Adsorption of CO to Characterize the Structure of a Pd/Ag(111) Single-Atom Alloy Surface

We have characterized the surface of a single-atom alloy consisting of a low coverage of Pd deposited onto a Ag(111) surface. For this purpose, we used reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) of adsorbed CO. At low temperatures, a single C-O stretch band at 2047 cm(-1) and a single CO desorption peak at 272 K were observed, corresponding to CO bound to isolated palladium atoms. At palladium coverages above 0.002 ML, palladium aggregates form, as revealed by a C-O stretch peak at 1950 cm(-1) and a desorption peak at 390 K, corresponding to CO bound at the bridge sites between two palladium atoms. The diffusion of palladium from the surface into the subsurface was monitored from the C-O stretches of CO bonded to palladium and to silver atoms. Through RAIRS and TPD of CO, the ratio of surface to subsurface palladium was determined. The results from TPD experiments following H-2 exposures at low temperature indicate that dissociation of H-2 at the palladium sites does not lead to spillover of atomic hydrogen to silver atoms.