590
Langmuir 1990, 6, 590-595
A Descriptive Model of Surface Sites on MoS, (WS,) Particles S. Kasztelan Institut Francais du Pgtrole, B.P. 311, 92506 Rueil-Malmaison, France Received June 27, 1989. I n Final Form: October 5 , 1989 A model is presented describing all the possible sites that can be generated by the structure of the exposed edge and basal planes of an MoS, or WS, single slab. It is shown that such a single slab contains only a limited number of elementary ensembles of Mo (W) ions interacting with a small molecule. Numerous different configurations can be generated for each of these elementary ensemble by changing the nature, number, and arrangements of adsorbates and vacancy. As a result, a list of all the configurations has been sorted out. It is proposed that catalytic sites for the various reactions performed by MoS,-based catalysts belong to that list.
Introduction Molybdenum disulfide and its homologue, tungsten disulfide, are compounds of considerable industrial importance, in particular, for their uses in the field of heterogeneous catalysis. Thus it is now recognized that MoS, or WS, is the active phase of so-called hydrotreating catalyst^.'-^ Other catalytic uses of MoS,-based catalysts have recently been described, such as alcohol synthesis from CO/H,." MoS, is also used for other purposes, such as a lubricant a t high temperature and pressures, and has also been investigated successfully as a cathode and anode material in photoelectrochemical cells for solar energy conversion. One interesting feature of these compounds is their layered structure where the S-Mo (W-S) layers are separated by a van der Waals gap." The absence of chemical bonds between the layers makes it possible to physically separate them one by one. This feature is also a t the origin of the lubricating property of MoS, as well of its ability to be intercalated. Separation of the layers to create a suspension of single MoS, layers, a process called exfoliation, has recently been d e m o n ~ t r a t e d . ' ~Fur'~~ thermore, single-layer MoS, particles can be found naturally on the surface of an alumina support such as in MoS, (WS,) based hydrotreating catalysts.14-17 (1)Schuit, G.C. A. Int. J. Quantum Chem. 1977,X I I , 43. (2)Furimski, E. Catal. Reu.-Sci. Eng. 1980,22, 371. (3)Ratnasamy, P.; Sivasanker, S. Catal. Reu.-Sci. Eng. 1980, 22, 401. (4)Chianelli, R. R. In Catalysis and Surface Science, Chemical Industries 1985,21,61. (5) Chianelli. R. R. In Surface Properties and Catalysis by NonMetals; Bonnelle, J. P., et al., Eds.; Doidrecht: Amsterdam, 198i;p 361. (6) Chianelli, R. R. Catal. Reo.-Sci. Eng. 1981,26, 361. (7)Tanaka, K. I. Adu. Catal. 1985,33,99. (8)Delmon, B. In Proceedings of the 3rd Intern. Conf. Chemistry and Uses of Molybdenum; Bary, H. F., Mitchell, P. C. H., Eds.; Climax molybdenum Co, 1979;p 73. (9)Topsoe, H.;Clausen, B. S. Appl. Catal. 1986,25, 273. (10)Murchinson, C. B.; Conway, M. M.; Stevens, R. R.; Quarderer, G. J. In Proceedings 9th Intern. Congress Catal. Phillips, M. J., Ternan, M., Eds.; 1988;Vol. 2, p 262. (11)Wilson, J. A.; Joffe, A. D. Adu. Phys. 1969,18, 193. (12)Joensen, P.;Frindt, R. F.; Morrison, S. R. Mater. Res. Bull. 1986, 21.457 -- , -- . . (13) Gee, M. A. M.; Frindt, R. F.; Morrison, S.R.; Joensen, P. Mater. Res. Bull. 1986,21, 543. (14)Sanders, J. V. Chem. Scr. 1979,14, 141. (15)Pollack, S. S.;Sanders, J. V.; Tisher, R. E. A p p l . Catal. 1983,8, 383.
