Structural and Spectroelectrochemical Study of Carbonate and Bicarbonate Adsorbed on Pt(111) and Pd/Pt(111) Electrodes

Spectroelectrochemical experiments and density functional theory (DFT) based calculations are combined to
elucidate the nature of the species adsorbed when a perchloric acid solution saturated with carbon dioxide is
in contact with well-defined electrode surfaces. Previous results reported for the Pt(111) electrode are revised
and completed with new data for a palladium monolayer deposited on the Pt(111) electrode. From the DFT
calculations and from the potential and pH dependent behavior of the main adsorbate vibrational bands, it is
suggested that, for potentials below 0.65 V, bicarbonate species are adsorbed at the Pt(111) electrode surface
in a short-bridge bidentate configuration. At higher potentials, the adsorbate adlayer is completed by the
formation of adsorbed carbonate also in a short-bridge configuration. The ratio between the surface coverages
of adsorbed bicarbonate and carbonate depends on both the electrode potential and the solution pH. Adsorbed
bicarbonate prevails at the palladium monolayer irrespective of the electrode potential. This behavior, together
with a concomitant lower adsorbate coverage, can be related to the competitive adsorption of hydroxylated
species.