J. Phys. Chem. 1994, 98, 13022-13027
13022
Adsorption and Reactions of Benzenethiol on the Ni(ll1) Surface T. S. Rufae1,f D. R. Huntley,*?$D. R. Mullins: and J. L. Gland+ Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received: July 13, 1994; In Final Form: September 22, 1994@
The adsorption and reactions of benzenethiol have been analyzed on the Ni(ll1) surface using TPD, XPS, LEED and HREELS. At 110 K, benzenethiol adsorbs on the surface through the sulfur atom. The vibrational data indicate that the S-H bond in benzenethiol breaks upon adsorption at 110 K for all exposures. The primary surface intermediate observed for high coverages of benzenethiol at 190 K is phenyl thiolate. HREELS results indicate that the aromatic ring in phenyl thiolate is neither parallel nor perpendicular to the surface, but tilted. For low coverages of benzenethiol, decomposition of the molecule to gaseous hydrogen, atomic sulfur, and carbon is the favored reaction pathway, while benzene formation is the dominant reaction for higher coverages. Two pathways are observed for the desorption of the benzene product. At intermediate coverages a desorption-limited process occurs near 400 K, while at high coverages benzene desorption is limited by hydrogenolysis of phenyl thiolate at 263 K. Above 300 K, a stable aromatic hydrocarbon intermediate, most likely a phenyl fragment, is observed on the sulfur-covered Ni(ll1) surface. This intermediate stays almost parallel to the surface at 450 K. Annealing benzenethiol overlayers on the Ni( 111) surface to 800 K results in a complex ( 4 3 9 x 439)-S LEED pattern possibly as the result of the reconstruction of the top Ni layer over a broad coverage range.
Introduction Despite the importance of aromatic thiol chemistry in catalytic desulfurization reactions, there have been relatively few studies on the adsorption of aromatic thiols on single-crystal surfaces. Studies of the adsorption and reaction of benzenethiol have been performedonNi(l10),1Ni(100),2Mo(110),3a n d C ~ ( l l O ) .In~ * ~ all systems, the thiol bonds through the S atom and the S-H bond is cleaved at low temperature to form adsorbed phenyl thiolate on the surface. High-resolution electron energy loss spectroscopy on Ni( 110)' indicates that the aromatic ring in the adsorbed phenyl thiolate is tilted away from the surface. Phenyl thiolate adsorbs on Ni(100) with the S in 4-fold sites and the C-S bond nearly perpendicular to the surface.* On Mo(llO), similar dissociative adsorption of the thiol occurs, and the aromatic ring in the phenyl thiolate is tilted 23" away from the surface normal, as determined by near-edge X-ray absorption fine structure analy~is.~ In addition, adsorbed benzyne ( C a ) was reported to exist as a stable surface intermediate on Mo(1 10). HREELS experiments on benzenethiol adsorption on Pt(1 11) and Ag( 111) electrodes indicate that phenyl thiolate is formed and that its ring is normal to the The reactions of benzenethiol have been 'studied on Ni( 110) and Mo( 1 In both cases, benzenethiol produces gas phase hydrogen and benzene. On both of these surfaces, the benzene reaction product desorbs over a fairly wide coverage range. This is in contrast to benzene itself, which is irreversibly adsorbed at all but saturation coverage^.^,^ Apparently, coadsorbed sulfur or thiolate species weaken the chemisorption interaction between the metal and the benzene. The weakened interaction causes both molecular desorption over a broader coverage range and a lower desorption temperature, compared to the case of benzene itself on a clean surface. An interesting difference in the reactivity of benzenethiol on Ni(ll0)' and M0(110)~has been observed. Studies on Mo-
* To whom correspondence should be addressed. University of Michigan. Ridge National Laboratory. Abstract published in Advance ACS Abstracts, November 1, 1994.
0 Oak @
0022-365419412098-13022$04.50/0
(1 10) indicate that the C-S bond in benzenethiol is considerably more stable than the C-S bond in alkanethiols such as methanethiol. This is evident in the desorption profiles of the hydrocarbon products. For example, methanethiol produces methane on Mo( 110) with a peak methane desorption temperature of 310 K,'O but benzene has a desorption peak maximum at 350 K.2 This data suggests that the C-S bond scission kinetics correlate with the gas phase bond dissociation energies (74 kcdmol for methanethiol and 86 kcdmol for benzenethiol)." In contrast, Ni( 110) shows comparable activity for C-S bond scission despite the differences in the C-S bond strengths, as indicated by the benzene and methane desorption peak maxima at 235 and 260 K, respectively. The goals of this study are to compare the reactions of benzenethiol on Ni( 111) with previous studies on Ni( 110). The Ni( 111) surface is the close-packed face of the fcc Ni structure and as such may be less reactive than the open troughs of Ni(1 10). The C-S bond scission reactivity will be compared to both benzenethiol on Ni( 110) and methanethiol on Ni( 111).l2 We examine the coverage and temperature dependence of the reactions of benzenethiol on clean Ni(lll), using TPD,X P S , AES,LEED, and HREELS. The vibrational and photoemission spectra of the thiol for various coverages indicate that the thiol adsorbs dissociatively at 110 K, resulting in the formation of phenyl thiolate on the Ni(ll1) surface. Evidence based on LEED patterns suggests that the surface may undergo reconstruction beginning at S coverages as low as 0.12 ML and temperatures near 300 K.