0743-7463/90/2406-0590$02.50/0
In addition to its structural anisotropy, MoS, is a catalytically anisotropic solid. This is now effectively accepted since the (0001) or so-called basal plane has been shown to be particularly unreactiveI8 except for the reactions via tertiary carbonium cation intermediates involving protons.lg On the contrary, the reactivity of the edge planes of the slab has now been emphasized on many occasions. Tanaka et al.'*19 have shown, for example, that the activity for the olefin hydrogenation of a single MoS, crystal cut perpendicularly to the basal plane is much greater than for a single uncut crystal. When cobalt is added to promote catalytic properties, it has been shown that a particular type of cobalt ion is created" and interacts preferably with the edge planes of MoS, slabs.,' Similarly, these edge planes have been shown to be very sensitive to oxidation.22 It can be noted, however, that, upon ion bombardment, reactive basal plane surfaces can also be ~ r e a t e d . ~ ~ ' ~ ~ It is now well accepted that sulfur vacancies are an essential part of the sites of MoS,. The old models of sites tend to consider only one vacancy as a requirement for having adsorption and catalytic sites. Tanaka et al.7'25 for sulfide catalysts and SiegelZ6for oxide catalysts have pointed out that several vacancies on one metal ion, i.e., coordinatively unsaturated ions (cus), should be considered in the description of catalytic sites. Both group of authors considered that isomerization and hydrogen scrambling of olefiis occurred on 2-cus metal ions whereas hydrogenation of olefins and hydrogen deuteration occurred on 3-cus metal ions. Campbell et alaz7have discussed the interaction of hydrogen and hydrogen sulfide with such cus Mo ions in MoS, catalysts. (16)Kasztelan, S.; Toulhoat, H.; Grimblot, J.; Bonnelle, J. P. Bull. SOC.Chim.Belg. 1984,93, 807.
(17)Topsoe, H.In Surface Properties and Catalysis by Non-Metals; Bonnelle, J . P., et al., Eds.; Dordrecht: Amsterdam, 1983;p 326. (18)Salmeron, M.; Somorjai, G. A,; Wold, A,; Chianelli, R. R.; Liang, K.S.Chem. Phys. Lett. 1982,90,105. (19)Okuhara, T.; Tanaka, K. I. J. Phys. Chem. 1978,82,1953. (20)Topsoe, H.;Clausen, B. S.;Candia, R.; Wivel, C.; Morup, S.J. Catal. 1981.68. 433. ----, --(21)Chianelli, R. R.; Ruppert, A. F.; Behal, S. K.; Kear, B. H.; Wold, A.; Kershaw, R. J . Catal. 1985,92, 56. (22)Tauster, S. J.; Riley, K. L. J. Catal. 1981,67,250. (23)Fleischauer, P.D.ASLE Trans. 1983,27,82. (24)Davis, S. M.;Carver, J. C. Appl. Surf. Sci. 1984,19, 193. (25)Tanaka, K. I.; Okuhara, T. Catal. Rev.-Sci. Eng. 1977,15,249. (26)Siegel, S. J. Catal. 1973,30, 139. (27)Campbell, K. C.;Webb, G. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1682.
