Observation of several different temporal patterns in the oxidation of formic acid at a rotating platinum-disk electrode in an acidic medium

Abstract

Results of an experimental study on the oxidation of formic acid at a rotating platinum-disk electrode are presented. Experiments were conducted under galvanostatic conditions using solutions containing sulfuric acid. By increasing the applied current slowly, the system was moved through sequences of oscillatory states. Most states exhibited temporal patterns consisting of different combinations of large and small oscillations and without a substantial range of intermediate amplitudes. The sequence found closest to the point where stationary behavior became unstable was ordered in such a way that the ratio of the number of small oscillations to the number of large oscillations increased with respect to increases in current. Each state that was identified contained one large oscillation and a number of small oscillations. Peak values in measured waveforms are consistent with the idea that a strongly adsorbed intermediate reacts with hydroxyl radicals. The first sequence was followed by a family of large-amplitude oscillations that exhibited periods with durations of up to several hours. Following the family of large-amplitude oscillations, a sequence of oscillatory states was found with an ordering reversed to that of the first sequence. This latter sequence was followed by a set of oscillatory states whose members could be identified as elements from a Farey sequence. The oxidation process also exhibited bistability: high-potential states and low-potential states were found within the same range of values of the current. Models are constructed from previously formulated kinetic steps and rate laws, and these models predict that potential oscillations accompany the oxidation process.