Electrochemical Oxidation of para -Substituted Benzyl Alcohols on Gold: Effects of Substrate Speciation and Interfacial Interactions

Abstract

The electrochemical oxidation of alcohols is a promising alternative anodic reaction to the kinetically slow oxygen evolution reaction in water electrolysis while also enabling the production of value-added chemicals. Here, we investigate how para-substituents influence the electrochemical oxidation of benzyl alcohol derivatives on gold (Au) electrodes in alkaline media. Benzyl alcohol (BA), 4-methoxybenzyl alcohol (4-MBA), and 4-(hydroxymethyl)benzoic acid (4-HMBA) were examined using cyclic voltammetry, chronoamperometry coupled with high-performance liquid chromatography (HPLC), electrochemical quartz crystal microbalance (EC-QCM) measurements, Raman spectroscopy, and density functional theory (DFT) calculations. Voltammetric measurements show broadly similar oxidation behavior for BA and 4-MBA, whereas 4-HMBA exhibits a substantially suppressed oxidation response. EC-QCM measurements further demonstrate that the three substrate molecules alter the oxidation and reduction behavior of the Au surface to different extents, indicating a strong coupling between molecular adsorption and surface redox dynamics. DFT calculations show that all three molecules preferentially adopt a ring-flat adsorption geometry on the Au(111) surface, but para-substitution changes the orientation of the benzylic alcohol group relative to the surface. BA places the reactive benzylic C–H bond closest to the Au surface and exhibits the longest calculated benzylic C–H bond within the computational model, providing a possible structural descriptor for the substrate-dependent electrochemical behavior. In contrast, the deprotonated carboxylate form of 4-HMBA binds strongly to Au and may reduce active-site availability. Increasing alkalinity generally enhances alcohol oxidation, although the response is substrate dependent. Together, these results show that the oxidation of para-substituted benzyl alcohols on Au is governed by the interplay among substrate speciation, adsorption geometry, benzylic C–H orientation, and substrate-induced changes in Au surface redox behavior rather than by substituent electronic effects alone.