Reactivity of CO and C H on Bimetallic Pt Ag /Pt(111) Surface Alloys Investigated by High-Resolution X-ray Photoelectron Spectroscopy.

We have investigated the adsorption and thermal reactivity of CO and C H on well-defined bimetallic Pt Ag /Pt(111) surface alloys (with x=0.67, 0.50, 0.38 and 0.32) using in situ synchrotron-based high-resolution X-ray photoelectron spectroscopy. During low-temperature (140 K) adsorption of CO, we find population of the energetically more stable Pt on-top site followed by the Pt bridge site, as known from clean Pt(111); CO does not adsorb on Ag facets under the applied conditions. The total CO coverage linearly decreases with Ag surface content. Upon heating, CO and CO desorb at higher temperatures than on bare Pt(111), for example, by ∼65 K for CO for an Ag content of 33 %. This shift is due to the well-known electronic ligand-effect of Ag atoms surrounding Pt atoms/clusters. For C H adsorption on the AgPt alloys at low temperature (120 K), we find two species in the C1s XP spectrum, each displaying a vibrational fine structure; this contrasts the situation on clean Pt(111), where only one species is found. Upon heating, ethylene reacts with ethylidyne being the dominant intermediate; in contrast to clean Pt(111), on the alloy surfaces the reaction to CCH occurs at much lower temperatures, e. g. by ∼60 K for the alloy with 68 % Ag, and no ethylene desorption is found.