Oxidation of ruthenium(II) tris(2,2'-bipyridine) ions by thallic ions in nitric acid, mediated by ruthenium dioxide hydrate: an example of reversible heterogeneous redox catalysis

Abstract

The kinetics of the oxidation of Ru(bpy)[sub 3][sup 2+] to Ru(bpy)[sub 3][sup 3+] by Tl[sup 3+] ions, catalyzed by a dispersion of RuO[sub 2][center dot]xH[sub 2]O in 3 mol dm[sup [minus]3] HNO[sub 3], are reported as a function of [Ru(bpy)[sub 3][sup 2+]], [Tl[sup 3+]], [Tl[sup +]], [RuO[sub 2][center dot]xH[sub 2]O], and temperature. The kinetics of Ru(bpy)[sub 3][sup 2+] oxidation fit an electrochemical model of redox catalysis involving electron transfer between the two electrochemically reversible redox couples, i.e., Ru(bpy)[sub 3][sup 3+]/Ru(bpy)[sub 3][sup 2+] and Tl[sup 3+]/Tl[sup +], mediated by the dispersion of microelectrode particles of RuO[sub 2][center dot]xH[sub 2]O. In this model, the rate of reaction is assumed to be controlled by the diffusion of Ru(bpy)[sub 3][sup 2+] toward, and Ru(bpy)[sub 3][sup 3+] away from, the catalyst particles. The Arrhenius activation energy for the catalyzed reaction is 25.9 [+-] 0.7 kJ mol[sup [minus]1], and the changes in enthalpy and entropy for the reaction are 36 [+-] 2 kJ mol[sup [minus]1] and 127 [+-] 6 J mol[sup [minus]1] K[sup [minus]1], respectively. This work describes a rare example of reversible heterogeneous redox catalysis.