Hydrogen activation on Mo-based sulfide catalysts, a periodic DFT study

Hydrogen adsorption on Mo-S, Co-Mo-S, and Ni-Mo-S (10 (1) over bar0) 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-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 MoS2 surface, hydrogen dissociation is always endothermic and presents a high activation barrier. On Co-Mo-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-Mo-S surfaces present Co-S sites for which the dissociation of hydrogen is exothermic and weakly activated. On Ni-Mo-S surfaces, Ni-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-D-2 exchange reactions.