Ab initio Study of Structural Stability of Mo−S Clusters and Size Specific Stoichiometries of Magic Clusters

Non-Dissociative Activation of Chemisorbed Dinitrogen on One or Two Vanadium Atoms Supported by a Mo6 S8 Cluster

Adsorption of N₂ on Mo₆ S₈ ^q _V_x clusters (x=0, 1, 2; q=0, ±1) were systematically studied by density functional theory calculations with dispersion corrections. It was found that the N₂ can be chemisorbed and undergo non-dissociative activation on single or double metal atoms. The adsorption and activation are influenced by metal types (V or Mo), N₂ coordination modes and charge states of the clusters. Particularly, anionic Mo₆ S₈ ^- _V₂ clusters have remarkable ability to fix and activate N₂ . In Mo₆ S₈ ^- _V₂ , two V atoms prefer to adsorb on two adjacent S-Mo-S hollow sites, leading to the formation of a supported V…V unit. The N₂ is bridged side-on coordinated with these two V atoms with high adsorption energy and significant charge transfer. The bond order, bond length and vibration frequency of the adsorbed N₂ are close to those of a N-N single bond.

Structural Properties of Gas Phase Molybdenum Sulfide Clusters [Mo3S13]2-, [HMo3S13]-, and [H3Mo3S13]+ as Model Systems of a Promising Hydrogen Evolution Catalyst

Amorphous molybdenum sulfide (MoS _x ) is a potent catalyst for the hydrogen evolution reaction (HER). Since mechanistic investigations on amorphous solids are particularly difficult, we use a bottom-up approach and study the [Mo₃S₁₃]^2- nanocluster and its protonated forms. The mass selected pure [Mo₃S₁₃]^2- as well as singly and triply protonated [HMo₃S₁₃]^- and [H₃Mo₃S₁₃]⁺ ions, respectively, were investigated by a combination of collision induced dissociation (CID) experiments and quantum chemical calculations. A rich variety of H _x S _y elimination channels was observed, giving insight into the structural flexibility of the clusters. In particular, it was calculated that the observed clusters tend to keep the Mo₃ ring structure found in the bulk and that protons adsorb primarily on terminal disulfide units of the cluster. Mo-H bonds are formed only for quasi-linear species with Mo centers featuring empty coordination sites. Protonation leads to increased cluster stability against CID. The rich variety of CID dissociation products for the triply protonated [H₃Mo₃S₁₃]⁺ ion, however, suggests that it has a large degree of structural flexibility, with roaming H/SH moieties, which could be a key feature of MoS _x to facilitate HER catalysis via a Volmer-Heyrovsky mechanism.

Small stoichiometric (MoS2)n clusters with the 1T phase

Stoichiometric (MoS₂)_n clusters (n = 1–6) were systematically studied by density functional theory calculations with hybrid B3LYP and pure GGA PW91 functionals.

Gas-Phase Reactivity Studies of Small Molybdenum Cluster Ions with Dimethyl Disulfide

Molybdenum sulfide is a potent hydrogen evolution catalyst, and is discussed as a replacement of platinum in large-scale electrochemical hydrogen production. To learn more about the elementary steps of MoS₂ production by sputtering in the presence of dimethyl disulfide (DMDS), the reactions of Mo_x⁺, x = 1–3, with DMDS are studied by Fourier transform ion cyclotron resonance mass spectrometry and density functional theory calculations. A rich variety of products composed of molybdenum, sulfur, carbon and hydrogen was observed. Mo_xS_y⁺ species are formed in the first reaction step, together with products containing carbon and hydrogen. The calculations indicate that the strong Mo-S bonds are formed preferentially, followed by Mo–C bonds. Hydrogen is exclusively bound to carbon atoms, i.e. no insertion of a molybdenum atom into a C–H bond is observed. The reactions are efficient and highly exothermic, explaining the rich chemistry observed in the experiment.

Bond Activation and Hydrogen Evolution from Water through Reactions with M3S4 (M = Mo, W) and W3S3 Anionic Clusters

Transition metal sulfides (TMS) are being investigated with increased frequency because of their ability to efficiently catalyze the hydrogen evolution reaction. We have studied the trimetallic TMS cluster ions, Mo₃S₄^-, W₃S₄^-, and W₃S₃^-, and probed their efficiency for bond activation and hydrogen evolution from water. These clusters have geometries that are related to the edge sites on bulk MoS₂ surfaces that are known to play a role in hydrogen evolution. Using density functional theory, the electronic structures of these clusters and their chemical reactivity with water have been investigated. The reaction mechanism involves the initial formation of hydroxyl and thiol groups, hydrogen migration to form an intermediate with a metal hydride bond, and finally, combination of a hydride and a proton to eliminate H₂. Using this mechanism, free energy profiles of the reactions of the three metal clusters with water have been constructed. Unlike previous reactivity studies of other related cluster systems, there is no overall energy barrier in the reactions involving the M₃S₄ systems. The energy required for the rate-determining step of the reaction (the initial addition of the cluster by water) is lower than the separated reactants (-0.8 kcal/mol for Mo and -5.1 kcal/mol for W). They confirm the M₃S₄^- cluster's ability to efficiently activate the chemical bonds in water to release H_2. Though the W₃S₃^- cluster is not as efficient at bond activation, it provides insights into the factors that contribute to the success of the M₃S₄ anionic systems in hydrogen evolution.

