Atomistic Study On The Microstructural Features Of Noble Metal-based Nanoparticles

Pt-and Pd-based alloy nanostructures usually exhibit unique catalytic activity and selectivity that are distinct from their parent metals,owing to their high surface-to-volume ratio,verious types of surface activity sites,novel structures and synergistic effects induced by alloying addition.Experimentally,it is still a challenge to glean the precise structural and compositional information of nanostructures,as well as the influences of different factors,including the particle size,morphology,composition,and temperature.On the other hand,Monte Carlo simulation(MCS)based on many-body potentials have recently evolved to effective tools to study the properties of nanostructure and complement ongoing experimental efforts.Nanostructured Cu–Pt alloys have received enormous attention as their excellent activity for oxygen reduction reaction(ORR).Ag–Pd alloys have been used as hydrogen separation membranes and selective hydrogenation catalysts,and are promising alternative ORR electrocatalysts because of their better catalytic properties in alkaline medium and lower cost than Pt-based catalysts.Meanwhile,the formation energy of alloys and different surface energy and atomic size of elements make Cu–Pt and Ag–Pd nanoalloys as ideal systems to study the competition or synergy between surface segregation and chemical ordering.In this work,MCS have been used to obtain the thermodynamically stable configurations of Cu–Pt and Ag–Pd nanoparticles(NPs)with different particle size,morphology or alloy composition.The composition and atomic ensembles on different surface sites,and the type,ratio,as well as spatial distribution of different chemical orderings in core were analyzed quantitatively.The present objective is to explore general trends of surface segregation and chemical ordering,understand the coupled effects of surface energy,formation energy,and strain energy on the structural features of NPs,and get insight into the surface structure-catalytic property relationships.The main results include:1.Developing and extending methods for identifing local structure in alloys.The overall mixing index was extended to the local mixing indexes that was used to identify ordered phases in Cu–Pt NPs.However,it may overestimate somewhat the amount of ordered phase and are unavailable to distinguish long period structures.Furthermore,a simple extension of the original common neighbor analysis method to the second nearest neighbors(2NN-CNA)is proposed to classify similar long range ordered structures.The bulk signatures of 2NN-CNA were given for seven ordered alloys.Several examples demonstrate this extended method,including nanocomposite and NPs.2NN-CNA can well distinguish ordered phases in Cu–Pt and Ag–Pd from each other.2.Understanding surface segregation and chemical ordering: general trends and competitive/synergistic relationship.The parameters of many-body potentials for Ag–Pd and Cu–Pt alloys were determined and tested.The surface energy of every element,the formation energy and ground states as well as relative stability of imaginary intermetallics in bulk alloys are well reproduced.Thus,they can be used for a wide range of composition for NPs,with reasonable accuracy.The surface segregation and chemical ordering patterns of Ag–Pd NPs as well as the roles of energetic factors and nanoscale effects were studied by performing MCS.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.Accordingly,small and Pd-rich nanoalloys display a continuous transition from Pd-core/ mixing-shell to mixing-core/ 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.The surface segregation and chemical ordering patterns of Cu–Pt NPs are studied for a broad range of sizes,shapes,composition,and temperature.Similar to Ag–Pd system,surface segregation of Cu is enhanced with increasing particle size or global Cu composition.For different morphologies,Cu segregation is enhanced with increasing surface openness.Despite their different morphologies,most of the types of ordered phases in core region are the same as bulk alloys.Due to the modification or suppression effects of surface segregation,the degrees of chemical ordering shift to the Pt-richer side and are more apparent in large-sized particle.It was found that both the Ag or Cu segregation on surface and the chemical ordering in core are the general rules and usually compete with each other.Thermodynamically,chemical ordering is favored by attractive Ag–Pd and Cu–Pt interactions,while the relative lower surface energy of Ag(Cu)to Pd(Pt)favor surface segregation.Furthermore,segregation of bigger atoms,Ag or Pt,onto the surface can release the strain energy effectively.As a result of the competition among these energetic factors,confined NPs can exhibit complicated and novel structures.Specially,for(truncated)octahedrons with near equimolar compositions,the core regions prefer the L11 bulk ordering characterized by alternative stacking(111)layers of Pt and Cu,and hence the transition from the L11 bulk ordering to multishell structure with alternative stacking of Cu and Pt {111} shells can be largely indeced by Cu facets strong Cu enrichment on {111} facets,illustrating a subtle synergy between the segregated Cu {111} facets and the L11 ordering.3.Getting insight into the structure-catalytic property relationships of nanoalloys.Special surface chemical patterns are formed as a subtle synergy between the preferential segregation of Ag and attractive Ag–Pd hetero-bonds,where Ag atoms not only isolate {111} Pd ensembles into monomers but also block low-coordinated Pd sites.For both the hydrogen evolution reaction and selective hydrogenation of acetylene,the correlations between the surface chemical structure and the improved catalytic performance were revealed from the viewpoints of active site isolation or blocking,implying that the composition of Pd can be largely reduced for these two reactions.Considering the multishell structures of Cu–Pt NAs with(truncated)octahedrons morphology,it is speculated that they can be conveniently transformed towards single Cu-rich core/Pt shell structures by selective etching the Cu species of as-prepared NPs.The most prevalent structure in dealloyed NPs gave Cu-rich core/Pt shell(2~5 monolayer)structure and will exhibite superior ORR activity due to the strain effects.The Cu50Pt50 ordered core of dealloyed NAs are most thermodynamically stable,where the closed shell or network of Pt will effectively protect the Cu species from further dissolution and thus can promote the electrochemical stability of dealloyed ORR catalysts.