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Promotional Effect of Co or Ni Impurity in the Catalytic Activity of MoS2: An Electronic Structure Study R. GÓMEZ-BALDERAS,1,2 J. M. MARTÍNEZ-MAGADÁN,2 R. SANTAMARIA,2,∗ C. AMADOR1 1 Departamento de Física y Química Teórica, Facultad de Química, UNAM, D.F. 04510, México Programa de Simulación Molecular, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, Apdo. Post. 15-805, D.F. 07730, México 2 Received 4 February 2000; revised 13 June 2000; accepted 14 June 2000 ABSTRACT: It has been observed that the catalytic activity of MoS2 crystals is enhanced when either Co or Ni atoms are added. The presence of these atoms leads to electronic rearrangements, which are considered the source of catalytic improvement. However, the relation between the electronic properties and the enhancement of the catalytic activity is not yet fully understood. In order to get insight into the electronic-level changes that affect the catalyst performance, a solid-state density functional study has been carried out for Mo, Co/Mo, and Ni/Mo sulfides, using bulk and surface models. The MoS2 crystallize in a well-known layered structure, which has been used together with the supercell model to simulate the (10 1̄ 0) edge surface of MoS2 . The binary sulfides were obtained substituting Co or Ni by Mo from the original MoS2 bulk model. The electronic structure in a nonmagnetic state is analyzed and, in particular, the density of states of metal and sulfur atoms for the surface and bulk are compared. Finally, we discuss the important role that these properties play in the hydrodesulfurization reaction and concluded that Mo at the surface remains the relevant reactive atomic center in the bimetallic systems, whereas Co and Ni are responsible for increasing the Mo reactivity at c 2000 John Wiley & Sons, Inc. Int J Quantum Chem 80: 406–415, 2000 the surface. Key words: hydrodesulfurization; MoS2 catalytic activity; Co and Ni promotional effects; LMTO; density functional theory Correspondence to: R. Gómez-Balderas. ∗ On sabbatical leave from the Instituto de Física, UNAM. Contract grant sponsor: DGAPA-UNAM. Contract grant sponsor: Instituto Mexicano del Petróleo. Contract grant number: FIES-96-15-III. International Journal of Quantum Chemistry, Vol. 80, 406–415 (2000) c 2000 John Wiley & Sons, Inc. CATALYTIC ACTIVITY OF MoS2 Introduction C rude petroleum contains a great variety of organic compounds, and many of them contain sulfur. The catalytic hydrodesulfurization (HDS) process consists in the transformation of organosulfurs into hydrocarbons plus hydrogen sulfide (H2 S). Among the transition-metal sulfides (TMS) that catalyze this process, the MoS2 -based catalysts are the most widely used in the petroleum refinement industry. It is known that addition of different transition metals to MoS2 , such as Co or Ni, leads to a pronounced enhancement of the MoS2 catalytic activity up to a factor of 10 with respect to that of the unpromoted MoS2 crystallites [1]; the doping of MoS2 allows a performance at similar rates as the Rh2 S3 and RuS2 structures, two of the most active HDS catalysts. The activity of these bimetallic sulfides is also known to be higher than that of CoS or NiS. A large number of investigations have been carried out in order to analyze the structural changes in the MoS2 crystals (because of the insertion of Co or Ni atoms) and their corresponding impact on the catalytic activity [2 – 5]. It has been observed that the presence of different metals to Mo in the MoS2 crystal can lead to the formation of different phases [6] due to several possible factors, like the promoter metal loading, impregnation procedure, calcination, or sulfiding temperatures of the catalyst [2]. General agreement has been reached on the formation of Co–Mo–S and Ni–Mo–S catalyst phases, and valuable experimental results [7, 8] suggest that a Co–Mo–S phase exists with a slab structure, similar to that of MoS2 . The metallic sites located on the catalytic surface represent the major part of active centers in the HDS process. The metal atoms are exposed, unsaturated, and available for binding reactant guest molecules. There is also experimental evidence [9] indicating that cobalt and nickel promoters can be found in the (10 1̄ 0) edge