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 MoS2, 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.