Experimental Section
This work was performed in a stainless steel ultrahigh-vacuum chamber equipped with a double-pass cylindrical mirror analyzer for X P S and A E S experiments, a HREEL spectrometer, a multiplexed quadrupole mass spectrometer for TPD, and a fourgrid retarding field analyzer for LEED. The nickel crystal was cut to within 0.5" of the (1 11) face and mechanically polished. The crystal was mounted on a precision manipulator, and crystal 0 1994 American Chemical Society
Benzenethiol on the Ni( 111) Surface
J. Phys. Chem., Vol. 98, No. 49, 1994 13023 283
1 .o
C,YSH
uptake on
nI
0.25
Ni(ll1)
A
-
.-0
0.8
1lOK
c
i
0.8
f 5
I
N(111)
0)
z
c
T,
/
0.20
L
B
C,H$H
0.4
%
-
N v)
0.2
c, 0.15
0
s
2 (0
f 0.05
0.00 0
100
4YW
m/e-llO
0.0
Benzenethlol
507
31 1
0.10
200
300
exposure
400
(sec)
Figure 1. Benzenethiol uptake as a function of exposure measured as the ratio of the S(152)/Ni(848) Auger intensities. Auger data were obtained after annealing the overlayers to 800 K. temperatures of 90-1100 K were attained through liquid nitrogen cooling and resistive heating. Sample temperature was measured using a chromel-constantan thermocouple spotwelded to the back of the crystal. The sample was cleaned by neon ion sputtering followed by annealing to 1000 K for 60 s. The common surface contaminants were sulfur and carbon, which are the decomposition products of thiols. Surface cleanliness and crystallographic order were confirmed by AES and LEED, respectively. All experiments were performed by adsorbing the gases on Ni( 111) surface at 110 K. The benzenethiol was stored over activated type 4A molecular sieves in a glass vessel and purified daily by several freeze-pump-thaw cycles using liquid nitrogen. The purity of the thiol was checked using mass spectrometry. Liquid benzenethiol was kept in an ice/water bath, and exposure of the benzenethiol vapors was accomplished by means of a directed doser through a 19 p m laser-drilled aperture. The dosing units refer to the exposure time in seconds. The Ni(1 l l ) crystal was positioned immediately next to the tip of the gas doser during exposure. The temperature-programmed desorption data were obtained with the sample biased at -70 V in order to avoid electron-stimulated decomposition reactions from the unshielded QMS ionizer.' The crystal was heated at a linear heating rate of 5.0 Us. The vibrational data were obtained with a high-resolutionLK2000-DAC EEL spectrometer operated at a resolution below 70 cm-' and beam energies 3-6 eV. All the EELS data were collected in the specular direction. Photoemission spectra were recorded in a separate UHV chamber using a VSW CLASS 100 hemispherical analyzer with emission collected normal to the surface. Excitation (250 eV) was obtained from beamline U13UA at the National Synchrtron Light Source. The instrumental resolution was 0.30 eV as determined by the width of the Fermi edge. The S 2p binding energies were measured with respect to the Fermi edge, and the analyzer was calibrated by comparing the measured photoemission peak positions from clean Cu and Au foils to literature values.13 The surface sulfur coverage was determined by comparing the S(152)/Ni(848) Auger peak intensity ratios with those obtained for methanethiol and H2S adsorption. Figure 1 shows this ratio after annealing the sample to 800 K for various
IO
(X20)
I
1
I
l
l
1
I
200
300
400
500
600
700
800
Temperature
(K)
Figure 2. Temperature-programmed desorption profiles following benzenethiol exposure. The exposure slightly exceeded that required to saturate the first monolayer. The heating rate was 5 IUS. benzenethiol exposures. Since no sulfur-containing products other than physisorbed benzenethiol desorb, the sulfur coverage measured in this way is equivalent to the initial uptake of benzenethiol. The coverage of sulfur at saturation was determined to be about 0.21 ML, somewhat lower than the 0.25 ML observed for methanethiol on the initially clean Ni(ll1) surface. l 2
