Positive Feedback Mechanism, Autocatalysis Mechanism, and Dependence on Atomic-Level Surface Structures in Electrochemical Oscillations for H2O2 Reduction on Pt Electrodes

Electrochemical reduction of H2O2 on Pt electrodes shows a variety of oscillations, depending on the electrode potential, electrode illumination, the atomic-level flatness of the electrode surface, and the presence or absence of metal atoms and halogen atoms adsorbed to submonolayer amounts on the electrode surface. This is a unique feature of this oscillating system, helpful for deeper understanding of oscillation phenomena. In this article, the fundamental behavior and mechanism of oscillations, named oscillations A and B, for polycrystalline Pt electrodes are first described. A mathematical simulation for oscillation A shows clearly the presence of a positive feedback mechanism and an important role of adsorbed hydrogen atoms, called upd-H. The behavior of oscillation A, especially the oscillation period, is changed by electrode illumination, application of weak external potential pulses, and coupling with other oscillation systems. The oscillation period is also altered by adsorption of foreign metal atoms such as Cu, Ag, Au, and Ru on Pt to sub-monolayer amounts. New-type oscillations called oscillations C and D appear when small amounts of halide ions are added to the solution. The appearance of oscillations C and D is explained by assuming a catalytic effect of adsorbed halogen atoms and an autocatalytic effect of adsorbed OH group on the H2O2 reduction. Another new-type oscillation is observed when an atomically-flat single-crystal Pt(111) electrode is used. These results clearly show an important role of atomic-level structures of the electrode surface in electrochemical oscillations.