Surface Structure And Thermal Stability Of Platinum Group Metallic Nanoparticles With High-index Facets

Platinum-group metallic nanoparticles are widely used as indispensable catalysts in fuel cells,petrochemical reforming and automotive catalytic converters due to their excellent catalytic performance.However,the rare reserve and high cost severely limit its further usage.In order to meet the increasing demand in industry,how to improve the intrinsic catalytic properties and utilization efficiency of them therefore becomes a key issue in the development of industrial catalytic fields.The successful synthesis of high-index-faceted nanoparticles provides a promising solution to this problem.High-index facets,denoted by a set of Miller indices {hkl}with at least one index greater than unity,possess a high density of low-coordinated atoms,which situated on steps,ledges and kinks with high reactivity required for high catalytic activity.Therefore,High-index faceted nanoparticles possess greatly potential applications for their enhanced catalytic performance.In this article,molecular dynamics methods were employed to systematically investigate the structural and thermal stabilities of platinum-group monometallic or multi-metallic nanoparticles.The article mainly includes following three aspects:In the first part,we have employed atomistic simulations to systematically investigate the structural,thermal stabilities and shape evolution of Pt nanoparticles with different high-index facets,that is,tetrahexahedra enclosed by {hk0} facets,trapezohedra by {hkk} ones,and trisoctahedra by {hhk} ones.The results show that {221} faceted trisoctahedral nanoparticles display the best structural and thermal stabilities while {410} faceted tetrahexahedral ones do the worst.The shape stability of these nanoparticles generally decreases in the order from trapezohedron and tetrahexahedron to trisoctahedron.For the same type of polyhedron,the structural,thermal and shape stabilities of the nanoparticles all decreases according to the order of {2kl},{3kl} and {4kl}facets.Further analyses have discovered that a large proportion of high-coordinated surface atoms are beneficial to enhancing both the thermal and the shape stabilities.This study provides an in-depth understanding of surface structures and thermodynamic evolutions of high-index-faceted metallic nanoparticles.In the second part,we have employed molecular dynamics simulations to investigate the thermodynamic evolution of tetrahexahedral Rh nanoparticles respectively covered by {210},{310} and {830} facets during heating process.Our results reveal that the {210} faceted nanoparticle exhibits better thermal and shape stability than the {310} and {830} faceted ones.Since the {830}facet consists of {210} and {310} subfacets,the stability of {830} faceted Rh nanoparticle is dominated by {310} subfacet,which possesses a relatively poor stability.This regularity could also be extended to other complex high-index-faceted metallic nanoparticles.Further analyses indicate that the surface atoms with higher coordination numbers display lower surface diffusivity,and thus being helpful for stabilizing the particle shape.This study offers an atomistic understanding of the thermodynamic behaviors of high-index-faceted Rh nanoparticles,and can be expected to have important implications on the exploitation and design of complex high-index-faceted metallic nanocatalysts.In the third part,we have employed molecular dynamics simulations to systematically explore the thermodynamic stability of Au@Pd@Pt core-shell trimetallic nanoparticles with different component ratios.The Lindemann index and common neighbor analysis were adopted to characterize their melting behaviors and structural evolutions during continuous heating.Our results reveal that the thermodynamic stability of Au@Pd@Pt core-shell NPs is strongly dependent on the Au and Pt ratios.Their melting points are prominently enhanced with rising Pt compositions when the Au core is the same.In contrast,the melting points are reduced with increasing Au composions when the Pt shell is the same.Besides,the atomistic snapshots demonstrated that for most nanoparticles,the melting processes start on the surface and propagate into the core.Specially,the melting processes start from interior zone when the proportion of Au core is large.Due to the lattice constant differences in Au,Pd and Pt,the stacking faults have been observed in the nanoparticles with large or moderate Au core.With the rising temperature,these stacking faults gradually decreased beyond 700 K and finally disappeared near the melting point.These results suggest that the thermodynamic stability of Au@Pd@Pt core-shell nanoparticles can be tuned by controlling the molar ratio of components,indicating a highly promising strategy to synthesize core-shell trimetallic even multimetallic nanoparticles with excellent catalytic performance and high stability.Due to the potential application of core-shell structures,this study is expected to be of significance not only to the exploitation of trimetallic nanoparticles but also to the further design of multimetallic nanostructures.