THE ELECTROCHEMICAL-BEHAVIOR OF IRREVERSIBLY ADSORBED PALLADIUM ON PT(111) IN ACID-MEDIA

The irreversible adsorption of submonolayer and monolayer coverages of palladium on Pt(111) has been investigated by means of cyclic voltammetry in sulphuric and perchloric acid. The so-called anomalous and normal hydrogen regions are always observed, irrespective of the number of palladium atoms adsorbed. However, subtle changes in the distribution of charge between the two regions and changes in their fine structure appear to contradict previous assertions concerning strongly bonded hydrogen on clean Pt(111). Depending on the electrolyte used, slight differences are also observed in the electrochemical characteristics of these features in the presence of palladium, particularly with reference to the reversibility of the peaks. This suggests that the anomalous peaks in perchloric and sulphuric acid have different origins. Coincidence of thermal Pd-O and Pt-O desorption with the anomalous peak in perchloric acid implies strongly that this feature arises from the adsorption and desorption of some oxygenated species, probably OH(ads) interacting weakly with the background electrolyte. The nature of the anomalous region in sulphuric acid is discussed in the light of these results. Finally, the role played by specifically adsorbed anions in facilitating surface mobility is again emphasised, particularly in relation to the stability of the palladium overlayer in acid media and the removal of surface heterogeneity.