Oscillatory Reaction Rates in the Electro-Oxidation of Formaldehyde on Palladium

Abstract

Formaldehyde has been used as a model system to investigate the electro-oxidation of small organic molecules, mainly on platinum[1]. This reaction is extremely dependent on the control conditions (potential and current) and it is also known to display self-organized oscillations on platinum[2]. In the present work we report a study of the oscillating reaction rates in the electro-oxidation of formaldehyde on polycrystalline palladium and in aqueous sulfuric acid media. The mapping of the existence region of potential oscillations was obtained by a slow current scan. Galvanodynamic oscillations were observed to range from 0.50 to 0.85 V. Further analysis was performed by galvanostatic measurements. The amplitude of potential oscillations increases in time. The potential-increase part of the cycle is formed by a rapid increase to about 0.60 V and then a slow one to 0.85 V, followed by an abrupt drop, with an average oscillation frequency of 0.15 Hz. Both the morphology and the frequency of oscillations differ from that observed for the electro-oxidation of formaldehyde on platinum, where the electrode potential varies from 0.40 to 0.90 V, and the it increases slowly until reaching 0.60 V followed by a rapid potential increase of up to 0.90 V, and then it decreases abruptly. Oscillations on platinum under comparable conditions were also about three times faster [3]. Overall, our results suggest the surface chemistry underlying the dynamics of formaldehyde elector-oxidation on palladium and platinum is considerably different, particularly under oscillatory regime. Some mechanistic aspects of the, far less studied, palladium system will be discussed.