Anion adsorption on Pd–Pt(111) electrodes in sulphuric acid solution

Anion adsorption at palladium-covered Pt(111) electrodes has been studied in sulphuric acid solutions using cyclic voltammetry, charge displacement and in situ FTIR experiments. The infrared spectra obtained for partially covered Pt(111) substrates show distinct bands for adsorbed (bi)sulphate on platinum and palladium surface atoms with frequencies similar to those observed for the bare and fully covered Pt(111) electrode surfaces, respectively. This observation is in agreement with the epitaxial growth of palladium on Pt(111). Increasing the palladium coverage in the (sub)monolayer range is shown to shift (bi)sulphate adsorption towards less positive potentials, whereas the total amount of adsorbed anions on the ensemble of palladium and platinum domains is approximately constant. Combination of CO displacement and voltammetric curves allows the determination of the potential of zero total charge (pztc) as a function of the palladium coverage. The pztc shifts from 0.33 to 0.235 V when the palladium coverage on the Pt(111) is increased up to a monolayer. The oxidation of the CO adlayer resulting from the charge displacement experiments has also been studied. The oxidation of adsorbed CO on palladium-covered Pt(111) electrodes takes place at higher potentials than on Pt(111). The pztc values have been used to calculate the contribution from anion adsorption to the charge obtained from the integration of the CO stripping voltammograms. Coverage values calculated for the CO adlayers generated during the CO displacement experiments at the palladium-covered Pt(111) electrodes diminish as the palladium coverage increases.