Formic Acid Electrooxidation on Palladium Nano-Layers Deposited onto Pt(111): Investigation of the Substrate Effect

Abstract

The influence of Pd nano-layers electro-deposited onto Pt(111) single crystal has been systematically studied toward the formic acid electrochemical oxidation reaction in H₂SO₄ and HClO₄. The studied Pd_xML/Pt(111) surfaces (x = 1, 2, 5, and 16 monolayers (ML)) are all more active than Pt(111) toward formic acid oxidation, even if the activity is very sensitive to the Pd film thickness and morphology. In sulfate solution, the competitive adsorption of long-range ordered (bi)sulfate on the pseudomorphic Pd terraces effectively hinders the formic acid oxidation only on the thinnest films. We could observe the different roles of the (bi)sulfate adsorption on the first and on the following deposited Pd layers. The sulfate adsorption competitive role rapidly fades away beyond about 5 ML of equivalent thickness, due to the surface roughness increasing and terraces width diminishing. In perchlorate media, anions do not adsorb competitively with formic acid intermediates, allowing a larger activity of the formic acid oxidation up to about 5 ML. At higher thicknesses, the difference in activity between the two electrolytic media is reduced, and it drops in both electrolytes close to 0.5 V vs. RHE, where Pd surface oxides are formed. Coupling the electrochemical results with the Pd layer structural description previously obtained from in situ SXRD experiments, the outstanding activity of Pd_1ML/Pt(111) observed in perchloric solution can be explained by the ligand effect of the underlying platinum atoms on the first pseudomorphic Pd layer. This advantageous effect is lost for Pd deposits thicker than 1 ML.

Graphical Abstract