Reactivity of CO and C₂ H₄ on Bimetallic Pt_x Ag_1-x /Pt(111) Surface Alloys Investigated by High-Resolution X-ray Photoelectron Spectroscopy

We have investigated the adsorption and thermal reactivity of CO and C₂ H₄ on well-defined bimetallic Pt_x Ag_1-x /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_on-top and CO_bridge desorb at higher temperatures than on bare Pt(111), for example, by ∼65 K for CO_on-top 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 C₂ H₄ 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 CCH₃ occurs at much lower temperatures, e. g. by ∼60 K for the alloy with 68 % Ag, and no ethylene desorption is found.