J. Phys. Chem. 1995,99, 9567-9575
9567
Reaction of S2 with NM/Mo(llO) (NM = Cu or Ag) Surfaces: Poisoning of Bimetallic Bonding and Noble-Metal-Promoted Sulfidation of Mo JosC A. Rodriguez* and Mark Kuhn Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973 Received: February 21, 1995; In Final Form: April 6, 1995@
TDS, XPS, and XAES were used to examine the reaction of S2 gas with NM/Mo(llO) (NM = Cu or Ag). On these surfaces S2 dissociates into atomic S at 300 K. At submonolayer coverages (0s 0 N M < 1 ML), S and Cu or Ag do not react to form noble-metal sulfides on top of Mo( 1lo). Instead, the S and noble-metal adatoms compete for making bonds with the Mo( 110) substrate. On the average, each S adatom diminishes the ability for bimetallic bonding of a minimum of three Mo surface atoms. At 0.4 < 0s < 0.8 ML, the weakening of the Mo-Cu and Mo-Ag bonds is very significant ( > 5 kcaymol), and the noble-metal adatoms form 3D clusters on the Mo( 110) surface. The exposure of NM/Mo( 110) surfaces to large amounts of S2 gas (0s > 1 ML) at 300 K produces noble-metal sulfides (CuS, or AgS,) and chemisorbed sulfur, without forming molybdenum sulfides. The sulfidation of molybdenum occurs after exposing NM/Mo( 110) surfaces to S:! at 600-700 K. Cu and Ag promote (or catalyze) the formation of molybdenum sulfides. By comparing the
+
results for the S2/NM/Mo(llO) systems with those reported for SdNM/Pt(lll) systems, it is found that the “promotional effect” of a noble metal on the rate of sulfidation of a transition metal depends on (a) the ability of the noble metal to dissociate S2, (b) the thermochemical stability of the transition-metal sulfide (which makes the process “downhill”), and (c) the atom-atom attractive interactions within the lattice of the transition metal (which make the process “uphill”).
I. Introduction Bimetallic catalysts that combine noble and transition metals are Frequently, the addition of a noble metal to a transition-metal surface produces drastic changes in the catalytic activity or selectivity of the surface for reactions that involve the conversion of hydrocarbon^.'-^ These changes can be the result of electronic perturbations produced by metal-metal bonding (“ligand e f f e ~ t ” ~ .or~ .a~consequence ) of a reduction in the number of active sites present on the transition-metal surface (“ensemble e f f e ~ t ’ ” , ~ .Usually, ~). the superior catalytic properties of the {noble transition} metal surfaces are very sensitive to sulfur poisoning.6 The lifetime of a catalyst can be drastically reduced in the presence of ppm levels of sulfur in the feedstream.6 This work is part of a research program focused on examining the adsorption of sulfur on well-defined bimetallic ~ u r f a c e s . ~ - ~ Al fundamental understanding of the effects of sulfur on the physical and chemical properties of {noble transition) metal surfaces may provide new ideas for improving the sulfur tolerance of these systems. In a previous study, we investigated the adsorption of S2 on Ag/Pt( 111) surfaces.I0 At 300 K, silver atoms in contact with Pt( 111) react with S2 to form noble-metal sulfides. The Ag-S bonds in these surface compounds break at high temperatures (750-850 K), producing sulfur and noble-metal adatoms that compete for making bonds with the platinum substrate. This competition leads to a weakening in the strength of the Pt-Ag bonds. In some cases, the weakening in the bimetallic bonds is so large (’5 kcavmol) that the Ag adatoms “ball up” on the surface instead of “wetting” the Pt substrate. A sulfur adatom diminishes the ability for bimetallic bonding of several (3-4) adjacent platinum atoms. In addition, the results of photoemission revealed that the presence of silver on the surface promotes the formation of several layers of platinum sulfide when the
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* To whom all correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, May 15, 1995.
AgPt(ll1) systems were exposed to large amounts of S2.I0 Experiments for the coadsorption of Ag and S on Ru(001) also showed a S-induced weakening of bimetallic bonding, but no evidence for a Ag-promoted sulfidation of the Ru substrate was found.8 In the present work, we examine the interaction of S:! with Ag/Mo( 110) or Cu/Mo(110) surfaces using thermal desorption mass spectroscopy (TDS),core and valence level photoemission, and X-ray excited Auger electron spectroscopy (XAES). Our noble metal interactions on main goal is to compare the S Mo( 110) with those found when these adsorbates were codeposited on surfaces of late-transition metals (Ru(OOl)* and Pt( 11l)Io). From a thermochemical viewpoint, the bimetallic surfaces under study combine a metal that forms sulfides with a low (Ag& AHf = -8 kcal/molI2) or moderate (CU~S, AHf = -19 kcaI/moll2) stability and a metal that has a very strong affinity toward sulfur (MoS2, AHf = -56 kcal/mol; Mo2S3, AHf = -93 kcal/molt2).