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0 1990 American Chemical Society
Surface Sites on MoS, (WS,) Particles Recently, Wambeke et al.,’ and Kasztelan et al.,’ have suggested that catalytic sites for diene hydrogenation and isomerization on alumina-supported MoS, catalyst involve unsaturated Mo ions belonging to the same edge plane (iOl0). Following Siege1 and Tanaka, they assigned the hydrogenation to 3-cus Mo ions2’ and the isomerization to 2-cus and also 4-cus Mo ion.,’ In addition, it has been noted that a couple of Mo ions rather than the isolated ion should be involved in a catalytic site on this edge plane.,’ The same use of two Mo ions for one adsorption or catalytic site has also been proposed by Delmon et al.30 to discuss the intimate mechanism of the hydrogenolysis step of thiophene hydrodesulfurization. Shabtai et al.31have also recently invoked the possibility of a complex catalytic site involving coordinatively unsaturated edge and basal plane Mo ions to explain the stereoselectivity observed for naphthalene hydrogenation. Interestingly, one isolated pieces of an MoS, layer or slab can be considered as a model of the active phase which is well suited for the study of the dependence of properties with structural features. Such a “slab model” has previously been shown to be useful for illustrating the effect of slab size and shape on the number of atoms and for correlating them with catalytic properties.16i32 These elements make it of interest to examine in detail the structure of the exposed crystallographic planes of an MoS, (WS,) slab in order to describe the possible structure of the sites. In this paper, we report on a method aiming at providing such a description by considering only structural features. The method consists of identifying (i) the shape of the particle, (ii) the nature of the surface crystallographic planes of this particle, and (iii) the different types of atoms and vacancies present on the surface. These three points forms the basis for the geometrical “slab model” already described el~ewhere,~, and only the main features of interest for this work will be recalled hereafter. With such a description of the particle, exposed surfaces and types of atoms in mind, we can then identify (iv) the elementary ensembles of ions interacting with a simple molecule after a d ~ o r p t i o n (v) , ~ ~the arrangements of atoms, vacancies, and adsorbed species within the elementary ensemble, Le., the so-called structure or architecture of the and (vi) the criteria needed to assign a catalytic function to a site. In the first part of this paper, the modeling of the MoS, slab and elementary ensembles is described, and a map of the different potential sites is sorted out. Then the various aspects of the description of the sites are discussed, and its consequences on the question of the number of different sites of MoS, or WS, are underlined.
Results Modeling of the MoS, Slab. The possible shapes of a single slab cut from an infinite layer of MoS, may be diverse, such as more or less regular triangles, rhombohedra, trapezoids, or hexagons.32 This last shape is intuitively thought to be the most probable one in view of the hexagonal structure of MoS,. Indeed, recent HRTEM (28) Wambeke, A.; Jalowiecki, L.; Kasztelan, S.; Grimblot, J.; Bon-
__nelle.-, .I. -P.J . &tal. - -
1988. . . -. - - - - ,-109. - - ,319. - ~ (29)Kasztelan, S.; Jalowiecki, L.; Wambeke, A.; Grimblot, J.; Bonnelle, J. P. Bull. SOC.Chim. Belg. 1987, 96,1003. (30)Delmon, B.; Dallons, J. L. Bull. SOC.Chim. Belg. 1988, 97, 473. (31)Shabtai, J.; Nag, N. K.; Massoth, F. E. In Proceedings of the 9th Intern. Congress on Catalysis; Phillips, M. J., Ternan, M., Eds.; 1988;Vol. 1, p 1: (32)Kasztelan, S.; Toulhoat, H.; Grimblot, J.; Bonnelle, J. P. Appl. Catal. 1984, 13, 127. (33)Kasztelan, S. C. R. Acad. Sci., Ser. 2 1988,307, 727. . .
Langmuir, Vol. 6, No. 3, 1990 591 (1010)
Figure 1. Top view of a hexagonal MoS, slab containing 37 Mo ions and 102 sulfur ions. The different positions of sulfur ions that can be removed to generate the elementary ensembles are indicated.
m-m tioioi
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@Figure 2. Schematic diagram of the three surface planes of regular MoS, slabs and description of the eight elementary ensembles generated by the removal of sulfur ions a t different positions.
