ELECTRODE-KINETICS OF [RU(NH3)6]3+/2+ COMPLEX CONFINED IN MONTMORILLONITE CLAY COATINGS ON GRAPHITE-ELECTRODES

The overall charge-transfer reaction at [Ru(NH3)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-degrees/cm s-1 and the cathodic transfer coefficient alpha-c) of the heterogeneous electron-transfer reaction of the [Ru(NH3)6]3+/2+ couple at the electrode/film interface and the apparent diffusion coefficient (D(app)/cm2 s-1) of the homogeneous charge-transport reaction within the film were evaluated at various concentrations (c-degrees = 1 mol dm-3) of the complex within the films. It was found that D(app), k-degrees and alpha-c change significantly with c-degrees in the range of c-degrees from 0.04 to 0.4 M; they increase with increasing c-degrees in the lower c-degrees range, reach their maximum values, and then decrease with increasing c-degrees in the higher c-degrees 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-degrees, no influence of the supporting electrolytes (Na2SO4, NaCl, NaBr, CH3COONa, CF3COONa, p-CH3C6H4SO3Na) on k-degrees, alpha-c and D(app) was observed. From the temperature dependences of the formal redox potential (E-degrees') and D(app), the reaction entropy (DELTA-S(rc)-degrees) of the [Ru(NH3)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(NH3)6]3+ (and [Ru(NH3)6]2+) and electrolyte ions, electrostatic cross-linking, single-file diffusion, structures of clay, electrode surface charge, etc.