surfaces, located at the same planes as molybdenum atoms. Still, important questions remain about the coordination of Co and Ni, whether it is tetrahedral, octahedral, or some other one. The induced rearrangement in the electronic structure of sulfides caused by the substitution of Co or Ni for Mo has been also the subject of considerable research efforts. There mainly exist two points of view: (1) the Co and Ni may act as charge donators on Mo [1, 10]. (2) The promoter atom itself may become the actual catalytic site [11]. In connection with the former point, it is believed that the two main factors responsible for the enhanced activity of Co(Ni)/Mo sulfide are: (a) the covalent contribution to the metal–sulfur bond strength and (b) the increase of d electrons associated with Mo. This second factor is considered as the dominant effect of the promoter. Although this conclusion was reached through a study with a low-level computational technique, the SCF-Xα scattered wave method, it may be expected that a more rigorous approach does not produce significant changes on the interpretation. It was also found that varying the transition metal, from Ti to Ni, next to a Mo atom results in downward shifts of the 3d-orbital energies, by an approximate amount of 1.5 eV, thus getting them closer to the 3p-sulfur orbital energies. In the presence of Co or Ni, Mo is reduced relative to Mo in MoS2 and, on the other hand, Mo is oxidized in presence of Cu relative to Mo in MoS2 [1]. Also, the donation/back-donation mechanism has been proposed for the HDS reaction. Raybaud et al. found [12] that charge is donated from the sulfur atom toward the nonbonding d-surface states of the metal atoms, and charge is back-donated from the bonding d-surface states to antibonding C–S–π states, for tiophene over MoS2 . Recently, Toulhoat et al. [13] proposed another explanation of the promotional effect. They introduce a definition of the metal–sulfur bond strength, which considers the cohesive energy of the bulk per metal–sulfur bond. High values of this parameter, as in MoS2 , imply a low HDS activity since the residence time of the guest sulfur would be greater over the metallic center. Low values of the metal– sulfur bond strength, as in Co or Ni sulfides, imply also a low HDS activity because the residence time would be now too small. Then, Toulhoat et al. [13] expect intermediate bond strengths for (Co, Ni)Mo bimetallic sulfurs between the two previous limits. As a consequence, the catalytic activity predicted for these mixed sulfurs should be in the range of that exhibited by RuS2 or OsS2 . The metal–sulfur bond strengths would explain the promoting effect of Co(Ni) impurities, allowing an optimal residence time of the guest sulfur over the active site in the HDS process. The catalytic phenomenon, which involves the breaking and formation of new chemical bonds, is considered to take place on the active surface of bimetallic TMS catalyst. Surface science can help us to understand the catalytic process by analyzing electronic rearrangements and their role in the sul- INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 407 GÓMEZ-BALDERAS ET AL. fur affinity for metals. In the present contribution, we consider the interaction between the Co and Ni promoters with Mo and investigate, with the use of solid-state density functional theory, the electronic structure modifications of Co/MoS2 and Ni/MoS2 edge surfaces. We assume a slab chemistry model with Co/Ni atoms positioned at the (10 1̄ 0) edge plane, which is expected to give a reasonable active surface model. It is well established that pure transition-metal sulfides such as CoS, NiS, and MoS2 have a lower catalytic activity than mixed crystals such as Co/ MoS2 and Ni/MoS2 . In order to get insight into the causes that lead to catalytic differences, we analyze the electronic structure of different systems through the study of their density of states (DOS). We will focus our attention on metallic atoms at the surface, which are the active sites responsible for catalytic transformations in the HDS process of organosulfur compounds. The empty intense states of unsaturated Mo atoms, located at the edge surface of MoS2 , present just above of the Fermi level, are responsible for the charge acceptor properties