studies reported by Hayden et al.34show that MoS, particules tend to be hexagonal. In addition, simple calculations using the Gibbs-Curie-Wulf law support a hexagonal shape for MoS, single slabs.36 Although a distribution of shape and size of the slabs should be taken into account to obtain a model closer to reality, in the following we shall consider only a regular hexagonal single slab as our “slab model” of the catalytic phase. An infinite layer of MoS, exposes only the basal (0001) plane composed exclusively of sulfur ions of the same type. An MoS, single slab can be cut from the infinite layer along two edge planes (iOl0) and (lOi0) whatever shape is ~ o n s i d e r e d .A~ ~schematic view of a hexagonal slab is shown in Figure 1. In addition to these two planes, the edge plane (1120) was also described by Farragher.36 However, such a plane will be more disordered and therefore less stable. Although this plane might contain catalytic sites, we shall consider only the more stable crystallographic edge plane hereafter. Another interesting feature of MoS, is the simplicity of its surface planes. The three planes of importance considered in this work have been schematically drawn in Figure 2 to illustrate this simplificity. The first characteristic of each of them is that they contain only one type of sulfur ion, i.e., a terminal sulfur ion bonded to one Mo ion in the (1010) edge plane, a bridged sulfur ion in the (TOlO) edge plane, and a tribonded sulfur ion in (34)Hayden, T. F.; Dumesic, J. A. J. Catal. 1987, 103, 366. (35)Toulhoat, H.; Kasztelan, S. In Proceedings of the 9th International Congress on Catalysis; Phillips, M. J., Ternan, M., Eds.; 1988; Vol. 1, p 152. (36)Farragher, A. L.Adu. Colloid Surf. Sci. 1973, 11, 3.
Kasztelan
592 Langmuir, Vol. 6, No. 3, 1990
a b the basal plane. These sulfur ions will be respectively symbolized by SI, SII,and SIII. The removal of these sull i i 1 1 fur ions will create different types of vacancy symbolized by VI, VII, and VI,,. 2 1 8 1 1 In addition, each of these planes also possesses only a b c d e f one type of Mo ion. In the (1010) edge plane, the Mo 1 E ion has two terminal sulfuj ions and is bonded to four 1 3 3 1 basal sulfur ions. In the (1010) plane, Mo ions are surSYMBOLS 2 rounded by four bridged sulfur ions and two basal sulMo 3 6 3 3 6 3 fur ions. In the basal plane, each Mo ion is bonded to B S 3 ~ I y j ~ ~ @ Sti@ j ~ six sulfur ions. Then, a t the intersections of the edge 3 3 6 6 3 3 0 Vacancy planes, corner Mo ions are found with two terminal two I 3 Ads. molecuie 4 FJJmmm bridged, and two basal sulfur ions. Needless to say, this f J H 1 3 3 1 description is valid only if no surface reconstruction or Figure 3. Classification of the configurations of ensembles 1 modification occurs, as will be assumed hereafter. (top) and 2 (bottom) according to the spatial arrangements of Description of the Elementary Ensembles. A site S2-,SH-, H*,and vacancy around the adsorbed molecule. cannot be described without considering the molecule that will be adsorbed or react on the surface. The case of a ulate the surface. The system can be viewed schematismall molecule requiring only one vacancy to be adsorbed cally as a number of positions or holes that will be comwill be the only one considered in the following for the petitively occupied by various adsorbates. By adsorsake of clarity. Within this limitation, only three types bates, we mean all the components of the surface such of adsorption will be possible, i.e., at a terminal, bridged, as S2-,SH-, adsorbed reactant, and adsorbed H* speor tribonded vacancy. Depending on the vacancy used cies. In addition to these species, the vacancies V must for adsorption, the local structural environment of the also be considered almost as a species by itself. Thus, molecule or site structure will be different. We therearound an adsorbed molecule many arrangements of these fore find a specific number of elementary ensembles of species, called configurations, should be found. atoms interacting with a molecule. These elementary The total number of possible configurations N , can be ensembles are identified from the position of the sulfur computed straightforwardly by using combinatorial analremoved, as shown in Figure 1, and are drawn schematysis. If k is the number of different species that can occupy ically in Figure 