Importance of Kinetics in Surface Alloying: A Comparison of the Diffusion Pathways of Pd and Ag Atoms on Cu(111)

Abstract

The microscopic details of how metals alloy have important consequences for both their material properties and their chemical reactivity. In this study, the initial stages of alloying of Pd and Ag with Cu(111) are compared. Low-temperature scanning tunneling microscopy reveals that physical vapor deposition of Pd and Ag at or above room temperature yields remarkably different surface alloys: Pd predominantly incorporates at the nearest ascending Cu step edge, whereas Ag appears to be able to traverse step edges rather easily and alloys into terraces both above and below its initial adsorption site. Density functional theory calculations reveal that even though Pd adatoms have a lower barrier than Ag for traversing step edges, unlike Ag they bind very strongly to ascending step edges and remain there permanently. This leads to a situation in which Pd atoms have at most a very small number of attempts to leave the terrace on which they are deposited before they are incorporated into the nearest ascending step edge. Ag adatoms, however, have many opportunities to cross step edges and can alloy at positions far from their initial starting point. This direct comparison demonstrates the importance in combining theory and experiment in order to understand complicated surface alloying mechanisms and illustrates how both the kinetics and the thermodynamics of the process must be considered to fully understand experimental observations.

An Atomic‐Scale View of Palladium Alloys and their Ability to Dissociate Molecular Hydrogen

“…The superior properties of alloys are often discussed both in terms of ligand (electronic) and ensemble (geometric) effects. [8,[21][22][23][24][25][26][27][28][29][30][31][32][33][34] The bulk alloys exist in both ordered and disordered phases; however, at most catalytically relevant temperatures and compositions, the binary Pd-Cu system is a disordered fcc alloy. Ensemble effects refer to the spatial distribution of atomic sites that host reactants.…”

Single Atom Alloy Preparation and Applications in Heterogeneous Catalysis

“…Understanding the nanocluster structure at the atomic level could help to establish much clearer structureproperty relationships, which further outlines effective principles to adjust the catalyst properties. Thanks to the stability of Au 25 and Ag 25 nanoclusters, they are considered as suitable backbones for mono-doped nanocrystals. Pd is the first element mono-doped into Au 25 nanoclusters, which affects the nanocluster properties tremendously.…”

“…And most importantly, researchers are very interested in investigating the electronic effects, which is imposed on metal nanoclusters by single dopant atoms. Stable metal nanoclusters such as Au 25 and Ag 25 are of paramount importance since they are the only existing platforms. Future works dedicated to developing new and stable metal nanoclusters would be very helpful for the quantitative determination of electronic effects.…”

“…The driving force for metal exchange process was hypothesized as the d 10 -d 10 interaction between metal ions and Au 25 NCs, since all metals that were successfully doped into Au 25 NCs have fully filled d orbitals. The metal exchange process provided an alternative method to dope single metal into Au 25 NCs by which the limitation of metal activity consequence could be bypassed compared to previous proposed methods. [4,6,8b] Cd 1 Au 24 (SC 2 H 4 Ph) 18 nanocluster such as optical and catalytic properties.…”

“…The atomically dispersion of Pd was achieved by Pd adatom diffusion over Cu substrate, since the ascending step edge on the surface of Cu(111) would trap and alloy with Pd single atom. [25] The as-prepared PdCu SAA therefore coined the definition of SAA that consisted mainly of two characteristics, 1) the more active metal was dispersed on the surface of another metal atomically, and 2) the dispersion of the more active metal was thermodynamically stable, namely no dimers or trimers.…”