INTERMITTENCIES AND RELATED PHENOMENA IN THE OXIDATION OF FORMALDEHYDE AT A CONSTANT-CURRENT

In the galvanostatic oxidation of formaldehyde on Pt at 43 degrees C, we have found type LII intermittency (in the Pomeau-Manneville classif ication) as well as type I intermittency. Potential oscillation patter ns change with time, and type I intermittency is usually observed afte r a periodic oscillation in a sequence of periodic oscillations, which appear in decreasing order of their periods and are interposed betwee n chaotic states. At a comparatively low current density (e.g., 0.23 m A cm(-2) or less for a 1 mol dm(-3) formaldehyde solution) in a soluti on with a formaldehyde concentration of I mol dm-3 or more, we observe type III intermittency before a periodic oscillation in the sequence. Under other conditions we observe a type-III-like intermittent patter n followed by a reverse period-doubling cascade. We confirm the charac teristics of type III intermittency with a period-k laminar flow by th e shape of kth and (2k)th return maps, by the relation between a lamin ar duration and the time from the period-k pattern, and by the relatio n between a maximal Lyapunov exponent and that same time. Here we rega rd time as a parameter related to some surface states. We also observe that type I and type III intermittencies coexist. We sometimes observ e a randomly alternating sequence of period-(n+1) and period-n pattern s without any intermittencies. When the sequence of periodic oscillati ons appear in increasing order of their periods, the order in which ty pe I and type In intermittencies appear is also reversed.