Results and Discussion Reaction Products of Benzenethiol with Ni(ll1). The only gaseous products formed as a result of the temperatureprogrammed desorption study of benzenethiol on Ni( 111) surface are hydrogen and benzene as shown in Figure 2. The thermal desorption of the condensed C&SH on the Ni(ll1) surface following adsorption at 110 K occurs at 199 K, in good agreement with the 198 K desorption temperature of multilayer benzenethiol from Ni(1 lO).l At saturation coverages of benzenethiol, benzene formation shows two desorption regions, a primary desorption peak at 263 K with a high-temperature shoulder around 290 K and small but complex desorption features in the temperature range 320-400 K. The corresponding hydrogen shows a desorption-limited peak around 310 K and a reaction-limited peak with a maximum rate at 507 K, accompanied by a broad high-temperature tail up to 700 K. Figure 3 shows the desorption profiles for hydrogen and benzene as a function of benzenethiol exposure to the clean Ni( 111) surface at 110 K. At low initial benzenethiol coverages ('0.10 ML) benzene desorbs primarily in a peak around 400 K. With an increase in the thiol coverage, the 400 K peak gets attenuated while a new low-temperature desorption feature appears in the temperature range 250-320 K. This feature appears to contain two separate desorption peaks around 260 and 290 K as can be seen in Figure 3 for coverages above 0.1 ML. The lowest-temperature peak becomes dominant with further increase in thiol coverage, while the 290 K peak saturates early and remains overshadowed by the strong peak at 263 K for high coverages. For exposures above monolayer, the sharp benzene peak at 199 K is due to a cracking fragment of benzenethiol in the multilayer desorption peak. The high temperature benzene desorption peak at 400 K observed for low coverages of benzenethiol is attributed to a desorption-limited process. Comparable low coverages of adsorbed benzene on
13024 J. Phys. Chem., Vol. 98, No. 49, 1994
Rufael et al. C,&SH / N i ( l l 1 )
C,H,SH I Ni(ll1)
I
10, I
'E Y
2.0
-a $
(MLI
I
a
0.71
1.5
8 1000 K
0.19
0.17
c
.g 1 . a
0.1s
v)
0.11
v)
g
0.11
0.5
o.ms 0.075
0.0
C,H, TPD
0 188
186
164 162 S 2p Binding Energy (ev)
1eo
Figure 4. S 2p core level spectra obtained after annealing a multilayer benzenethiol exposure to the indicated temperatures for 60 s. Temperature
(K)
Figure 3. Coverage dependence of the H2 and C& desorption profiles. All exposures were done at 110 K. The heating rate was 5 Us.
the clean Ni(ll1) surface desorb in a similar broad feature around 400 K.14 The low-temperature benzene desorption at 263 K is a reaction-limited peak as the presence of sulfur on the Ni( 111) surface prevents the benzene formed from remaining chemisorbed. On the basis of X P S and HREELS data (vide infra), we propose then that the C-S bond in phenyl thiolate is broken below 300 K, resulting in benzene formation which remains chemisorbed on clean regions of the surface at low coverages but which desorbs in a reaction-limited peak as the sulfur coverage increases. For the lowest thiol exposure shown in the top panel of Figure 3, hydrogen desorbs mainly at 450 K with visible overlapping peaks at 400 and 478 K. As the benzenethiol exposure increases, the intensity of the 450 K hydrogen peak diminishes and by saturation coverages it totally disappears. The peak at 478 K shifts toward higher temperature with increasing coverage and becomes dominant around 507 K. The peak observed at 400 K must be a desorption-limitedpeak since hydrogen desorbs from the clean Ni( 111) surface around 400 K.15 As the thiol exposure increases (-0.10 ML), the 400 K shoulder becomes broad and shifts toward low temperature and appears to diminish in intensity. At still higher coverage, a new desorption peak appears at 318 K. As the thiol exposure is further increased, this desorption feature exhibits slight shifts to lower temperature. The peak is also assigned to a desorption-limited peak but from a sulfur-modified surface. Similar