Ab Initio Study of the Structural and Electronic Properties of a Real Size MoS₂ Slab:  Mo₂₇S₅₄

Abstract

It is shown that ab initio MO approaches are suitable to simulate the relatively large hexagonal MoS 2 slab corresponding to the average initial size (15-20 Å) of MoS 2 on alumina-supported catalysts used in hydrodesulfurization (HDS) and hydrogenation (HYD) of petroleum fractions. By using a general ab initio MO program, GAMESS, the structure optimization of the Mo 27 S 54 slab is possible at the UHF/MINI/ECP)SBK level of theory, providing helpful information on the structural and electronic properties of the slab. The improvement, compared to other theoretical calculations based on simplified structural models, is that the different bonding and electronic properties of the Mo and S atoms in basal plane and on different edges and corners of the slab can clearly be distinguished. The structure of the slab is relaxed toward the edges. The 2-fold coordinatively unsaturated sites (CUS) on corners have both electron-donating and -accepting properties, making them potentially active for HDS (and HYD) reactions. The 2-fold CUS's on the edges ("reduced edges") seem to permit only the HYD reaction or CO hydrogenation. The removal of the basal plane S atoms requires activated atomic hydrogen species. The unreduced (1 h010) edges do not seem to be potentially active catalytically. This deepens the understanding of the catalytic nature of the MoS 2 structure for several reactions, especially HDS and HYD (hydrotreating reactions) and CO hydrogenation.

Formation and adsorption properties of the bridging sulfur vacancies at the (1¯ 0 1 0) edge of Mo27S(54−x): A theoretical study

“…Compared to the other cases, reactions (3) and (7) show high positive enthalpy (14.7 and 41.5 kcal/mol), indicating that this type of sulfur vacancy is relatively difficult to form. The negative enthapies of reactions (2), (4), (6), (8) and (10) show that formations of corresponding reduced structure are thermodynamically favorable for atomic hydrogen.…”

“…Furthermore, it has been shown experimentally that the removal of S atoms from a MoS 2 slab starts from the edge on which the formed CUS has strong adsorption affinity for thiophene and H 2 S and is responsible for the thiophene HDS [5,6]. Although it is easy to form 2-fold CUS on the (1 0 1 0) edge of MoS 2 even under the atmosphere of H 2 , the 2-fold CUSs may not activate the thiophene to the degree of cleaving S C bond of thiophene [7,8]. More specifically, 3-and 4-fold CUSs formed at the edge of (1 0 1 0) together with SH groups seem to be responsible for the activation of HDS [7,9].…”

“…In addition, the roles of atomic hydrogen and proton are not also taken into account. Using cluster model, Li et al [8] optimize the structure of Mo 27 S 54 cluster whose size is similar to that in supported catalysts. They pointed out the Mo sites situated in (1 0 1 0) edge are more active for HDS reaction when the sulfur atoms blocking the Mo atoms are removed.…”

Application of computational methods to catalytic systems

“…It was found that the reaction of H 2 with MoS 2 clusters was endothermic. Analyzing a bigger cluster, Mo 27 S 54 [58], the authors concluded that there are two types of twofold unsaturated sites (CUS): those on the corners are potentially active sites for HDS reactions while the others on the (1010) edges are only reactive to -bond activation.…”

How to find an optimum cluster size through topological site properties: MoSxmodel clusters

“…There are in the literature several works on MoS 2 clusters using a variety of cluster sizes, with methods ranging from semiempirical 17,[41][42][43][44][45][46] to ab initio and DFT. 3,18,30,[47][48][49][50][51][52][53][54] In particular, the investigation of Wen et al 52 concentrated in the study of structure and stability of MoS x clusters with the change of sulfur coverage on both Mo and S edges and the effect of cluster sizes on the surface structures from a thermodynamic point of view.…”