of the surfaces, although a detailed study of the bonding with the adsorbed molecule would be necessary in order to quantify the acceptor properties. Our analysis of the enhanced catalytic activity, induced by promotional effects of inserted Co/Ni metals in MoS2 , will be made on that basis, namely, the intensity of the states near the Fermi level, which are understood to indicate reactivity indexes. Even when the definition of local quantities in extended systems may be arguable, they are useful for the interpretation of our results. Therefore, we compute the charge projection onto atomic sites for valence states in the following way: Z EF Nlν (ε) dε, Qlν = −∞ where l is an angular momentum index, ν indicates the atomic site, and Nlν is the density of states projected onto the atomic site and angular momentum. Hereafter we will refer to Qlν as the atomic orbital population. Methodology We employ first-principles density functional theory (DFT) to compute the electronic structure of sulfide crystals assuming a nonmagnetic state. We resort in particular to the Kohn–Sham version 408 of DFT, where monoelectronic Schrödinger-type equations for periodic systems are solved selfconsistently. The electronic exchange and correlation energy terms are treated in the local density approximation, using a parameterized form of the exchange and correlation potentials of the homogenous electron gas, initially proposed by Hedin and Lundqvist [14] and von Barth and Hedin [15]. We employed the linear muffin-tin orbitals (LMTO) method in the atomic sphere approximation (ASA) [16] in our ab initio calculations, as implemented in the ESOCS program [17], version 4.0.0. In the solution of the Schrödinger equation, ESOCS adopts atom-centered potentials of the muffin-tin type, with spherical symmetry, and a constant potential in the interstitial region. The solutions are given as linear combinations of muffin-tin orbitals and obey the Bloch theorem to produce a periodic wave function suitable for extended systems like crystals and surfaces. Once the solutions are obtained, the density of states can be calculated and projected onto the different atomic sites. The analysis of results is focused on the behavior of the density of states, but special attention is given to those states around the Fermi level. We note that usually valence electrons for sulfur include 3s and 3p orbitals, nevertheless, our preliminary calculations showed that the band associated with the 3s orbital is far away from the Fermi level (−13 eV) and, therefore, we do not take it into consideration in the foregoing discussion. The 3s orbital is contemplated as a core state and the 4s, 3p, and 3d orbitals constitute the sulfur valence states. For the case of transition-metals cobalt and nickel, we employ 3d, 4s, and 4p valence orbitals, and for the case of molybdenum the 4d, 5s, and 5p orbitals are used as valence states. Our preliminary calculations also showed that spin-polarized results differ slightly from the spin-compensated results, practically giving no variation in the density of states, and maintaining our conclusions essentially unchanged. Modeling We have analyzed bulk and surface models of the MoS2 , Co/MoS2 , and Ni/MoS2 structures. In the pure MoS2 crystal (space group P63 /mmc) the Mo atom is located at the center of a trigonal bipyramid formed by S atoms. The bulk structure is composed of two layers of molybdenum at z = 14 and z = 34 positions in the cell, and each Mo layer is sandwiched between sulfur layers. The S–Mo–S sheets VOL. 80, NO. 3 CATALYTIC ACTIVITY OF MoS2 FIGURE 1. Representation of the bulk model, a = 9.468 Å, b = 9.468 Å, and c = 12.275 Å. Selected atoms for DOS analysis are labeled and displayed as balls. are stacked in the (001) direction in the sequence . . . BCBCBC. . . (layers of Mo in bold) and are held together by weak van der Waals interactions between S atoms. Our bulk model for the pure MoS2 crystal is shown in Figure 1. Three rows of trigonal prisms in the y direction and three rows in the x direction form the unit cell, which has 54 atoms in the composition Mo18 S36 , and is periodically repeated in the x, y, and z directions. The unit cell of the Co(Ni)/MoS2 bulk is obtained by replacing a Co(Ni) by a Mo atom. The lattice parameters that were