9452
J. Phys. Chem. 1995,99, 9452-9457
Nature of Adsorption Sites on Sulfided Mo Catalysts and Their Selectivity in Chemisorption of Probe Molecules Zongxuan Hong*9+and John R. Regalbuto Department of Chemical Engineering, The University of Illinois at Chicago, 810 S. Clinton St., Chicago, Illinois 60607 Received: June 22, 1994; In Final Form: February 6, 1995@
Oxygen chemisorption has been studied on sulfided Mo catalysts supported on Si02 with differing dispersions as a function of pretreatment. It was found that presulfided samples purged in He at 350 "C chemisorb about half as much 0 2 as samples reduced first in H2 at 350 "C, followed by He purge at the same temperature. From this study in combination with other studies of chemisorption of probe molecules such as H2S, CO, 0 2 , and NO on sulfided Mo catalysts, a two-site chemisorption model is proposed. The adsorption sites are intrinsically related to the geometric and electronic character of the MoS2 two-dimensional layer structure. Frontier orbital concepts and Hoffmann's theory of bonding on surfaces are employed to postulate the selectivity in chemisorption of probe molecules on the two adsorption sites and to correlate quantitatively the chemisorption amount of various probe molecules. The model suggests that Mo-S sites with unsaturated coordination located in S-Mo-S layers are adsorption sites for probe molecules HzS, CO, 02, and NO, while S sites located in S - S layers only adsorb 0 2 and NO.
Introduction Much work is being conducted to understand the nature of catalytic processes in molybdenum-containing hydrotreating catalysis and to gain insight into the active sites on the catalyst surface. This knowledge will help in the design and synthesis of future catalysts. With the help of spectroscopy techniques such as MES and EXAFS, Topsoe et al.'-3 found that sulfided Mo catalysts are predominantly present in MoSz-like structures in which MoS2 crystallizes in a layered structure parallel to the basal plane. Especially for unsupported catalysts, threedimensional MoS2 structure is present; Le., several MoS2 slabs are stacked on top of each other as in bulk MoS2. HREM studies also showed that the predominant morphology of MoS2 in operating catalysts is as a single Detailed observations demonstrated different activities and adsorptivities between basal planes and edges, and plane edges were shown to be strong adsorption sites for thiophene adsorption and for activated h y d r ~ g e n a t i o n . ~It, ~is, ~now generally accepted that coordinatively unsaturated positions where the sulfur andor anionic vacancies are associated with Mo and/or Co ions on the edges of the MoS2 slabs are reactive sites for hydrodesulfurization and hydr~genation.~-' Although these studies have given some descriptions of the nature of active sites in coordination structure and chemical function, they have not provided a quantitative measurement of the number of those active sites. Selective chemisorption with probe molecules such as 02, CO, and NO has been found to be an effective way of characterizing and measuring specific catalytic surface areas or active sites of su1fides.l2 On the basis that edge sites of MoS2 crystals should selectively adsorb 0 2 in preference to basal sites,I3 and edge sites of MoS2 microcrystallites are believed to be active sites for HDS reaction, there should be an intrinsic correlation between 0 2 uptake and HDS activity. Chemisorption using different probe molecules has been studied in order to get information about the active sites Present address: Department of Chemical Engineering, University of Wisconsin, Madison, WI 53706. * To whom correspondence should be addressed. Abstract published in Advance ACS Abstracrs, April 1, 1995. @
0022-365419512099-9452$09.00/0
and to measure the active site concentrations on HDS catalysts, such as chemisorption of 02,14-24NO C0,28-30and H2SF4q3' Especially 0 2 chemisorption &dies have shown that 0 2 uptakes correlate well with HDS activities for unpromoted Mo catalyst^,'^-^' except for a few poor correlations observed for promoted ~ a t a l y s t s . ~ Tauster ~ , ~ ~ , ~et~al.' ' , I 4 claimed 02 adsorbs selectively on the edge sites which they considered to be active for HDS reaction. The basic notion is that the adsorption sites are anion vacancies created by Mo containing multiple coordinative ~ n s a t u r a t i o n . ~However, ~ - ~ ~ insight into the nature of sites measured by 0 2 chemisorption is not yet clear, although some qualitative explanations have been put forth. Regalbuto et al.37have produced three series of MoO3/Si02 catalysts with differing dispersions: a sintered hexagonal series, a sintered orthorhombic series with somewhat better dispersion, and a well-dispersed hexagonal series. Our group further found that the dispersions of these series of catalysts did not change much after sulfurization under normal condition^.