shifts in the hydrogen desorption which occur for thiols on Ni(ll0)' and Ni(100)16 have been attributed to the destabilization of the adsorbed hydrogen by coadsorbed sulfur. However, the large shift in temperature and the change in HZpeak shapes above about 0.1 1 ML of S may also be correlated to changes in the surface structure as indicated by the weak ( 4 3 9 x 439) LEED pattern observed. Support for this includes the fact that the hydrogen desorption peak is not shifted to as low a temperature in the course of methanethiol reactions on Ni( 111).l2 Sharp desorption features have been previously observed at structural transitions. 14,17,18 The high-temperature H2 desorption tail above 520 K is attributed to the stepwise dehydrogenation of the aromatic ring
upon heating. Steinriick et al. made a detailed TPD study of benzene adsorption and decomposition on the Ni( 111) surface.14 They observed a hydrogen desorption peak at 444 K for low benzene coverages and at 454 K for high coverages, which was accompanied by a broad shoulder in the 500-650 K range due to stepwise dehydrogenation of the benzene molecule. The fact that hydrogen desorption occurs above 400 K in the C6Hfli(1 11) system suggests that the source of the Hz desorption near 300 K in the present study is likely to be the sulfhydryl hydrogen although some C-H scission could be occumng concomitant with C-S bond scission, while the high-temperature peak is the primary result of C-H bond scission in the ring. Deuterium preadsorbed TPD experiments with benzenethiol on the Pt( 111) surface clearly show that the origin of the low-temperature hydrogen desorption peak is the S-H bond while the broad high-temperature feature arises from the ring hydrogen.lg The 450 K H2 peak observed for submonolayer thiol coverages is also assigned to C-H bond cleavage in the ring. This conclusion is supported by the fact that for low coverages, where decomposition reactions dominate, the 450 K HZpeak is very intense. For saturation benzenethiol coverage, this reaction pathway becomes inactive. The competing 450 and 500 K H2 formation channels could be due to two different surface intermediates or to a change in the orientation of the intermediate with increasing benzenethiol coverage. Identification of Surface Intermediates. The sulfur 2p core level photoemission data provide information as to the number and nature of sulfur-containing species on the surface. Figure 4 shows a series of W S spectra obtained for a thin multilayer of benzenethiol adsorbed at 100 K and annealed to the indicated temperatures for 60 s. For simplicity, the binding energy of the S 2~312component of the S 2p doublet will be used to refer to the various peak positions. The spectrum shown in Figure 4 following adsorption at 100 K shows both the physisorbed thiol with a binding energy at 164.00 eV and a second molecular state with a binding energy of 162.4 eV. This state is assigned to the phenyl thiolate intermediate on the basis of the vibrational spectra (vide infra) and by analogy to other surfaces. Annealing the surface to 190 K results in desorption of the multilayer benzenethiol, and the spectrum is dominated by the phenyl thiolate intermediate. No evidence of atomic sulfur, with binding energy of < 162 eV, is
J. Phys. Chem., Vol. 98, No. 49, 1994 13025
Benzenethiol on the Ni( 111) Surface
TABLE 1: Vibrational Mode Assignments during Thermal Processing of Multilayer Benzenethiol on Ni(ll1Y
C,H,SH / Ni(ll1)
CsHsSH CsHsS
CsH5
CsHs
CsHsS1
C6Hd
assignmentb (110 K) (190 K) (300 K) (450 K) Ni(1 l0Y Ni(l1 l)d v(Ni-SC&) v(Ni-S) v(Ni-C a s ) W-H) B(C-H) + v(C-C) W-H) v(C-C) v(C-C) v(S-H) v(C-H)
150
n
‘0 r
100
v)
C
I
0
0
465
465
449 413 517 745 1004
403 782
3 10 743 956
320 7301820
738 930
731 1011
1166 1467 1572 2561 3063
1173 1476
1173
1122 1328
1185 1496
1130 1430
3063
3026
3011
3070
3000
All frequencies are in cm-’. Y = stretching mode; y = “out-ofplane” and /3 = “in-plane” bending modes. At 200 K, ref 1. At 280 K. ref 22.