used to build the unit cell were a = 9.468 Å, b = 9.468 Å, and c = 12.275 Å, which correspond to experimental parameters of the MoS2 crystal [18]. The atomic sphere radius of sulfur is rS = 1.5261 Å, of molybdenum is rMo = 1.4677 Å, and of cobalt and nickel are rCo = rNi = 1.2711 Å. In order to satisfy the ASA condition, empty spheres are added in such a way that the final packing of atoms plus empty spheres is a hexagonal closed packed structure. The radii of the empty spheres are adjusted for both bulk and surface models, accordingly with the atomic sphere radii. The maximum allowed sphere overlap is 15% of the total cell volume. For instance, the bulk empty sphere radius is 1.3061 Å and the surface empty sphere radius is 1.4376 Å, for the MoS2 system. The (10 1̄ 0) edge surface is modeled using supercells built from two S–Mo–S sheets stacked in the z direction. Each sheet consists of trigonal prism rows (two in the y direction, plus terminal molybdenum atoms, and three in the x direction). In the y direction the slabs are separated by 10.52 Å, and empty spheres fill this space. The entire supercell structure, including the empty spheres, is hexagonal closed packed and has the following dimensions: a = 9.468 Å, b = 18.936 Å, and c = 12.275 Å. Figure 2 shows the main features of our surface FIGURE 2. Representation of the (10 1̄ 0) surface, a = 9.468 Å, b = 18.936 Å, and c = 12.275 Å. Side view shows layers 1, 2, and 3 as L1, L2, and L3. Atoms for which DOS is analyzed are labeled and displayed as balls. INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 409 GÓMEZ-BALDERAS ET AL. model. In the bimetallic surfaces the promoter atom is located in a molybdenum position. The surfaces studied expose unsaturated metal atoms as well as sulfur atoms. Toulhoat et al. claim that reconstruction of the surface is expected to be moderate for transition-metal sulfides with respect to that observed in the transition metal [13]. Using ab initio molecular dynamics simulations, Raybaud et al. have demonstrated that the (10 1̄ 0) edge surface suffers small relaxations and remains stable at the usual temperatures of HDS reactions [19]. Hence, in the present approach, atoms at the surface are fixed in the bulk-terminated geometry. Results and Discussion MoS2 The MoS2 compound is an important catalyst component used in the refining industry, and previous studies exist on its crystal structure and electronic features. For instance, Raybaud et al. [19] have performed band structure calculations of the MoS2 bulk, by working within the generalized gradient approximation of DFT and using a plane wave basis set. They have shown the mixture of the 4d bands of molybdenum with the 3p bands of sulfur and identified the MoS2 bulk as a semiconductor with a gap of Eg = 0.89 eV. We have performed similar calculations for the MoS2 bulk, and our results are in good agreement with the density of states obtained by Raybaud et al. [19]. The fact that ESOCS reproduces previous results that were obtained with a different approach gives us confidence to describe with similar accuracy the electronic structure of bimetallic compounds. Figure 3 shows the MoS2 bulk total density of states projected onto the Mo and S atoms, as well as the global DOS. This figure also gives the Mo 4d and S 3p states (represented by dotted lines in the graph), which appear at the same energy interval, and essentially determine the band structure. A gap of 1.3 eV between occupied and unoccupied states is shown at the Fermi level, the experimental value for this gap being 1.3 eV [19]. This fortuitous match, possibly due to error cancellation, should not be regarded too seriously given the well-known usual underestimation of band gaps by local density methods. Nevertheless, the existence of this gap characterizes the MoS2 crystal as a semiconductor, if we consider that most semiconductors such as germanium, silicon, or GaAs have energy gaps lower than 2 eV [20]. Above the Fermi 410 FIGURE 3. Atomic site projected DOS for pure MoS2 in the bulk model. level, molybdenum exhibits unoccupied states with higher intensities than sulfur, consequently, Mo has the highest probability to accept electrons or, equivalently, Mo is more electrophilic than S. In regard to the MoS2 (10 1̄ 0) crystal surface, Raybaud