J. Phys. Chem. 1994, 98, 264-269
264
IRAS Study of CO Adsorption on Mixed Co and S Overlayers on Mo(ll0) W. Kevin Kuhn,+ J.-W. He,# and D. Wayne Goodman' Department of Chemistry, Texas A&M University, College Station, Texas 77845 Received: July 12, 1993; In Final Form: October 25, 1993'
Infrared reflection absorption spectroscopy (IRAS) was used to study the adsorption of carbon monoxide on mixed S and Co overlayers on a Mo(l10) surface. The results of this study indicate that there is a migration/ segregation of the adatoms into 2-D and 3-D islands and that the different domains are distinguishable by the stretching frequency of the adsorbed CO. Clean Mo(l10) as well as sulfided Co, (8x2)Co, p(2X2)S, and ~ ( 2 x 2 ) domains s on the Mo( 110) surface are all observable. In addition, sulfur was seen to act as both a site blocker and electron-withdrawing agent on these surfaces. Finally, data obtained in this study suggest that the active adsorption site in a Co-MoS2 catalyst could be a Co-promoted M0-S site.
Introduction Co-promoted MoS2 hydrodesulfurization (HDS) catalysts are extensively used in refineriesto remove S from crude oil feedstock. MoS2 itself can be used for this reaction, but the addition of Co can increase the activity of the catalyst by an order of magnitude. There have been several different explanations given for this increase in activity upon Co addition. Two of these involve whether the Co-MoS2 interaction is one of the Co truly acting as a promoter that perturbs the MoS2 catalyst forming the socalled C o - M o S structure at the edges of the MoSz crystallitesI4 or whether the Co-MoS2 interaction actually forms a highly dispersed CO& phase that is the active component of the ~ a t a l y s t . ~These . ~ studies have all attempted to determine the nature of the active component in the supported catalysts. There have been very few studies attempting to understand the effects of cobalt on more tractable model single-crystalbimetallic systems. A recent study7 of Co/S/Mo(100) has shown that annealing causes the sulfur to migrate on top of the Co. This migration, however, did not significantly affect the thermal desorption of Co from the surface.' The studies of CO adsorption on S/Co/Mo( 110) and Co/S/ Mo(l10) surfaces presented in this work are an attempt to understand the CoS-Mointeraction using a model single-crystal system. In the experimentson S/Co/Mo( 110) surfaces,S appears to act almost entirely as a site blocker. For the Co/S/Mo( 110) surfaces, however, effects attributable to both site blocking and electron withdrawal are observable.
Experimental Section The experiments were performed in an ultrahigh-vacuum chamber (base pressure 1 5 X Torr) equipped for IRAS, Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD). This apparatus has been described in detail elsewhere.8 The sample was spot-welded to two Ta wires on the back face of the crystal, allowing resistiveheating to 1500K and liquid nitrogen cooling to 90 K. An electron beam assembly allowed heating to 2000 K with the sample temperature being monitored with a W-5% Re/W-26% Re thermocouple. The crystal surface was cleaned using a relatively simple procedure: oxidation at 1500 K in 2 X le7Torr of oxygen followed by annealing in vacuum at a temperature of 2000 K. After this procedure, C, 0,and S Present address: Frank J. Seiler Research Laboratory (FJSRL/NC) 2354 Vandenberg Dr., Suite 2A35, USAF Academy, CO 80840-6272. Present address: DuPont Chemicals, Research & Development, Sabine River Laboratory, P.O. Box 1089 (FM-1006). Orange, TX. * To whom correspondence should be addressed. Abstract published in Aduonce ACS Abstracts. December 15, 1993.
