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.

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.

No abstract

No abstract

No abstract

We investigate theoretically, by quantum DFT calculations, the adsorption of H2 molecules on the [100] MoS2 surfaces, considering various edge sulfur stoichiometries. Depending on the nature of the gas phase, the adsorption energies vary from strongly positive values to strongly negative ones. Using these energies, we have constructed a thermodynamic diagram, which gives the stoichiometry of the edges and the nature of the adsorbed hydrogen atoms as a function of the total pressure and of the P H 2 S/P H 2 partial pressure ratio and determines the best conditions to examine the S−H groups using spectroscopic techniques.

In this work is reported a periodic density functional theory study of the vacancy formation mechanism on the [10−10] and the [−1010] edges of MoS2 nanocrystallites that are the active phases in hydrodesulfurization catalysis. It has been previously shown that, from a thermodynamic point of view, there should be only very few vacancies on these edges and that their number is only slightly influenced by an increase in the hydrogen partial pressure. The kinetics of the vacancy creation is now considered through a detailed analysis of the intermediates and transition states found on the different pathways for the extraction of a surface sulfur atom by a hydrogen molecule of the gas phase. Only on one of the crystallite edges, the (10−10) (metallic edge), does the activation energy of the rate determining step of the vacancy formation remain smaller than 1 eV. This value allows us to consider that a dynamic equilibrium takes place on this edge. The rate-determining step is the heterolytic dissociation of the H2 molecule, leading to the formation of one S−H and one Mo−H group. The activation energy for the H2S departure is estimated to be 0.50 eV, in nice agreement with the value deduced from sulfur exchange experiments. The vacancy formation is possible on the [−1010] edge of the crystallite (the sulfur edge) but the rate determining step, which is the displacement of one S−H group on the surface, has an activation energy of 1.25 eV. This kind of vacancy formation on the sulfur edge does not imply the departure of one H2S molecule from the surface.

Experimental IR spectra of carbon monoxide adsorbed on a series of Mo/Al2O3, CoMo/Al2O3, and NiMo/Al2O3 sulfided catalysts have been compared to ab initio DFT calculations of CO adsorption on CoMo and NiMo model surfaces. This approach allows the main IR features of CO adsorbed on the sulfide phase to be assigned with an uncertainty of 15 cm(-1). On the CoMo system, the band at 2070 cm(-1) is specific of the promotion by Co and is assigned to CO interacting either with a Co atom or with a Mo atom adjacent to a Co atom. On the NiMo system, CO adsorption on Ni centers of the promoted phase leads to a high-wavenumber band at approximately 2120 cm(-1) that strongly overlaps the band at 2110 cm(-1) characteristic of nonpromoted Mo sites. For NiMo and CoMo catalysts, broad shoulders at low wave numbers (below 2060 cm(-1)) are characteristic of Mo centers adjacent to promoter atoms, indicating a partial decoration of the MoS2 edges by the promoter.