^^ This renders the three catalyst series ideal and, to our knowledge, novel for comparing 0 2 chemisorption on different dispersions of MoS2 on Si02. It was discovered that 0 2 uptake varied greatly but systematically with the pretreatment conditions; presulfided samples purged in He at 350 "C chemisorb about half as much 0 2 as same samples reduced first in H2 at 350 "C, followed by He purge at the same temperature. Although there has been many studies on chemisorption of different probe molecules on sulfided Mo catalysts, there is no such study attempting to correlate them with surface sites quantitatively. In this study, with this result and utilizing the results of studies of chemisorption of probe molecules H2S, CO, 0 2 , and NO in the literature, we have proposed a general relation goveming the chemisorption of different probe molecules on MoS2 slab edges and, in so doing, hope to further define the intrinsic nature of adsorption sites on sulfided Mo catalysts. 18325-27
Experimental Section Catalysts. The preparation of various dispersion MoO3/SiO2 catalysts followed the method of Regalbuto et al.37 Three 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99,No. 23, 1995 9453
Adsorption Sites on Sulfided Mo Catalysts
TABLE 1:
0 2
Chemisorption Measurement Data 0 2 adsorption amount (He purge)
sample MoSkSiO, - -~IMSSO)
MoSzlSi0~(MSDH)
MoS2/SiOz (MSSH)
Mo loading
0 2 adsorption amount (H2 reduction)
adsorption amount 02/Mo (moVmol)
0 2
(Mo atom/nm2)
(moYmo1)
On/Mo
Odcatalyst Qmollg)
OzMo (moYmol)
Odcatalyst WmoYg)
ratio He purge/ H2 reduction
difference
type I1
0.2 0.8 2.0 4.0 0.2 0.8 2.0 4.0 0.2 0.8 2.0 4.0
0.0602 0.0490 0.0268 0.0123 0.057 1 0.0608 0.0364 0.021 1 0.0566 0.0169 0.0122 0.0084
7.23 22.48 27.84 22.07 6.89 28.04 37.93 37.67 6.77 7.76 12.68 15.00
0.1476 0.1027 0.0595 0.0262 0.1501 0.1206 0.0840 0.0464 0.1469 0.0374 0.0219 0.0150
17.73 47.1 1 61.87 46.91 18.12 55.61 87.50 82.70 17.57 17.18 22.78 26.93
0.41 0.48 0.45 0.47 0.38 0.50 0.43 0.45 0.39 0.45 0.56 0.56
0.0874 0.0537 0.0327 0.0139 0.0930 0.0598 0.0476 0.0253 0.0903 0.0205 0.0097 0.0066
1.45 1.10 1.22 1.13 1.63 0.98 1.31 1.20 1.60 1.22 0.80 0.80
different morphology oxide catalysts-sintered orthorhombic Moo3 (SO), dispersed hexagonal Moo3 (DH), and sintered hexagonal Moo3 (SH)-were prepared with four different Mo loadings: 0.2, 0.8, 2.0, and 4.0 Mo atoms/nm2. The oxide catalysts were sulfided in 10 vol % H2S/H2 flow at a rate of 40 mL/min. The samples were first kept at room temperature for 10 min, then ramped to 400 "C in 1 h (heating rate 6 "Chin), and finally kept at 400 "C for 2 h. The sulfided catalysts from oxide catalysts SO, DH, and SH are defined respectively as MSSO, MSDH, and MSSH. 02 Chemisorption Measurements. The dynamic pulse method was utilized for 0 2 chemisorption measurements. It has been found by Bodrero et a1.I2 and Tauster et al.I4-l6 that 0 2 chemisorption measurements by the dynamic pulse method at 25 "C are surface selective. Bodrero et a l . I 2 observed that saturation coverage could be obtained after exposure of the catalyst to a few pulses of oxygen at 25 "C and stated that the effect of a slow continual 0 2 uptake near saturation as indicated by Tauster et al.15 was minimized by conducting the experiment over a short period of time. It appears that 0 2 adsorption on unsupported MoS2 and supported sulfided Mo catalysts obtained by static or continuous flow techniques at -78 "C and those obtained by a pulse flow technique at 25 "C are comparable and are a measure of saturation coverage of the surface.'2,20 To conduct an experiment, about 0.52 g of the MoOdSiOz sample (0.30 g of MoO3/SiO2 for the Mo loading at 4.0 Mo atoms/nm2) was first sulfided in situ followed by two different pretreatments prepared for 0 2 adsorption measurement: (i) He purge-the sulfided sample was purged in situ with He at a rate of 60 mL/min at 350 "C for 6 h; (ii) H2 reduction-the sulfided sample was first reduced in situ with H2 at a rate of 40 " i n at 350 "C for 6 h, followed by He purge at a rate of 60 " i n at 350 "C for 2 h. The reduction temperature of 350 "C was established by an initial set of experiments at increasing reduction temperatures to determine where 0 2 uptake attained a plateau. Results showed that the 0 2 adsorption amounts are about the same for reduction temperatures from 350 to 400 "C and a reduction time from 6 to 8 h. This is in agreement with the work of Liu et al.39and Lindner et al.@ Results and Discussion The results of 0 2 chemisorption measurements with the two different pretreatments (He purge and H2 reduction) as well as the difference and ratio in 0 2 uptakes for the two pretreatments are listed in Table 1. A distinct feature we can find from Table 1 is that, for all different loadings and dispersions of MoS2 supported on Si02, 0 2 uptake is different for pretreatments of He purge and H2 reduction, and the ratio of 0 2 uptakes from He purge to HZ reduction is about 0.5 for Mo loadings higher than 0.8 MoInm2 and about 0.4 for Mo loading of 0.2 Mo atom/ nm2.