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11. Experimental Section The experiments were performed in an ultrahigh-vacuum (UHV) system with a base pressure of less than 3 x Torr. The chamber was equipped with a quadrupole mass spectrometer for thermal desorption studies and a hemispherical electron energy analyzer with multichannel detection for photoemission studies. The photon source was unmonochromatized Mg K a radiation. The sample was positioned at a takeoff angle of 30’. The total experimental resolution for the photoemission studies was approximately 0.8 eV. The binding energy scale in the photoemission spectra was calibrated using the Cu 2 p and ~ Mo 3d512 peaks of pure Cu and Mo, which were set at binding energies of 932.5 and 227.9 eV, re~pective1y.I~ The sample manipulator allowed liquid nitrogen cooling to around 80 K and resistive heating to 1550 K. Heating to 2400 K was achieved by electron bombardment from behind the sample. A W-5% Re/W-26% Re thermocouple was spot-
0022-3654195/2099-9567$09.00/0 0 1995 American Chemical Society
Rodriguez and Kuhn
9568 J. Phys. Chem., Vol. 99, No. 23, 1995
welded to the edge of the sample for temperature measurements. The Mo( 110) surface was cleaned following procedures reported in the literature. l 4 The typical cleaning procedure involved successive cycles of oxidation (02 background pressure of 5 x Torr) at high temperature (1000-1300 K), followed by annealing to 2400 K. The cleanliness of the surface was verified using X-ray photoelectron spectroscopy (XPS). The deposition of Ag and Cu was performed by resistively heating W filaments wrapped with high-purity wires of the noble metals. The Ag and Cu coverages were determined by TDS area a n a l ~ s i s . ’S2~ gas was generated inside the UHV chamber by decomposing Ag2S in a solid-state electrochemical cell: Pt/ Ag/AgL/Ag2S/Pt.‘5 After applying the voltage across the cell, sulfur evolved predominantly as SZwith a minor “contamination” of S , clusters.Isb For small doses of gaseous sulfur, the coverage of sulfur on the sample was determined by measuring the area under the S 2p peaks, which was scaled to absolute units by comparing to the corresponding area for the saturation coverage of sulfur on Mo(llO), which is known to be 0.91 m o n ~ l a y e r . ’ ~In. ’this ~ work, coverages are reported with respect to the number of Mo(l10) surface atoms (1.43 x lOI5 atoms/ cm2). One adatom ( S , Cu, or Ag) per substrate surface atom corresponds to 8 = 1 monolayer (ML).
S 2p: SIMo(1I O )
I
168
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I
’
166
I
164
’
I
162
’
I
~
160
Binding Energy (eV)
Mo 3d: S/Mo( II O )
(E3
e,=o.gi
111. Results 111.1. Adsorption of Sulfur on Mo(ll0). The properties of S adlayers on Mo(l10) have been the focus of several I ( I ’ I ’ I / previous works.I6-l9 The two-dimensional (2D) phase diagram 234 232 230 228 226 for the S/Mo(llO) system shows several ordered structure^.^^^'^ Binding Energy (eV) At sulfur coverages of 0.25 and 0.50 ML, ~ ( 2 x 2 and ) ~(2x2) Figure 1. S 2p (A) and Mo 3d (B) spectra for S/Mo(110) surfaces. In LEED patterns are observed. For a ~ ( 2 x 2 overlayer, ) the S part A, the dotted traces correspond to spectra taken after dosing sulfur atoms are adsorbed on hollow sites, off-center, displaying a at 300 K and annealing of the sample to 1200 K in order to improve bonding configuration that is close to a 3-fold coordination.’* the long-range order of the S adlayer.]’ The solid traces correspond TDS experiments indicate that below 8, = 0.5 ML desorption to spectra acquired after exposing Mo( 110) to S2 at 700 K. of sulfur occurs at temperatures above 2100 K, while for coverages larger than 0.5 ML a new desorption peak develops those of Mo( 110) and Ag(l1 l).24 Our XPS data for Ag atoms between 1700 and 2000 K.” in contact with Mo( 110) showed Ag 3dw2 binding energies that were shifted less than 0.1 eV with respect to that of bulk Ag. Figure 1A shows S 2p XPS spectra for several S/Mo( 110) The corresponding TDS spectra displayed Ag desorption surfaces. The solid traces correspond to spectra taken after temperatures that increased from 1020 to 1075 K when the Ag saturating Mo( 110) with sulfur at 700 K. As the sulfur coverage coverage was raised from 0.2 to 1 ML. For Ag multilayers is increased, the position of the S 2~312level varies between deposited on Mo( 1lo), the onset of desorption was at -875 K 161.4 and 161.6 eV. These values are in good agreement with those found in previous XPS studies for s / M ~ ( l l O ) . ’Figure ~ (see Figure 2A). 1B displays Mo 3d XPS spectra for clean Mo( 110) and surfaces 111.3. Adsorption of Silver and Copper on S/Mo(llO). saturated with sulfur. The presence of sulfur induces very small Figure 2 displays Ag thermal desorption spectra acquired after positive shifts (0.10-0.15 eV) in the position of the Mo 3d5/2 depositing Ag on clean Mo( 110) and on surfaces precovered by 0.25, 0.40, and 0.60 ML of sulfur. For these experiments, level. These results indicate that no molybdenum sulfide was formed.20~21 The chemisorbed layer of sulfur “passivates” the XPS measurements showed that the amount of S present on metal toward further adsorption of S2. the surface after taking the TDS spectra was identical to the S 111.2. Adsorption of Copper and Silver on Mo(ll0). The coverage seen before the deposition of Ag. The results in Figure solubility of copper and silver in molybdenum at temperatures 2 indicate that S adatoms induce a significant weakening in the below 1300 K is negligible.22 LEED,23s24AES,23,24XPS,14aand strength of the Mo-Ag bond. The larger the amount of S TDS25have been employed to examine the interaction of Cu present on the surface, the lower the activation energy for with Mo(ll0). At room temperature, Cu grows on Mo(ll0) desorption of Ag. By analyzing the data in Figure 2B, we can layer by layer up to a coverage of at least 3 ML.23,24The first conclude that at 8s = 0.25 ML approximately 40% of the Mo Cu monolayer is pseudomorphic to the Mo s u b ~ t r a t e . * ~ > ~ sites ~ are weakly affected by the presence of sulfur, while the During the growth of the second layer the film reorganizes, rest of the Mo sites (-60%) are strongly perturbed. On these exhibiting a packing density that is closer to that of Cu( 11l).23 sites the desorption temperature of Ag is very close to that of a Ag multilayer, suggesting the formation of three-dimensional Cu atoms bonded to Mo( 110) have Cu 2~312binding energies that are very similar (differences 50.1 eV) to that of bulk C U . ’ ~ ~ (3D) clusters of Ag on the surface. For surfaces with 0.40 ML Our TDS results for Cu/Mo( 110) showed desorption of the Cu of S and submonolayer coverages of Ag (Figure 2C), all the monolayer at -1230 K, with the onset for desorption of the Ag adatoms desorb at temperatures below 1000 K. This implies multilayer appearing around 1050 K. This behavior agrees with a decrease of 70-100 K with respect to the Ag desorption that found in previous studies.25 temperature of Ag/Mo(l 10). In the (8s = 0.40, @ag< 1 ML} On Mo(ll0) at 300 K, silver grows layer by layer.24 The surfaces, the line shape of the Ag desorption peak is different from that expected for zero-order desorption, indicating that first monolayer displays a LEED pattern that is different from
Reaction of
S2
with Cu/Mo( 110) and Ag/Mo(llO)
J. Phys. Chem., Vol. 99, No. 23, 1995 9569
@
4g/Mo(llO)
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Ag/S/Mo( 110)
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Temperature (K)
Figure 2. Ag thermal desorption spectra acquired after dosing Ag to Mo(l10) and S/Mo(l 10) surfaces at 300-350 K (heating rate = 5 Ws).The S M o ( l 1 0 ) surfaces were prepared by exposing Mo(l10) to SZgas at 300 K, with a subsequent brief annealing to 1200 K in order to improve the long-range order of the S adlayer." The vertical bar in parts B, C, and D indicates the desorption temperature for a Ag monolayer supported on clean Mo(l10).
