ELECTROCHEMICAL STRUCTURE-SENSITIVE BEHAVIOR OF IRREVERSIBLY ADSORBED PALLADIUM ON PT(100), PT(111) AND PT(110) IN AN ACIDIC MEDIUM

Irreversibly adsorbed palladium from solution dosing on platinum has been investigated on the three basal orientations by voltammetric study of the effect of this adspecies on hydrogen and oxygen electrosorption. The study shows that hydrogen adsorption at ad-palladium occurs reversibly, giving rise to a sharp reversible adsorption state growing with coverage, while the amount of hydrogen adsorbed on the substrate decreases. The potential of the adsorption state depends on the orientation of the platinum substrate. For Pt(100) and Pt(111), a second pair of peaks develops at potentials respectively 100 and 40 mV positive of the previous states. This second state begins to grow before a complete disappearance of the adsorption states of the substrate, and its growth occurs at the expense of the hydrogen bonded to palladium already adsorbed as well as of hydrogen bonded to the substrate. This indicates that the growth of the second palladium adlayer begins before the first monolayer is completed. From analysis of the distribution of hydrogen bonded to palladium and platinum, it is seen that there is a 1 : 1 surface atom replacement, which is indicative of epitaxial growth of palladium on Pt(111) and Pt(100). The stability of irreversibly adsorbed palladium is high outside the oxygen adsorption range. Oxygen adsorption is structure sensitive. In the case of a Pd-Pt(111) electrode, oxygen adsorption is well correlated with conclusions obtained from hydrogen adsorption regarding the way the monolayer and second palladium layers are formed.