AN EX-SITU STUDY OF ELECTRODEPOSITED LEAD ON PLATINUM(111) .2. VACUUM CHARACTERIZATION AND THERMAL-DESORPTION OF THE EMERSED ADLAYER

The vacuum and thermal properties of Pt(111) electrodes emersed from Pb-containing HClO4 solutions were studied with Auger electron spectroscopy, thermal desorption spectroscopy, Ar ion sputter depth profiling, and low energy electron diffraction. Following emersion of the electrode and transfer to the vacuum chamber the adsorbate consists of Pb, ClO4, O, OH, and Cl, which form a number of (square-root 3 x square-root 3)R30-degrees diffraction patterns. Lead is the primary thermal desorption product of submonolayer deposits and evolves in a single peak that shifts from 1300 K in the limit of low coverage to 1150 K at saturation of the first layer. The activation energy for Pb desorption varies linearly from 324 kJ/mol in the limit of zero coverage to 273 kJ/mol at monolayer saturation. Heating bulk Pb deposits produces, in addition to Pb desorption, a number of Cl- and O-containing Pb compounds, the most prominent being PbO and PbO2, which desorb at 975 and about 800 K, respectively. PbO appears to result from oxidation during the emersion step, whereas PbO2 forms during the temperature ramp by a thermally driven reaction with coadsorbed oxidants (e.g., ClO4). Multilayer Pb desorbs in a peak at 960 K with an activation energy of 180 kJ/mol. Auger and depth profile measurements gave reasonably consistent estimates of emersed adlayer thickness ranging from 2.5 to 25 angstrom depending on emersion potential.