Influence of chloride ion adsorption on the kinetics and mechanism of Ru(NH3)63+/2+ electrode reactions

Abstract

In the experiment, we prepared metal electrodes with sub-micron to nanometer for the scanning electrochemical microscopy to study the kinetics of Ru(NH3)63+/2+ redox system. The rates of reduction of Ru(NH3)6(NO3)3 were found essentially independent of the nature of the electrode material, but mildly dependent upon chloride ion concentration. We observed that the ‘outer-sphere’ reactant of Ru(NH3)6(NO3)3 reduced at relatively slower rates in low-chloride solutions. This phenomenon appears to be compatible with the expected variations in a ligand-bridged mechanism in which the electrostatic repulsions among the specifically adsorbed chloride anions that dominate would have an adverse influence on the adsorption of inner-sphere cationic reactants and thus cause a decrease in the reaction rate. However, as 0.1 M supporting electrolyte KCl was added into the target solution, a significant increase in the reaction rate was measured as a result of the ionic double-layer effect. Cationic complexes will experience the same electrostatic attraction from adsorbed chloride anions as will outer-sphere reactants. It is suggested that the reaction pathway of Ru(NH3)63+ reduction can change from the anion-bridged inner-sphere to the outer-sphere with increasing chloride ion concentration up to 0.1 M.