2 vis-&vis the drawing of each of the crysthe n - 1 position of the elementary ensemble (one of tallographic planes. the positions, the central one indicated by a cross in FigFor adsorption on a terminal vacancy in the plane (lOiO), ure 2, is the one used by the molecule), the total number two possibilities arise because adsorption can occur on of configurations will be an edge Mo ion giving ensemble 1 or on a corner Mo ion giving ensemble 2. For adsorption on a bridged vacancy N, = k(n-1) in the (iOl0) plane, adsorption may occur on a corner The number obtained for a system based on S2-, SH-, Mo ion (ensemble 3) or on an edge Mo ion (ensemble 4). H*, and V, i.e. k = 4, is 4 for ensemble 1, 64 for ensemNote that in this case the adsorbed molecule is in fact in ble 2, 1024 for ensembles 3 and 4,and 16 384 for enseminteraction with a pair of Mo ions. Ensembles 3 and 4 bles 5 to 8. correspond to the two couples of corner-edge and edgeThrough the use of the symmetry and the equivalence edge Mo ions, respectively. In Figure 2, these ensembles of position relative to the adsorbed molecule, the numappear to have a similar structure. However, the distincber of structurally different configurations can be notation has to be made because the edge Mo ion is different bly reduced, and a map of all the different configurafrom the corner Mo ion in terms of type of vacancy, and tions can be established. In Figure 3, the various configthus an adsorbed molecule will be in a different environurations for ensembles 1 and 2 are drawn. Ensembles 3 ment in ensemble 3 and 4. A completely strict descripand 4 are schematically equivalent, and only ensemble 4 tion of these edge ensembles would have taken into account has been considered in Figures 4-7. For this ensemble, the basal sulfur present in the coordination sphere of the the various configurations were obtained as follows. When edge Mo ions considered. However, in view of the wellonly S2- and V are the species considered, 32 combinaknown high stability of the basal plane sulfur ions, it is tions are found which can be reduced to 12 different conassumed that, for ensembles 1 to 4 the basal plane sulfigurations (Figure 4, first row). Then the configurafur ions are permanently present, holding the corner and tions including SH- are sorted out. The addition of one edge Mo ions to the slab. hydrogen species to the free vacancies gives the configLikewise, for adsorption on a tribonded vacancy in the urations in Figure 5, and then the addition of two or three basal plane, several possibilities arise depending on the to five hydrogen species gives the configurations reported location of this vacancy and give ensembles 5 to 8. Now in Figures 6 and 7, respectively. Note that in these figthe adsorbed molecule interacts with three Mo ions. In ures for each configuration the number of equivalent comfact, ensembles 6, 7, and 8 are not likely catalytic sites binations is given. in normal uses but are of interest for basal plane vacancy Thus, Figures 3-7 contain all possible different configgeneration through, for example, ion b ~ m b a r d m e n t . ~ ~ urations of the elementary ensemble generated by the Configurations of the Elementary Ensembles. The edge planes of MoS, for a catalytic system where only elementary ensemble defined above refers essentially to S2-, SH-, H*, and V, are taken into account. It is clear the arrangements of Mo and sulfur ions with no considthat, starting from a very limited number of structurally eration of the degree of surface population. However, different atoms in the MoS, structure, a great but finite rather than having only sulfur and vacancy, it is evident number of configurations can be identified. that different types and numbers of adsorbates will popThe above description has been made only for the case of a small molecule needing only a single vacancy to be (37) Linee, J. R.;Carre, D. J.; Fleischauer, P. D.Langmuir 1986, 2, adsorbed. It is clear, however, that larger molecules may 805. i
Surface Sites on MoS, (WS,) Particles a
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Langmuir, Vol. 6, No. 3, 1990 593 h
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need several contiguous vacancies to be absorbed. In this case, other elementary ensembles would have to be defined. In addition, the surfaces considered were regular ones, but the presence of defects or reconstructed planes can be considered if necessary and will simply lead to the addition of new and different elementary ensembles to
