Hydrogen adsorption on bimetallic PdAu(111) surface alloys: minimum adsorption ensemble, ligand and ensemble effects, and ensemble confinement

Abstract

The adsorption of hydrogen on structurally well defined PdAu-Pd(111) monolayer surface alloys was investigated in a combined experimental and theoretical study, aiming at a quantitative understanding of the adsorption and desorption properties of individual PdAu nanostructures. Combining the structural information obtained by high resolution scanning tunneling microscopy (STM), in particular on the abundance of specific adsorption ensembles at different Pd surface concentrations, with information on the adsorption properties derived from temperature programmed desorption (TPD) spectroscopy and high resolution electron energy loss spectroscopy (HREELS) provides conclusions on the minimum ensemble size for dissociative adsorption of hydrogen and on the adsorption energies on different sites active for adsorption. Density functional theory (DFT) based calculations give detailed insight into the physical effects underlying the observed adsorption behavior. Consequences of these findings for the understanding of hydrogen adsorption on bimetallic surfaces in general are discussed.

Direct synthesis of hydrogen peroxide using concentrated H2 and O2 mixtures in a wall-coated microchannel – kinetic study

“…Therefore, it is widely accepted that "dilution" of Pd surface with Au is a cause of higher selectivity in the direct synthesis observed over proportional to square route of hydrogen partial pressure. The pairwise hydrogen dissociation mechanism indicated as is indicated by the good agreement with the experimentally collected kinetic data is in agreement with the conclusion made by Takehiro et al [54] on hydrogen dissociation on AuPd surfaces. The heat of adsorption is also influenced by the surface coverage i.e.…”

“…As established by the kinetic model, direct formation of hydrogen peroxide from hydrogen and oxygen seems to involve hydrogen adsorption, dissociation, but not spill-over of H-species over the AuPd surface. It is most likely that reaction occurs between non-dissociated oxygen and hydrogen species adsorbed pairwise as suggested by several studies on hydrogen adsorption over AuPd surfaces [53,54] .…”

“…Surface diffusion of H species between isolated Pd ensembles is inhibited considering the high energy of H ad adsorbed on Au surface sites. This further implies that dissociative adsorption of H 2 molecules is only possible if both H species can be stably adsorbed on the same Pd ensemble, in a way that hydrogen adsorption occurs pairwise [54] . TPD and STM imaging studies show that H 2 uptake mainly occurs on Pd islands.…”

“…The formation of Pd ensembles is crucial for dissociative adsorption of hydrogen, considering that H ad is the most stable on Pd 3 threefold hollow sites, less stable on Pd 2 bridge sites and least stable on Pd monomers [54] . Surface diffusion of H species between isolated Pd ensembles is inhibited considering the high energy of H ad adsorbed on Au surface sites.…”

Structure, chemisorption properties and electrocatalysis by Pd3Au overlayers on tungsten carbide – A DFT study

“…One of the most common electrocatalytic reactions, having a great impact in practice, is hydrogen evolution reaction (HER) in which H þ ions (acidic solutions) or water (neutral and alkaline solutions) is reduced to gaseous H 2 . In general, the structures of practical HER catalysts are determined by the possibilities of common surface alloy preparation techniques: diffusion controlled in situ co-deposition, electrochemical deposition of thick overlayers or pulse deposition of metal clusters on appropriate support [5]. One of the common approaches in the preparation of bimetallic electrocatalysts for hydrogen electrode reactions is the deposition of thin layers of noble metal on foreign host supports.…”