*
0022-3654/94/2098-0264%04.50/0
surface impurities were determined to be less than 1%. The cleanlinessand long-range order of the surface were verified with AES and LEED, respectively. The IRAS spectra were acquired for 256 scans in the single reflection mode at a resolution of 4 cm-l and an incidence angle of 85' from the surface normal. The spectra shown are raw data, corrected only for the base line. Co was deposited onto the crystal surface via evaporation from a resistively heated W wire wrapped around a high-purity Co wire. To ensure film cleanliness, the source was outgassed prior to each deposition. AES showed that no impurities accumulated on the surface during the metal deposition. All adsorbate coverages are referenced to the number of Mo surface atoms (1.428 X 1015 atoms/cm*) with one Co atom per Mo atom corresponding to Oc0 = 1.00 monolayer (ML). To deposit sulfur onto the surface, the sample was dosed with H2S. Heating to >500 K caused the H2S to decompose with the hydrogen desorbing, leaving only S on the surface. The HIS was cleaned by first freezing in liquid nitrogen and then vacuumdistilling the dissolved gases. The CO used in this work was obtained from Matheson, was 99.99% pure, and was used without further purification. Gas exposures in this work are given in terms of langmuirs (1 langmuir = 1 X 1W Torres). Results The Co/Mo( 110) system has been studied in detail prev i o ~ s l y ; ~however, - ~ ~ to help delineate the specific effects that S and Co have on the adsorption of CO, spectra of CO adsorption on Co/Mo( 110) are shown in Figures 1 and 2. (Spectra for CO adsorption on S/Mo( 1 10) are presented in the preceding article.) Figure 1 shows the IR spectra of CO on Co/Mo(l10) at the indicated Co coverages (Oc,). The Co was deposited onto the Mo( 110) surface at 100 K, heated to 900 K,and then dosed with 10 langmuirs of CO at 90 K, and the IR spectra were acquired. The pseudomorphic to (8 X 2) phase transition is clearly reflected in the spectra as Oc0 increases from 1.02 to 1.69 ML. Co is known to form a pseudomorphic structure on Mo(l10) for Co coverages less than 1 ML; Le., the Co atoms assume the Mo( 110) substrate l a t t i ~ e . ~ -At l ~8co= 1 ML, an ordered structure appears and is assigned as (8x2) from its LEED pattern. The Cocoverage of the (8x2) structure is 1.250 ML9-I1 and is, therefore, 25% more densely packed than the pseudomorphic structure and 2% less densely packed than the Co(OOO1) plane. A similar transition (pseudomorphic to (8x2)) has also been observed for the Co/ W(110) system.12.13 CO adsorbed on the pseudomorphic and (8x2) phases shows peaks at 2074 and 2052 cm-I, respectively. As the intensities of the two peaks change, the peak frequencies remain essentiallyunchanged. The peak at 2035 cm-I corresponds to CO adsorbed on the Mo surface. The small peak at 2095 cm-l 0 1994 American Chemical Society
CO Adsorption on Co and S Overlayers on Mo( 1 10)
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 265
CO/Co/Mo(l 10)
CO/S/Co/Mo(110) UNANNEALED
I
eco = 0.5 ML
0.0005
29“
81
OCO -
1.55 ML
-
1.35 ML 1.00 ML 0.80 ML 0.50 ML
I
0.20 ML
2200
I
I
2100
2000
WAVENUMBER
0.00 ML
1900
1800
(cm-’)
2200
0.0 ML
2100
2000
1900
I
1800
WAVENUMBER (cm-l) Figure 3. IR spectra of CO adsorbed on S/Co/Mo(llO) at 90 K with varying S coverages.
Figure 1. IR spectra of CO on Co/Mo( 110) at 90 K a t the indicated Co coverages.
CO/S/Co/Mo(110)
I
1 Ip.ooos
CO/Co/Mo(llO)
ANNEALED
,e,
I
0.5 ML OS -
370K
0.4 ML
W
U 2010 0.3 ML
ee
2060 --
r&;-l
m
2025
2200
2100
2000
1900
2200
2100
2000
1900
1800
WAVENUMBER ( c m - l ) 1800
WAVENUMBER (cm-‘1 F i e 2 IRspectraofCOon0.35 MLofCoonMo(l10). COexposure was at 90 K with spectral acquisition at the indicated temperatures.
