118
Langmuir 1988,4,118-120
Adsorbed Sulfhydryl (SH) on the Mo(100) Surface John L. Gland and Edward B. Kollin Corporate Research Laboratory, Exxon Research and Engineering Co., Annandale, New Jersey 08801
Francisco Zaera* National Synchrotron Light Source, Brookhauen National Laboratory, Upton, New York 11973 Received May 18, 1987. In Final Form: July 20, 1987 Thermal decomposition of H a on the Mo(100) surface results in the formation of an adsorbed sulfhydryl
(SH)species that is stable up to about 240 K. Adsorbed sdfkydryl was characterizedby using high-resolution
electron energy loss spectroscopy (HREELS) and temperature programmed desorption ("I'D). Molecular HzSdesorbs from the surface below 180 K. Decomposition of adsorbed SH yields adsorbed atomic sulfur above 300 K. As part of a program to develop a fundamental understanding of the surface chemistry of H2S on clean and sulfur-modified metals, we have undertaken a study of H2S adsorption and decomposition on the Mo(100) surface. Despite the importance of H2S chemistry in catalytic processes, studies of stable surface species and reaction mechanisms have only recently become available.l+ Koestner et al.l observed SH formation on both the clean and sulfur-modified Pt(ll1) surface using high-resolution electron energy loss spectroscopy (HREELS) in combination with several other surface spectroscopies. Baca, Schulz, and Shirley2 have performed a vibrational study of H2S decomposition on the Ni(100) surface and propose that a sulfhydryl species may form below 170 K at intermediate coverages. Fisher3 proposed SH formation at intermediate H2Scoverages on the Ru(001) surface on the basis of ultraviolet photoemission spectroscopy (UPS).We report here vibrational results which indicate that adsorbed SH is formed well below 190 K and remains stable above 240 K on an H2S-saturated Mo(100) surface. These experiments were performed in an UHV system equipped with a high-resolution electron energy loss spectrometer (HREELS), a multiplexed quadrupole mass spectrometer for temperature programmed desorption (TPD), low-energy electron diffraction (LEED) optics, and a hemispherical electron energy analyzer for Auger electron spectroscopy (AES)and X-ray photoemission spectroscopy (XPS).The temperature of the Mo(100) sample could be controlled between 100 and 2200 K by resistively heating two 0.5-mm T a wires spot-welded across the back of the crystal. The sample temperature was monitored with a 0.1-mm W/5% ReW/25% Re thermocouple spot-welded to the edge of the crystal. The sample was cleaned by using alternate sputter-1300 K oxygen treatment-2000 K annealed cycles. Surface cleanliness and order were confirmed by using LEED and electron excited AES before each adsorption experiment. The vibrational spectra shown in Figures 1 and 2 were obtained after the sample was heated to the specified temperature and then allowed to cool to 100 K. Thermal desorption spectra were obtained by using a 10 K/s heating rate. The HREELS and TPD spectra shown in Figure 1 were obtained following H2S saturation of the clean Mo(100) surface at 95 K. As indicated in the right panel of Figure 1, H2S and H2 desorb from the surface following saturation with HzS. Molecular HzS desorption occurs at 100 K with
* Present address: Department of Chemistry, University of California, Riverside, CA 92521.
