Bistability and oscillations in the electrocatalyzed oxidation of formaldehyde

Abstract

A variety of nonlinear phenomena, which was observed during the electrochemical oxidation of formaldehyde at a rotating platinum disk, is described. The experiments were conducted under galvanostatic conditions and the applied current was treated as a bifurcation parameter. The oxidation process exhibited bistability; both high-potential and low-potential states were found within the same range of values of current. However, most of the low-potential stationary behavior was either nonexistent or unstable in the region of coexistence and, instead, oscillations were observed. Period-doubling bifurcations, leading to chaos, were found by changing the current. Waveforms were recorded that represented dynamical states belonging to a periodie-chaotic sequence which was previously characterized under other conditions. The results presented here include dynamical states that were measured further into the sequence. New types of states within the sequence were also discovered. These latter states possessed characteristics that made them appear similar to dynamical states recently measured in an experimental investigation of the oxidation of formic acid. However, an examination of their symbolic patterns, which were obtained from one-dimensional mappings, reveals that they were different. The observation of yet other types of oscillations suggests that the oxidation of formaldehyde is coupled to more than one of the stages of the process in which oxide layers are formed on the platinum electrode.

Regeneration Oscillation Observed during Oxidation of Methanol, Formic Acid, and Formaldehyde with Chloride Ions

“…Although the regeneration oscillation may be one of mixed-mode oscillations, 18,19 it has a distinctive feature that the metastable high potential is higher than 1.0 V, which potential value is a sign of the cease of ordinary oscillations, and the duration of the metastable state is long, sometimes longer than 1 h, as shown later. The regeneration oscillation is similar to the "drastically" changed oscillation observed by Chen et al 31,32 with 5.0 © 10 ¹6 M chloride ions and 0.10 M HCOONa in a 0.50 M perchloric acid solution at room temperature, although the duration of the high potential state is very short.…”

“…It is also established [14][15][16][17] that the indirect path involves adsorbed CO in common for oxidations of the three C 1 -compounds. It is well known 8,[18][19][20][21][22] that adsorbed CO plays a crucial role in the oscillation. Gojuki et al 9 have recently proposed that adsorbed water is also involved in the oscillation.…”

Temporal Instabilities in Anodic Oxidation of Small Molecules/Ions at Solid Electrodes

“…In the above cited work, Xu and Schell [65] have reported the period-doubling phenomena, In more recent studies, the single-crystal, low-index Pt electrodes were employed, for which the oscillatory instabilities were reported under potentiostatic conditions and discussed in terms of several possible mechanisms [26,[71][72][73][74][75][76]. In these papers a very rich set of experimental data was collected, including the role of concentration of formic acid, the magnitude of ohmic drops, and of the solution stirring intensity.…”

“…Later studies of the oscillations and bistability on solid electrodes were reported by Wojtowicz et al [61], Bockris et al [63], and Schell et al [64][65][66][67]. In addition to classical electrochemical methods, Inzelt and Kertész [68] have employed electrochemical quartz crystal microbalance (EQCM) technique to show that the potential oscillations in the course of galvanostatic oscillations of formic acid at Pt electrode are associated with accumulation and consumption of strongly bound species.…”

“…The source of instabilities in the electrooxidation of formic acid, proposed by Strasser et al [26,76], is based on the earlier, well-established electrochemical mechanism of this process, involving the "direct" and "indirect" paths [78,79], similarly as proposed by Wojtowicz et al [61] and Schell et al [64][65][66][67].…”