is associated with trace surface C from dissociated CO on the Co overlayers.11 The fwhm for the 2052-cm-1 peak at Oc0 = 1.55 ML is 10 cm-l. IR spectra of CO on 0.35 ML of Co/Mo(llO) as a function of temperature are shown in Figure 2. The Co was deposited at 100 K and annealed to 1100 K prior to a saturation CO exposure at 90 K. The two features present in the 100 K spectrum at 2079 and 2025 cm-l correspond to CO adsorbed on Co and the clean Mo substrate, respectively. The decrease in fwhm and increase in peak height of the CO-Co feature as the temperature is increased from 100 to 180 K is again due to either an increase in the order of the CO adsorbed on these domains or a reduction in the dephasing and dipole4ipole coupling that occurs at high CO coverages. As the sample temperature is increased to 340 K,the C O C Ofeature disappears,indicatingthat the CO adsorbed on these sites has desorbed. The CO remaining on the clean Mo surface desorbs at -380 K. The data discussed in the preceding paper on CO adsorption on S/Mo(l10) were a starting point for the studies of CO adsorptiononCo/S/Mo(llO). Asasecondstepintheevaluation
Figure 4. IR spectra of CO adsorbed on an annealed S/Co/Mo(llO) surface at 90 K with various sulfur coverages.
of the interactions involved in the Co/S/Mo(llO) system, S deposition on Co/Mo( 110) surfaces was studied. Figures 3 and 4 show the IR spectra of CO adsorbed onto a S/Co/Mo( 110) surface with various S coverages. In Figure 3,0.5 mL of Co was deposited onto a clean Mo( 110) surface at 100 K and flashed to 1 100 K prior to H2S exposure at 90 K. The resulting surface was flashed to 500 K to desorb the hydrogen prior to CO exposure and spectral acquisition at 90 K. The two features at 2073 and 2040 cm-’ for the non-sulfided surface are due to CO adsorption on 2-D Co islands and the Mo substrate, respectively (see Figure 1).11 As the S coverage is increased, both peaks shift toward lower frequency, broaden, and are reduced in intensity. This likely results from a site-blocking interaction of the S on the Co/Mo substrate. Figure 4 shows the IR spectra of CO adsorbed on the same surfaces shown in Figure 3 after they were annealed to 1100 K with a subsequent CO exposure at 90 K. As the S coverage is increased, the peak corresponding to CO adsorbed on Co is rapidly attenuated. The CO-Mo feature, on the other hand, shows an increase in intensity at low S coverages before being attenuated as the S coverage is further increased. This implies that annealingcauses theS atoms to migrateon thesurface such that they poison the Co sites preferentially to the Mo sites.
266
Kuhn et al.
The Journal of Physical Chemistry, Vol. 98, No. 1 , 1994 CO/Co/S/Mo(110)
CO/S/Co/Mo(110)
I:
I
0.001
,e, = 0.5 ML e, = 0.25 ML
UNANNEALED
*'yo
eo = 0.15 ML
ANNEAL TEMP.
1005K . u
1090
1.5 ML
]E1 1
L
1.0ML
U
8
m 4
0.3 ML 8 . 1 ML
, 0.00ML
L2200
2100
..J
500K I900 1800
2000
, .
WAVENUMBER (cm") Figure 5. IR spectra of CO adsorbed on S/Co/Mo(l 10) at 90 K after annealing the surface to the indicated temperatures.
2200
T
'7
1800
'
CO/Co/S/Mo(l10) I
1
ANNEALED
jT
e, = 0.15 ML
I
f.002
2052
1100K
Y
0
3K
k*271
8m a
1900
I
1.5 ML 0.25 ML
W
2000
WAVENUMBER (cm-l) Figure 7. IR spectra of CO adsorbed on unannealed Co/S/Mo( 110) surfaces at 90 K as a function of Co coverage.
CO/S/Co/Mo(l10)
ec, e,
2100
e0.1 ML
.\I?
2200 500K
21 1
0.00 ML
700K
2100
2000
1900
I
WAVENUMBER (cm-l) Figure 6, IR spectra of CO adsorbed at 90 K on S/Co/Mo( 110) as a function of annealing temperature.