type
A similar phenomenon has been observed with other probe molecules such as CO. On bulk MoS2 catalysts, Lindner et ale4'found the amount of CO adsorption to be about half of the amount of 0 2 adsorption. Studies of H2S adsorption by Burch et al.24 and Wright et al.31found H S may strongly adsorb on the surface of MoS2. HzS was evacuated under vacuum at temperature at about 387 OC during the adsorption pretreatment. It has also been shown that very little H2S is removed by purging with an inert gas at low temperature while removal of adsorbed H2S requires temperatures above 350-450 0C.42943Bodrero et al.I2 stated that removing H2S from the catalyst surface after sulfiding is critical in obtaining meaningful chemisorption results or, in other words, in freeing all the sites for 0 2 adsorption. The results of Lindner et al!O and our condition tests at varying reduction temperatures have c o n f i i e d that H2 reduction at 350 OC for 6 h is sufficient for removing adsorbed H2S molecules. The 0 2 uptakes attained a plateau when the sample was reduced with enough high temperature and long time, suggesting that all the adsorption sites are free from occupation of adsorbents at this status. This result also implies that there is a maximum amount in adsorption sites, and this amount is a constant independent of reduction conditions, which also means that H2 reduction will only remove chemisorbed H2S species but unlikely remove S atoms from MoS2 lattice structure creating more adsorption sites, or say S depletion from lattice is not significant. This is because that, if H2 reduction would remove lattice S atoms, the adsorption sites might change with H2 reduction conditions like temperature and time, and there would not be a constant amount of 0 2 uptake. This consideration is also consistent with thermodynamics data in which AGO for the reaction of MoS2 with H2 to form H2S and M02S3 is highly positive at the conditions of H2 reduction pretreatment (350 "C and Hz pressure of 1 atm). The observed phenomenon raises two questions. First, are there two types of sites for 0 2 chemisorption present in about a 1:1 ratio, and what is their nature? Second, by what manner are the two sites selective to a particular chemisorbent? It is natural to suggest that there are two types of adsorption sites on the edge planes of MoS2 crystallite slabs based on the experimental results. One type of site is still occupied by strongly chemisorbed H2S from the sulfiding step after the He purge and not available for adsorption of other molecules due to space hindrance. These sites will be referred to as type I adsorption sites. The other type of site does not or only weakly adsorbs H2S, and is unoccupied after the He purge, and so able to adsorb 0 2 . These sites will be referred to as type 11adsorption
Hong and Regalbuto
9454 J. Phys. Chem., Vol. 99, No. 23, 1995
S-Mo-S Layer
t I
3
4 ,
-1
I
S-S Layer
(-#y-JJ ‘kb‘ -:-a
S-Mo-S Layer
S-Mo-Slayer - Type A sites
f
S-S layer - Type B sites
0.6 15
4
(ii)
S-Mo-Slayer - Type A sites
0S atom Mo atom
0S atoms which construct
S-S layer - Type B sites
the octahedral sites in S-S layer
Figure 1. Lattice structure of hexagonal MoS2 crystallite. sites. When the MoS2/Si02 catalysts are first reduced in H2, chemisorbed H2S species are removed, leaving both type I and type I1 sites unoccupied. In this case, 0 2 will adsorb on both sites when introduced onto the surfaces of MoS2. Accordingly, the difference of the two 0 2 uptakes (Table 1) will be the H2S adsorption amount onto type I sites. The ratio of type I to type I1 sites is about 1, except for 0.2 Mo atom/nm2loading samples where it is greater than 1. Active sites for adsorption of 02, H2S, and CO have been described as edge or comer sites of MoS2 layers containing anion vacancies with multiple coordinative ~ n s a t u r a t i o n . ~However, ~.