The adsorption and oxidation of carbon monoxide at the Pt(111)/electrolyte interface: atomic structure and surface relaxation

Using in-situ X-ray diffraction we have studied the adsorption of CO onto Pt(111) in pure perchloric acid. The adsorption of CO at an applied electrode potential of 0.05 V (versus the reversible hydrogen electrode) causes expansion of the topmost Pt atomic layer by ∼4% of the bulk lattice spacing. At this potential the adsorbed CO forms a compressed p(2×2) adlayer which fully covers the Pt surface. As the potential is stepped positively, the p(2×2) structure undergoes an order–disorder transition which appears to be triggered by oxidation of a small fraction of the adsorbed CO adlayer and is reversible or irreversible according to the overpressure of CO.