To more clearly illustrate the effects of annealing, IR spectra acquired after annealing the S/Co/Mo( 110) surface to different temperatures are shown in Figure 5. The clean Mo( 110) surface was dosed with 0.5 ML of Co and annealed to 1100 K. This was followed by an exposure to 1.5 langmuirs of H2S (enough to form a 0.25 ML S layer after heating) and annealing to the indicated temperatures. Saturation CO exposure and spectral acquisition were at 90 K. This figure shows that increasing the annealing temperature from 500 to 800 K increased the intensity of the C W o f e a t u r e a t theexpenseof the CO-Mo feature. The feature at -2100 cm-1 is due to small amounts of C on the Mo surface due todissociative CO adsorption on the Coadatoms. Annealing to higher temepratures (95CLllOO K), however, caused a marked reduction in the intensity of the CO-Co feature with an increase in theCO-Mofeature. Thisseems toindicatethat the preferential poisoning of the Co sitesdoes not occur until the surface is annealed to a sufficiently high temperature. Another direct example of the interaction between S and Co on a Mo( 110) surface is shown in Figure 6. A 1.5 ML sample of Co was deposited onto the clean Mo( 110)surface and annealed to 1100 K. This procedure creates the (8x2) Co overlayer structure discussed earlier (see Figure 1). This surface was exposed to 1.5 langmuirs of H2S at 90 K, flashed to the indicated
2100
2000
1900
1800
WAVENUMBER (cm-l) Figure8. IRspectraofCOadsorbedonannealedCo/S/Mo(1lO)surfaces at 90 K as a function of Co coverage.
temperature, and cooled back to 90 K for CO exposure and spectra acquisition. The feature at 2048 cm-1 is due to CO adsorption on the (8x2) Co overlayer. The frequency shift (4 cm-1) and reduced intensity of this peak compared to the peak at 2052 cm-1 for CO on the clean (8x2) Co overlayer (see Figure 1) is related to the site-blocking nature of the unannealed S overlayer. Annealing the surface to higher temperatures drastically attenuates the peak at 2048 cm-' with two new peaks growing in at 2085 and 2016 cm-l. The peak at 2016 cm-1 correlates well with the feature observed previously for a 0.25 ML S coverage on Mo( 110) (see Figure 1 in preceding article). It is, however, also possible that this feature could correspond to CO adsorbed on domains of clean Mo(l10). Figures 1 and 2 imply that if -1/2 of the surface were clean, exposed Mo( 1lo), that the frequency of CO adsorbed on these domains would be -2020-2010 cm-1. Eitherway, theappearanceoftho featureat 2016cm-1necessitates 3-Dcluster formation of the Co overlayer. Thus, the peak at 2085 cm-1 is likely due to CO adsorption on S modified 3-DCo clusters. The actual S coverage on these Co clusters depends on whether or not there is any S remaining on the Mo( 110) surface. Again, the featureat -2100cm-lisduetoslight Ccontamination due to a small amount of dissociative CO adsorption on Co. The complex interactions between Co, S, and Mo were furtber studied by depositing Coon S/Mo( 110) surfaces. Figures 7 and 8 show the IR spectra of CO on unannealed and annealed Co/ S/Mo( 110) surfaces as a function of Co coverage, respectively.
CO Adsorption on Co and S Overlayers on Mo( 1 10)
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 267
CO/Co/S/Mo(llO) 2072ir2068
1*,To. M
CO/Co/S/Mo(110)
UNANNEALED
ANNEALED
e,
e, = 0.5 ML
0.5 ML
-
@cO 1.5 ML 1.5 ML 1.0 ML 0.5 ML
1.0 ML
8 2 3K E3m 4
0.00 ML
2200
2100
2000
1900
IS00
WAVENUMBER (cm-l) Figure 9. IR spectra of CO adsorbed at 90 K on unannealed Co/S/ M( 110) surfaces at various Co coverages.
