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

Irreversibly adsorbed palladium from solution dosing on platinum has b een investigated on the three basal orientations by voltammetric study of the effect of this adspecies on hydrogen and oxygen electrosorptio n. The study shows that hydrogen adsorption at ad-palladium occurs rev ersibly, giving rise to a sharp reversible adsorption state growing wi th coverage, while the amount of hydrogen adsorbed on the substrate de creases. The potential of the adsorption state depends on the orientat ion 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 comple te disappearance of the adsorption states of the substrate, and its gr owth occurs at the expense of the hydrogen bonded to palladium already adsorbed as well as of hydrogen bonded to the substrate. This indicat es that the growth of the second palladium adlayer begins before the f irst monolayer is completed. From analysis of the distribution of hydr ogen 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 adsor bed palladium is high outside the oxygen adsorption range. Oxygen adso rption is structure sensitive. In the case of a Pd-Pt(111) electrode, oxygen adsorption is well correlated with conclusions obtained from hy drogen adsorption regarding the way the monolayer and second palladium layers are formed.