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Figure 3. Ag 3d XPS and Auger MVV spectra for Ag/Mo( 1 10) and AglS/Mo( 110) surfaces at 300 K. The spectra in parts B and D were acquired after dosing Ag to a Mo( 110) surface precovered with 0.40 ML of S. This S/Mo(110) system was prepared following the methodology described in Figure 2. The primary electrons were excited using Mg K a radiation.
some of the Ag adatoms were still "wetting" the Mo substrate when the desorption process started. In contrast, the Ag desorption peaks for the (0s = 0.60, OAg .c 1 ML} surfaces in Figure 2D show trends and a line shape that match those for desorption of Ag multilayers. For these surfaces, the large coverage of S leads to a big weakening in the Mo-Ag bonds, and the desorption of Ag is preceded by the formation of 3D clusters of the noble metal. It is likely that the strength of the Mo-Ag bond has been reduced by more than 4 kcdmol, which is the difference between the desorption activation energies of the Ag multilayer (63 kcal/mo126)and Ag monolayer (67 k c d molz7)on clean Mo( 110). Figure 3 displays representative Ag 3d X P S and MVV Auger spectra for Ag/Mo( 110) and Ag/S/Mo(110) surfaces at 300 K. The Ag 3d levels and Auger transitions are sensitive to bonding between Ag and S.28 For example, the formation of Ag2S from Ag induces a large decrease (-1.1 eV) in the binding
energy of the Ag M4VV transition and a small reduction (-0.15 eV) in the binding energy of the Ag 3dw2 peak.28 The photoemission spectra in Figure 3 show peak positions for the Ag/S/Mo( 110) surfaces that are almost identical to those of Ag/ Mo( 110) surfaces or Ag multilayers. This indicates that no silver sulfide was formed in the { 0s = 0.40 ML, OAg = 0.1 1.5 ML} systems. In general, for the deposition of Ag on S/Mo(110) (0s .e 0.7 ML) surfaces, the Ag 3d XPS and MVV Auger spectra showed no evidence for significant bonding between silver and sulfur. Under these conditions, the strong Mo S interactions led to weak Ag S interactions, and it is likely that on the surfaces small islands of sulfur coexisted with patches of silver. The results for the adsorption of Cu on S/Mo( 110) surfaces show trends similar to those found for the deposition of Ag. Figure 4 displays Cu TDS spectra taken after depositing several coverages of Cu on a Mo( 110) surface precovered by 0.30 or
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Rodriguez and Kuhn
9570 J. Phys. Chem., Vol. 99, No. 23, 1995
I Cu/Mo(l 10)
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Figure 4. Cu TDS spectra acquired after dosing copper to clean Mo(l10) (A) and Mo(l10) surfaces precovered with 0.30 (B) and 0.45 ML of S (C). Heating rate = 5 Ws. The S h l o ( l l 0 ) surfaces were prepared following the methodology described in Figure 2.
Figure 5. Cu 2p3,~XPS and L3VV Auger spectra for C u h l o ( l l 0 )
0.45 ML of S. No desorption of sulfur was observed during
Mo, and domains of Cu in which the noble-metal adatoms are either bonded to Mo sites weakly perturbed by S or forming 3D clusters. In summary, the TDS results in this section indicate that S induces a large weakening in the strength of the Mo-Ag and Mo-Cu bonds. The Mo-Cu bond is -10 kcaVmol stronger than the Mo-Ag bond, but it still is not able to compete with the effects of the S Mo interaction. This interaction is so strong that at submonolayer coverages of sulfur there is no significant bonding between the S and noble-metal adatoms. 111.4. Reaction of S2 with Ag/Mo(llO) and Cu/Mo(llO). In this section, we will focus on the interaction of relatively large amounts of sulfur (0s > 1 ML) with Ag/Mo(llO) and Cu/Mo( 110) surfaces at temperatures between 300 and 700 K. TDS spectra acquired after dosing small amounts of S2 to Mo( 110) surfaces with submonolayer coverages of Ag or Cu (0s 8 N M < 1 ML) exhibited trends that were similar to those seen in Figures 2 and 4 for the Ag/S/Mo( 110) and Cu/S/Mo(110) systems. Figures 6 and 7 show photoemission spectra taken after exposing a Mo( 110) surface precovered with 1.4 ML of Ag to S2 gas at 320 K. Adsorption of sulfur induces a small decrease (-0.2 eV) in the Ag 3dw2 binding energy (Figure 6A) and a large reduction (-1 eV) in the kinetic energy of the Ag MVV features (Figure 6B). This is consistent with the formation of Ag2S.28 (On Ag(ll1) at 300 K, the reaction of SZ with the surface leads to the fast formation of a A p S film.32) An analysis of the corresponding Mo 3d XPS spectra (Figure 7B) indicates that no molybdenum sulfide has been formed at 320 K,20,21although a significant amount of sulfur (0s > 1 ML) is present in the system. Annealing of the Ag$/Mo( 110) surface from 320 to 700 K produced a large reduction in the intensity
these experiments. The sulfur adatoms induce a significant reduction in the thermal stability of the Mo-Cu bonds. In Figure 4B,the Cu desorption temperatures for the (0s = 0.30 ML, OAg < 1.1 ML} surfaces are 50-100 K smaller than those seen for submonolayer coverages of Cu on clean Mo( 110). Cu desorption peaks with a temperature and line shape that match those of Cu multilayers were observed after adsorbing Cu on a Mo(l10) surface with 0.45ML of sulfur (Figure 4C). For these systems, the Cu adatoms “ball up” before desorbing from the S-covered Mo surface. From these results, we can estimate that there was a weakening of at least 6 kcaVmolZ9in the strength of the Mo-Cu bond. Figure 5 shows Cu 2~312XPS and Cu L3VV Auger spectra for Cu/Mo(llO) (dotted traces) and Cu/S/Mo(llO) surfaces (solid traces). For the systems containing sulfur we can see peak positions very similar to those found for Cu/Mo( 110) and Cu multilayers. This fact indicates the absence of significant bonding between copper and sulfur on top of Mo( 110).30331 The formation of Cu2S from Cu is accompanied by a small binding and a large energy shift of -+0.2 eV in the Cu 2~312peak30*31b kinetic energy shift of --1.4 eV in the Cu L3VV tran~ition.~’ An analysis of photoemission data reported in the literature for bulk copper sulfides shows that a Cu 0.5s~ CuS transformation induces a small reduction of -0.3 eV in the binding energy of the Cu 2p3/2 level and a large decrease of -0.8 eV in the kinetic energy of the Cu L3VV feature^.^^,^^ None of these peak shifts are observed in the photoemission data of Figure 5 for the Cu/S/Mo( 110) surfaces. These systems probably consist of small islands of S atoms strongly bound to