50
0 1000
Energy
2000
Loss
,3000
4
(cm’ )
Figure 5. HREEL spectra as a function of exposure of benzenethiol. All spectra were measured at the adsorption temperature of about 110 K. The peak at about 1750 cm-’ observed in the lowest coverage spectra is amibuted to small amounts of adsorbed CO. apparant for temperatures less than 190 K, in contrast to previous studies on Ni(llO).’ This suggests that the close packed Ni(1 l l ) surface is less reactive toward C-S bond scission than the structurally open Ni(ll0) face. Substantial C-S bond scission occurs between 190 and 250 K as shown by the growth of the atomic sulfur peak at 161.20 eV and the attenuation of the peak ascribed to phenyl thiolate. By 300 K, C-S bond scission is essentially complete. At 500 K, the atomic S peak is shifted to higher binding energy due to the presence of surface carbon from the decomposition of the aryl fragments. Annealing to higher temperatures results in the absorption of carbon into the bulk and the shift in the S 2p position back to lower binding energy. The shift in the S 2p due to coadsorbed carbon has also been observed during the decomposition of methyl thiolate on Ni( 111). The identity and structure of the adsorbed species that form following benzenethiol adsorption on the Ni( 111) surface have been investigated as a function of coverage and temperature using high-resolution electron energy loss spectroscopy. Figure 5 shows vibrational spectra for a range of initial benzenethiol coverages at 110 K. The spectrum of the physisorbed, benzenethiol, curve a, exhibits peaks at 465, 738, 930, 1166, 1467, 1562, 2561, and 3063 cm-’. The loss peak at 2561 cm-’ is due to the S-H stretch mode of the intact molecular thiol, and the 3063 cm-’ peak is assigned to C-H stretch. The 738,930, and 1166 cm-’ peaks are associated with the C-H modes, and the 1467 and 1562 cm-’ modes are associated with C-C stretch modes of the ring. The very intense mode observed in the loss spectra at 738 cm-’ corresponds to the out-of-plane C-H bending mode which is a strong indicator of the presence of an aromatic ring. This mode has a dipole moment perpendicular to the aromatic ring. The mode at 930 cm-’ is assigned to an in-plane C-H bending mode which may have some contributions from the C-C ring breathing mode. Table 1 lists the assignment of the observed vibrational losses. The intensities of the vibrational modes for submonolayer exposures of the thiol are generally attenuated relative to the multilayer spectra. However, the S-H stretching mode near 2600 cm-l is absent for submonolayer spectra, indicating that the S-H bond is
broken upon adsorption at 110 K. Similar dissociative adsorption for benzenethiol has also been reported on Ni( 1lo), Ni(loo), and Mo(ll0) near 100 K.1-3 The C-S stretching frequency, which should be a useful indicator of the integrity of the carbon-sulfur bond, is obscured by the strong ring mode at 738 cm-l. Alternatively, a C-S bond scission would result in a Ni-S stretching mode near 400 cm-1,12,20*21 which is not observed, suggesting no C-S bond scission occurs at low temperature even at low coverage. The loss peak observed near 470 cm-’ for the subsaturation coverages shown in Figure 5 is instead assigned to the Ni-SC& stretching mode since it disappears on annealing the surface to 300 K or higher and is replaced with a new mode at 417 cm-’. An interesting difference between the high coverage spectra (Figure 5b) and the low coverage spectra (Figure 5e) is the shift and splitting in the C-H stretching region. At low coverage, the C-H stretching region consists of two peaks at 2915 and 301 1 cm-’ while at high coverage a single symmetric peak is observed at 3060 cm-’. This shift may be related to partial rehybridization of the aromatic ring in phenyl thiolatesZ2 at low coverage, indicative of a strong interaction of the aromatic ring with the surface. The C-H stretching frequencies of adsorbed cyclohexane on Ni(ll1) appear at 2900 cm-1.23 Electron energy loss spectra were also recorded at various annealing temperatures for a saturated coverage of benzenethiol on the Ni( 111) surface as shown in Figure 6. The vibrational spectra of the intermediates formed by thermal decomposition of the adsorbed benzenethiol were obtained by annealing to the specified temperatures for 60 s and then quenching the sample below 150 K before data were collected. The mode assignments are summarized in Table 1 for the various species observed as a function of annealing temperature. The vibrational spectrum for the multilayer benzenethiol is reproduced in the bottom of Figure 6. Annealing the multilayer covered surface to 190 K for 60 s removes the physisorbed benzenethiol. The vibrational spectrum recorded at 190 K is similar to the multilayer except that the S-H stretching mode is absent. This observation confirms the presence of adsorbed phenyl thiolate (C6HsS) on the Ni( 111) surface. The single S 2p state observed in the S X P S suggests that phenyl thiolate is the only intermediate present at this temperature. Since the vibrational data were collected in the specular direction, applying the “surface selection rule” to the observed losses can provide information on the general orientation of the ring relative to the surface. According to the selection rule, only modes that have a component of their dynamic dipole moment normal to the surface are active.24If the aromatic ring in the phenyl thiolate at 190 K were parallel to the Ni surface, the strong out-of plane y(C-H) bending mode
13026 J. Phys. Chem., Vol. 98, No. 49, 1994
Rufael et al.