a broad shoulder extending up to about 180 K. The substantial hydrogen desorption peaks observed following H2S exposure indicate dissociation of adsorbed HzS. The hydrogen desorption spectrum is complex and unusual, with several peaks occurring below the temperature for H2 desorption from the clean surface (285, 325, and 400 K) or sulfur-modified surfaces (275-295,380 K).' We have shown that sulfur atoms block hydrogen chemisorption sites, but their electronic interaction with the surface is not significant and no new hydrogen sites are created.' Sulfur island formation can be ruled out since chemisorbed atomic sulfur on Mo(100) forms several LEED patterns as a function of ~0verage.l~ The low-temperature Hz peaks therefore suggest that hydrogen from H2Sdecomposition is recombining without becoming chemisorbed on the Mo surface. The vibrational transitions observed as a saturated H2S overlayer was heated to a series of annealing temperatures are illustrated in the left-hand panel of Figure 1. These transitions along with their deuteriated counterparts from Figure 2 are summarized in Table I. The vibrational assignments are consistent with the observed isotope shifts observed on deuteriation. Reference spectra for HzSsolid: H a gas: atomic sulfur adsorbed on the Mo(100) surface,'O and SH adsorbed on the Pt(lll)-(2X2)S surface1are also included in Table I. The vibrational spectrum observed following H2S adsorption at 100 K indicates that molecular H2Sis present, as evidenced by the stretching modes at 2260 and 2510 cm-' and the scissors mode at 1170 cm-'. This latter peak is greatly attenuated above 165 K, indicating that only small amounts of molecular H2S remain on the surface after heating to that temperature. Bending modes at 545 and 635 cm-' are also seen, but a definite assignment cannot be made for these peaks (see below). Our spec(1)Koestner, R. J.; Salmeron, M.; Kollin, E. B.; Gland, J. L. Chem. Phys. Lett. 1986,126, 134. (2)Baca, A. G.; Schulz, M. A,; Shirley, P.A. J.Chem. Phys. 1984,81, 8304. __ .. .
(3) Fisher, G. B. Surf. Sci. 1979,87,215. (4)Hedge, R. I.; White, J. M. J. Phys. Chem. 1986,90,296. (5) Battacharava, A. K.: Clarke, L. J.: Morales de la Garza, L.J. Chem. Soc., Faraday Trhns. 1981,77,2223. (6) Bonzel, H. P.; Ku, R. J. Chem. Phys. 1973,58,4617. (7)Zaera, F.;Kollin, E. B.; Gland, J. L.Surf. Sci. 1986, 166, L149. (8)Anderson, A.; Binbrek, 0. S.; Tang, H. C. J. Raman Spectrosc.
1977,6, 213. (9) Vibrational Spectra of Polyatomic Molecules; Sverdlov, L. M.; Kovner, M. A.; Krainov, E. P., Eds.; Wiley: New York, 1974;p 536. (IO) Zaera, F.;Gellman, A.; Gland, J. L., unpublished results.
0743-7463f 88f 2404-0118$01.50/0 0 1988 American Chemical Society
Langmuir, Vol. 4,No.I, 1988 119
Adsorbed Sulfhydryl on Mo(100)
H2S Over Mo(l00)
300K
H2S x 20
I
0
1000 2000 3000 Frequencylcm- 1
I
I
300 500 TemperaturelK
100
Figure 1. HREEL spectra for saturation H2Son Mo(100) after annealing to the indicated temperatures. The right-hand side shows the thermal programmed desorption spectra obtained after H2S saturation at 80 K. Table I. Vibrational Transitions for H2S (DzS)Adsorbed on the Mo(lO0) Surface (cm-') assignment Mo-S stretch S-H (S-D) bend S-H (S-D) scissors S-H (S-D) stretch temp, K 100 165 190 210 300 H2S(s)*
HzSW
SH/(2X2)S-Pt(lll)' S/Mo(lOO)10
290 300 300 310 310 310 310 315 320 320
H2S D2S H2S D2S H2S D2S H2S D2S H2S D2S H2S H2S SH
530,600 -, 430 545, 635 -, 450 615 450 610 450 580 -
-
-
Mo-S stretch 280 290
585 coverage 0.5 0.67
330 340
0.75 1.00
troscopic results are in agreement with the H2S desorption spectrum in Figure 1, where most of the H2S is shown to desorb molecularly below 180 K. The primary species on the surface above 165 K are SH and atomic sulfur. The adsorbed SH has an intense bending mode at 610 cm-' and a weaker S-H stretching mode near 2500 cm-l. The vibrational spectrum for SH adsorbed on the Mo(100) surface is very similar to the spectrum observed for SH adsorbed on the Pt(ll1) and (2X2)S-Pt(lll) surfaces.' The M d - H bending mode at 610 cm-l has a deuterium shift of 1.35, similar to the PtS-H bending modes at 585 and 605 cm-' observed on the
1170 860
-
-
-
1180 -
2260, 2505 1670, 1850 2260, 2510 1660, 1830 2510 1800 2500 1830
-
2550, 2560 2615, 2635 2500
LEED pattern 42x21
S-modified and clean Pt(ll1) surface, respectively.' The S-H stretching mode at 2500 cm-' with a deuterium shift of 1.39 is also reminiscent of the 2500-cm-' transition observed on the Pt surface. This frequency closely corresponds to the frequency of the S-H observed in (PH,)Ni(SH)2 at 2535 cm-l, although no S-H bending mode was reported for the organometallic compound.'l The large (11) Schmidt, M.; Hoffmaun, G. G.; Holler, R. Znorg. Chem. Acta 1979, 32, L19. (12) Demuth, J. E.; Ibach, H.; Lehwald, S. Phys. Reu. Lett. 1978,40, 1044.