~ there is still not a clear picture of the site structure and location. As we know, the nature of sites is intrinsically related to the surface geometric structure and electronic properties of the materials. Many s t ~ d i e shave ~ . ~confirmed ~ that, in sulfided Mo catalysts, the MoS2 exists in the hexagonal phase. The hexagonal MoS2 crystal structure, shown in Figure 1, is formed from layers consisting of two close-packed sheets of S atoms with a sheet of metal atoms occupying trigonal prismatic sites between the S sheets. Each Mo atom is covalently bonded to six S atoms with trigonal prismatic coordination. Between layers, there are large open areas with octahedral sites coordinated by S atoms. Layers are held together by van der Waals forces. Cleaved edge planes are drawn schematically in Figure 2, noting that the depicted edge surface structure does not take into account charge neutrality and relaxation effect^.'^.^.^^ Most stable sets of edge planes are [lOiO] and [2ii0].Io Figure 2 indicates that there are two major types of structure formed by Mo and S atoms whichever edge planes are exposed. Type A sites are located in S-Mo-S layers, which are built by Mo and S atoms where Mo is covalently bonded to S with unsaturated coordination on the surface. Type B sites are in the space between S-S layers where there are unsaturated octahedron sites coordinated by S atoms. There are some differences in type A sites between the [ l O i O ] and [2iiO] planes since the Mo-S distances and Mo coordinative unsaturation are different. But these differences in electronic properties and geometric structure are taken to be small compared to those between the S-Mo-S layer sites and the S-S layer sites. Also when exposed at surfaces, atoms tend to restructure to reach the lowest surface free energy, and the differences are diminished. So it is reasonable to assume that these differences in type A sites are insensitive for the chemisorption of small molecules such as H2S, 02, and CO. Considering MoS2 as a semiconductorin the two-dimensional layer, the band structure has been constructed based on electrical
(2iio)
Figure 2. Schematic dr_a_wingof edge planes of MoS2 layer structure: (i) [1100] plane, (ii) [2110] plane [Hayden, T. F.; Dumesic, J. A. J. Catal. 1987, 103, 3661.
and optical measurement^.^^ The conduction band is based on Mo 4d orbitals (4dV and 4dX2+) and approximately 3 eV wide; nonbonding Mo 4dZ2orbitals are 1.75-2 eV below the conduction band and about 1 eV wide; the S 3p valence band overlaps with Mo 4dZ2orbitals and is about 5 eV wide.48 K a ~ o w s k i ~ ~ calculated surface potential by employing Slater exchange and overlapping charge densities of Mo(4d55s’) and S(2s22p4)and expanded the surface potential in spherical harmonics within an atomic Wigner-Seitz cell at all sites including the empty octahedral sites. It is concluded that the empty-site potential results from the overlap of the charge-density tails from neighboring sites.49 According to these results and the band structure of the S-Mo-S layer, it is reasonable to consider that the band structure of the S-S layer picks the S 3p valence band energy of S-Mo-S layers and extends into the region of the S-S layer. Therefore, it is reasonable to postulate that the upper electron-filled level of the potential energy of surface states is approximately in the shape of MoS2 layer structure, and it is shown schematically in Figure 3. The Fermi level of the S-Mo-S layer in which most electrical and optical properties are measured is a little above the Mo 4dZ2energy level. On the basis of the geometric and electronic properties of the edge surfaces of MoS2 layers structure, we suggest that there are two types of sites with differences in structure and compositions as well as band energy level, in which type A sites are in S-Mo-S layers and type B sites are in S-S layers. The quantities of these two types of sites are in the ratio of 1:1 from the layer structure of MoS2 and are nearly independent of MoS2 particles size and how the edge surface planes are composed and distributed. The question now is to associate sites A and B with sites I and I1 or, in other words, to correlate sites derived from MoS2 geometric structure and electronic property to sites measured by chemisorption of probe molecules. To our knowledge, the vast amount of chemisorption data with different probe molecules such as H2S, CO, 0 2 , and NO have not been correlated with the adsorption sites in a rigorous way. Although there are some qualitative descriptions based on coordination of adsorption site^,^^,^ intrinsic relations between physical proper-
J. Phys. Chem., Vol. 99, No. 23, 1995 9455
Adsorption Sites on Sulfided Mo Catalysts
C . c) e
-0 7
6
Type A sites Type B sites Type A sites Type B sites
-0- 0
- 0-
0--
Type A sites
s
EF ~
Energy Level
Figure 3. Schematic drawing of potential energy level of surface states corresponding to the geometric dimension of MoS2 crystallite. Type A sites are located in S-Mo-S layers; type B sites are located in S-S layers.
