title: Site blocking and CO/sulfur site exchange processes on stepped Pt surfaces.
abstract: The influence of preadsorbed sulfur on the adsorption of CO on Pt(355) and Pt(322) is investigated systematically for sulfur precoverages between 0.02 and 0.30 ML by in situ x-ray photoelectron spectroscopy of the C 1s and S 2p core levels. The two surfaces have the same nominal terrace width of 5 atomic rows, but different step orientation. For both, at low temperatures (130 and 150 K), S preferentially adsorbs at the steps and passivates them for CO adsorption. The required S precoverage is significantly smaller for Pt(322), because of the lower number of steps as a result of the S-induced double-step formation. Upon heating, population of step sites with CO occurs due to irreversible CO/S site exchange. At low S precoverages, the characteristic transition temperatures of 165 K for Pt(355) and 245 K for Pt(322) are significantly different, indicating a higher activation barrier for the CO/S site exchange process for Pt(322). For medium to high S precoverages the transition temperature stays unchanged for Pt(322) but increases for Pt(355). The observed behavior is attributed to a kinetic passivation of the steps by sulfur at low temperatures, which is lifted upon heating.
authors.affiliation: Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91054 Erlangen, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Streber
authors.forename: C
authors.initials: C
authors.lastname: Papp
authors.forename: M P A
authors.initials: MP
authors.lastname: Lorenz
authors.forename: A
authors.initials: A
authors.lastname: Bayer
authors.forename: S
authors.initials: S
authors.lastname: Wickert
authors.forename: M
authors.initials: M
authors.lastname: Schöppke
authors.forename: R
authors.initials: R
authors.lastname: Denecke
authors.forename: H-P
authors.initials: HP
authors.lastname: Steinrück