C,YSH I Ni(ll1)
I
1000
Energy
S
I
2000
Loss
I
,3000 (cm' )
40
Figure 6. HREEL spectra obtained after annealing a multilayer benzenethiol exposure to the indicated temperatures for 60 s. The spectra were measured near 110 K.
near 730 cm-' would be essentially the only mode observed, since its dipole moment would be perpendicular to the surface, and studies of benzene confirm that the ring modes are dipole s ~ a t t e r e d . ~The ~ , ~fact ~ . ~that ~ the C-H stretching and ring modes which have parallel dipole moments with respect to the ring are also observed suggests that, unlike benzene itself, the aromatic ring in phenyl thiolate is not parallel to the surf a ~ e . ~ If~ the , ~phenyl ~ s ~ thiolate ~ were adsorbed in an upright configuration, the modes at 3063, 1467 and 1011 cm-' should give rise to strong dipole losses and the y(C-H) bending mode should be weak. The fact that the out-of-plane y(C-H) bending mode is very intense suggests that the ring is not in a perpendicular configuration. We propose that the phenyl ring in the adsorbed phenyl thiolate is tilted relative to the Ni( 111) surface. This geometry has been proposed for phenyl thiolate on Ni(llO), also based on HREELS data and on Ni(100) and Mo( 110) based on NEXAFS studies.23 On the Pt(111) and Ag (111) electrodes a perpendicular orientation for the phenyl thiolate is o b ~ e r v e d . ~ . ~ Heating the surface to 300 K causes the loss of surface species due to the formation of benzene, which desorbs at 263 K. A clear indication of C-S bond activation is observed at 300 K by the intense Ni-S stretch mode at 413 cm-' due to the formation of atomic sulfur, consistent with the XPS data which indicate essentially complete C-S bond scission by 300 K. A number of changes occur regarding the ring vibrations compared to the 190 K spectrum. First, the C-H stretch loss is red-shifted by about 37 cm-I. Second, the ratio of the y(C-H) to the v(C-H) intensities is reduced, indicating a tilt away from the surface. The identity of the species at this temperature is ambiguous. However, the adsorbed species is aromatic with its ring tilted further from the surface compared to the phenyl thiolate. This configuration can be either a phenyl (C&) or benzyne (c6H4) group adsorbed along with atomic sulfur. Benzene itself is an unlikely intermediate at such high sulfur coverage and temperature in hydrogen-poor conditions. The hydrocarbon content of the surface is maintained upon annealing to 450 K, since no desorption occurs in the 300450 K temperature range. Therefore, the surface intermediates in the 300 to 500 K range seems likely to be unchanged.
However, the HREELS data suggest that changes in the adsorbate orientation do occur. The evidence lies in the comparison of the 300 and 450 K spectra. Apart from the Ni-S stretching mode at 403 cm-', the spectrum, after annealing to 450 K, resembles that of n-bonded benzene on Ni( 111),22925-27 which exhibits strong and overlapping modes at 730/820 cm-', a v(Ni-C) mode at 320 cm-l and very weak modes at 1130, 1430, and 3000 cm-1.22 The spectrum obtained by annealing saturated benzenethiol to 450 K is strikingly similar. The y(C-H) loss is shifted to 782 cm-', and most importantly, the relative intensities of the C-H out-of-plane bending mode and the C-H stretching modes suggest a reorientation so that the aromatic ring is more parallel to the surface. This parallel geometry may be required for the further C-H bond scission which occurs above 500 K, as shown in the H2 desorption traces. The reorientation from the near perpendicular to parallel adsorption geometry is not accompanied by bond scission or bond formation, suggesting that structural changes in the substrate, such as the onset on the 4 3 9 x 2/39 structure, may drive the transition. Structure of the Annealed Surface. A complex (439 x 439)-S LEED pattem was observed following annealing of high coverages of C&IsSH on Ni( 111) to 800 K. This LEED pattem is thought to be due to a reconstructed Ni surface similar to the ( 5 4 3 x 2)-S structure which is best described by a distorted Ni(100)-c(2 x 2)-S top Ni l a ~ e r . ~ ~ -The ~ O ( 4 3 9 x 439) structure has been previously observed in the S/Ni(111) system; however, there appears to be some controversy regarding the absolute S coverage for this structure. Perdereau and OudarZ8 reported the absolute sulfur coverage of the ( 4 3 9 x 439)-S structure obtained from annealing H S on the Ni( 111) surface as 0.23 ML using radioactive S assay. Recently, Ku and Overbury30reported that the ( 4 3 9 x 439)-S structure, prepared by annealing H2S on the Ni( 111) surface, occurs at a S coverage of 0.32 ML and is reconstructed. In the present study the S coverage of the observed (439 x 439) structure was found to be variable over a wide range between 0.11 and 0.21 ML of S. However, direct comparison to the coverages previously reported for the ( 4 3 9 x 439) structure is difficult since carbon was also present from the decomposition of the benzenethiol. Above about 600-800 K, the surface carbon apparently absorbs into the subsurface. This may induce