+
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(dotted traces) and CulShlo(ll0) surfaces (solid traces). The spectra were taken after dosing Cu at 300 K. The S h l o ( l l 0 ) surfaces were prepared following the procedure mentioned in Figure 2.
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J. Phys. Chem., Vol. 99, No. 23, 1995 9571
Reaction of Sz with Cu/Mo( 110) and Ag/Mo( 110)
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Ag 3d: S/Ag/Mo(llO)
@I
S 2p S/Ag/Mo(llO) 8,=1.40
378
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Binding Energy (eV)
Binding Energy (eV)
Mo 3d: S/Ag/Mo(llO)
03
4g M W Auger S/Ag/Mo(llO)
eA8=1.40
ga 1
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345
350
355
360
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234
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232
1
230
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Binding Energy (eV)
Kinetic Energy (eV)
Figure 6. Ag 3d XPS (top) and Auger MVV spectra (bottom) acquired after dosing S2 to a Mo( 1 10) surface precovered with 1.40 ML of silver. In the first step the bimetallic surface was exposed to S2 gas at 320 K. Then, the system was annealed to 700 K with additional dosing of sulfur at this temperature.
Figure 7. S 2p (top) and Mo 3d (bottom) XPS spectra acquired in the same set of experiments that produced the photoemission spectra in Figure 6. In the final step the AgS,/MoS,Mo(llO) system was annealed to 1100 K to induce the desorption of silver.
+
of the Ag 3d signal (Figure 6A) and an increase in the Mo 3d signal without affecting its position and line shape (not shown). This behavior indicates a change in the morphology of the system, in which 3D clusters of silver sulfide are formed uncovering part of the Mo( 110) substrateG8Further exposure of the system to S2 gas at 700 K produces an increase in the amount of adsorbed sulfur (Figure 7A) and the formation of molybdenum sulfide (Figure 7B). At this point, sulfides of silver and molybdenum (AgS, and MoS,) coexist in the sample. Heating to 1100 K induces the desorption of Ag, and only the molybdenum sulfide is left on the surface (Figure 7). By comparing the results of Figures 1 and 7, we can conclude that the presence of silver on the surface promotes (or catalyzes!) the formation of molybdenum sulfides from metallic molybdenum and S2 gas. The bottom of Figure 8 shows Ag and S2 TDS spectra acquired during the thermal decomposition of a {AgS, MoS,} film. The film was prepared by exposing a Mo(ll0) surface precovered by 1.2 ML of Ag to S2 at 700 K. In Figure 8B, silver desorbs from the surface at temperatures between 900 and lo00 K, whereas desorption of S2 is observed between 1200 and 1300 K. After heating to 1400 K, the results of XPS indicated that only chemisorbed sulfur and metallic Mo were present in the system. The desorption peak seen for Ag is very similar to those observed for the desorption of Ag multilayers from Mo(l10) and other metals.*-I0 In previous studies,8 we found that thick Ag2S films decompose at temperatures between 800 and 900 K following the pathway AgzS(s) S2(g) Ag(s). For the {AgS, MoS,} film no desorption of S2 was observed below 900 K. This behavior suggests that the reaction AgS,(s)
+
+
'
-
+
-
+
MoS,(s) Ag(s) MoS,+,(s) has taken place below 900 K, leaving 3D clusters of silver on top of a molybdenum sulfide film. In another set of experiments we investigated the reaction of Sz with Cu/Mo(110) surfaces. Figure 9 displays photoemission spectra taken after dosing sulfur to a Mo( 110) surface precovered with 1.3 ML of Cu at 320 K. The shifts in the positions of the Cu 2~312level (Figure 9A) and L3VV Auger transition (Figure 9B) indicate the formation of a copper sulfide (CUS,).~O,~~ Due to the lack of change in the Mo 3d features (Figure 9D), we can conclude that no molybdenum sulfide has been formed at this point. On the other hand, when the CuS,/Mo(l 10) system is exposed to S2 at 700 K, one can see a large uptake of sulfur (Figure 9C) and the appearance of a molybdenum sulfide (Figure 9D). Thus, it appears that Cu promotes the sulfidation of molybdenum. The reaction between S2 gas and surfaces precovered with submonolayer coverages of copper also led to the formation of molybdenum sulfide at high temperatures, but not at room temperature. Photoemission results for a typical case are shown in Figure 10. After exposing a Mo( 110) surface with 0.73 ML of Cu to sulfur at 320 K, only copper sulfide, chemisorbed sulfur, and metallic molybdenum were present in the system. On the other hand, when a similar experiment was carried out at 700 K, a significant amount of molybdenum sulfide was formed. An increase in temperature from 320 to 700 K enhances the mobility of the metal atoms in the molybdenum lattice, favoring the penetration of sulfur into the bulk of the sample. In summary, we have found that at 300 K sulfides of Ag and Cu are formed on top of Mo( 110) when the sulfur coverage exceeds 1 ML. Annealing to 700 K produces a change in the
Rodriguez and Kuhn
9572 J. Phys. Chem., Vol. 99, No. 23, 1995
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Mo 3d: AgSJMoSJMo(1 I O )
-gv) ._ C
. A e C
-g!
