On the problem of potential shifts at glassy carbon electrodes: Part II. Voltammetric analysis of the surface interactions involved in the electrode processes of nitrobenzenes at freshly polished and electrochemically pretreated electrodes

The contribution of the heterogeneous (involving non-adsorbed solution species) and surface path (involving adsorbed species) to the electrode processes of aromatic nitro compounds has been studied as a function of the surface conditions of glassy carbon (GC-20) by linear sweep voltammetry (LSV) and cyclic voltammetry (CV). The influence of adsorption was negligible at the freshly polished electrodes. Oxidative electrochemical pretreatment led to the formation of active sites at which the adsorption of the reactant resulted in enhanced electron-transfer rates. The nature of the adsorption process and the characteristics of the electrode reaction in the adsorbed state were studied in detail for p-nitrotoluene. The adsorption followed the Frumkin isotherm with an interaction parameter of 1.2, indicating moderate attractive interactions between the adsorbed molecules. The rate constant for the adsorbed species was of the order of 10⁵, higher than the apparent heterogeneous rate constant of p-nitrotoluene in the solution phase. The magnitude of the potential shift and the strength of adsorption varied with the relative position and electron-withdrawing power of the substituent in the aromatic ring of nitrobenzene. Free radical coupling was selectively catalysed at electrochemically pretreated electrodes.