120 Langmuir, Vol. 4, No. 1, 1988
6,3:
Gland et al. D2S Over Mo(l00)
ooo
r:
300K
x 3333
x 333
21OK
190K
1OOK
0
1000
2000
3000
Frequencylcm - 1
Figure 2. HREEL spectra for saturation D2Son Mo(100)after annealing to the indicated temperatures.
intensity of the Mo-S-H peak a t 610 cm-' suggests that the bending mode is dipole active as proposed previously on the Pt(ll1) surface.' A large intensity for this mode in the context of the dipole selection rule suggesh that the adsorbed S-H is tilted relative to the surface normal since (13) Clarke, L.J. Surf. Sci. 1981, 102,331.
the transition would be dipole forbidden if the SH were adsorbed normal to the surface. An alternate electron impact scattering mechanism would not result in such large scattering intensity. The low-temperature spectra taken at 100 and 165 K contain two Mo-S-H bending modes at 635 and 545 cm-l. These two modes may correspond to two separate bending modes for H$ or may be caused by two forms of molecular H2S or by coexisting SH and H2S. We may expect up to four low-frequency modes for adsorbed SH and up to five for H2S (including rocking and bending modes), and even though some modes may be degenerate when chemisorbing on a high symmetry site, they may split when adsorbed in a lower symmetry configuration. We are unable to distinguish between theae possibilities at present. The low-frequency S-H stretching mode at 2260 cm-I for H2S adsorption has not been previously observed.'+ We suggest that this mode may be caused by H2S adsorption in a low-symmetry configuration with one H closer to the surface than the other. Substantial interaction may be occurring between hydrogen in molecular H2Sand the Mo surface, causing the lower frequency mode to appear. It is unlikely that this mode is associated with adsorbed SH, since SH has a S-H stretching mode at 2510 cm-l. Complete decomposition of chemisorbed H2S occurs below 190 K,and the vibrational spectra obtained after annealing at 190 and 210 K only show features due to the SH moiety, namely, peaks at 310 (Mo-S stretch), 610 (S-H bend), and 2500 cm-' (S-Hstretch). No peaks around 1200 cm-' were detected, indicating that H,S is no longer present on the surface. Finally, annealing above 240 K results in the decomposition of adsorbed SH leaving adsorbed atomic sulfur and hydrogen. The adsorbed atomic sulfur has a Mo-S stretching mode near 320 cm-l as observed previously.'O The vibrational transitions at 580 and 1100 cm-' are caused by Mo-H modes from hydrogen atoms created after H2S and SH decomposition.' This adsorbed hydrogen desorbs near 350 K as indicated by the TPD spectrum. In summary, H2S, SH, S, and H have all been detected from H2Sadsorbed on the Mo(100) surface. The adsorbed SH has a S-H stretching mode at 2500 cm-I and an intense Mo-S-H bending mode at 610 cm-'. The intensity of the SH bending mode suggests that it is tilted with respect to the surface normal. Heating above 300 K results in the formation of a (2x2) overlayer of atomic sulfur with a Mo-S stretching mode at 320 cm-'.
