On the nature of electroactive sites in clay-modified electrodes

Abstract

The electrochemical properties of montmorillonite clay films deposited on pyrolytic graphite electrodes have been studied by cyclic voltammetry in order to elucidate the nature of the electroactive sites. Electrodes coated with pre-exchanged ML 2+ 3 -montmorillonite (M = Fe, Ru, Os and L = bpy; M = Fe and L = phen), and MV 2+-montmorillonite (MV 2+ = methylviologen) do not exhibit a voltammetric response when immersed in solutions containing only electrolyte. This result shows that the cations bound electrostatically to the exchange sites, whether intercalated within the galleries or held at the external surfaces, are rigorously electroinactive. However, these films incorporate ML 2+ 3 and MV 2+ ions from solution readily and exhibit voltammetric responses characteristic of diffusional processes. Os(bpy) 3 2+-exchanged montmorillonite films immersed in Fe(bpy) 3 2+ solutions show a voltammogram only for the Fe(bpy) 3 3+/2+ couple, and peak current values indicate that Fe(bpy) 3 3+ does not accept electrons from Os(bpy) 3 2+ gallery cations. The electroactivity for films immersed in ML 3 2+ or MV 2+ solutions is attributed in part to cations which are bound at the clay surface in excess of the cation exchange capacity by an ion pairing mechanism. Experiments using synthetic clays of different particle sizes suggest that binding of the ion pairs is related to the presence of edge surface sites.