▎ 摘　　要

Rational design of single-atom catalyst (SAC) presents a promising route to precise heterogeneous catalysis, yet it requires us to understand the catalytic activity that often can be correlated with the adsorption energies. Here, we investigate the hydrogen adsorption on a series of 3d transition metal (TM) SACs anchored on the N-4-coordination site of N-doped graphene (MN4/C), and find that the adsorption energies present a volcano curve that violates the d-band theory. By decomposing the adsorption energies into two distinctive contributions, i. e., the orbital interaction (Delta Eorbit ) and electrostatic interaction (Delta Eelstat ), we find that it is the competition between the two that results in the volcano curve. We further identify that the trend of Delta Eorbit is dictated by the TM 4 s orbital that is governed by bonding with the substrate, while Delta Eelstat is regulated by the charge transfer from TM single-atom to the N-4/C substrate, which originates mainly from the bonding between the TM 3d(xy) orbital and the substrate. Furthermore, we establish the intrinsic dipole moment of active site as a quantitative descriptor for both Delta Eorbit and Delta Eelstat in the adsorption on MN4/C, and apply it to understanding the trends of adsorption energies on 4d and 5d TM-based MN4/C SACs. Our findings provide deep insights into understanding the adsorption and thus the catalytic activity on TM-based SACs.