Structural effects during CO adsorption on Pt–bimetallic surfaces: I. The Pt(100) electrode

Synchrotron surface X-ray scattering (SXS) and rotating ring disk electrode (RRDE) measurements have been performed during the coadsorption of metal adatoms and carbon monoxide on the Pt(100) surface in an electrochemical environment. Whereas RRDE experiments give a macroscopic view of the coadsorption process, via the adsorption isotherms of deposited metal adatoms, the SXS measurements give insight into the structural changes that occur on the atomic level. The results show that both underpotentially deposited (UPD) Cu and Pb are displaced by CO from the Pt(100) surface, the driving force being the resulting negative shift in the surface free energy. When bromide is present in a c(2×2) structure on top of the Cu monolayer there is an activation barrier to the displacement effect. Coadsorption of CO then enhances the c(2×2) structure by displacing disordered Cu or Br that is present on the surface. Cycling the electrode potential, during which the Cu monolayer is desorbed, leads to complete blocking of the UPD process by CO. In contrast to these results, the reversible formation of a Bi c(2×2) UPD adlayer is unaffected by the presence of solution CO.