ELECTROCHEMICAL AND VACUUM COADSORPTION OF CARBON-MONOXIDE AND LEAD ON PT(111)

Coadsorption of CO with Pb adlayers on Pt(111) was examined by in situ electrochemical and ex situ surface science methods. CO alters the Pb surface redox reaction, reducing the potential for the Pb(ad)2d+ doub le-line arrow pointing left and right Pb(ad)0 reaction from 0.65 V for Pb in 0.1 M HClO4 to 0.52 V with coadsorbed CO. Pb adlayers do not ap preciably affect the potential for CO oxidation at moderate Pb coverag es. At high Pb coverages, oxidation of CO occurs over a broad potentia l range, of 0.3-0.6 V. The CO/Pb/Pt(111) system exhibits dynamic emers ion with a constant emersion potential of about 0.5 V that is set by t he Pb surface redox reaction, Pb(ad)2+ double-line arrow pointing left and right Pb(ad)0. Pb adatoms act as site blockers to CO adsorption, a monolayer of Pb completely blocking CO adsorption in vacuum. The the rmal desorption behavior of CO from Pb/Pt(111) surfaces was modeled us ing three parameters: E(a) the CO activation energy for desorption in the limit of zero CO coverage, E(b) the coverage dependent CO activati on energy term, and DELTAE the change in E(a) and E(b) with respect to Pb coverage.