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Abstract
The influence of preadsorbed sulfur on the adsorption of CO on Pt(355) and Pt(322) isinvestigated systematically for sulfur precoverages between 0.02 and 0.30 ML by in situ x-rayphotoelectron spectroscopy of the C 1s and S 2p core levels. The two surfaces have thesame nominal terrace width of 5 atomic rows, but different step orientation. For both, atlow temperatures (130 and 150 K), S preferentially adsorbs at the steps and passivates themfor CO adsorption. The required S precoverage is significantly smaller for Pt(322), becauseof the lower number of steps as a result of the S-induced double-step formation. Uponheating, population of step sites with CO occurs due to irreversible CO/S siteexchange. At low S precoverages, the characteristic transition temperatures of165 K for Pt(355) and 245 K for Pt(322) are significantly different, indicating ahigher activation barrier for the CO/S site exchange process for Pt(322). Formedium to high S precoverages the transition temperature stays unchanged forPt(322) but increases for Pt(355). The observed behavior is attributed to a kineticpassivation of the steps by sulfur at low temperatures, which is lifted upon heating.
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