Voltammetry in the presence of ultrasound: Surface and solution processes in the sonovoltammetric reduction of nitrobenzene at glassy carbon and gold electrodes

The application of power ultrasound in electrochemistry may promote or usefully modify electrode reactions. In the work reported here the four-electron reduction of nitrobenzene in alkaline (pH 13) aqueous media was studied as a model system. The electrochemical reduction is known to follow a complex mechanism [E. Laviron, A. Vallat and R. Meunier-Prest, J. Electroanal. Chem., 379 (1994) 427], involving protonations as well as a dehydration step; both surface and solution pathways for this reduction may be observed [C. Nishihara and H. Shindo, J. Electroanal. Chem., 221 (1987) 245], depending on the nature and state of the electrode. Both under silent and ultrasonic conditions nitrobenzene is reduced on glassy carbon electrodes in a chemically reversible one-electron process followed by an irreversible three-electron reduction. At sufficiently negative potentials the reduction process remains overall four-electron, with phenylhydroxylamine as the major product even at the high current densities obtained with intense ultrasound. Glassy carbon electrodes are shown to be suitable for kinetic studies, although damage, as endorsed by an increase in roughness and capacitance and probably initiated by mechanical damage at very short electrode-horn distances, was detected by a.c. impedance, voltammetric and atomic force microscopy (AFM) methods. On gold electrodes a more complicated mechanism due to a surface reaction pathway arises. A comparison of sonovoltammetric and rotating disk voltammetric results gives evidence for the homogeneous pathway being dominant under applied ultrasound conditions. The transition between surface and solution pathways is mass flux as well as concentration dependent, and an estimate for the rate of the surface catalyzed reaction for a concentration of 1.44 mM nitrobenzene (k = 4 ± 2 × 10^-2 cm s^-1), attributed to protonation of the nitrobenzene radical anion, is obtained from combined rotating disk and sonovoltammetric data. Damage to the gold surface as monitored by various techniques is small, but manifests itself by an interesting decrease in double layer capacitance.