CHAOTIC AND PERIODIC POTENTIAL OSCILLATIONS IN FORMALDEHYDE OXIDATION

The appearance of chaos and windows of periodic oscillations in the ga lvanostatic oxidation of formaldehyde on platinum at 43 degrees C is s tudied using various applied currents and formaldehyde concentrations. Potential oscillation patterns change with time in such a way that, a fter a period-doubling cascade, a sequence of periodic oscillations in terposed between chaotic states appears. The sequence of periodic osci llations occurs in two ways, that is, in descending and ascending orde rs of their periods. The descending-order sequence is observed at comp aratively low formaldehyde concentrations (0.003-10 mol dm(-3)), and t he ascending one is observed at comparatively high concentrations (0.3 -18 mol dm(-3)). At formaldehyde concentrations between 0.3 and 10 mol dm(-3), the ascending-order sequence always appears at higher current s than the descending one. We plot bifurcation diagrams for these two order sequences where one axis is time, which is related to surface st ates. The next-minimum return map for chaotic oscillations shows a ten dency to become more nonuniform with time, which tendency is confirmed by calculating the variance in local Lyapunov exponents. Current osci llation is related to the potential oscillation. Negative resistance h idden in an ordinary cyclic voltammogram is found when the potential s equence before oscillation occurs (an induction period) is simulated. The species responsible for the negative resistance is thought to be a dsorbed water adjacent to adsorbed carbon monoxide.