ИСТИНА

Electrocatalysis is important to many applications in the electrochemistry. A large number of works have been devoted to the experimental and theoretical study of homogeneous redox electrocatalysis. The theory of catalytic currents can be illustrated by the reduction of hydrogen peroxide in the presence of ferric ions [1]. In this system iron, changing its valence, seems to be the transfer agent for electrons from the electrode to hydrogen peroxide. The slow stage of the process is the reaction between H2O2 and Fe2+, and therefore the height of the catalytic wave is governed by the rate of this reaction. The current increases due to hydrogen peroxide, which by itself is not reduced electrolytically until much more negative potentials are reached, although chemically it is a more powerful oxidizing agent than ferric iron [2]. The theory of homogeneous electrocatalysis is most well-developed for polarography; however, in some works, variously shaped electrodes and chemical reactions of various orders were considered. In all works devoted to the theoretical study of homogeneous redox electrocatalysis, the ionic migration was ignored. In many cases, the supporting electrolyte is present in the solution, which almost completely suppresses the migration of electroactive ions. However, in some cases, the concentration of supporting electrolyte can be rather low or it is absent at all [3]. In this case, the migration transfer of all types of ions, which are present in the solution, should be taken into account. This work is devoted to the theoretical study of the effect of migration on homogeneous redox electrocatalysis of electrochemical reactions at the rotating disk electrode. The mathematical model involves the Nernst-Planck equations that take into account diffusion, migration, convection, homogeneous chemical reactions, and electroneutrality condition. For the convenience of solving and analyzing the results, the mathematical model was reduced to the dimensionless form. The numerical solution was performed by the finite volume method on the nonuniform grid. Based on the results of computational experiments, for the electrocatalytic reduction of hydrogen peroxide in the presence of ferric ions, three zones were observed in the plane of concentrations of H2O2 – H2SO4. These zones differ in the nature of the limiting current: for H+ ions, Fe+2 ions, and hydrogen peroxide H2O2. The dependences of the limiting current on the concentrations of H2SO4 and H2O2 were determined with and without consideration of the ionic migration. It was found that the ionic migration may be ignored only in the cases that the concentration of H2SO4 is significantly higher than that of H2O2. It was shown that the strongest effect of ionic migration is reached on the boundaries between zones that differ in the nature of the limiting current.