Chemisorption Can Reverse Defect-Defect Interaction on Heterogeneous Catalyst Surfaces.

Atomic-level understanding of roles of defect-defect interaction in the bonding of adsorbates on surfaces is critical for tailoring catalysts atom-by-atom and designing new catalysts. Here, from first-principles calculations, we propose a microscopic mechanism for the role of sulfur vacancy-vacancy interaction in hydrogen bonding on surfaces of MoS, a nonprecious two-dimensional catalyst for hydrogen evolution reaction. We find that before hydrogen adsorption the interaction of a sulfur vacancy with others is repulsive, originating from the antibonding-like coupling of occupied in-gap vacancy states. When the sulfur vacancy is adsorbed by a hydrogen atom, its interaction with other unadsorbed sulfur vacancies becomes attractive, which can be attributed to the decoupling of repulsive vacancy-vacancy interactions and the occupying of bonding-like coupling states between the in-gap vacancy states that are unoccupied before hydrogen adsorption. This repulsive-to-attractive reverse of vacancy-vacancy interaction reduces the hydrogen adsorption energy and explains why the hydrogen adsorption energy decreases with increasing sulfur vacancy concentration. The emerging picture enables a more general discussion of local defect effects on the adsorption of various adsorbates at different surfaces, providing guidance to improve catalytic performance through defect engineering.