Adatom Coverage Dependences and Specificity Effects on the Rate of Formic Acid Electro-oxidation at Polycrystalline Platinum

The catalysis and inhibition of formic acid electro-oxidation that may accompany metal-submonolayer formation on polycrystalline platinum was studied quantitatively using the rotating ring-disk electrode (RRDE) technique. Under controlled flux conditions one can attain limiting current behavior for the oxidation of formic acid and thereby determine the upper limit for the optimum adatom coverage for lead and bismuth. These values are found to be 30-35% of the real area of the platinum electrode. The electro-oxidation experiments in this paper are correlated with the literature dealing with polycrystalline and single-crystal studies of hydrogen adsorption and formic acid oxidation at platinum to explain differences of the behavior of various underpotential deposited (UPD) species. The poisoning of the electro-oxidation process of formic acid requires the presence of adsorbed hydrogen or unhindered access to the first adsorbed intermediate COOH that is produced. During the initial stages of UPD, submonolayers of silver or copper do not deposit significantly on Pt(100), the plane that is responsible for most of the electro-oxidation of formic acid. Therefore, UPD silver or copper cannot inhibit poisoning of the formic acid oxidation process by decreasing hydrogen adsorption, or restricting access to COOH via a third body mechanism. Bismuth, lead, and other UPD species with radii larger than platinum do deposit significantly on Pt(100) at low UPD coverages, and can therefore slow the poisoning of the formic acid process on this plane.