New Insights into the Formic Acid Oxidation Mechanism on Platinum Electrodes

In the formic acid oxidation reaction only two electrons are exchanged. Despite the apparent simplicity of this reaction, the oxidation mechanism is still a subject of debate. The major issue is the role of formic acid, solution formate and adsorbed formate in the oxidation mechanism. In order to shed light into this issue, formic acid oxidation on platinum single crystal electrodes has been studied to determine the effects of pH and specific anion adsorption in the oxidation mechanism. It has been found that the solution pH has an important effect in the oxidation currents. As has been already observed for polycrystalline electrodes [1], currents are proportional to the solution pH. Thus, it can be proposed that the active species in the oxidation mechanism is formate. On the other hand, the specific adsorption of sulfate on the Pt(111) electrode has a catalytic effect on the oxidation since the onset potential is displaced towards more negative values. This unexpected result indicates that the adsorbed anions have an important role in the oxidation mechanism as DFT calculations suggest [2]. In order to get further insight into the oxidation mechanism, activation energies have been calculated, both for the route through the active intermediate and for the route through CO. The activation energy of the process diminishes in the presence of adsorbed sulfate, which confirms the active participation of the adsorbed anion in the oxidation mechanism. Using previous data obtained with fast voltammetry [3], an oxidation mechanism is proposed for the direct route. In this mechanism, the oxidation through the active intermediate route requires an adsorbed anion (formate or sulfate) and a free platinum site where a solution formate species can bond. The currents calculated with this mechanism are compared with the experimental currents. An excellent agreement is found between both results, which supports the validity of the proposed oxidation mechanism.

References

1. Joo J, Uchida T, Cuesta A, Koper MTM, Osawa M (2013) J Am Chem Soc 135:9991-9994

2. Wang H-F, Liu Z-P (2009) J Phys Chem C 113:17502-17508

3. Grozovski V, Vidal-Iglesias FJ, Herrero E, Feliu JM (2011) ChemPhysChem 12:1641-1644