Electrode kinetics of [Ru(NH 3) 6] 3+/2+complex confined in montmorillonite clay coatings on graphite electrodes

Abstract

The overall charge-transfer reaction at [Ru(NH 3) 6] 3+/2+-containing montmorillonite clay film-coated graphite electrodes was examined quantitatively by cyclic voltammetry, potential-step chronoamperometry (chronocoulometry) and normal pulse voltammetry. The relevant kinetic parameters (i.e. the standard rate constant k°/cm s −1 and the cathodic transfer coefficient α c) of the heterogeneous electron-transfer reaction of the [Ru(NH 3) 6] 3+/2+ couple at the electrode/film interface and the apparent diffusion coefficient ( D app/cm 2 s −1) of the homogeneous charge-transport reaction within the film were evaluated at various concentrations ( c° = 1 mol dm −3) of the complex within the films. It was found that D app, k° and α c change significantly with c° in the range of c° from 0.04 to 0.4 M; they increase with increasing c° in the lower c° range, reach their maximum values, and then decrease with increasing c° in the higher c° range. These kinetic parameters were also found to depend in different ways on the solution pH (pH 1–10). Under the condition of constant c°, no influence of the supporting electrolytes (Na 2SO 4, NaCl, NaBr, CH 3COONa, CF 3COONa, p-CH 3C 6H 4SO 3Na) on k°, α c and D app was observed. From the temperature dependences of the formal redox potential ( E°') and D app, the reaction entropy (ΔS° rc) of the [Ru(NH 3) 6] 3+/2+ couple within the clay film and the activation parameters of the homogeneous charge-transport reaction were evaluated. The mechanism of the charge-transport process within the clay film and the effects of the polymer domain, pH and supporting electrolyte on the rates of the heterogeneous electron-transfer and homogeneous charge-transport processes are discussed on the basis of an electron-hopping reaction, physical diffusion of [Ru(NH 3) 6] 3+ (and [Ru(NH 3) 6] 2+) and electrolyte ions, electrostatic cross-linking, single-file diffusion, structures of clay, electrode surface charge, etc.