ELECTROCHEMICAL AND VACUUM BEHAVIOR OF CARBON-MONOXIDE AND LEAD COADSORBED ON PLATINUM(111)

The electrochemical and vacuum behavior of coadsorbed CO and Pb on Pt(111) was examined in situ by cyclic and linear sweep voltammetry and ex situ by Auger electron spectroscopy and thermal desorption spectroscopy (TDS) in ultrahigh vacuum (UHV). Lead was deposited electrochemically from 0.1 M HClO4 with 1 mM Pb2+, and CO was adsorbed by either solution dosing in situ or vapor dosing in UHV. Coadsorbed CO alters the Pb surface redox reaction Pb(ad)2+ reversible arrow Pb(ad)0, reducing its potential from 0.65 V/RHE in 0.1 M HClO4 without CO to 0.52 V with CO. The CO + Pb/Pt(111) system exhibits dynamic emersion with a constant emersion potential of about 0.5 V established by the CO-shifted Pb surface redox reaction. The electro-oxidation potential of CO, which is in the range 0.7-0.8 V in the absence of Pb, is largely unaffected by small and moderate coverages of Pb. A near-saturated monolayer of Pb (theta(Pb) = 0.60 expressed as Pb atoms per Pt surface atom) substantially reduces the overpotential for CO oxidation, with the reaction occurring over the range 0.3-0.6 V. In agreement with earlier studies, substantially more CO can be adsorbed by solution dosing than by vapor dosing. The maximum local CO coverage for solution dosing is approximately theta(CO,loc) = 0.7 (molecules per Pt surface atom) for Pb coverages ranging from zero to 0.4. For vapor dosing the maximum local CO coverage is theta(CO,loc) = 0.5 independent of Pb coverage. Either species, when present as a saturated monolayer, blocks adsorption of the other, and Pb at lower coverages acts as a site blocker towards CO. The activation energy for CO desorption was measured with TDS and fitted to a three-parameter model over the entire range of allowed Pb and CO coverages. This model shows that coadsorbed Pb destabilizes CO by reducing its activation energy for desorption by up to 15%.