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Discussion The configurations shown in Figures 3-7 are all the possibilities of arrangements of the adsorbates and vacancies in the elementary ensemble around the adsorbed molecule. The identification of a site responsible for a given catalytic reaction among all the reported configurations is a difficult task. For this purpose, we need criteria for choosing, among all the configurations listed, the configuration(s) able to perform a given reaction. The important criteria that can be considered are the nature and number of species and the arrangements of these species
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Kasztelan
594 Langmuir, Vol. 6, No. 3, 1990
in the elementary ensemble. These structural criteria will define the environment of the adsorbed molecule or "site structure" according to Tanakaa7We shall note, however, as was done by Tanaka et al.,38that the "site structure sensitivity" of a reaction appears only when the ratecontrolling step of the overall reaction is sensitive to the structure of the site. Purposely, the energetic and kinetic aspects are not considered in this paper. However, it is evident that these factors should not be forgotten since the satisfaction of the proper structural criteria does not necessarily mean that the reaction will proceed. The structural criteria are therefore to be considered as necessary but not sufficient to get adsorption or reaction. The number of vacancies or the number of coordinative unsaturations possessed by the active metal center has been a criterion invoked by several authors in the past, Thus Siege126 has postulated that the nature of the elementary reaction involving hydrogen is dependent on the number of coordinative unsaturations (cus) present on the metal ion. Two simple or elementary reactions were considered, namely, isomerization (ISOM) and hydrogenation (HYD) of olefins. According to Siegel, ISOM will proceed on originally 2-cus ions whereas HYD will need 3-cus ions. Tanaka et ala3' has used the same approach for sulfides and also considered that ISOM would occur on 2-cus sites and HYD on a 3-cus active metal center. Recently, Kasztelan et al.,' have proposed that ISOM may also occur on 4-cus sites on MoS,. The fact that hydrogenation initially needs three vacancies on the active metal ion is clearly a criterion for the selection of configurations. Thus, no hydrogenation sites can be found within the configurations of ensemble 1, Le., in the (1010) edge plane. Therefore, if we consider the edge planes as the only ones involved in the catalysis, hydrogenation sites should be found only withi_nthe configurations of ensembles 2 , 3 , and 4, i.e., in the (1010) edge plane. Such a conclusion was reached by Wambeke et aL2' and Kasztelan et aL2' on the basis of the experimental observation that the variation in catalytic activity for dienes HYD and ISOM occurred only when a particular type of sulfur ion with medium heat of adsorption and identified as SII, Le., belonging to the (Tolo) edge plane, was removed. These numbers of cus are in fact the initial number of unsaturations, without taking into account the hydrogen species and the adsorbed molecules filling these vacancies. It is clear that rather than the bare number of initial unsaturation it is the nature and number of species adsorbed and vacancies present on the metal ion around the adsorbed molecule that are the important criteria. For example, it is evident that, once adsorbed, an olefin will be hydrogenated if two hydrogen species are available. They may be either two H*, two SH-, or one H* and one SH-, depending on the reaction mechanism chosen, but they should be there. Nomenclature has been introduced either by Siege126or Tanaka et aL7p3' to distinguish between metal centers with different number of cus and hydrogen species. We shall use in the following the nomenclature recommended by Tanaka et al.T3' where the initial number of cus is indicated on the left side of the symbol M for metal ion and the nature and number of adsorbate are indicated as in a chemical formula. The list of symbols corresponding to the configurations of ensemble 2 is given in Table I.
