Switching Hydrogen Transfer Pathway Over Paddle-Wheel Dicopper Molecular Catalysts for Efficient Electrocatalytic Acetylene Semihydrogenation

Elucidating the hydrogen transfer pathway of electrocatalytic acetylene semihydrogenation (EAH) is crucial for understanding the catalytic mechanism and designing high-performance catalysts, but faces grand challenges. Herein, we unprecedentedly switch the hydrogen transfer pathway between Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism over paddle-wheel dicopper molecular catalysts, where the charge density of the Cu sites is effectively modulated by the ligands with different electron-withdrawing ability. Extended X-ray absorption fine structure (EXAFS), operando characterizations, and theoretical calculations together reveal that the moderate charge density of Cu sites in benzoic acid-Cu catalyst (BA-Cu) inhibits H₂O dissociation and reduces C₂H₂ adsorption energy, promoting the co-adsorption of C₂H₂ and H₂O and steering the EAH through the Eley-Rideal mechanism. In acetic acid-Cu (AA-Cu) with low charge density and monofluorobenzoic acid-Cu (MFBA-Cu) with high charge density, the Cu sites facilitate H₂O dissociation to *H and hydrogenate C₂H₂ via the Langmuir-Hinshelwood mechanism. In 1 M KOH aqueous solution, the BA-Cu delivers an ethylene partial current density of 328 mA/cm² with an ethylene Faradaic efficiency (FE) of 96.4% at –0.9 V versus RHE, which is about 2-fold and 1.4-fold higher than AA-Cu and MFBA-Cu. This work provides mechanistic insights for the rational design of high-performance electrocatalysts by regulating the hydrogenation kinetics.