et al. [19] used a supercell model consisting of a slab of 72 atoms (in a Mo24 S48 composition). Their layer separation was 12.8 Å and they only allowed the first MoS2 layer to relax to simulate a stable surface structure. They found that the surface remains metastable in the bulk-terminated geometry, except for small relaxations that have little influence on the electronic DOS. Even when we use a smaller model without surface relaxation, a three-layer slab supercell model with 54 atoms (in a Mo18 S36 composition) with a layer separation of 10.52 Å, the relevant features of the system are reproduced when we compare with the result of Raybaud. In Figure 4, we give the Mo and S density of states for each layer of sheet B for the slab model, together with the DOS of the bulk. If we compare layer 1 molybdenum atoms at the surface (referred hereafter as Mo1) with molybdenum in the bulk, the bulk shows a gap of 1.3 eV, contrary to the surface, which hardly shows any gap. The lack of a gap at the surface is associated to the shift of the Mo1 empty states toward the Fermi level, due to VOL. 80, NO. 3 CATALYTIC ACTIVITY OF MoS2 tron donor/acceptor character. This feature is related with the donation/back-donation mechanism, which is due to the surface. Although, it would not be determinant to discuss chemical trends when charge differences are of the order of 0.01, the population analysis seems to be consistent with this electron donor/acceptor character provided by the DOS results. From Table I, Mo2 in the bulk has 4.18, 0.58, and 0.37 electrons in the d, p, and s shells, and Mo1 at the surface has 4.23, 0.40, and 0.33 electrons in the d, p, and s shells, respectively. Consequently, the surface raises the electron donor/acceptor character, diminishing the s and p populations (also participating in donation) and increasing the d population (back-donation). We also stressed the considerable amount of charge allocated on the empty spheres sites, whose radius is 1.3061 Å. In the bulk, as shown in Table I, the total charge on these sites is 0.37 electrons; this charge counterbalance the atomic sites charge ensuring the electroneutrality of the system. In the case of interstitial spheres in the surface model, whose radius is now 1.4376 Å, they have 0.56 electrons. In connection with the empty spheres located between slabs, we observe for the surface model a variation from 0.27 electrons just above the surface to 4.5 × 10−5 electrons a half distance between slabs. This fact validates our model since the electronic density is mainly spread in the slab region. FIGURE 4. Atomic site projected DOS for (10 1̄ 0) surface of pure MoS2 . The Mo and S densities of states of MoS2 bulk are shown for comparison. the lower coordination of Mo at the surface. The increased amount of occupied and unoccupied surface states around the Fermi level associated to Mo1 suggests that this type of atom has a elecTABLE I Projected charges by angular momentum and total charges by atomic sites.a MoS2 Mo1 Bulk l 0 1 2 3 Total Surface l 0 1 2 3 Total a Total 0.33 0.40 4.23 0.10 5.06 Mo2 S1 0.37 0.58 4.18 0.14 5.27 0.03 3.74 0.31 0.10 4.18 0.42 0.64 4.23 0.15 5.44 0.04 3.57 0.36 0.11 4.08 Co/MoS2 Sphere Mo1 0.37 0.34 0.39 4.22 0.10 5.05 Mo2 S1 Co 0.38 0.57 4.20 0.15 5.30 0.04 3.70 0.28 0.09 4.11 0.38 0.44 7.46 0.03 8.31 0.42 0.64 4.25 0.15 5.46 0.04 3.59 0.32 0.10 4.05 0.31 0.29 7.56 0.02 8.18 Ni/MoS2 Sphere Mo1 0.39 0.34 0.39 4.21 0.10 5.04 Mo2 S1 Ni Sphere 0.38 0.57 4.20 0.15 5.30 0.04 3.71 0.28 0.09 4.12 0.44 0.48 8.36 0.03 9.31 0.39 0.42 0.64 4.25 0.15 5.46 0.04 3.61 0.32 0.09 4.06 0.38 0.34 8.40 0.02 9.14 empty sphere charges are also included in the case of bulk models. INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 411 GÓMEZ-BALDERAS ET AL. The molybdenum and sulfur bands of layer 2 (as seen in Fig. 4) start to transform onto the DOS of the bulk, but it should take deeper layers in a thicker slab model to resemble that of the bulk. Layer 3 is equivalent by symmetry to layer 1, thus when analyzing layer 3 sheet B, we will have the complete electronic structure picture of layer 1 with surface and subsurface Mo and S sites. In contrast with layer 1, the particular importance of layer 3 sheet B is that its main contribution comes from sulfur at the surface, which