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 structure 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 measurements were done using a Canberra 13-element germanium detector. X-ray absorption near-edge structure (XANES) measurements indicated that, in the undepotential deposition (UPD) region, the Pb species are essentially neutral Pb atoms. At all potentials positive to the main UPD peak in the cyclic voltammogram the Pb is in the Pb(II) state. Analysis of the extended X-ray absorption fine structure (EXAFS) data for potentials more negative than 0.0 V vs. SCE required a two-shell fit involving Pb-Pb and Pb-Pt interactions. At more positive potentials, in the UPD region, the data could be fitted to a single Pb-Pt shell, indicating a high degree of lateral disorder in the layer. There is no evidence of Pb interaction with oxygenated species in the UPD region. On stripping the UPD layer the Pb was present as hydrated Pb2+ ions. At more positive potentials there was clear evidence of incorporation of Pb into the platinum oxide layer. Repeated cycling changes the nature of the UPD layer.