Potential-Driven Evolution of Coordination and Oxidation State in Cu–N–C Delivers Efficient and Selective Acetylene Semihydrogenation

Electrocatalytic acetylene semihydrogenation (eASH) offers a sustainable route for ethylene purification, and the Cu single-atom catalyst (SAC) anchored on N-doped carbon (Cu-N-C) delivers exceptional performance in catalyzing eASH. However, identifying its true active sites under operating conditions remains a challenge. Here, using grand canonical ensemble density functional theory, we reveal that the Cu-N-C SAC undergoes a drastic, potential-driven structural evolution that fundamentally governs its catalytic performance. We construct a surface Pourbaix diagram to map the thermodynamic landscape of the active centers, demonstrating that the square-planar Cu(II)N4 motif reconstructs into low-coordinate, linear Cu(I)-hydride species under reducing potentials. This operando-generated Cu(I)-hydride center exhibits exceptional chemoselectivity, favoring acetylene adsorption via soft-soft acid-base interactions while kinetically suppressing overhydrogenation of ethylene and the hydrogen evolution reaction. Furthermore, we uncover a potential-dependent switch in the reaction mechanism: at low overpotentials, the reaction is mediated by interfacial water, whereas at high overpotentials, a pendant hydrogenated nitrogen ligand on the catalyst surface serves as an intramolecular proton shuttle, facilitating a highly efficient surface-enabled pathway. These findings establish a unified mechanistic paradigm linking applied potential to coordination geometry, oxidation state, and proton-transfer kinetics, providing critical insights for the design of high-performing eASH SACs.