Adsorption and dissociation of hydrogen molecules over S-vacancies in a Nb-doped MoS 2 monolayer.

Motivated by the recent interest in the hydrogen energy, we have carried out a complete study of the catalytic activity of a defective molybdenum disulfide monolayer (MoS2) by means of density functional theory (DFT) calculations. The MoS2 monolayer is characterized by a nonreactive basal plane. In principle, its catalytic activity is concentrated at the edges, but an alternative way to increase such activity is obtained by creating active sites where the molecules can dissociate. These defects can be easily produced experimentally by different techniques. In our study, we have performed an atomic, energetic and electronic analysis of a hydrogen molecule adsorbed on a MoS2 monolayer. In a first step, we have found that the H2 molecule remains physisorbed over both doped-free and Nb-doped MoS2 monolayers, showing that the Nb atom does not increase the poor reactivity of the clean MoS2 layer. 
Interestingly, our energetic results suggest that the vacancies will prefer to be formed close to the Nb atoms in the doped monolayer, but the small energy difference would allow the formation in non-doped like sites. Theoretically, we found out the conditions for the molecular dissociation on a S vacancy. In both cases, with and without Nb, the molecule should rotate from the original perpendicular position to an almost parallel orientation jumping an energetic barrier. After that, the atoms are separated binding to the Mo atoms around the missing S atom. Our ab initio molecular dynamics simulations show that for low pressure conditions (using one single molecule in the system) the H2 prefers to desorb from the vacancy, while for larger pressures (when additional H2 molecules are added to the system) the molecule is finally dissociated on the vacancy. Our long simulations confirm the great stability of the structure with the two H atoms binding to the Mo atoms close to the vacancy. 
Finally, the inclusion of a third (or a fourth) H atom in the vacancy leads to the formation and desorption of a H2 molecule, leaving one (or two) atoms in the vacancy.