For these figures, the clean Mo( 110) surface was exposed to 0.8 langmuir of H2S (enough to form 0.1 5 ML of S after heating) and flashed to 1400 K prior to Co deposition at 90 K. For Figure 8 the sample was flashed to 1100 K prior to CO exposure and spectral acquisitionat 90 K, whereas for Figure 7 the sample was not annealed prior to CO exposure. Figure 7 clearly shows the buildup of Co on the surface with the feature due to CO-Co replacing the feature due to CO-Mo. There is no substantial interaction between the Co overlayer and the S/Mo( 110) substrate; the Co just covers the surface. In Figure 8, on the other hand, it is clear that the 1100 K anneal causes intermixing of the Co and the S on the Mo( 110) surface. This is evident from the fact that the feature at -2025 cm-1 never disappears but just shifts in frequency as the Co coverage is increased up to 1.5 ML. The sharp feature at 2052 cm-1 corresponding to domains of (8x2) Co on clean Mo( 110) (see Figure 1) also supports the migration of S on the surface. In addition, the feature at 2085 cm-1 is in good agreement with the peak at 2085 cm-1 in Figure 6 corresponding to CO adsorbed on sulfided 3-DCo clusters. All of these features point to the fact that annealing the surface to 1100 K induces migration of the Co and S into distinct domains. The IR spectra of CO adsorbed on unannealed and annealed Co overlayers on the c(2X2)S/Mo( 110) surface are shown in Figures 9 and 10. For these figures, the Mo(l10) sample was saturated with H2S and flashed to 1400K to desorb the hydrogen and anneal theS overlayer to form thec(2X2)S film. This surface was dosed with the indicated Co coverage and either annealed to 1100 K or not prior to exposure and spectral acquisition at 90 K (Figures 9 and 10, respectively). In Figure 9, the buildup of Co on the surface is clearly evident with the feature at -2070 cm-' due to CO-Co replacing the feature due to CO-Mo. This figure is quite similar to Figure 7 in that there is apparently no substantial interaction between the Co overlayer and the S/Mo(1 10) substrate at 90 K; the Co just covers the surface. The results shown in Figure 10, however, are quite different. Just as was observed in Figure 8, annealing the surface to 1100 K causes the CO and S to segregate and/or interact on the Mo(1 10) surface. An intriguing aspect of this figure is that, up to a Co coverage of -0.3 ML, there are no CO stretching features that appear to be due to CO adsorbed on Co. The two peaks present in the 0.3 ML Co spectrum are most likely due to CO adsorption on the c(2X2)S/Mo(llO) surface and ~ ( 2 x 2 ) s domains. For higher coverages of Co, a feature due to CO adsorption on Co is finally observable (-2060 cm-1). The two features present in the 1.O and 1.5 ML Co spectra are due to CO
0.0 ML
2200
2100
2000
I900
IS00
WAVENUMBER (cm-l) Figure 10. IR spectra of CO adsorbed at 90 K on annealed Co/S/Mo(110) surfaces at various Co coverages. CO/Co/S/Mo(l10)
e, = 0.15 ML eco 1.55 ML TEYP. 400K
1880
350K 300K
II
2WK 200K IWK 100K 2200
2100
2000
1900
1800
WAVENUMBER (cm-') F i g w e l l . IRspectraofCOon 1.5 MLofCoonS/M(110). COexposure was at 100 K with spectral acquisition at the indicated temperatures.
adsorbed on S-modified Co domains and on p(2X2)S/Mo domains. The spectra for 1.O and 1.5 ML of Co in Figure 10 are similar to the 0.4 ML S/Co/Mo(llO) spectrum observed in Figure 4. The spectra in Figure 10 have a higher intensity for the CO-Co peak, however, due to the larger amount of Co on the Mo( 110) surface in this figure. The temperature dependenceof the features in the 1.5 ML Co spectrum from Figure 8 is shown in Figure 11. The CO stretching features in these spectra show the expected red shifts as the CO coverage in each domain is reduced due to desorption. As the sample temperature is increased from 100 to 250 K,the feature at 2086 cm-1 decreases in intensity and finally is no longer visible. This implies that the CO adsorbed onto sulfided 3-DCo clusters desorbs between 200 and 250 K. Concomitant with the decrease in intensity of the 2086 cm-1 is an increase in the intensity of the feature at 2025 cm-'. This likely does not represent a migration of CO to the Mo sites but probably represents a reduction in the screening of these sites due to the desorption of the CO on the sulfided 3-DCo cluster sites. Increasingthe sample temperature from 250 to 350 K c8uses the CO adsorbed on the (8x2) Co
268 The Journal of Physical Chemistry, Vol. 98, NO. 1 , 1994
Kuhn et al.
