Structural effects during CO adsorption on Pt-bimetallic surfaces. II. The Pt(111) electrode

The structural behavior of coadsorbed CO and metal adatoms on the Pt(111) surface in acid electrolyte has been explored by means of surface X-ray scattering (SXS) measurements to elucidate the potential-induced changes at the atomic level. Close to the Nernst potential for bulk metal deposition, CO displaces both Cu and Pb from the Pt(111) surface. In the latter case, the Pb (3×3) structure, observed in CO-free solution, initially increases its surface coverage and long-range order until the PbCO repulsion weakens the PbPt bond and displacement of Pb in the (3×3) structure occurs. In the presence of bromide anions, the CuBr adlayer formed at negative potential is not initially displaced by CO. The energetic barrier to displacement is overcome by cycling the potential whereby Cu is desorbed from the surface and Cu UPD is then blocked by adsorbed CO. In contrast, displacement of PbBr by CO at negative potential occurs immediately. For both CuBr and PbBr, after displacement by CO, the p(2×2)-CO structure is observed at negative potential and undergoes the same potential-dependent order–disorder transition that is observed in a solution free of metal ions.