Halide Electrolyte Effects in the Electrochemical Hydrogenation of Ketones on Copper.

Electrochemical hydrogenation of biomass-derived ketones offers a sustainable route to value-added chemicals but is often limited by competition from the hydrogen evolution reaction (HER). Here, we investigate how electrolyte composition, and specifically halide identity, governs the selective electrochemical hydrogenation of acetophenone to 1-phenylethanol on copper electrodes under near-neutral aqueous conditions. Potassium halide electrolytes introduce specific halide-surface interactions that modulate hydrogen adsorption, delay HER onset, and influence organic reduction behavior. Among the halides studied, chloride- and bromide-containing electrolytes exhibit higher activity and selectivity toward 1-phenylethanol formation than iodide, highlighting a strong dependence of catalytic performance on halide identity. Systematic variation of applied potential, electrolyte identity, halide composition, and acetophenone concentration reveal strong correlations between catalytic performance and electrolyte-dependent interfacial chemistry. Kinetic analysis, hydrogen scavenging experiments, and electrochemical in situ surface-enhanced Raman spectroscopy suggest a surface-mediated hydrogenation pathway involving adsorbed acetophenone and surface-derived hydrogen species, consistent with a Langmuir-Hinshelwood-like mechanism. Density functional theory calculations of hydrogen and halide coadsorption on Cu(111) provide molecular-level insights into these observations, showing that iodide remains more stable on the copper surface at cathodic potentials than chloride or bromide. This persistent iodide coverage limits surface accessibility for acetophenone adsorption, resulting in reduced hydrogenation activity despite delayed HER. Together, these results demonstrate how halide electrolyte identity tunes surface coverage, reaction kinetics, and selectivity during electrochemical hydrogenation, highlighting electrolyte engineering as an effective strategy for enabling selective ketone reduction under mild, near-neutral conditions.