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234
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Binding Energy (eV)
@
TDS AgSJMoSJMo(ll0)
II
the behavior of the NM/S/Mo(llO), NM/S/Ru(OOl), and NM/ SPt( 111) surfaces are the product of changes in the S metal substrate interaction. Mo has a much larger affinity for S than Ru or Pt,12,34 making it more difficult to form AgS, and CuS, on Mo( 110) than on Ru(001) or Pt( 111). Coadsorption with sulfur substantially decreased the bonding energy of silver or copper on Mo( 110). In principle, sulfur can reduce the stability of a noble-metal layer bonded to Mo by weakening the strength of the NM-Mo interactions (a “ligand effect”) and by preventing NM-NM lateral interactions (steric blocking). The first type of perturbation is more important than the second one. In Figures 2 and 4, most of the Mo sites show a clear perturbation in their ability to form bimetallic bonds when the S coverage is close to 0.25 ML. These results are very similar to those observed for the coadsorption of sulfur and noble metals on R u ( O O ~and ) ~ ~Pt( ~ 11l).Io The morphology of the NM/S/Mo( 110) surfaces is complex, making it hard to determine how many Mo atoms are perturbed by a S adatom. On the average, each S adatom “poisons” the bonding ability of three Mo surface atoms. Thus, it seems that S adatoms affect bimetallic bonding only locally. An important question is how sulfur “poisons” bimetallic bonding in these systems at a microscopic or electronic level. To answer this question, the first step is to examine the nature of the bimetallic bond in the {noble transition) metal surfaces. The results of quantum-mechanical calculations dealing with the interaction of noble metals with Pt, Ru, or Mo show that the net charge transfer in the {noble f transition} metal bonds is very small (c0.15 e l e ~ t r o n ) . ~ ~These - ~ ’ bimetallic bonds are best described as mainly “metallic”, with a low degree of ionic ~ h a r a c t e r . ~ ~By - ~ ’comparing the results for the Cu/S/Mo( 110) and Au/S/R~(001)~ systems, one finds that independently of the direction of charge transfer within the bimetallic bond (Cu M O ( ~ ~ or O )Au ~ ~ R u ( O O ~ ) ~sulfur ~ ) always reduces the adsorption energy of the noble metal. Thus, it appears that any effect of sulfur on the metal-to-metal charge transfer plays only a secondary role in the poisoning mechanism, and we must focus now our attention on how sulfur changes the ability of the transition-metal surface to form “metallic” or “covalent” bonds. S has an electronegativity that is much larger than that of Mo, Ru, or Pt?O a fact that suggests a net substrate S charge transfer. Indeed, molecular orbital calculations for Monsmand Runsmclusters show Mo S and Ru S charge transfer^.^' Thus, one can imagine that a S atom bonded to Mo(llO), Ru(OOl), or Pt( 111) will “polarize” the electrons of the metal substrate toward itself, making more difficult in this way the formation of bonds between the transition-metal substrate and the noble-metal adatoms. Band-structure calculations for a S layer on a Rh(100) slab42indicate that a S atom modifies the chemical behavior of adjacent Rh sites (first and second nearest neighbors) by reducing their contribution to the density of states at the Fermi level. S could be inducing similar electronic perturbations on the Mo( 1lo), Ru(OOl), and Pt( 111) surfaces, weakening in this way their ability to respond to the presence of noble-metal adatoms. IV.2. Silver- and Copper-Promoted Sulfidation of Molybdenum. From a thermochemical viewpoint, the formation of MoS2, MoS3, or Mo2S3 from S2 gas and metallic Mo should occur spontaneously. l 2 For example
+
I
800
900
’
I
’
1000
I
1100
’
I
1200
’
I
1300
’
1400
Temperature (K)
Figure 8. (A) Mo 3d XPS spectra for clean Mo( 1lo), a AgSJMoSJ Mo(l10) system prepared by dosing S2 to a Mo(l10) surface precovered with 1.2 ML of Ag at 700 K, and finally, a SMo( 110) system generated by annealing the AgS,JMoS,Mo(llO) system from 700 to 1400 K. (B) Thermal desorption spectra acquired during the annealing of the AgSJ MoSJMo(ll0) system in part A (heating rate = 5 Ws).
morphology of the noble-metal sulfide films, inducing the formation of 3D clusters of AgS, or CuS,. At 600-700 K, the noble-metal sulfides promote the reaction between S2 and Mo to yield molybdenum sulfides.
