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 a datoms and carbon monoxide on the Pt(100) surface in an electrochemical env ironment. Whereas RRDE experiments give a macroscopic view of the coadsorpt ion process, via the adsorption isotherms of deposited metal adatoms, the S XS measurements give insight into the structural changes that occur on the atomic level. The results show that both underpotentially deposited (UPD) C u and Pb are displaced by CO from the Pt(100) surface, the driving force be ing the resulting negative shift in the surface free energy. When bromide i s present in a c(2 x 2) structure on top of the Cu monolayer there is an ac tivation barrier to the displacement effect. Coadsorption of CO then enhanc es the c(2 x 2) structure by displacing disordered Cu or Br that is present on the surface. Cycling the electrode potential, during which the Cu monol ayer is desorbed, leads to complete blocking of the UPD process by CO. In c ontrast to these results, the reversible formation of a Bi c(2 x 2) UPD adl ayer is unaffected by the presence of solution CO. (C) 2000 Elsevier Scienc e B.V. All rights reserved.