The stability of the (100) MoS2 surface has been studied using periodic DFT calculations taking into account various parameters such as the temperature and the partial pressure ratios of H2 and H2S present in the surrounding atmosphere. It appears that the sulfur coverage of the surface is strongly dependent on the H2/H2S ratio and that under working conditions, the most stable surface does not contain any coordinately unsaturated sites (CUS). Direct comparisons with experimental literature data such as EXAFS or TPR measurements show a good agreement between calculations and these experiments. The second part of the study deals with the behavior of hydrogen on the surfaces. The endothermic dissociation always leads to Mo−H and S−H groups. This implies that hydrogen is not stable on the MoS2 surface unless at very high pressure or very low temperature. Furthermore, H2 dissociation on the surface will not lead to the formation of CUS.

Both cubic and hexagonal phases of ZnS are modeled using interatomic potential based simulations and density functional theory. A new set of potential parameters is derived, showing improved behavior compared with the previous ones. Results obtained with this new potential model show very good agreement with those obtained with density functional theory calculations and with experimental results when available. To calculate crystal morphologies for both phases, we perform an extensive study of the surface energies. In the cubic phase we take into account all the nonpolar surfaces with Miller indexes 0, 1, 2, 3, and 4, and all the polar surfaces with indexes 0, 1, and 2. The nonpolar (110) surface is the most stable surface in this phase and entirely dominates the crystal morphology, which is a dodecahedron showing only the (110) surface and its equivalents. In the hexagonal phase we find that it is necessary to take into account polar surfaces to obtain the crystal morphology, which has a highly anisotropic, cylindrical-like shape, with nonpolar surfaces on the sides and polar surfaces closing the cylinder.

Hydrogen adsorption on Mo[bond]S, Co[bond]Mo[bond]S, and Ni[bond]Mo[bond]S (10 1 macro 0) surfaces has been modeled by means of periodic DFT calculations taking into account the gaseous surrounding of these catalysts in working conditions. On the stable Mo[bond]S surface, only six-fold coordinated Mo cations are present, whereas substitution by Co or Ni leads to the creation of stable coordinatively unsaturated sites. On the stable MoS(2) surface, hydrogen dissociation is always endothermic and presents a high activation barrier. On Co[bond]Mo[bond]S surfaces, the ability to dissociate H(2) depends on the nature of the metal atom and the sulfur coordination environment. As an adsorption center, Co strongly favors molecular hydrogen activation as compared to the Mo atoms. Co also increases the ability of its sulfur atom ligands to bind hydrogen. Investigation of surface acidity using ammonia as a probe molecule confirms the crucial role of sulfur basicity on hydrogen activation on these surfaces. As a result, Co[bond]Mo[bond]S surfaces present Co[bond]S sites for which the dissociation of hydrogen is exothermic and weakly activated. On Ni[bond]Mo[bond]S surfaces, Ni[bond]S pairs are not stable and do not provide for an efficient way for hydrogen activation. These theoretical results are in good agreement with recent experimental studies of H(2)[bond]D(2) exchange reactions.