Electrocatalytic Oxidation of Alcohols at Gold Electrodes in Alkaline Media

The electrooxidation in a pH 11 carbonate/hydrogen carbonate buffer of an aromatic (benzyl alcohol (BA)) alcohol and of several aliphatic alcohols on massive gold and on gold/quartz electrodes has been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM). The activity of gold is high for BA, lower for ethylene glycol (EG), and very low for propanol and butanol. Based on the fact that the double-layer capacity with BA is about one third of that for the aliphatic alcohols, which points to an expulsion of water from the Au surface due to the hydrophobic character of BA, we suggest that BA removes water molecules from the electrode surface, this removal facilitating its electrooxidation. The electrooxidation of BA started at the same potential, at -0.1 V, at which a surface redox process, corresponding to the adsorption-desorption of OH, appeared in the cyclic voltammogram of gold in the pH 11 buffer at higher temperatures, which indicates that the oxidation of BA occurs by its reaction with surface OH groups, in spite of their very low coverage at room temperature. BA adsorbs on Au, as shown by the EQCM: when BA was added to the cell while the Au/Q electrode was held at -0.2 V, no mass change was observed with 4.3 mM BA, while with 23.3 mM BA the mass increased by 70 ng cm(-2). The fairly high activity of Au for EG electrooxidation is attributed to the presence of two OH groups in the molecule. For the monohydroxylic alcohols the activity increases in the order ethanol<propanol<butanol, which we attribute to an increasing hydrophobic character that would facilitate the displacement of water molecules from the interface. The apparent activation energy for the oxidation of 2-propanol on Au was 45 kJ mol(-1), while that for ethylene glycol was only 30 kJ mol(-1), probably because the presence of two OH groups in the molecule facilitates its oxidation. Benzaldehyde was detected by HPLC as the main product of BA electrooxidation, reaching a maximum concentration at 10 min, at which the faradaic efficiency was 39%. At longer times the concentration of benzaldehyde decreased, probably because of its oxidation to benzoic acid.