Origin of adsorption trends in two-dimensional single-atom catalysts via d-state filling

Single-atom catalysts supported on two-dimensional materials have attracted significant attention due to their tunable local bonding environments and unique electronic structures. In this work, we systematically investigate the adsorption of light atoms (H, C, N, O, Si, P, and S) on transition-metal sites anchored on N-doped graphene and WS₂ monolayers using density functional theory. We find that adsorption energies across different adsorbates and supports exhibit a unified trend governed by the filling of metal d-states. Specifically, the adsorption strength follows the progressive occupation of bonding and nonbonding states, followed by the onset of antibonding state filling, which weakens adsorption. This picture is consistent with previously proposed electron-counting concepts and is shown here to be robust across different two-dimensional supports, indicating that the adsorption characteristics are primarily controlled by the local electronic structure of the metal center. Finally, we demonstrate the practical relevance of this understanding by applying it to the rational design of catalysts for the nitrogen reduction reaction, offering general insights for optimizing the activity of two-dimensional single-atom catalysts.