175 J. Electroannl. Chem., 315 (1991) 175-189 Elsevier Sequoia S.A., Lausanne zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG JEC 01652 Electrode kinetics of [Ru(NH,) 6]3+‘2+ complex confined in montmorillonite clay coatings on graphite electrodes Takeyoshi Okajima I, Takeo Ohsaka ’ and Noboru Oyama Department of Applied Chemistry, Tokyo University of Agriculture and Technology, 2-24-16 Nakamachi, Koganei, Tokyo 184 (Japan) (Received 4 January 1991; in revised form 15 May 1991) The overall charge-transfer reaction at [Ru(NH~)~]~+/~+ -containing montmorillonite clay film-coated graphite electrodes was examined quantitatively by cyclic voltammetry, potential-step chronoamperome- try (chronocoulometry) and normal pulse voltammetry. The relevant kinetic parameters (i.e. the standard rate constant k O/cm s-’ and the cathodic transfer coefficient olc) of the heterogeneous electron-transfer reaction of the [Ru(NH,),] 3+/2+ couple at the electrode/film interface and the apparent diffusion coefficient ( DaP,/cm2 s-‘) of the homogeneous charge-transport reaction within the film were evaluated at various concentrations (co = 1 mol dmm3) of the complex within the films. It was found that DaPp, k o and (Y= change significantly with co in the range of co from 0.04 to 0.4 M; they increase with increasing co m the lower co range, reach their maximum values, and then decrease with increasing co in the higher co range. These kinetic parameters were also found to depend in different ways on the solution pH (pH l-10). Under the condition of constant co, no influence of the supporting electrolytes (Na,SO,, NaCl, NaBr, CHsCOONa, CFsCOONa, p-CH&H,SO,Na) on k”, a, and DaPP was observed. From the temperature dependences of the formal redox potential (E o ‘) and DaPP, the reaction entropy (A&z ) of the [Ru(NH&,]~+/~+ 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~)~]~’ (and zyxwvutsrqponmlkjihgfedcba [Ru(NHs)s]*’ ) and electro- lyte ions, electrostatic cross-linking, single-file diffusion, structures of clay, electrode surface charge, etc. INTRODUCTION Since montmorillonite clay, which is an inorganic cation-exchanger, was used as an “electrode modifier” by Ghosh and Bard [l], clay-modified electrodes have 1 Present address: Department of Electronic Chemistry, Graduate School at Nagatsuta, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan. OO22-0728/91/$03.50 0 1991 - Elsevier Sequoia S.A. All rights reserved