Electrochemistry of alumina pillared clay modified electrodes

A simple method for preparing metal oxide pillared clay films on graphite electrodes, based on the use of preformed pillared structures, is described. The films exhibited very good electroactivity when immersed in solutions containing redox active cationic species, e.g. Fe(bpy)(3)(2+), Os(bpy)(3)(2+), Ru(NH3)(6)(3+) and methyl viologen. Aging a pillared clay film, loaded with electroactive cations, in de-ionized water or varying the concentration of the supporting electrolyte have pronounced effects on the voltammetric behavior of the pillared clay films. These effects demonstrate clearly the key role of ion-paired electroactive cations for the electroactivity of pillared clay films. The pH dependence of the observed electroactivity for an alumina pillared clay film arises in part from the binding of the electroactive cations at the surface hydroxyl groups of the alumina pillars. This binding favors further formation of electroactive ion pairs. The voltammetric response from anionic redox couples, observed in acidic solutions, is attributed to the amphoteric nature of the alumina pillars. Discrepancies in the voltammetric behavior of silica and alumina pillared montmorillonite are explained in the light of different degrees of attachment of the redox active cations to the surface hydroxyl groups of the two metal oxide pillars. A model describing the charge transport in metal oxide pillared clays is also presented.