ties of sites and chemisorption of probe molecules are unclear. To address this question, the frontier orbital concepts and Hoffmann’s theory on bonding on surfaces are applied.50 As depicted in Figure 4, a bond forms between a molecule and the surface when the electron charge shifts from the surface to the molecule or vice versa according to the frontier orbital energy level of the molecules and the Fermi level of the surface. Admittedly, when a molecule adsorbs on the surface, its molecular structure and orbital energy levels will change. However, we might still utilize molecular frontier orbitals and their energy levels of the probe molecule in the gas phase to judge its chemisorption trend on a surface. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of probe molecules H2S, CO, 02, and NO are calculated by the MNDO method and listed in Table 2. Since the ionization energies of these molecules are high (the HOMO energy levels are very low), it is unlikely that these molecules would transfer electron density from their bonding HOMO orbitals to the surface of MoS2 in chemisorption. Most probably, electron density of the surface tends to shift to the antibonding LUMO orbitals of the probe molecules to form bonding (interaction 2 of Figure 4). Therefore, this is considered to be the driving force for chemisorption of molecules H2S, CO, 02, and NO on the surface of MoS2. Interactions 3 and 4 take into consideration the changes in the energy levels of the molecular orbitals when molecules approach the surface. Because of a lack of detailed information for molecules approaching the surface, and for simplicity, we do not take them into account. The bonding model based on Hoffmann’s theory to explain the selectivity in chemisorption of probe molecules H2S, CO, 02, and NO is shown in Figure 5. If the potential energy of the upper electron-filled level of surface states of the S-Mo-S layer is higher than the LUMO energy level of H2S, CO, 02, and NO, then the electron density of the S-Mo-S layer can shift from the surface of MoS2 edge planes to respective LUMO orbitals resulting in chemisorption of these probe molecules. The potential energy of upper electron-filled level of surface states of the S-S layer is about 1 eV lower than that of the S-Mo-S layer. If they are higher than the LUMO energy levels of 0 2 and NO, but lower than those of H2S and CO, only 0 2 and NO can chemisorb on these region of the surface of MoS2 edge planes. Type A sites, the sites located in the S-Mo-S layer, are therefore associated with type I sites which can adsorb probe molecules H2S, CO, 02, and NO when unoccupied. Type B sites, the sites located in the S-S layer, are associated with type I1 sites which can adsorb 0 2 and NO,
but not H2S and CO. It is likely that for different Mo surface planes like [lOiO] and [2iiO] there might be not large enough energy and geometry difference for H2S or CO to differentiate for adsorption. We should note here that even though delocalized energy bands are used to explain the electronic properties of the surface, the adsorption sites in fact should be localized and formed by several atoms in a special coordination structure with some unsaturation. These sites have similar energy levels as the delocalized band structure. Actually, the band structure for the S-Mo-S layer is consistent with the orbitals of octahedral coordinated Mo clusters from ligand field theory, and the band structure for the S-S layer is similar to S2- orbitals. When the adsorption sites are occupied by adsorbents like probe molecules, they cannot further adsorb other molecules because of spatial hindrance by the adsorbed molecules and/or saturation of coordination. Saturation of chemisorption is reached when all the sites are occupied by adsorbed molecules and/or all the dangling coordinations are saturated. As mentioned before, the amount of sites A and sites B is about equal according to the geometric