First-principles modeling of the highly dynamical surface structure of a MoS catalyst with S-vacancies.

Vacancy sites, , S-vacancies, are essential for the performance of MoS catalysts. As earlier studies have revealed that the size and shape of the S-vacancies may affect the catalytic activity, we have studied the behavior and mobility of such vacancies on MoS using DFT calculations and kinetic Monte-Carlo (kMC) simulations. The diffusion barriers for the S-vacancies are highly dependent on the immediate environment: isolated single S-vacancies are found to be immobile. In contrast, small S-vacancies formed from = 2 to 5 neighboring S-vacancies are often highly dynamic systems that can move within a confined area. Large extended S-vacancies are generally unstable and transform quickly into alternating patterns of S-atoms and vacancy sites. These results illustrate the importance of recognizing MoS (but also other catalysts) as dynamic structures when trying to tune their catalytic performances by introducing specific defect structures.