surface reconstruction at lower sulfur coverages than required if only sulfur were present. Both the Ni-S stretch at 400 cm-' and the S 2~312binding energy of 161.10 eV suggest that after annealing to 800 or 1000 K the sulfur is adsorbed in 3-fold hollow sites on the Ni( 111) surface?0 However, the complex (439 x 439) LEED pattem is observed, indicating some reconstruction. For the fully reconstructed ( 5 4 3 x 2) surface, produced by adsorbing H2S, the Ni-S stretch shifts to 325 cm-' and the S 2p core level peaks are at higher binding energy,20indicating that the (4?39 x 439) structure produced from benzenethiol does not have the same pseudo-c(2 x 2)-S/Ni(lOO) "4-fold" S bonding sites as the higher S coverage surface. Comparison to Benzenethiol Reactions on Other Surfaces. Phenyl thiolate is the intermediate formed following adsorption of benzenethiol on Ni( 11l), Ni( 100) and Ni( 1 In all three cases, at high coverages, the thiolate is stable below about 230 K, where C-S bond scission commences. The details of the reactions of benzenethiol have been more completely studied on Ni( 110) and are qualitatively similar to those reported here for Ni(ll1). However, there are differences which can be understood in terms of the structural differences between the two surfaces. The Ni(ll0) surface is clearly more reactive
Benzenethiol on the Ni( 111) Surface toward C-S bond scission than the Ni( 111) surface. X P S data from Ni(ll0) show evidence of some C-S bond scission occurring near 100 K, although at high coverage, the C-S bond is stabilized so that phenyl thiolate is present up to the benzene production temperatures near 230 K. Hydrogenolysis occurs more readily on Ni(ll0) with the main benzene desorption occumng near 235 K while on Ni(l11) the desorption temperature of benzene is 265 K. In both cases the phenyl ring in the phenyl thiolate is thought to be tilted relative to the surface normal, and stable aromatic moieties are proposed above 300 K. Phenyl thiolate was proposed to undergo a hydrogen-assisted desulfurization mechanism on Ni( 1lo), and independent kinetics data to be reported elsewhere on phenyl thiolate decomposition on Ni( 111)31support a similar conclusion. This is particularly interesting since surface hydrogen on Ni( 111) is inactive toward hydrogenating either methyl groups or ethylene,32while the surface hydrogen on Ni(ll0) may be more active [18]. The activation of surface hydrogen on Ni( 111) toward thiolate hydrogenolysis may be due to a weakened Ni-H bond due to coadsorbed S, which is also reflected by the shift of the Hz desorption to lower temperature on sulfided surfaces. Hydrogen may also be more active toward hydrogenation of thiolates than alkyl species since the steric hindrance present for the alkyl groups is absent for the thiolates. Furthermore, neither Ni( 110) nor Ni( 111) exhibits a correlation between C-S bond energy and C-S bond cleavage kinetics since methane is produced from methanethiol (Dc-s =74 kcal/ mol)" at a somewhat higher temperature than benzene from benzenethiol (Dc-s = 86 kcal/mo1)l1 on both surfaces. This is in contrast to Mo(llO), where benzene is formed from benzenethiol some 40 K higher than methane from methanethiol. This lack of correlation for the nickel surfaces may be a reflection of the hydrogen-assisted desulfurization mechanism proposed for the aromatic thiols which is probably not operative for alkanethiols. The difference between Mo and Ni may be related to the higher hydrogenation activity of Ni.
Conclusion The adsorption and reaction of benzenethiol on the clean Ni(1 l l ) surface have been studied using TPD, AES, and HREELS. The vibrational data indicate that the S-H bond in benzenethiol breaks upon adsorption at 110 K for all exposures. Total decomposition is the preferred reaction path way for low coverages of benzenethiol, while benzene formation is the dominant reaction for higher coverages. The primary surface intermediate for the formation of benzene is phenyl thiolate, which undergoes C-S bond scission between 200 and 300 K. Benzene is observed in a desorption-limited peak near 400 K until the S coverage reaches 0.14 ML when a second, reactionlimited, benzene desorption occurs with a peak at 265 K. The hydrogen desorption features above 450 K are due to the stepwise decomposition of aromatic moieties. HREELS results indicate that the aromatic ring in phenyl thiolate is tilted. Stable aromatic hydrocarbon species with differing orientation are observed at 300 and 450 K. Above 300 K, a stable aromatic hydrocarbon intermediate, most likely phenyl, is observed on the sulfur-covered Ni(ll1) surface. Comparison of the HREELS spectrum of this intermediate after
J. Phys. Chem., Vol. 98, No. 49, 1994 13027 annealing to 450 K with that of benzene on clean Ni(ll1) suggests that the intermediate bonds with its ring parallel or near parallel to the Ni surface at 450 K. Complete decomposition of benzenethiol on the Ni( 111) at 800 K results in a complex (439 x 439)-S LEED pattern possibly as a result of the reconstruction of the top Ni layer over a broad coverage range.