understand the CoS-Mo interaction using a model single-crystal system. IntheexperimentsonS/Co/Mo(l lO)surfaw,Sappears to act almost entirely as a site blocker. In Figures 3 and 4, it is Discussion obvious that increasing the sulfur coveragedecreases the intensity Again, it should be mentioned that IR peak intensities of the CO features and leads to red shifts in their stretching frequently do not accurately represent the true concentration of frequencies. In addition, Figure 4 strongly suggests that this surface species. "Intensity transfer"l6l6 and "~creening"~~-2~ site-blocking effect occurs preferentially at the Co sites. There effects can significantly perturb the observed IR peak intensities. does appear, however, to be an activation energy barrier to the As mentioned previously, the screening effect has been observed migration of the S on the surface that leads to the preferential for CO adsorbed on submonolayer Cu and Ag on Pt(l1 l),17Jg poisoning of the Co sites at lower S coverages. Figure 5 shows Cu on Ru(0001),21,22 and Ni on M 0 ( 1 1 0 ) . I ~ . ~Due ~ to the that a t an annealing temperature of 800 K the majority of the polarizability of the overlayer adatoms upon CO adsorption, the Cosites are still available for CO adsorption. But upon annealing CO-adatom ensembles are effective a t screening the intensity of to 950 K or higher, the intensity of the Co peaks is drastically chemisorbed CO on the overlayer-free ~ubstrate.'~Jg This has reduced. In addition to the major role S plays as a site blocker been clearly shown in a previous study of CO adsorption on Cu/ on S/Co/Mo( 110) surfaces, there is evidence in Figure 6 that it Rh( lOO)23 where the CO-Rh peak on the unannealed surface is can also exhibit electron-withdrawing effects. The feature at attenuated to zero at a Cu coverage that is at least 0.2 ML lower 2085 cm-I in this figure that is assigned to CO adsorption on than necessary for an annealed surface. The polarizability of the sulfided Co 3-D clusters is at a higher frequency than is seen for overlayer adatoms makes "screening" the most likely cause of the CO adsorbed on clean Co clusters. intensity attenuation; however, in certain systems (Cu/Ta( 110) For the Co/S/Mo( 110) surfaces, it was possible to see effects for examplez4)the proximity of the peaks means that "intensity attributableto both site blocking and electron withdrawal. Figure transfer" cannot be ruled out. In order to interpret the IRAS 8 shows the effects of increasing the Co coverage on a S/Mospectra obtained, it is not crucial to determine with absolute certainty which of these effects is occurring. It is only necessary (110) surface with a relatively low S coverage (0.15 ML). At to note that it is occurring so the intensity perturbations can be a cobalt coverage of 1.5 ML, the annealed surface has three qualitatively evaluated to allow a meaningful interpretation of features observable that can be attributed to C O adsorption onto the data. clean Mo( 1lo), (8X 2)Co/ Mo( 1lo), and sulfided 3-D Co clusters. The ability to use CO as a molecular probe for phase transitions The assignment of the peak a t 2025 cm-I in the 1.5 ML Co obviously results from a different CO bonding strength and/or spectrum in Figure 8 to CO adsorption on clean Mo( 110) rather configuration to the different metal overlayer structures, as well than on the p(2X2)S/Mo(110) was made based on the shift of as the intrinsic high resolution of IRAS. During the structural this feature toward higher frequency as the C O coverage was phase transitions, the changes in electronic and structural increased from 0.5 to 1.5 ML. Figure 1 in the preceding article properties of the metal overlayers induce a frequency shift in the shows that a S coverage of 0.15 ML causes a 5-10-cm-1 red shift. adsorbed CO. The phase change may result in an alteration of Thus, the 5-cm-I blue shift seen here is consistent with the the CO-CO intermolecular spacing and consequently molecule migration of S away from the Mo(l10) surface. In addition, molecule interactions, such as the dipole-dipole c0up1ing.l~ In since the peak at 2085 cm-l is associated with sulfided Co clusters, addition, changes in the electronic properties of the overlayer there has to be a depletion of sulfur from another region on the could alter the substrate-CO interactions, such as charge backsurface. Thus, it appears that essentially all of the S on this donation from the metal overlayersto the adsorbed CO. However, surface has migrated to the Co clusters. The assignment of the any effect due to the overlayer geometric change is expected to peak a t 2052 cm-I to CO adsorption on domains of (8X2)Co on benegligiblesince thespectra in Figure 1 show that theintegrated the Mo(l10) substrate was corroborated by LEED observations intensity and the full width at half-maximum for the spectra at of this surface which did indeed show a faint (8x2) multidif1.00and 1.35 M L are invariant during the phase transition. Thus, fraction