IV. Discussion IV.l. Repulsive Interactions between the Sulfur and Noble-Metal Adatoms at Submonolayer Coverages. Scanning tunneling microscopy has been employed to examine the coadsorption of Au and S on Mo(100) at 300 K.33 Repulsive interactions led to the segregation of Au and S into separate domains. At 0s > 0.5 ML, the Au adatoms formed 3D clusters on the surface instead of “wetting” the Mo substrate as happens in the case of A ~ / M o ( ~ O O This ) . ~ ~type of behavior is similar to that found for the Ag/S/Mo( 110) and Cu/S/Mo( 110) systems. For the coadsorption of Au and S the formation of sulfides is not e x p e ~ t e d . ~On , ’ ~ the other hand, thermochemical considerations indicate that silver and copper can form stable sulfides.I2 In our previous studies for the coadsorption of submonolayer coverages of S and Ag or Cu on Ru(001)8 and Pt( 11l),lo.llwe found clear evidence for the formation of noblemetal sulfides at 300 K. These sulfides decomposed at high temperatures (600-1000 K), producing S and noble-metal atoms that competed for making bonds with the Ru and Pt substrates.8,10.1’The lack of reactivity of the sulfur atoms bonded to molybdenum is a consequence of a very strong Mo S interaction. This interaction probably induces mixing of the s and p valence levels of S, producing hybrid orbitals that are oriented toward the Mo surface and preventing lateral bonding between the S and noble-metal adatoms. The differences in
-
-
-
-
S2(gas)
-
-
+ Mo(so1id) - MoS2(solid), AG = -73
kcal/mol
In spite of this “downhill” pathway, the reaction of S2 with Mo( 110) produces only a layer of chemisorbed sulfur. This sulfur layer “passivates” the metal toward further adsorption of
Reaction of
S2
with Cu/Mo(l 10) and Ag/Mo( 110)
I Cu Zp,:
I
I
936
'
I
934
I S 2p: S/Cu/Mo(110)
@I
SICulMo(ll0)
,
938
J. Phys. Chem., Vol. 99, No. 23, 1995 9573
'
932
I
~
930
I
928
01
'
926
168
166
Binding Energy (eV)
164
162
160
Binding Energy (eV)
@I
I Mo 3d: S/Cu/Mo(llO)
Cu L,W Auger: SICu/Mo(110)
eC,=1.3o
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,
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908 910 912 914 916 918 920 922 924 Kinetic Energy (eV)
I
I
234
'
I
232
'
I
'
230
I
228
'
226
Binding Energy (eV)
Figure 9. Photoemission spectra acquired after dosing S2 to a Cu/Mo(llO) surface (8cu = 1.30 ML) at 320 and 700 K. The top spectra in parts A and B correspond to the "clean" Cu/Mo( 110) surface. The spectrum of clean Mo( 110) is included in part D for comparison. Cu L,W Auger: Cu/S/Mo(llO)
I
Y 908
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912
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916
918
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922
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Kinetic Energy (eV)
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Mo 3d Cu/S/Mo(llO)
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'i 8
.
For the SlMo(ll0) systems, two factors make difficult the penetration of S into the bulk of the sample. First, the surface free energy of sulfur (0.08 J m-2 43) is much smaller than that of molybdenum (2.88 J m-243). And second, the cohesive energy of metallic Mo is very large.44 The influence of these factors is somehow suppressed by the "promotional effects" of Ag and Cu. These noble metals have relative low surface free energies (Ag, 1.30 J m-2; Cu, 1.93 J m-2 43), and their presence on the Mo surface probably frees sulfur for migration into the bulk of the sample. Silver and copper sulfides can also promote the formation of molybdenum sulfides by inducing changes in the structural geometry of the surface that enhance the diffusion of S into the lattice of metallic Mo. For the adsorption of S 2 on AglMo(ll0) surfaces, we propose the reaction scheme displayed at the bottom of Figure 11. A similar reaction scheme is also valid for the Cu/Mo(110) surfaces and for other {noble transition} metal surfaces examined before.'O.ll In this reaction scheme the rate of sulfidation of the metal substrate depends on the following factors: (1) the ability of the admetal to dissociate S2, (2) the surface free energy of the admetal, (3) the difference in thermochemical stability between the S-admetal and S-metal substrate bonds, and (4) the cohesive energy of the metal substrate. By comparing the results for the interaction of S2 with several {noble transition} metal surfaces, one can get an idea of the relative importance of these factors. The results of Figures 7B and 9D indicate that, for similar coverages of Ag and Cu and the same dose of S2 at 700 K, the amount of MoS, formed in the Cu/Mo(l 10) system is larger than that formed in the Ag/Mo( 110) system. This reflects the fact that copper is more reactive toward S2 than silver8~'2(Le., factor 1). Figure 12 shows Pt 4f XPS spectra acquired after dosing S2 to Pt(l1 l ) , I o AgPt(l1 l),Io and surface alloys of Pt-Cull and Pt-Zn." The Pt 4f7/2 regions of the systems that contain Ag and Cu exhibit two features which correspond to PtS, (peak at higher binding energy) and metallic Pt (peak at lower binding energy). The noble metals promote the sulfidation of Pt.lO,'l In contrast, the exposure of Pt-Zn alloys to SZgas results in the breakdown of the alloys and the formation of zinc sulfide
+
+
234
232
230
228
2
Binding Energy (eV)
Figure 10. Cu L3VV Auger and Mo 3d XPS spectra for the coadsorption of copper and sulfur on Mo(1 IO). In the first step, 0.73 ML of Cu was deposited on a Mo(l10) surface precovered with 0.54 ML of S at 320 K. Then, the sample was exposed to Sz gas at 320 and 700 K. For comparison, we also include the Cu L3VV Auger spectrum for a "clean" Cu/Mo( 110) surface (dotted traces in part A) and the Mo 3d X P S spectrum for clean Mo( 110) (dotted traces in part B). SZ. The large kinetic barrier associated with the penetration of
S into Mo(l10) is overcome only when Ag or Cu is present on the surface.