Acknowledgment. Research sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. (DRH,DRM), and Contract DE-FG02-91ER14190 at the University of Michigan (JLG,TSR). The National Synchrotron Light Source at Brookhaven National Laboratory is supported by the Division of Chemical Sciences and Division of Material Sciences of the U.S. Department of Energy under Contract DE-AC02-76CH00016. References and Notes (1) Huntley, D. R. J. Phys. Chem. 1992, 96, 4550. (2) Takata, Y.; Yokoyama, T.; Yagi, S.; Happo, N.; Sato, H.; Seki, K.;Ohta, T.; Kitajima, Y.; Kuroda, H. Surf:Sci. 1991, 259, 266. (3) Roberts, J. T.; Friend, C. M. J. Chem. Phys. 1988,88,7172. Stohr, J.; Outka, D. A. Phys. Rev. B 1987, 36, 7891. (4) Agron, P. A.; Carlson, T. A.; Dress, W. B.; Nyberg, G. L. J. Electron Spectrosc. Relat. Phenom. 1987, 42, 21. (5) Shen, W.; Nyberg, G. L.; Liesegang, J. Surf: Sci. 1993, 298, 143. (6) Kahn, B. E.; Chaffins, S. A.; Gui, J. Y.; Lu, F.; Stem, D. A.; Hubbard, A. T. Chem. Phys. 1990, 141, 21. (7) Gui, J. Y.; Stern, D. A.; Frank, D. G.; Lu, F.; Zapien, D. C.; Hubbard, A. T. Langmuir, 1991, 7, 955. (8) Huntley, D. R.; Jordan, S. L.; Grimm, F. A. J. Phys. Chem. 1992, 96, 1409. (9) Liu, A. C.; Friend, C. M. J. Chem. Phys. 1988, 89, 4396. (10) Uvdal, P. C.; Wiegand, B. C.; Friend, C. M. Surf:Sci. 1992,279, 105. (11) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 493. (12) Rufael, T. S.; Mullins, D. R.; Gland, J. L.; Huntley, D. R. J. Phys. Chem., submitted. (13) M. P. Seah, Surf: Interface Anal. 1989, 14, 488. (14) Steinriick, H.-P.; Hiiber, W.; Pache, T.; Menzel, D. Surf:Sci. 1989, 210, 293. (15) Winkler, A.; Rendulic, K.D. Surf:Sci. 1982, 118, 19.22.M. (16) Castro, M. E.; White, J. M. Surf: Sci. 1992, 257, 22. (17) Christmann, K. Surf: Sci. Rep. 1988, 9, 1. (18) Glines, A. M.; Anton, A. B. Surf Sci. 1993, 286, 122. (19) Rufael, T. S.; Gland, J. L., unpublished results. (20) Mullins, D. R.; Huntley, D. R.; Overbury, S. H. Surf: Sci., in press. (21) Chesters, M. A.; Lennon, D.; Ackermann, L.; Hdberlen, 0.;Kriiger, S.; Rosch, N. Surf: Sci. 1993, 291, 177. (22) Lehwald, S.; Ibach, H.; Demuth, J. E. Surf: Sci. 1978, 78, 577. (23) Demuth, J. E.; Ibach, H.; Lehwald, S. Phys. Rev. Lett. 1978, 40, 1044. (24) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic Press: New York, 1982. (25) Bertolini, J. C.; Rousseau, J. Surf:Sci. 1979, 89, 467. (26) Bertolini, J. C.; Dalmai-Imelik, G.; Rousseau, J. Surf:Sci. 1977, 67, 478. (27) Dalmai-Imelik, G.; Bertolini, J. C. J. Vac. Sci. Technol. 1971, 9, 677. (28) Perdereau, M.; Oudar, J. Surf: Sci. 1970, 20, 80. (29) Edmonds, T.; Macarroll, J. J.; Pitkethly, R. C. J. Vac. Sci. Technol. 1971, 8, 68. (30) Ku,Y.; Overbury, S. H. Surf Sci. 1992, 276, 262. (31) Huntley, D. R.; Mullins, D. R.; Lyman, P. F. to be submitted. (32) Daley, S. P.; Utz,A. C.; Trautman, T. ; Ceyer, S. T. J. Am. Chem. SOC. 1994, 116, 6001.
