Mechanisms of two electrochemical oscillations of different types, observed for H2O2 reduction on a Pt electrode in the presence of a small amount ofhalide ions

Two electrochemical oscillations, previously called oscillation C and D, ar e observed for H2O2 reduction on a Pt electrode in an acidic solution when a small amount of halide ions is added to the solution. Detailed studies, i ncluding impedance analyses and in-situ light reflectance measurements as w ell as mathematical simulations, have revealed that both oscillations C and D fall into hidden negative differential resistance (HNDR) oscillators, th ough oscillation D can be classified into a new-type HNDR oscillator not re ported thus far. The H2O2 reduction on Pt has two-type NDR's: One arises fr om a decrease in the coverage of adsorbed OH (acting as an autocatalyst for the H2O2 reduction) with a negative potential shift, and the other arises from suppression of the H2O2 reduction by formation of under-potential depo sited hydrogen (upd-H) in further negative potentials. Oscillation C appear s from hiding of the former NDR by a decrease in the coverage (Ox) of adsor bed halide ions (acting as a site blocking agent) with the negative potenti al shift. Oscillation D appears from hiding of the latter NDR by not only t he decrease in Ox but also an addition of a transient cathodic current due to the upd-H formation, indicating that oscillation D is really a new-type HNDR oscillator.