Surface Hydroxyl Network Promoted Hydrogen Transfer Dynamics for Efficient Photocatalytic Acetylene Semi-Hydrogenation

Photocatalytic semi-hydrogenation of acetylene (C₂H₂) to ethylene (C₂H₄) is seriously limited by the inefficient generation and directional transfer of active hydrogen species. Here, we report a proton-coupled electron transfer (PCET) mechanism for photocatalytic acetylene semi-hydrogenation by establishing a hydroxyl network over hydroxyl-modified carbon nitride (C₃N₄-OH)/Ni(OH)₂ composite. Such a hydroxyl network not only enhances photogenerated charge separation but also establishes a strong hydrogen-bonding microenvironment for adsorbing interfacial water and facilitating hydrogen transfer dynamics. Femtosecond transient absorption (fs-TA) spectroscopy, in situ photochemical infrared spectroscopy, kinetic isotope effect (KIE), and active hydrogen (H*)-trapping reveal that the fast proton transfer via a PCET mechanism, rather than a conventional hydrogen atom transfer (HAT) pathway. Eventually, the C₃N₄-Ni(OH)₂ achieves an exceptionally high C₂H₄ production rate of 15.7 mmol g_cat⁻¹ h⁻¹ with a C₂H₄ selectivity of 98.2% under simulated solar irradiation. For purifying a crude C₂H₄ stream containing 0.5 vol% C₂H₂, the C₂H₂ conversion remains ∼98% over a long-term continuous-flow operation. This work elucidates the pivotal role of surface hydroxyl networks in governing hydrogen kinetics and paves a new avenue for the design of high-performance photocatalysts.