AN EX-SITU STUDY OF ELECTRODEPOSITED LEAD ON PLATINUM(111) .1. EXAMINATION OF THE SURFACE REDOX BEHAVIOR OF LEAD AND DYNAMIC EMERSION

Electrochemical deposition of lead onto a Pt(111) electrode was studied ex situ by means of a combined electrochemical and ultrahigh vacuum system. The Pb/Pt(111) system was examined with cyclic voltammetry in the electrochemical cell and Auger spectroscopy, high energy resolution X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry, thermal desorption spectroscopy, and low energy electron diffraction in the vacuum system. Electrodeposition of Pb from 0.1M HClO4 involves both classical electrodeposition (i.e., underpotential deposition) and a surface redox reaction. X-ray photoelectron spectra show two distinct types of adsorbed lead, Pb0 and Pb(ad)2+, in amounts that vary with emersion potential. The standard reduction potential for the reaction Pb(ad)2+ + 2 e- <-- --> Pb(ad)0 , as determined by XPS, is E0 = 0.65 +/- 0.05 V(RHE), in excellent agreement with the set of reversible peaks at 0.63 V in the cyclic voltammogram. A consequence of the surface redox reaction is that emersion is a dynamic process; discharge of the double layer after emersion results in a final emersed potential close to E0, regardless of the emersion potential. XPS measurements of the shifts of adsorbate binding energies as a function of emersion potential show that the double layer fully collapses upon emersion at potentials of -0.3 to 0.55 V(RHE), whereas a partially intact double layer remains after emersion at other potentials. We propose a model of the Pb surface redox process in which the oxidized species Pb(ad)2+ must be accompanied by some form of oxygen-containing species, such as O(ad) or OH(ad).