Valentin Alexiev

Ab initio study of MoS2 and Li adsorbed on the (100) face of MoS2

Periodic Hartree–Fock methods were used to calculate the geometric and electronic properties of 2H-MoS2 , 1T-MoS2, the (100) surface of MoS2 and Li adsorbed thereon. For the calculations, the structures were generated by an extension of unit cells to the respective bulk structures (1T- and 2H-MoS2) or by cutting sections, each consisting of six or eight layers of sulfur and molybdenum, from a crystal ((100) surface of MoS2 with and without adsorbed Li). Structural optimization was performed with a post Hartree–Fock DFT correlation correction. The calculated structures of 2H-MoS2 and 1T-MoS2 are in good agreement with experimental data and the metastable and metallic properties of 1T-MoS2 are also described correctly. The relaxation of the (100) surface of 2H-MoS2 leads to a minor reconstruction of the surface accompanied by the formation of S2 species and an inward relaxation of Mo atoms. Adsorption of Li on this surface is favoured in the high symmetry positions above the van der Waals gap. Relaxation of the Li-covered (100) surface of 2H-MoS2 shifts the Li atoms towards the S2 pairs and closer to the surface. Upon adsorption, the system becomes metallic and delocalized surface states form at the Fermi level due to electron transfer processes from the Li atoms to the surface layers of MoS2.

DFT Study of MoS2 and Hydrogen Adsorbed on the (1010) Face of MoS2

DFT calculations were employed to investigate the properties of the catalytically important (100) edge of MoS2 and the adsorption of hydrogen thereon. The electronic properties of the bulk and surface as well as the relaxed positions of surface atoms are calculated by two different techniques, namely all-electron and plane wave pseudo-potential DFT methods. Hydrogen adsorption is studied by means of a (2×1) surface cell to account for H–H interactions and the different configurations of adsorbed hydrogen atoms. Our calculations demonstrate that the electronic structure and the positions of the relaxed surface atoms obtained by the two methods are identical, with the exception of minor discrepancies which are attributed to the different relaxation procedures. The (100) surface of MoS2 contains weakly coupled S–S pairs formed by relaxation of its sulfur atoms. The results of the hydrogen adsorption show that hydrogen is adsorbed on the sulfur pairs but not on Mo atoms on the surface. Only one of the possible configurations of adsorbed H atoms on a (2×1) cell is energetically favoured, while the energy of the other configurations is higher with respect to the energy of H2 and MoS2.

Studies on intermolecular interactions of metal chelate complexes. V. copper(ii) dithiophosphate complexes: an example of an inner self-redox reaction

SummaryStudies of copper dithiophosphate (dtp) complexes by various physical methods, in the solid and the molten state as well as in solution, are reported. In the solid state all the complexes of dithiophosphate (RO)2PS2− [R = CnH2n+1 (n=1–4), Ph, orcyclo-C6H11] are diamagnetic but in the molten state and in solution they are paramagnetic. Interconversions were found to be reversible, and the effect was ascribed to an inner self-redox reaction. Only the bulkyo-tolyl derivative is paramagnetic in the solid and molten states and in solution. It is proposed that the self-redox reaction involves association between two molecules of CuII(dtp)2 during crystallization, followed by formation of [CuI(dtp)]2, and (RO)2P(S)S-S(S)P(OR)2, and then [CuI(dtp)]4. The molecular structures of complexes with R = isopropyl ando-tolyl confirm these inferences.

Ab initio study of 2H MoS2 by using Hay and Wadt effective core pseudopotentials for modelling the surface structure

Periodic Hartree–Fock and DFT methods were employed to calculate the geometric and electronic properties of bulk 2H-MoS2 and of its catalytically important (100) edge structure. The core electrons of molybdenum and sulfur were represented by the effective core pseudo-potentials, developed by Hay and Wadt. For the calculations (100) type surface structures were generated by cutting sections from an 2H-MoS2 crystal, which consists of four, six or eight rows of molybdenum and sulfur atoms. The calculated elastic constants follow the experimental constants and show that 2H-MoS2 is an anisotropic covalent compound held together by weak dispersion interactions between neighbouring S-Mo-S units. The relaxation of the (100) surface of 2H-MoS2 leads to an inner relaxation of the Mo atoms and the formation of weakly coupled surface S-S species. As a result of the surface relaxation, the electronic charge of the surface states is different to that of the bulk states: empty Mo d states move into the band gap, and the surface becomes a better electron acceptor. In all calculations the model consisting of six rows of sulfur and molybdenum atoms is the most favourable one, because it provides an accurate description of the 2H-MoS2 surface structure and enables calculations at reasonable computation time. The calculations based on pseudo-potentials are in good agreement with all-electron calculations and confirm that, in contrast to the semiconducting bulk, the (100) 2H-MoS2 surface has metallic properties.

Studies on intermolecular interactions of metal chelate complexes. VI. On the selfredox reaction of copper(II) dithiophosphinates

SummaryThe kinetics of the inner selfredox reaction of Cu(dtph)2 (dtph = dithiophosphinate) was studied by e.p.r. The reaction is second order in CuII(dtph)2 with effective rate constant 5.5 dm3 mol−1s−1 (at 300 K); the plot of log keff/(1/T) is not of Arrhenius type, suggesting that keff depends on the equilibrium constant for the formation of a precursor between two molecules of CuII(dtph)2. On the other hand, the differences in a0 and aav (a0<aav) extracted from e.p.r. spectra in solution and frozen solution or magnetically dilute samples suggest that CuII(dtph)2 is associated in solution, whereas in the other two forms it is a monomer. The observed instability of CuII(dtph)2 towards inner selfredox reaction is compared to that of other sulphur-containing CuII complexes and discussed in terms of the relatively low redox potential of the ligand.

Density Functional Theory Calculations of the Dissociation of H2 on (100) 2H-MoS2 Surfaces: A Key Step in the Hydroprocessing of Crude Oil

Hydrogen activation on the (100) surface of MoS2 structures was investigated by means of density functional theory calculations. Linear and quadratic synchronous transit methods with a conjugate gradient refinement of the saddle point were used to localize transition states. The calculations include heterolytic and homolytic dissociation of hydrogen; that is, an H2 molecule dissociates on an MoS2 catalyst surface into two hydrogen atoms, which react further with the catalyst surface under formation of either one Mo-H and one S-H (heterolytic) or of two S-H surface groups (homolytic). Our results favor the heterolytic adsorption of hydrogen. Ni- and Co-promoted MoS2 have been considered to investigate the secondary promotional effect on the H2 dissociation. The authors observed a negative secondary promotional influence on the H2 dissociation in the case of Ni-promoted MoS2, whereas Co shows a positive effect.

Energetics and electronic properties of defects at the (100) MoS2 surface studied by the perturbed cluster method

The properties of defects on the (100) MoS2 surface have been investigated by the perturbed cluster method. The perturbed cluster method provides an accurate description of the local defect properties while taking into account the interaction between the defect and the surrounding crystal. The surface energies, including correlation correction, of different defect structures of various sizes on the (100) MoS2 surface are reported and compared with the energy of a reference surface cluster. The results, in conjunction with calculations of the electronic properties and electrostatic potential of the different defect sites, show that the chemistry of the defects differs from that of the perfect (100) “as-cleaved” surface. The enhanced reactivity of the defects is ascribed to the anisotropy in the electrostatic potential. The presence of “nodes” in the surface electrostatic potential suggests that the adsorption of small polarizable molecules will preferentially take place in the vicinity of these defects.