Catalysis and inhibition of charge transfer rate of cadmium ions at a partially blocked dropping mercury microelectrode by butanol

A classical problem of the effects on charge transfer rate constants by foreign adsorbed species was re-investigated by the dropping mercury microelectrode technique with rapid and reproducible response. A question is why reaction rates generally have a non-linear relation with the fraction of the blocked electrode area. Reduction rate constants of cadmium ions in the presence of 1-butanol were evaluated in 1 mol dm(-3) KF as a function of coverage of 1-butanol and the electrode potential, where the coverage obeyed a Frumkin type isotherm including a quadratic dependence of the potential. The rate constants showed a non-linear relation with the coverage. The rate was enhanced by the adsorption at potentials E < -0.85 V vs. SCE whereas it was blocked at E > -0.75 V. A statistical model interpreting both the catalytic and the inhibiting effects was proposed, in which the cadmium ion interacts with several adsorbed 1-butanol molecules at the nearest neighboring reaction sites on the electrode surface. The model gave a non-linear relation with the coverage as a function of the interaction energy. The interaction energy was estimated from another statistical mechanical model of an adsorbed rigid dipole in the electric field on the assumption that the dipole interacts coulombically with the cadmium ion. The model was analyzed quantitatively and explained the experimental data approximately. (C) 1997 Elsevier Science S.A.