shows a relatively large intensity of states at the Fermi level. This region of the (10 1̄ 0) surface is supposed to be not active in the HDS reactions. Surface generation produces a decrease in the charge populations of surface and subsurface atoms, but an increase in the bulklike atoms of this model. From Table I, the total charge of Mo1 at the surface diminishes 0.21 valence electrons, and the subsurface S1 charge decreases 0.10 electrons with respect to the corresponding atoms in the bulk. In contrast, Mo2 has 0.23 valence electrons more than in the bulk. Co/MoS2 The main results for Co embedded in the MoS2 bulk are presented in Figure 5. The density of states for the S1 sulfur (first neighbor) and the Mo2 molybdenum (second neighbor) are also depicted there. These atoms are the closest neighbors to Co and are expected to show relevant changes in their electronic structure. If we compare the graphics of MoS2 bulk (Fig. 3) with those of MoS2 bulk doped with Co, the presence of new intense states are evident around the Fermi level, which partially fill the former gap that characterized the MoS2 bulk. As we approach the Fermi level from the occupied states region, Co depletes the density of states of Mo2 and S1, producing a gap just below the Fermi level. The largest peaks belong to Co compared with those of Mo2 and S1. FIGURE 5. Atomic site projected DOS for Co/MoS2 and Ni/MoS2 bulk models. 412 VOL. 80, NO. 3 CATALYTIC ACTIVITY OF MoS2 FIGURE 6. Atomic site projected DOS for Co/MoS2 and Ni/MoS2 (10 1̄ 0) surfaces. In regard to the effects produced by Co inserted at the MoS2 surface (Fig. 6), we have analyzed the DOS of Co, Mo1, and S1 in layer 1 and Mo2 in layer 2. They all are localized in the B sheet (see Fig. 2). Energy gaps are not observed for any atom in Figure 6. In the occupied DOS portion, Co shows the largest intensity values in comparison with the other atoms, but Mo1 has the most intense peak in the unoccupied portion, certainly an effect produced by the presence of Co. The largest Mo1 DOS in this region gives a charge-acceptor character to this atom, favoring the interaction with sulfur. On the other hand, Mo2 has a charge-donor character and seems to be less affected than Mo1 because its spectrum looks similar to that of Mo2 in the pure MoS2 crystal (Fig. 4). If we compare the DOS of Co in the bulk with Co at the surface, the gap that appears before the Fermi level in the bulk (Fig. 5) disappear when Co is at the surface (Fig. 6). This feature is also observed for Mo1 and S1 and may be taken as a clear fingerprint to discriminate between the surface and bulk for this doped crystal. In analyzing the charge distribution, Table I shows that Mo2 in the Co/MoS2 bulk system has a total valence charge of 5.30 electrons. By comparing this value with molybdenum in the pure MoS2 (5.27 electrons), Mo2 is chemically reduced because of the Co impurity. The chemical reduction of Mo in Co/MoS2 and in Ni/MoS2 relative to Mo in pure MoS2 has been also previously observed [1, 10]. On the other hand, the total valence charge of S1 is 4.11 electrons, 0.07 less than in the pure MoS2 . Now, when the surface is present, as we observe in the undoped MoS2 , the total valence charge also decreases in the surface and subsurface atoms of the Co/MoS2 system, from a value of 5.30, 4.11, and 8.31 electrons to 5.05, 4.05, and 8.18 electrons for Mo1, S1, and Co, respectively. In analyzing the populations by angular momentum, Mo2 in the bulk has 4.20, 0.57, and 0.38 electrons for the d, p, and s shells, and Mo1 at the surface gives 4.22, 0.39, and 0.34 electrons for the INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 413 GÓMEZ-BALDERAS ET AL. d, p, and s shells, respectively. Correspondingly, Co in the bulk gets 7.46, 0.44, and 0.38 valence electrons for the d, p, and s shells, while Co at the surface has 7.56, 0.29, and 0.31 electrons for the d, p, and s shells, respectively. Again, the surface raises the electron donor/acceptor character, diminishing s, and p populations (involved in the donation) and increasing the d population (back-donation) in the Mo and Co metallic atoms at the surface. Ni/MoS2 The results for Ni embedded in the MoS2 bulk are presented in Figure 5, as well as for Mo2 molybdenum and the S1 sulfur. Although the total