Rodriguez and Kuhn
9574 J. Phys. Chem., Vol. 99, No. 23, 1995
Interaction of S, gas with molybdenum
* Scheme I: Clean
Mo(ll0)
chemisorbed layer of sulfur prevents reaction with S, gas
* Scheme 11: \r
Ag
s
Ag
Ag-promoted Mo(ll0)
$
s
i+g
Mo
s
S
.+g .$
Mo
3
+
Ag adatoms favor reaction with S, gas and migration of S into the bulk of Mo
Figure 11. Schemes for the reaction of S2 gas with clean and Agpromoted Mo(l10).
I Pt 4f: S, at 550 K
induce the formation of MoS, or RuS,, although the heats of formation of MoS2 (-56 kcal/molI2)and RuS2 (-47 kcal/molI2) are considerably larger than that of PtS2 (-26 kcallmoll*). For Ag/Mo( 1lo), sulfidation of the metal substrate occurs after dosing S2 at 600-700 K, but the amount of MoS, formed is much smaller than the amount of PtS, formed in AgPt( 111) under similar conditions. The cohesive energies of Mo and Ru are -20 kcallmol bigger than that of Pt,” making it more difficult for the penetration of S into the bulk of the Mo or Ru sample (Le., factor 4). Thus, it appears that the rate of sulfidation of a transition-metal substrate depends on a delicate balance between the thermochemical stability of the transitionmetal sulfide (which makes the process “downhill”) and the attractive forces within the lattice of the transition metal (which make the process “uphill”). After studying the trends in the behavior of the {noble transition} metal surfaces in the presence of S2, one finds that the following properties are necessary conditions for an admetalpromoted sulfidation of the metal substrate: (1) the metal substrate must form sulfides that are more stable than those formed by the admetal, and (2) the admetal must have a surface free energy that is smaller than that of the metal substrate. These properties probably will provide sufficient conditions for the sulfidation of the metal substrate in the cases in which the metal substrate has a cohesive energy and an affinity toward sulfur that are comparable to those of Pt. For example, this will apply to bimetallic systems that combine a noble metal and transition metals like Fe, Ni, and Pd,’2,” which are commonly used as catalysts.
I
*
V. Conclusions
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I
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82
l
80
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I
18
’
l
16
’
1
’
14
1
72
’
l
10
.
I
68
Binding Energy (eV)
Figure 12. Pt 4f XPS results comparing clean Pt(ll1) with a series of X/Pt(l11) systems (X = Ag, Cu, or Zn) which have been exposed to Sz at 550 K.’Osl’ films on top of Pt, without the production of platinum sulfides.” In these cases, the Zn-S bond is much stronger than the Pt-S prohibiting the migration of S to form a platinum sulfide. For the bimetallic systems in which the admetal promotes the sulfidation of the substrate (NM/Mo(l 10) and NM/ Pt( 11l), with NM = Cu or Ag), the metal substrate is able to form sulfides that are more stable than those formed by the admetal.I2 Once sulfur is adsorbed on a noble metal, the large difference between the thermochemical stability of the NMS, and MoS, or PtS, compounds favors the migration of sulfur to form bulklike molybdenum or platinum sulfides (Le., factor 3). The “rate” of substrate sulfidation displayed by AgSJ Mo( 1lo), AgS,/Ru(001), and AgS,Pt( 111) surfaces upon annealing to high temperature reflects changes in the cohesive energy of the metal substrates (i.e., factor 4). For AgSxl Pt( 11l),l0 annealing from 300 to 500 K induces the formation of platinum sulfide as a consequence of the reaction AgS, f Pt AgS,-, PtS,. On the other hand, annealing of AgSJ Mo(ll0) and AgSX/Ru(001)*surfaces to 500-700 K does not
-
+
+
1. At submonolayer coverages (Os eNM< 1 ML), S and Ag or Cu do not react to form noble-metal sulfides on top of Mo(l10). The strong S Mo interaction prevents the formation of lateral bonds between the S and noble-metal adatoms. The formation of AgS, and CuS, compounds takes place only after saturating the Mo surface with sulfur (0s > 1 ML). 2. The S and noble-metal adatoms compete for making bonds with the Mo( 110) substrate. On the average, each sulfur adatom diminishes the ability for bimetallic bonding of a minimum of three Mo surface atoms. At 8s > 0.4 ML, the weakening of the Mo-Ag and Mo-Cu bonds is very significant ( > 5 kcaY mol), and the noble-metal adatoms form 3D clusters on the Mo(l10) surface before desorbing at -980 K (Ag case) or 1140 K (Cu case). 3. At temperatures between 300 and 700 K, Sz gas reacts with Mo( 110) producing a chemisorbed layer of sulfur, without forming bulklike molybdenum sulfides. The sulfidation of molybdenum occurs after exposing Ag/Mo( 110) or Cu/Mo( 110) surfaces to S2 gas. Silver and copper promote the formation of molybdenum sulfides by providing sites for the dissociation of S2 and by favoring the migration of sulfur from the surface into the lattice of the molybdenum substrate. The “promotional effect” of copper is larger than that of silver. This is a consequence of differences in the reactivity of the noble metals toward S2 (CU > Ag). 4. A comparison of the behavior observed for S2/Ag/Mo(110) with that reported for SdAg/Pt( 111) indicates that the “promotional effect” of the noble metal on the rate of sulfidation of the transition metal substrate is larger in the platinum systems. This reflects differences in the cohesive energies of the metal substrates that make more difficult the penetration of S into the bulk of Mo than into Pt. 5. In general, the “promotional effect” of a noble metal on the rate of sulfidation of a transition metal depends on (a) the
Reaction of
S2
with Cu/Mo( 110) and Ag/Mo( 110)
ability of the noble metal to dissociate S2, (b) the thermochemical stability of the transition-metal sulfide (which makes the process “downhill”), and (c) the atom-atom attraction within the lattice of the transition metal (which makes the process “uphill”).
