Electrochemical Alkyne Semi-Hydrogenation via Proton-Coupled Electron Transfer on Cu(111) Surface.

Electrocatalytic alkyne semi-hydrogenation (EASH) powered by renewable electricity using water as a hydrogen donor provides a sustainable alternative to conventional thermocatalysis. However, the current EASH systems predominantly follow hydrogen atom transfer (HAT) pathways, which are prone to over-hydrogenation and at the same time compete with the hydrogen evolution reaction. In this work, we report a proton-coupled electron transfer (PCET) mechanism enabled on Cu(111) surface for highly efficient and selective EASH. Well-defined two-dimensional Cu nanosheets with exposed (111) facets achieve > 98% selectivity for electrochemical semi-hydrogenation of 4-aminophenylacetylene to 4-vinylphenylamine in a membrane electrode assembly reactor. The Cu nanosheets can also efficiently remove 1%-8% alkyne impurities in alkene and exhibit broad substrate scope, stereoselectivity, as well as operational stability. In situ Raman spectroscopy measurements reveal that, during the PCET-mediated EASH, the covalent adsorption of alkynes and their conversion to weakly bound planar intermediates facilitate the EASH process and suppress over-hydrogenation. Interfacial K-structured and linearly hydrogen-bonded water species further enhance EASH selectivity via proton supply and steric modulation. Radical scavenging and kinetic isotope effect studies, along with theoretical calculations, corroborate a PCET-dominated mechanism on Cu(111) surface. This work establishes a PCET-driven paradigm for selective hydrogenation beyond the conventional HAT pathways.