Oxidation of some reactive small molecules on copper and platinum in solutions of high pH

Abstract

The oxidation of four compounds—hydrazine, the formate anion, formaldehyde and methanol —at copper electrodes in aqueous sodium hydroxide and potassium carbonate solutions, and at platinum in aqueous potassium carbonate solution, was investigated using cyclic voltammetry techniques. Despite the use of a slightly elevated temperature (60°C), methanol and the formate ion were inactive, at potentials less than ca 1.3 V ( rhe), at copper at high pH; the electrode is apparently a poor catalyst for activating these molecules with respect to oxidation at low potentials. While formaldehyde and hydrazine were both active with respect to oxidation, their behaviour on copper was quite different. Formaldehyde oxidized at low potentials, < ca 0.3 ( rhe), a region where hydrazine gave no response; however, the situation was reversed at higher potentials. Such behaviour was interpreted in terms of the incipient hydrous oxide mediator theory of electrocatalysis, the mediator for formaldehyde oxidation being a Cu(I) species while that for hydrazine oxidation was a Cu(II) species. A more powerful Cu(III) mediator, capable of oxidizing even CH 3OH and HCOO −, was produced above ca 1.3 V ( rhe). Some evidence for the presence or formation of these mediators was obtained using ac voltammetry. All four compounds oxidized on platinum in the aqueous carbonate solution. However, with the exception of hydrazine, the oxidation currents below ca 0.4 V ( rhe) were quite small, probably due to the formation of CO ads poisoning species.