Role of the Adatoms in the Oxidation of Formic Acid on Platinum Electrodes

Oxidation of formic acid on platinum electrodes is catalyzed by the modification of the surface with different adatoms. Although a correlation between the activity of the modified surface and properties of the adatom can be found, the exact role of the adatom is still subject of debate. In the present communication, a detailed mechanism will be described establishing the roles of the adatom and the platinum surface with the aide of new experimental results and DFT calculations. The electrochemical results show that higher currents and lower onset potentials are recorded for Bi or Pb on Pt(111) electrodes, whereas Se act as a mere third body. Additionally, fast voltammetry experiments indicate that the increase in curretn observed for Bi is not related to the adsorption of formate on the platinum sites. Temperature dependent experiments shows that  the increase in currents as the Bi coverage increases is parallel to a diminution of the measured activation energy. To determine the role of the adatom, DFT calculations were carried out. These calculation show that presence of an adatom causes a reorganization of the electronic structure of the surface, where the adatom can have an excess of positive or negative charge. A direct correlation between the excess of positive charge and the catalytic effect is observed. Thus, the adatoms with the highest catalytic effect are those with a largest positive charge. Moreover, these positive charge favors the adsorption of formate on the adatom and the dissociation of formate into CO₂ and adsorbed H on Pt. For the most active adatoms, the activation energy of this dissociation process is negligible. These results justify the observed diminution of the experimental activation energy with the coverage.