Choosing the exact number of adsorbates needed by the adsorbed molecule for a given reaction to occur, i.e., assigning an elementary reaction to a configuration, cannot be done without taking into account the reaction mechanism. The map of configurations shown in Figures 3-7 has been conceived to be independent of them and to allow various assignments to be made. However, for the purpose of giving an example of interest of such a map, we shall consider the existence of a heterolytic dissociation of hydrogen as the basic feature of the reaction mechanism. Recent theoretical c a l c ~ l a t i o n and s ~ ~ organometallic studies41point to such a mechanism with the hydride ion formed being the active hydrogen species. This has also been suggested, in particular, by Tanaka et and Wambeke et al.,' This choice means that to get an active site we need absolutely one vacancy, to adsorb the molecule, and one active H* species. It is the nature of the other adsorbates that will determine the reaction performed. The different cases are proposed in Table I for ensemble 2, the more appropriate for this discussion. For isomerization, we shall get one adsorbed molecule, one H*, and no vacancy, i.e., two other adsorbates (H*, SH-, S2-) to fit with the initial 2-cus metal center condition and one adsorbed molecule, one H*, and no sulfur or SH- to fit with the 4-cus metal center condition. For hydrogenation, we would need one adsorbed molecule, one H*, and one SH- to provide the molecule with the second hydrogen species and one vacancy or another H* to fit the 3cus active metal center condition. With these simples structural rules, the assignment of the ISOM and HYD elementary reactions to the configurations of ensemble 2 can be made as reported in Table I. We have also considered in this table the site which does not lead to a catalytic reaction but for which the hydrocarbon molecule will stay adsorbed (ADS). This underlines the fact that the requirements to get adsorption or reaction are different. A direct consequence of this observation is that there cannot be any linear correlations between surface
(38) Tanaka, K.I,; Okuhara, T. J. Catal. 1980, 65, 1. (39) Okuhara, T.; Tanaka, K. I.; Miyahara, K. J. Catal. 1977,48,220.
(40) Anderson, A. B.; Al-Saigh, 2.; Hall, W. K. J. Phys. Chem. 1988, 92,803. (41) Angelici, R. J. Acc. Chem. Res. 1988, 21, 387.
Table I. Symbols and Tentative Assignments of Elementary Reactions Such as Isomerization, Hydrogenation, or Adsorption Site to Configurations of Elementary Ensemble 2 of M o S Z P symbol
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ISOM
HYD
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IC
2a 2c 2b 3a 3b 4a
a M = Mo, V = vacancy, S = Sz-, SH = SH-, H = H*, and the number on the upper left side of M is the initial number of cus according to SeigelZ6and Tanaka e t al.7*38.
Surface Sites on MoS, ( WS,) Particles phenomena requiring different environments of the molecule. An obvious example is the comparison between catalytic data and adsorption data since the latter need, for a simple adsorption, a vacancy whatever its environment may be, whereas the former need many more criteria to be satisfied. One important feature that has not been taken into account by SiegeP and Tanaka et a1.7,25,38,39 is that a molecule can be adsorbed on a vacancy bridged between two or several metal ions and therefore be in interaction and eventually react with these metal ions. This is found for ensembles 3 and 4, where two Mo ions are involved and for ensembles 5 to 8, where the adsorbed molecule is now in interaction with three Mo ions. This adds on a new dimension since each active metal center is not independent, and therefore arrangements of these adsorbates over all the metal ions of the elementary ensemble have to be considered. Furthermore, the interaction of the molecule with several Mo ions may lead to complex surface reactions. It is tempting to imagine that each of these Mo ions can eventually work in concert, and thus two or several elementary reactions can be performed on the molecule. Such an eventuality was recently postulated by Delmon et al.,30 and a combinaison of elementary reactions should result in a catalytic function as defined by Massoth et al.42for hydrotreating reactions. Clearly this feature increases the diversity of the sites and of the catalytic function performed. In particular, starting from two elementary reactions such as ISOM and HYD as considered above, new functions or combinations of elementary reactions can be imagined, such as concerted double ISOM, concerted double HYD, and concerted HYD-ISOM when two Mo atoms are involved. These functions will result from the combination of the elementary reactions that each active metal center will be able to perform according to its own environment. Accordingly, for the configurations of ensembles 3 or 4 in Figures 4-7, we can use the rules defined above for an isolated Mo ion and apply them successively to each of the metal centers. In Figure 4, the configurations have no hydrogen, and therefore an adsorbed molecule will not react. In Figure 5, the configurations possess one hydrogen, and the reaction performed will be deduced from the rules defined above or from identification using Table I. In Figures 6 and 7, the two possibilities arise, i.e., either one Mo or two Mo participate in the reaction. In the case of one Mo, two assignments are possible for each configuration, whereas in the case of two Mo, the combinaison of two elementary reactions can be proposed. As examples, it can be seen in Figure 6 t h a t confi uration 6a can be symbolized by 3M(V),(H)(S)- M(V)(H),(S) and lead to ISOM-ISOM, configuration 6c can be symbolized by 3M(V)z(H)(SH)3M(V)(H),(S) and lead to HYD-ISOM, and configuration 6f can be symbolized by 3M(V),(H)(SH)3M(V)(H)2(SH)and lead to HYD-HYD. Evidently, such combination of elementary reactions is speculative. However, these examples shows that all configurations can be given a role by using simple structural criteria. In addition, they show that even when seemingly the Mo ions are all structurally similar, numerous different reactions or combination of reactions might be performed by them because they operate with different environments.