Acknowledgment. This work was carried out at Brookhaven National Laboratory and supported by the U.S. Department of Energy (DE-AC02-76CH00016), Office of Basic Energy Sciences, Chemical Science Division. References and Notes (1) Sinfelt, J. H. Bimetallic Catalysts; Wiley: New York, 1983. (2) (a) Clarke, J. K. A. Chem. Rev. 1975, 75,291. (b) Ponec, V. Adv. Catal. 1983, 32, 149. (3) Campbell, C. T. Annu. Rev. Phys. Chem. 1990, 41, 775. (4) Rodriguez, J. A.; Goodman, D. W. Surf. Sci. Rep. 1991, 14, 1. ( 5 ) Sachtler, W. M. H. Faraday Discuss. Chem. Soc. 1981, 72, 7. (6) Bartholomew, C.H.; Agrawal, P. K.; Katzer, J. R. Adv. Catal. 1982, 31, 135. (7) Kuhn, M.; Rodriguez, J. A.; Hrbek, J. Surf. Sci. 1994, 314, L897. (8) Kuhn, M.; Rodriguez, J. A. J . Phys. Chem. 1994, 98, 12059. (9) Kuhn, M.; Rodriguez, J. A. Chem. Phys. Lett. 1994, 231, 199. (10) Kuhn, M.; Rodriguez, J. A. J . Catal., in press. (1 1) Kuhn, M.; Rodriguez, J. A. Catal. Lett., in press. (12) Lunge’s Handbook of Chemistry, 13th ed.; McGraw-Hill: New York, 1985; pp 9-21, 9-37, 9-53, 9-59. (13) Williams, G. P. Electron Binding Energies of the Elements, National Synchrotron Light Source, Brookhaven National Laboratory, Version 11, Jan 1992. (14) (a) Rodriguez, J. A.; Campbell, R. A.; Goodman, D. W. J . Phys. Chem. 1991, 95,5716. (b) Campbell, R. A,; Rodriguez, J. A,; Goodman, D. W. Surf. Sci. 1991, 256, 272. (15) (a) Heegemann, W.; Meister, K. H.; Betchtold, E.; Hayek, K. Surf. Sci. 1975, 49, 161. (b) Xu,G.-Q.; Hrbek, J. Catal. Lett. 1989, 2, 35. (16) Peralta, L.; Berthier, Y.; Oudar, J. Surf. Sci. 1976, 55, 199. (17) Sanchez, A.; De Miguel, J. J.; Martinez, E.; Miranda, R. Surf. Sci. 1986, 171, 157. (18) Toofan, J.; Tinseth, G. R.; Watson, P. R. J. Vac. Sci. Technol. A 1994, 12, 2246. (19) Roberts, J. T.; Friend, C. M. J. Chem. Phys. 1988, 88, 7172. (20) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-Ray Photoelectron Spectroscopy; PerkinElmer: Eden Praire, MN, 1978; p 104. (21) Jaegermann, W.; Schmeisser, D. Surf. Sci. 1986, 165, 143. (22) Hansen, M. Constitution of Binary Alloys, 2nd ed.; McGraw-Hill: New York, 1958; pp 34, 600. (23) Tikhov, M.; Bauer, E. Surf. Sci. 1990, 232, 73. (24) Bauer, E.; Poppa, H. Thin Solid Films 1984, 121, 159.
J. Phys. Chem., Vol. 99, No. 23, 1995 9575 (25) Xu,X.;Goodman, D. W. Appl. Phys. Lett. 1992, 61, 1799. (26) For Ag multilayers, a plot of the logarithm of the desorption rate against Wyielded a straight line, with an apparent activation energy of 63 kcaVmol. (27) (a) This value was estimated using the Redhead’s equation for firstorder desorption kinetics?7b with a preexponential factor of IOi3 s-’ and a Ag desorption temperature of 1075 K at a heating rate of 5 Ws (see Figure 2A). (b) Redhead, P. A. Vacuum 1962, 12, 203. (28) Kaushik, V. K. J . Electron Spectrosc. Relat. Phenom. 1991, 56, 273. (29) For Cu/Mo(l IO), the difference between the activation energies for desorption of the multilayer and monolayer states is -6 k ~ a l l m o l . ~ ~ ” ~ ~ ~ This value provides a good estimate for the difference between the strength of the Mo-Cu and Cu-Cu bonds on clean Mo( 1IO). The fact that Cu is “balling up” in the (0s = 0.45 ML, Ocu < 1 ML} surfaces indicates that the Mo-Cu bond has become less stable than the Cu-Cu bond ( i t . , at least 6 kcaVmol weaker than the Mo-Cu bond on clean Mo( 110)). (30) Nakai, I.; Sugitani, Y.; Nagashima, K.; Niwa, Y. J . Inorg. Nucl. Chem. 1978, 40, 789. (31) (a) Klein, J. C.; Proctor, A.; Hercules, D. M.; Black, J. F. Anal. Chem. 1983, 55, 2055. (b) Perry, D. L.; Taylor, J. A. J . Mater. Sci. Lett. 1986, 5, 384. (32) Schwaha, K.; Spencer, N. D.; Lambert, R. M. Surf. Sci. 1979, 81, 273. (33) Dunphy, J. C.; Chapelier, C.; Ogletree, D. F.; Salmeron, M. B. J . Vac. Sci. Technol. B 1994, 12, 1742. (34) On Ru(001) and Pt( 11 l), half a monolayer of sulfur is stable on the surfaces up to temperatures of 10009and 700 K,Io respectively. For a similar coverage of sulfur on Mo( 1lo), S desorption takes place above 2000 K.I7 (35) Rodriguez, J. A.; Hrbek, J. Surf. Sci. 1994, 312, 345. (36) Rodriguez, J. A.; Kuhn, M. J . Phys. Chem. 1994, 98, 11251. (37) Rodriguez, J. A.; Kuhn, M. Surf. Sci. 1995, 330, L657. (38) Copper induces a decrease in the work function of Mo( 1 a phenomenon that suggests a Cu Mo charge transfer. Ab initio SCF calculations for the deposition of Cu on early-transition metals36 show a small net charge transfer (
