IN-SITU X-RAY-ABSORPTION SPECTROSCOPIC STUDY OF ADSORBED PB ON CARBON-SUPPORTED PT

In situ X-ray absorption spectroscopy (XAS) was used to study the stru cture of adsorbed Pb on carbon-supported Pt in 1 M HClO4 + 5 x 10(-3) M Pb2+, in the potential range -0.24 to 1.15 V vs. SCE. The XAS measur ements were done using a Canberra 13-element germanium detector. X-ray absorption near-edge structure (XANES) measurements indicated that, i n the undepotential deposition (UPD) region, the Pb species are essent ially neutral Pb atoms. At all potentials positive to the main UPD pea k in the cyclic voltammogram the Pb is in the Pb(II) state. Analysis o f the extended X-ray absorption fine structure (EXAFS) data for potent ials more negative than 0.0 V vs. SCE required a two-shell fit involvi ng Pb-Pb and Pb-Pt interactions. At more positive potentials, in the U PD region, the data could be fitted to a single Pb-Pt shell, indicatin g a high degree of lateral disorder in the layer. There is no evidence of Pb interaction with oxygenated species in the UPD region. On strip ping the UPD layer the Pb was present as hydrated Pb2+ ions. At more p ositive potentials there was clear evidence of incorporation of Pb int o the platinum oxide layer. Repeated cycling changes the nature of the UPD layer.