Reactivity of CO and C₂H₄ on Bimetallic Pt_xAg_1‐x/Pt(111) Surface Alloys Investigated by High‐Resolution X‐ray Photoelectron Spectroscopy

Abstract

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.

Stability, Electronic Properties and CO Adsorption Properties of Bimetallic PtAg/Pt(111) Surfaces

“…36,37 As an extreme example, multicarbonyl species with more than one CO bound to a Pt atom had been proposed by Tsang et al and Oduro et al for Co-doped Pt catalysts. 38,39 Combining these data with earlier results, 25,34 we will also try to get further information on the role of electronic ligand and strain effects. Finally, aiming also at a more general understanding, we will compare the results obtained here with comparable data reported earlier for the structurally rather similar PdAg/Pd(111) system.…”

“…This agrees perfectly with previous experimental findings from temperature programmed desorption experiments where peak broadening with increasing coverage was rather small, 25 and also from equilibrium measurements. 34 Third, the fact that CO adsorption on these pseudomorphic Ptn ensembles embedded in a matrix of Ag surface atoms is significantly stronger than adsorption on Pt(111), indicates that adsorption on Pt atoms in the surface alloy is dominated by lateral ligand effects rather than strain effects. In the present case of a pseudomorphic overlayer, we expect compressive strain induced by the larger Ag surface atoms, which would lead to a destabilization of the Pt-CO bond.…”

“…The structure, formation and stability of PtAg/Pt(111) surface alloys had been characterized previously by a variety of different techniques, [29][30][31][32] the results were summarized previously by Jankowski et al 33 and by Bauer et al 34 These studies found that deposition of submonolayer amounts of Ag on Pt (111) and subsequent annealing to about 620 K results in the formation of monolayer PtAg surface alloys, where the Ag atoms are confined to the topmost layer 24,29,30,32 with a tendency to phase separation to form two-dimensional, homoatomic ensembles rather than a random distribution. 25 Thermal and CO induced intermixing of Pt and Ag in Ag films on Pt(111) and in Pt film covered Ag(111) surfaces were investigated by Jankowski et al 33 experimentally and by Hua et al theoretically.…”

“…25 A later high-resolution XPS study, which could distinguish also bridge-bonded CO from on-top adsorbed CO based on their slightly different C1s binding energies, essentially confirmed these findings. 34 In the present study, we are particularly interested in identifying possible highly stable local structures and in correlations between electronic properties of specific Ptn ensembles and their CO adsorption strength. Considering the CO adsorption behavior we are most interested in changes in the adsorption characteristics with increasing local coverage, i.e., with increasing number of COad per Pt surface atoms in Ptn ensembles in bimetallic surfaces.…”

Formation of Catalytically Active Nanoparticles under Thermolysis of Silver Chloroplatinate(II) and Chloroplatinate(IV)

“…The highest activity is achieved with the composition Ag 0.38 Pt 0.62 [11]. The use of AgPt as a catalyst to synthesise H 2 O 2 improves the selectivity up to 50%, unlike pure platinum, which yields only 6% [14]. The Ag 2 Pt/Al 2 O 3 catalyst also shows a high activity when compared to monometallic analogues during CO oxidation under ambient temperature [15].…”