f
(42) Muralidhar, G.; Massoth, F. E.; Shabtai, J. J. J. CataE. 1984,85, 44; 1984, 85, 53.
Langmuir, Vol. 6, No. 3, 1990 595 The list of all the configurations given in Figures 3-7 in fact covers the variation of the population of the edge surface from totally unsaturated up to totally saturated by the various adsorbates present in the system. Therefore, it is clear that two MoS, slabs with different adsorbate populations will have different distributions of sites and therefore different selectivities. Thus, it can be suggested that selectivity is defined not only by kinetic and mechanistic features but is also strongly dependent on structural features. The first one is the different type of sites generated by different elementary ensembles sensitive to particle structures, and the second and more important one is the different type of site generated by different populations of a given ensemble. The above conclusion is an important point to be noted and solves the debate on the number of different types of sites present in hydrotreating catalysts. This debate resulted in fact from two apparently contradictory observations. On one hand, it has long been known that competition exists between, for example, hydrodesulfurization and hydrodenitrogenation, leading one to conclude that different molecules are competing for the same site. On the other hand, no linear correlations are found, most of the time, between these functions, in particular when compositions vary. From this, it has been postulated that each function has its own site located at a different position on the exposed surface planes of the active phase. The description given above clearly shows that all the molecules compete for the same vacancy or adsorption site, but once adsorbed it is probable that they need different environments to be converted, Le., different catalytic sites. It can therefore be postulated that molecules involved in so-called hydrotreating reactions such as olefins, aromatics, sulfides compounds, and nitrogen compounds as well as hydrogen, hydrogen sulfide, and ammonia all compete for the occupation of the vacancies. However, in the case of the organic molecules, reactions such as hydrogenation, hydrodesulfurization, and hydrodenitrogenation will probably have different “site structure sensitivity”. Only numbers, types, and arrangements of species on a given surface have been considered in this paper. Among other structural factors that have not be considered is the way the molecule is adsorbed since, for a given configuration, several orientations of the adsorbed molecule may be possible with more or less probability leading eventually to stereoselectivity. The reactivity aspects of the sites have also not been considered. Parameters such as the Mo ion oxidation number and the nature, number, and arrangement of adsorbed species in the elementary ensemble may have a strong influence. Finally, we can note that the complexity of the surface phenomena on MoS, catalyst does not result from the complexity of surface structure but from the great number of possibilities of site structure. It is believed, however, that, by taking structural factors into consideration, the model developed in this paper is a first step toward a more accurate description of surface phenomena on MoS,-based catalysts. Acknowledgment. Fruitful discussions on the nature of the sites in MoS,-based catalysts with Drs. J. P. Bonnelle, J. Grimblot, A. Wambeke, L. Jalowiecki, P.C.H. Mitchell, and H. Toulhoat are gratefully acknowledged. Registry No. MoS,, 1317-33-5; WS,, 12138-09-9.