density of states of Ni in the bulk differs in quantitative form from that of Co, the curves of Ni follow a similar pattern to the Co curves, as it would be expected from a rigid band model. The Mo2 and S1 total density of states also resemble that of Co/MoS2 . In regard to the Ni/MoS2 surface, Ni does not show relevant peaks in or near the Fermi level, and we can hardly associate a charge acceptor or donor character to the Ni atom. In general the DOS curves of Mo1, Mo2, and S1 resemble the corresponding curves of the Co/MoS2 surface. Due to the fact that the Ni/MoS2 and Co/MoS2 surface DOS are similar, it is likely that these systems show an equivalent chemical performance, except at the specific sites where the Ni or Co atoms are located. A charge analysis also reveals a similar behavior of Co/MoS2. From Table I it is observed that total charge decreases in the Mo1 and S1 surface atoms and increases in Mo2 subsurface atoms when bulk and surface models are compared. A projected angular moment population also gives the same trends as in the case of Co/MoS2; that is, the s and p populations decrease in the metal atoms, but the d population is increased. Promoter atoms in the bulk produce a slight increase of 0.02 electrons in the Mo 4d population for both bimetallic systems, as compared with that of the pure MoS2 . However, we observe a small decrease in the 4d population of Mo1 if we compare the pure MoS2 surface with bimetallic surfaces. Hence, the back-donation character of Mo1 at the bimetallic surfaces would be reduced by the presence of impurity atoms. In contrast, Co and Ni 3d populations are increased 0.10 and 0.04 electrons or, equivalently, the back-donation character of Co and Ni is increased at the surface. 414 Conclusions The MoS2 crystal structure is an effective catalyst component commonly used in the oil-refining industry. The insertion of Co or Ni into the MoS2 structure is known to increase the catalytic activity. In order to understand the improved catalytic performance of the doped MoS2 crystal, we carried out electronic structure calculations for MoS2 bulk and surface models. Our results confirm the picture in which Co and Ni atoms transfer charge to Mo, increasing its 4d electron density. A relative chemical reduction of Mo subsurface atoms of the (10 1̄ 0) surface was also shown in the presence of Co and Ni atoms. As long as the density of states is a relevant parameter for describing reactivity, our results indicate that the electronic structure of Mo at the surface is very similar in both bimetallic sulfides Co/MoS2 and Ni/MoS2 , which have a similar catalytic activity. This fact suggests that molybdenum at the surface is the relevant atomic center for the reactivity in the hydrodesulfurization process. On the other hand, Co and Ni exhibit different electronic structure at the (10 1̄ 0) surface; however, they are responsible for increasing the reactivity of the MoS2 surface. Then, they can not be considered as the central atomic sites in the hydrodesulfurization process. The catalytic reaction mechanism involves charge redistribution induced by the adsorption of the guest sulfide molecule onto the MoS2 surface. In fact, charge flows from molecular orbitals of the donor to the molecular orbitals of surface atoms. Such a step is favored by the insertion of promoters that produce a shift of unoccupied states from higher energies to the Fermi level. On the other hand, it is established that back-donation, from metal to antibonding molecular states, facilitates the weakening of a carbon–sulfur bond. Our results suggest that the back-donation process is not favored by molybdenum 4d orbitals but favored by promoter 3d orbitals at the surface. ACKNOWLEDGMENTS One of us (R.G.B.) acknowledges support from DGAPA-UNAM. Financial support from Instituto Mexicano del Petróleo (Ref. num. FIES-96-15-III) is also acknowledged. VOL. 80, NO. 3 CATALYTIC ACTIVITY OF MoS2 References 1. Harris, S.; Chianelli, R. R. J Catal 1986, 98, 17. 2. Topsoe, H.; Clausen, B. S.; Massoth, F. E. in Catalysis: Science and Technology, Vol. 11; Boudart, M.; Anderson, J. A., Eds.; Springer: Berlin, 1996; p. 57. 3. Chianelli, R. R.; Daage, M.; Ledoux, M. J. Adv Catal 1994, 40, 177. 4. Chianelli, R. R. 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