Surface Segregation and Chemical Ordering Patterns of Ag–Pd Nanoalloys: Energetic Factors, Nanoscale Effects, and Catalytic Implication

Abstract

We performed Monte Carlo simulations to determine the roles of energetic factors and nanoscale effects in the surface segregation and chemical ordering patterns of Ag-Pd nanoalloy particles.Ag atoms significantly segregate onto the surface and preferentially occupy the low-coordinated sites, which significantly reduce the surface and strain energies of the nanoalloys. The segregation isotherms reveal that surface Ag composition is enhanced with increasing particle size or Ag concentration to circumvent the finite matter effects. Chemical ordering favored by attractive hetero-bonds can coexist and compete with surface segregation. Accordingly, small and Pd-rich nanoalloys display a continuous transition from Pd-core/ mixing-shell to mixingcore/ Ag-shell, where an ordered core is absent as a result of surface segregation and limited Ag supply. By contrast, large nanoalloys with equimolar or Ag-rich concentration exhibit the strong core ordering characteristics of bulk alloys. Particularly, surface patterns with partially alloyed facets and Ag-blocked vertices and edges are formed. This study also discussed the effects of isolating and blocking surface Pd active sites by Ag on the hydrogen evolution reaction and selective hydrogenation of acetylene.

Pd–Ag/SiO2 bimetallic catalysts prepared by galvanic displacement for selective hydrogenation of acetylene in excess ethylene

“…Indeed, the surface free energy of Ag metal is lower than that of Pd metal based on calculated and experimental results from others. [35][36][37][38] Mezey 37 reported surface free energies (SFE) of 2043 and 1302 ergs cm −2 for Pd and Ag metallic surfaces at 298 K, respectively, indicating that it is thermodynamically preferred for Ag to diffuse to the surface of a Ag-Pd bimetallic particle. Tang 38 has also calculated the SFE values of specific Ag facets and estimated the free energies of the two most common facets of Ag particles, the (111) and (100) surfaces, are 881 and 948 ergs cm −2 , further showing the thermodynamic favorability for Ag diffusion to the surface of Pd-Ag bimetallic particles.…”

“…[35][36][37][38] Mezey 37 reported surface free energies (SFE) of 2043 and 1302 ergs cm −2 for Pd and Ag metallic surfaces at 298 K, respectively, indicating that it is thermodynamically preferred for Ag to diffuse to the surface of a Ag-Pd bimetallic particle. Tang 38 has also calculated the SFE values of specific Ag facets and estimated the free energies of the two most common facets of Ag particles, the (111) and (100) surfaces, are 881 and 948 ergs cm −2 , further showing the thermodynamic favorability for Ag diffusion to the surface of Pd-Ag bimetallic particles. In support of the first possibility, Xia et al 39 reported that when Na 2 PdCl 4 was added to a suspension of Ag nanocubes, etch pits developed on the surfaces of the nanocubes, primarily at the corners.…”

Catalyst Site Selection via Control over Noncovalent Interactions in Self-Assembled Monolayers

“…Most other approaches to specific site blocking, including those involving Pd catalysts, have employed additional metals that will hypothetically have different affinities for different surface facets 44 . Monte-Carlo and DFT calculations have demonstrated that the second metal with lower surface energy (Cu, Ag and Au) preferentially occupies the low coordination sites on Pd particle surface, such as high index planes, corners and edges [45][46][47] . In addition to simulation results, the strong tendency of Cu atoms to locate at low-coordinated surface sites was supported by low energy ion scattering spectroscopy and FTIR spectroscopy after CO adsorption on the surface; Cu addition was found to cause Pd catalyst selectivity improvement for the hydrogenation of 1,3-butadiene to butene and the dehydrogenation of ethanol to ethanal at the cost of activity 48,49 .…”

Atomic overlayer of permeable microporous cuprous oxide on palladium promotes hydrogenation catalysis

“…20 . During the optimization, the upper three layers (including the Cu 12 O 8 layer) were allowed to be relaxed and the bottom two layers of Pd were frozen 50 .…”