulfur coverage of 0.56 f 0.1 monolayer. Finally, assuming the van der Waals diameter of S (3.7 A), the close-packed sulfur coverage on Rh(100) would be 0.53 monolayer. These results point to a saturation sulfur coverage near 0.5 monolayer. On Ni(100),3 which is structurally similar to Rh( loo), H2S adsorption results in the formation of 4 2 x 2 ) LEED pattern indicating a saturation coverage of 0.5 monolayer. The highly effective sulfur coverages reached with heating to 600 K (Figure 1) may be due to the migration of sulfur beneath the surface (see below). The thermal desorption spectra of molecular H2Sfrom Rh( 100) exhibits low- and high-temperature peaks (see Figure 5). The low-temperature peak at 130 K is narrow and does not shift with coverage. Moreover, it saturates quickly and, thus, is not due to multilayers of H2S. We assign this peak at 130 K to physisorbed H2S. This is consistent with results for H2S adsorption on Pt2 and NL5 Normally, we expect adsorption into sites with strong bonds to saturate before adsorption into weakly bonding sites. The fact that the low-temperature peak saturates before the hightemperature peak probably indicates that activated rearrangement (dissociation, ordering, movement to new sites) of the adsorbed species occurs during TPD. Assuming a simple first-order Redhead analysis,28the desorption activation energy for physisorbed H2S is estimated as 7.5 kcal/mol, a value close to the molar heat of vaporization (7.7 kcal/mol). The high-temperature H2S peak appears at 300 K for low coverages and shifts to 220 K for high coverages. This type of behavior can be attributed to either second-order desorption kinetics or a bond energy that is coverage-dependent. In view of the fact that coadsorbed D2 and H2S showed negligible isotope exchange into the desorbing H2S, it seems unlikely that the desorption is second order. Instead we attribute the shift of the peak temperature to a first-order desorption process with a coveragedependent desorption energy associated with repulsive lateral interactions between the adsorbed molecules. If a preexponential factor of 1013 s-l is assumed, the desorption energy for the chemisorbed H2S is 18.0 kcal/mol in the limit of low coverage. At saturation coverage, the desorption energy drops to 13.0 kcal/mol. At low H2S exposure the H2 desorption shows a single peak centered at 290 K (Figure 4). With increasing exposure the H2 desorption spectra become quite broad and shift to lower temperature indicating that they probably involve second-order H-H recombination-desorption kinetics. The kinetics are complicated
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(23) Hengrasmee, S.; Watson, P. R.; Frost, D. C.; Mitchell, K. A. R. SurJ Sci. 1979, 87, L249. (24) Greenlief, C. M.; Hegde, R. I.; White, J. M. J . Phys. Chem.,in press. (25) Ho, P.; White, J. M. Surf. Sci. 1984, 137, 117. (26) Peebles, D. E.; Peebles, H. C.; White, J. M. Surf.Sci. 1984, 136, 463. (27) Maurice, V.; Peralta, L.; Bertheir, Y.; Oudar, J. Surf. Sci. 1984, 148, 623. (28) Redhead, P. A. Vacuum 1962, 1 2 , 203.
by the dissociation and rearrangement of H2S. The coverage-dependent thermal desorption peak areas (Figure 3) show a trend toward a decreasing fraction of dissociation of H2S as the coverage increases. Because of the low decomposition temperature for H2S on Rh( loo), thermal desorption always generates a sulfided surface from which any remaining molecular H2S desorbs. It is interesting to note the similarities between H2S adsorption on Pt( 11 1),2 Ru(1 lO),I Ni( and the present work. In all these cases, there is complete dissociative adsorption at low temperatures and low coverages with hydrogen remaining on the surface. At low temperatures and higher coverages on Ru(1 lo),' Pt(111),2 and Ni(100),3 first SH and then H2S were observed. While we have no direct evidence for SH, its formation and decomposition are consistent with our results. H2S on Sulfded Rh(100). After a near-saturation H2S dose, the desorption of molecular H2Sfrom the sulfided Rh( 100) surface shows the same two peaks as from the clean surface. This indicates that neither the high nor the low-energy molecular states are influenced significantly by saturating the Rh( 100) surface with sulfur. Evidently, as noted above, exposing a clean surface to H2S at 100 K generates sulfide during adsorption and/or the early stages of TPD. This is consistent with the isotope exchange results of Figure 9. H2 desorption (Figure 6) following HIS adsorption is strongly diminished on sulfided Rh and the low-temperature region is preferentially attenuated. Clearly, dissociation of H2Sis inhibited by sulfur but even when the S/Rh AES ratio is at its maximum value (0.9), a small amount of dissociation still occurs and leads to relatively tightly bound H atoms on or under the surface. These give the higher temperature H2 peak. We speculate that some sulfur migration beneath the surface is involved. D2 Desorption Behavior. When 40 langmuirs of D2 was adsorbed on a clean Rh( 100) surface followed by 0.8 langmuir of H2S, there was a drastic change in the D2 desorption behavior (see Figure 9). In the absence of H2S, D2 has a low-temperature peak at 150 K (15% of total) and a high-temperature peak at -275 K (85% of total) in agreement with previous work.21 When 0.8 langmuir of H2Swas postdosed on the D-saturated Rh( 100) surface, the D2 desorption increased near 170 K. At the same time, the high-temperature (275 K) peak intensity decreased, but its peak temperature did not change. Moreover, the low-temperature peak in Figure 9b is also retained as a shoulder when H2Sis postdosed. The total amount of deuterium desorbing (D2 0.5 HD) remained unchanged within experimental error, indicating that H2S does not displace adsorbed deuterium under these experimental conditions. Decomposition of Figure 9b into three peaks gives 34%of the desorbing D2 in the high-temperature peak (-275 K), 54% in the new lower-temperature peak (-170 K), and 12% in the lowest-temperature peak (150 