structure of MoS2 layer edges; i.e., the amount of type I sites and type I1 sites is about equal. Therefore, this model explains the results that the amount of 0 2 adsorption is about twice as much as the amount of CO adsorption found by Lindner et al.4’ This model also agrees with the results that 0 2 and NO chemisorbed on the same sites by Jung et al.I8 and Hall et al.27 According to the model, H2S can only adsorb on type A sites while 0 2 can adsorb on both types of sites. This agrees with the result of H2S chemisorption study showing that H2S adsorbs before, at strongly on the surface of M o S ~ . ~As~ discussed *~~ ambient pressure, H2S can only be removed under H2 reduction at high enough temperature and long time and will remain on the MoS2 surface with only He purge. So when samples were pretreated with He purge, type A sites on the surface of MoS2 were still covered by H2S molecules and not available for adsorption of other probe molecules like 02. In this condition, 0 2 only adsorbed on type B sites when introduced to the surface. While in the case that samples were pretreated with H2 reduction on the surface of which all chemisorbed H2S were removed, 0 2 adsorbed on both types of sites. Because sites A and sites B are about equal in amount, it is obvious that 0 2 uptake with He purge pretreatment should be about half as much as 0 2 uptake with H2 reduction pretreatment. Or in other words, the amount of H2S chemisorption is about the same as the amount of 0 2 chemisorption with pretreatment of He purge and about
Hong and Regalbuto
9456 J. Phys. Chem., Vol. 99, No. 23, 1995
t
antibonding in
anti bonding in
qrface >@
A
LUMO orbital
h
F
bonding in A
s"
bonding
in
-&F 1.85eV 1.39 eV
sur face
3P A
-0.11eV
Surface
- 1.72eV
MoS 2
6
*
ICC no effect
attract ion
(a>
(b)
Bonding weakened
c I
strengthened
weakened
Surface (3)
Figure 4. (1) Schematic drawing of basic molecule-surface interaction. Various interactions between the adsorbate and the surface are indicated by 1-4. Interactions 1 and 2 are the driving force for molecule binding to the surface in the process that electron density of molecule or surface transfers from generally bonding orbital of one component to antibonding orbital of the other. This results in forming a bond between the adsorbed molecule A and surface with bondings within the surface and within molecule A weakened. Interactions 3 and 4 refer to changes in the energy level of the molecular orbitals. When interactions between molecule and surface grow, and antibonding or bonding states are swept past the Fermi level, interaction 3 or 4 provides molecule-surface bonding. ( 2 ) Schematic drawing of interaction between adsorbed molecule and surface: (a) two molecules have no effect with each other because there are no electron density transfemng to the interaction levels; (b) electron density of surface transfers to the interaction levels enhancing molecule-surface bonding. (3) Schematic drawing of the overall effects of these interactions. Molecule-surface bonding is accomplished at the expense of bonding within the molecule and within the surface [Hoffmann, R. Rev. Mod. Phys. 1988,60,601]. half the amount of 0 2 chemisorption with pretreatment of H2 reduction. This model explains the results in Table 1 that the type I sites to type I1 sites ratio is about 1; Le., the amounts of H2S and 0 2 chemisorption have a ratio of about 1 for all high loading samples. For the highly dispersed 0.2 Mo/nm2 samples, the MoSz layered structure does not develop completely.
Figure 5. Schematic bonding model of chemisorption of probe molecules on MoS2 surface. Chemisorption is formed by electron density transferring from surface to LUMO orbital of probe molecules. TABLE 2: Frontier Orbital Energy Levels of Probe Molecules (in eV) probe molecule HOMO LUMO -9.976 1.850 H2S co -13.32 1.394 -11.49 -1.719 0 2 -4.768" -0.114 NO a
For HOMO of doubly occupied orbital: -16.59 eV.
