Formic acid decomposition on polycrystalline platinum and palladized platinum electrodes

This is a comprehensive study in which a formic acid decomposition reaction is examined as a probe of catalytic properties of polycrystalline platinum and palladized platinum electrodes. The electrode potential varies in a broad range, and the reaction is carried out in perchloric acid and sulfuric acid solutions containing different concentrations of HCOOH. Analytical methods used to access the decomposition reaction are chronoamperometry and cyclic voltammetry. At very short times, we prove that only a negligible amount of surface CO is formed, and the CO unaffected decomposition reaction, leading to CO2 formation, can be interrogated. Surprisingly, the decomposition reaction displays Tafel behavior only in a very narrow potential range. This observation, made with both clean Pt and Pt/Pd electrodes, suggests that water-surface interactions, and/or (bi)sulfate-surface interactions, increase with increasing electrode potential and create a steric/electronic barrier for the decomposition of formic acid (and methanol, J. Phys. Chem. 1994, 98, 5074). We therefore offer a pessimistic view about platinum as a universal material for heterogeneous catalysis applications involving rearrangements of organic molecules. Such rearrangements may only be fulfilled with a low electrochemical driving force, at least at room temperature, but at higher potentials, the electrode becomes deactivated due to the unique attributes of the double layer structure on the platinum electrode. We have also found that the deceleration of formic acid oxidation (to CO2) is primarily due to CO chemisorption only at potentials overlapping with those from the hydrogen adsorption range, or not too positive from this range. At more positive potentials, the decay in formic acid decomposition is neither due to CO formation nor to solution mass transfer limitations. The presence of interfacial CO2 (J. Electroanal. Chem. 1994, 376, 151) or adsorption of formic acid and/or formate anion, could account for the decay. Finally, a detailed analysis of kinetic isotherms involved in the two pathways, CO2 formation and CO chemisorption, is made and the mechanism of formic acid decomposition on platinum is discussed. The electrolyte anion effects involved in formic acid oxidation in HClO4 and in H2SO4 solutions are also presented.