First principles modeling of Mo6S9 nanowires via condensation of Mo4S6 clusters and the effect of iodine doping on structural and electronic properties

By employing first principles DFT calculations, we propose a new stable model for Mo6S9 nanowires (NWs) obtained by condensing tetrahedral Mo4S6 clusters rather than octahedral Mo6S8 clusters, which are known as magic clusters in the Mo-S polyhedral cluster family. The pristine NW is found to be metallic and its local structure and physical properties can be tuned by doping of iodine atoms. This doping increases the number of Mo-Mo bonds in the NW, thus, Mo4 tetrahedra are initially fused to the Mo6 octahedron, and then, to the Mo8 dodecahedron. Further, a close correlation among the Mo-Mo bonding in the local structure, mechanical and electronic properties, is observed from our study. Finally, the stability of the pristine and iodine doped Mo8S12-xIx NW structures obtained from condensation of Mo4 tetrahedra are found to be quite comparable with that of already reported Mo6S9-xIx NWs with Mo6 octahedra as building blocks.

Theoretical study on the reaction mechanism of reverse water–gas shift reaction using a Rh–Mo6S8 cluster

The reverse water gas shift (RWGS) reaction catalyzed by a Rh–Mo₆S₈ cluster is investigated using density functional theory calculations.

Molybdenum oxides versus molybdenum sulfides: geometric and electronic structures of Mo₃X(y)⁻ (X = O, S and y = 6, 9) clusters

We have conducted a comparative computational investigation of the molecular structure and water adsorption properties of molybdenum oxide and sulfide clusters using density functional theory methods. We have found that while Mo₃O₆⁻ and Mo₃S₆⁻ assume very similar ring-type isomers, Mo₃O₉⁻ and Mo₃S₉⁻ clusters are very different with Mo₃O₉⁻ having a ring-type structure and Mo₃S₉⁻ having a more open, linear-type geometry. The more rigid ∠(Mo-S-Mo) bond angle is the primary geometric property responsible for producing such different lowest energy isomers. By computing molecular complexation energies, it is observed that water is found to adsorb more strongly to Mo₃O₆⁻ than to Mo₃S₆⁻, due to a stronger oxide-water hydrogen bond, although dispersion effects reduce this difference when molybdenum centers contribute to the binding. Investigating the energetics of dissociative water addition to Mo₃X₆⁻ clusters, we find that, while the oxide cluster shows kinetic site-selectivity (bridging position vs terminal position), the sulfide cluster exhibits thermodynamic site-selectivity.

Sulfur-based redox reactions in Mo3S7(4+) and Mo3S4(4+) clusters bearing halide and 1,2-dithiolene ligands: a mass spectrometric and density functional theory study

The gas phase fragmentation reactions of sulfur-rich [Mo(3)S(7)Br(6)](2-) (1(2-)), [Mo(3)S(7)(bdt)(3)](2-) (2(2-)), and [Mo(3)S(4)(bdt)(3)](2-) (3(2-)) (bdt = benzenedithiolate) complexes have been investigated by electrospray ionization (ESI) tandem mass spectrometry and theoretical calculations at the density functional theory level. Upon collision induced dissociation (CID) conditions, the brominated 1(2-) dianion dissociates through two sequential steps that involves a heterolytic Mo-Br cleavage to give [Mo(3)S(7)Br(5)](-) plus Br(-) followed by a two-electron redox process that affords [Mo(3)S(5)Br(5)](-) and diatomic S(2) sulfur. Dianion [Mo(3)S(7)(bdt)(3)](2-) (2(2-)) dissociates through two sequential redox processes evolving diatomic S(2) sulfur and neutral bdt to yield [Mo(3)S(5)(bdt)(3)](2-) and [Mo(3)S(5)(bdt)(2)](2-), respectively. Conversely, dianion [Mo(3)S(4)(bdt)(3)](2-) (3(2-)), with sulfide instead of disulfide S(2)(2-) bridged ligands, remains intact under identical fragmentation conditions, thus highlighting the importance of disulfide ligands (S(2)(2-)) as electron reservoirs to trigger redox reactions. Regioselective incorporation of (34)S and Se at the equatorial position of the Mo(3)S(7) cluster core in 1(2-) and 2(2-) have been used to identify the product ions along the fragmentation pathways. Reaction mechanisms for the gas-phase dissociation pathways have been elucidated by means of B3LYP calculations, and a comparison with the solution reactivity of Mo(3)S(7) and Mo(3)S(4) clusters as well as closely related Mo/S/dithiolene systems is also discussed.

Assembling nanowires from Mo-S clusters and effects of iodine doping on electronic structure

Using ab initio calculations, we find high stability of octahedral Mo6S8 clusters, which can further be condensed to form Mo3nS3n+2 (n, an integer) nanowires. These linear structures are energetically more favorable compared with other closed-packed polyhedral isomers of Mo-S clusters. The octahedral units in nanowires are stabilized by strong Mo-Mo interactions and p-d hybridization between Mo 4d and S 2p orbitals. There is a free electron-like band that crosses the Fermi energy in infinite nanowires and leads to their metallic character. Iodine doping acts as electron donor and can be used to tailor the electronic conductivity. For Mo12S8I4 nanowires, both electrons and holes are found to contribute to conduction. These nanowires are energetically more favorable than the experimentally obtained Mo12S6I12 nanowires.