Relatively more single S-Mo-S layers should form, increasing the ratio of A sites to B sites. This is why the ratio of type I sites to type I1 sites or the ratio of H2S uptakes to 0 2 uptakes in these samples is higher than 1. This ratio is not a constant, and it should depend on how the MoS2 dispersed and developed on the surface of Si02 support. As for highly dispersed sulfided Mo catalysts supported on supports in which there are strong interactions between the support and Mo phase like Al2O3, the adsorption of probe molecules is somewhat complicated by the interference from support^.'^ Also, as for some highly dispersed Mo catalysts on A1203, the morphology of sulfided Mo phase on support surfaces is also complicated by the formation of Al2(M004)3.~ The application of this model, which is based on the catalysts having MoS2 layer structure character, to such cases should await further study of morphology of sulfided Mo catalysts on A1203 and investigation of support-Mo phase interactions. Finally, we should point out that the present model has been proposed for chemisorption on the MoS2 surfaces on which there are no promoter atoms such as Co. For promoted catalysts, doping Co into the lattice of S-Mo-S layer will alter the band structure of MoS2 and also affect local site structure. This might especially change the adsorption of probe molecules like H2S and CO, because their LUMO energy levels might be close to the potential energy of the upper electron-filled level of surface states of catalysts. Since the LUMO energy level of 0 2 is much lower, however, the change in band structure of the promoted surface may not greatly affect 0 2 chemisorption, and this model might still be valid. Also, even though this adsorption model explains our experimental results and correlates quantitatively the data from other studies of chemisorption of probe molecules H2S, 0 2 , CO, and NO, a correlation of the number of these sites to the activity of hydrodesulfurization or hydrogenation reactions cannot be made as of yet.
Adsorption Sites on Sulfided Mo Catalysts Conclusion The difference in 0 2 uptake between samples purged in He and reduced in H2 is explained by the presence of two types of sites for the chemisorption of probe molecules. The 1:l ratio of these sites can be explained crystallographically, while their selectivity in chemisorption of probe molecules can be explained from the potential energy level of surface states and frontier orbital energies of probe molecules using Hoffmann’s theory of bonding on surfaces. The sites capable of chemisorbingH2S, CO, 0 2 , and NO are considered to be Mo-S sites located in S-Mo-S layer edges, while the sites capable of chemisorbing 0 2 and NO, but not H2S or CO, are thought to be empty sites located in the S-S layer edges with S in an octahedral coordination. References and Notes (1) Topsee, H.; Clausen, B. S. Catal. Rev.-Sci. Eng. 1984, 26, 395. (2) Candia, R.; Clausen, B. S.; Bartholdy, J.; Topsge, N.-Y.; Lengerler, B.; Topsge, H. In Proceedings of the 8th International Congress on Catalysts; Verlag-Chemie: Weinheim, 1984; p 11-375 (3) Clausen, B. S.; Tops@, H.; Candia, R.; Villadsen, J.; Lengeler, B.; Als-Nielsen, J.; Christensen, F. J. Phys. Chem. 1981, 85, 3868. (4) Pratt, K. C.; Sanders, J. V.; Christov, V. J. Cafal. 1990, 124, 416. (5) Pollack, S . S.; Sanders, J. J.; Fischer, R. E. Appl. Cafal. 1983, 8, 383. (6) Topsge, H. In Surface Properties and Catalysis by Non-Metals: Oxides, Sulfides, and Other Transition Metal Compounds; Bonnelle, J. P., Pelmon, B., Derouane, E. G. Eds.; Reidel: Dordrecht, 1983; p 329. (7) Topsge, H.; Topsge, N.-Y.; Sofensen, 0.;Candia, R.; Clausen, B. S.; Kallesoe, S.; Pedersen, E. Prepr. Am. Chem. SOC.Div. Pet. Chem. 1983, 28, 1252. (8) Sgrensen, 0.;Clausen, B. S.; Candia, R.; Topsere, H. Appl. Catal. 1985, 13, 363. (9) Voorhoeve, R. J. H.; Stuiver, J. C. M. J. Catal. 1971, 23, 243. (10) Hayden, T. F.; Dumesic, J. A. J. Caral. 1987, 103, 366. (1 1) Tauster, S . J.; Pecoraro, T. A.; Chianell, R. R. J. Catal. 1980, 63, 515. (12) Bodrero, T. A,; Bartholomew, C. H. J. Catal. 1983, 84, 145. (13) Bahl. 0. P.: Evans. E. L.: Thomas. J. M. Proc. R. SOC.London, Ser. A’1968,306, 53. (14) Chianell. R. R.: Tauster, S. J. J. Card 1981, 71, 288. (15) Tauster, S . J.; Riley, K. L. J. Catal. 1981, 67, 250. (16) Tauster, S. J.; Riley, K. L. J. Catal. 1981, 70, 230. (17) Bachelier, J.; Duchet, J. C.; Comet, D. J. Phys. Chem. 1980, 84, 1925. (18) Jung, H. J.; Schmitt, J. L.; Ando, H. Presented at the 4th Intemational Conference on Chemistry and Uses of Molybdenum, Golden, CO, Aug 9-13, 1982.
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