pattern. it is unlikely that the CO adsorbed on the two phases has The surface used for the spectra shown in Figure 10 was the significantly different intermolecular interactions. It is well-known that as the CO coverage increases, the increase closest to a typical Co-MoSz HDS catalyst that was obtainable in molecular interactions usually results in a decrease in IR cross in this study. The substrate was an annealed c(2X2)S/Mo( 110) section (due to depolarization) and an upshift in frequency (due surface which can be thought of as roughly approximating the to dipoledipole coupling and/or a reduction in back-donation surface of a MoS2 crystallite. Increasing Co coverages were then from the substrate).14 Theabrupt frequency shifts observed during added to this surface and annealed (sintered) prior to CO exposure the phase transitions are, therefore, attributed mainly to changes to probe the available adsorption sites. Figure 10 shows that up in the electronic properties of the metal overlayers." Figure 1 to a Co coverage of -0.3 ML there are no CO adsorption features shows a red shift of 20 cm-l as the Co surface adlayer changes associated with Co adatoms. At coverages of 0.5 ML and above, from a relatively open (1X 1) structure to a relatively close-packed features due to Co appear and at a Co coverage of 1.5 ML, (8x2) structure. The stretching frequency of C O adsorbed on sulfided Co makes up the main adsorption feature. It is unlikely each phase remains virtually unchanged during the phase that this heavy promoter loading would ever be used in an actual transitions, however (see Figure 1 and ref 11). This strongly catalyst. A typical value for a technical catalyst is a 1:3 Co to implies a 2-D island growth mechanism of the phases, with twoMo ratio' which implies a Co surface coverage of 33% or 0.3 ML. dimensional phase expansion at the island edges. The configFor this Co coverage, as stated above, there are no observable IR uration and bond strength of C O at the island interiors would features due to CO adsorbed on Co. It is, therefore, quiteunlikely thus be preserved during the phase transition. In an early study that a Co site on this type of surface would be the active catalytic of C O adsorption on a Ru(0001) surface, the CO within the site. Therefore, these results strongly suggest that the active ordered C O islands of the ( d 3 X d 3 ) R 3 0 ° structure showed a center in a CO-MoS2 catalyst is a Co promoted M& site. different frequency from the CO at the island edges.25 In the As a final point, it is important to again restate the fact that present work, however, each ordered phase is represented by a despite the sensitivity of the vibrational frequency of CO to its single peak. Therefore, the contribution of CO at the island electronic and chemical environment, the peak areas should not edges is apparently negligible. be assumed to be representative of the relative populations of the These studies of CO adsorption on S/Co/Mo(l 10) and Co/ S/Mo(l 10) surfaces presented in this work are an attempt to indicated features. domains to desorb. Finally, the remaining C O on the Mo( 110) domains desorbs between 350 and 400 K.
-
CO Adsorption on Co and
S Overlayers on Mo( 110)
Summary and Conclusions In this study, the complex interactions between Co and S on the Mo(1 10) surface have been evaluated using the IRAS spectra of adsorbed CO as a probe of the surface structure. It is noteworthy that, for all the figures shown in this work, multiple peaks were observed in the IRAS spectra. These results imply that there is a segregationof the adatoms into 2-D (and sometimes 3-D) islands and that the different domains are distinguishable by the stretching frequency of adsorbed CO. It is the inherent high spectral resolution of IRAS combined with the localized nature of the CO-substrate interaction that makes these observations possible. Several specific interactions were observed in this study and can be summarized as follows: (1) Sulfur can act as both a site blocker and an electron-withdrawing agent on these surfaces. (2) Annealing mixed S and Co overlayers results in the migration and segregation of the components into distinct domains with Co being preferentially sulfided at high annealing temperatures. (3) Clean Mo( 110) as well as sulfided Co, (8x2)Co, p(2X2)S, and ~ ( 2 x 2 ) domains s on the Mo( 1 10) surface are all observable. (4) The spectra obtained from the 0.3 ML C0-~(2X2)S/Mo(l10) surface suggestthat theincreasedcatalyticactivityofthe industrial Co-MoSz catalyst is due to a Co-promoted Mo-S site as the active site. Acknowledgment. We acknowledge with pleasure the support of this work by the Department of Energy, Officeof Basic Sciences, Division of Chemical Science. References and Notes (1) Topee, H.; Clausen, B. S.;Topsac, N.-Y.; Pedersen, E.; Niemann, W.; MLiller, A.; Btigge, H.; Lengeler, B. J. Chem. Soe., Faraday Trans. I 1987,83, 2157, 2169.
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