K). From this analysis it is clear that the D2 desorbing in the 170 K region is derived from the high-temperature peak. The coadsorption interactions observed here are similar to those for PH3/D2,21NO/D2,2Sand CO/D226coadsorption on Rh( 100) in that postdosing shifts D2 desorption to lower temperature, affecting more strongly the high-temperature D2 state. This is indicative of repulsive interactions between the coadsorbed species. However, unlike H2S, partial displacement of D(a) was observed when PH3, NO, and C O were postdosed. Since the desorption temperature (and the heat of adsorption) of molecular HIS lies below the desorption temperature of D2, it is not particularly surprising that H2S does not displace D2. However, any S formed by decomposition of H2Sduring adsorption (see below) is expected to lower the desorption temperature of D2 and perhaps even partially displace it. That it does not suggests that there are small but important steric and kinetic barriers inhibiting displacement. Isotope Exchange. Since HDS and D2S desorbed with 100 X less intensity than H2Swhen H2S and D2 were coadsorbed (Figure 8) and since a major fraction of the adsorbed H2S does dissociate (Figure 3), we conclude that H2S dissociation is irreversible under the conditions of our experiments. As shown in Figure 9, for saturation D(a) coverage followed by exposure to 0.8 langmuir
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300 The Journal of Physical Chemistry, Vol. 90, No. 2, 1986 of HIS, all the isotopically labeled hydrogen molecules were observed in significant amounts. This scramblintg is the result of H2S decomposition to give H(a), which recombines with D(a). In the thermal desorption spectra, the H D peak appears at 170 K, so H(a) from H2Sis available for reaction with D at very low temperatures. This implies that H2S dissociates either during adsorption or below 170 K. Baca et aL3 have also observed the dissociation of H2S on Ni( 100). Their temperature-dependent EELS shows dissociation of the molecular H2S on Ni(100) at -170 K. H2S Desorption Behavior. As shown in Figure 7, neither the amount of HIS desorbing molecularly nor the H2S desorption energetics are significantly altered by preadsorbed saturation coverages of D(a). However, the total amount of hydrogen desorbing decreases by 25% in the presence of D(a). Thus, preadsorbed deuterium decreases the fraction of H2S that decomposes. This was further supported by AES results which show a 50% smaller S/Rh ratio after flashing the H2S/D2/Rh(100) surface than the H,S/Rh( 100) surface (Figure 7). It is of interest to compare the behavior of H2S, H 2 0 , and PH3 on Rh( 100). Both hydrogen sulfide and phosphine20-21are characterized by significant amounts of irreversible dissociation at low temperatures. The decomposition process is characterized by the lack of significant incorporation of D in the desorption of the parent molecule in coadsorption experiments. There is, however, very little irreversible decomposition of water under the conditions of these experiment^.^',^^ The relatively facile decomposition of H2Sand PH3 on Rh( 100) can be related to much weaker S-H and P-H bond energies compared to that of 0-H bonds. Further, low-temperature, low-pressure hydrogen titration of chemisorbed oxygen occurs readily on Rh( 100) whereas adsorbed phosphorus and sulfur do not react with hydrogen under these conditions. This underscores the care which must be exercised in attempts to correlate the effects of additives with their intrinsic electropositive or electronegative character. Certainly, the chemical activity of these additives themselves must be considered. The reactivity toward hydrogen is only one example. Another is apparent in the differing chemical states of sulfur and oxygen in various compounds. Sulfur compounds of widely varying formal oxidation state are wellknown and range from Se to S6+. Oxygen,
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Hegde and White on the other hand, is almost always 02-.Moreover, sulfur-metal bonds are much more covalent than might be predicted on the basis of electronegativity tables.27 Thus, while varying additives from electronegative to electropositive will show some systematic correlations, careful consideration of the chemistry introduced by the additive itself must also be considered.
Conclusions On the basis of this work, we draw the following conclusions: I . The sticking coefficient for H2Son clean Rh( 100) is relatively large (we estimate > 0.5) and constant up to ca. 0.6langmuir exposure, indicating the involvement of a kinetically important weakly held molecular precursor state in adsorption. 2. The only species observed in desorption are H2S and H,. Residual sulfur is not desorbed. 3. The approach to saturation of atomic sulfur coverages is very slow and requires exposures greater than 50 langmuirs at either 100 or 300 K. 4. Preadsorbed sulfur decreases the extent of decomposition of H2S. 5. With or without preadsorbed sulfur, there are two molecular H2Sdesorption states. The high-temperature state is attributed to chemisorbed H2Sinteracting directly with sulfided rhodium. The low-temperature peak is attributed to weakly held H2Sbonded within islands of chemisorbed HIS. 6. Preadsorbed D(a) inhibits decomposition of H2S but does not change the HIS desorption energetics. 7. In the presence of coadsorbed H2S, D2 desorbs from the Rh(100) surface at lower temperatures than it does from the clean surface. 8. H2S does not displace D(a) from the surface at 100 K. 9. Coadsorption of D(a) and H2S leads to an insignificant amount of D incorporation in molecular hydrogen sulfide desorption, showing that H2S decomposition is irreversible. IO. H from dissociating H2S is available for reaction with coadsorbed D at temperatures below 170 K, showing that dissociation of H2S occurs at or below this temperature. Acknowledgment. This work was supported in part by the U. S. Army Research Office. Registry No. H,S, 7783-06-4; H2, 1333-74-0; Rh, 7440-16-6.
