Abstract

The electro-oxidation of formic acid has been extensively studied, mainly because of two reasons. From the applied perspective, formic acid has been pointed out as one of the most promising molecules to be used in fuel cells. On the other hand, from the fundamental perspective, it has been used as a model system to study the electro-oxidation of other small organic molecules [1,2]. Despite the large amount of studies concerning the electro-oxidation of formic acid, some mechanistic issues remain to be well understood. In this work, we report a comparative study of the electro-oxidation of formic acid on polycrystalline platinum and palladium electrodes under oscillatory regime. The experiments were performed under potential control mode, with an external resistance coupled between the working electrode and the potentiostat. All results are shown in terms of Phi potential, that is, the electrode potential compensated by the external resistance. On platinum, the oscillations develop between ~0.58 and ~0.80 V, with an oscillatory frequency in the range of 0.78 – 1.49 Hz. As a general trend, the potential slowly increases from ~0.58 to ~0.62 V, followed by a sudden increase to ~0.80 V. On palladium, a very different behavior was observed in oscillation’s waveforms and frequencies. The oscillations develop between ~0.25 and ~0.55 V, with an oscillatory frequency in the range of 0.17 – 0.20 Hz. At the beginning, there is an abrupt increase in the electrode potential from ~0.25 to ~0.38 V. After that, the potential slowly increases until ~0.55 V, when it falls abruptly to ~0.25 V, and the process restart. Overall, the results suggest that the surface chemistry underlying the dynamics of formic acid electro-oxidation on palladium and on platinum is considerably different, particularly under oscillatory regime. Results suggest that the surface poisoning due carbon monoxide adsorption on palladium system is different when compared to platinum, even though the reaction steps for oxidation is similar in both systems. The low frequency oscillations observed on palladium indicates a slower poisoning process on its surface. Consequently, provide us perspectives on mechanistic aspect of this system, not yet treated in much detail. Thus, a broad discussion about the mechanistic issues of those systems will be presented.

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