Molecular Simulation Of Structure And Melting Behavior Of Nano-scaled Metallic Cluster

Nano-sized metallic clusters process large specific surface and high vacancies of coordination of surface atoms. In addition, the heterogeneous structural characteristics and homogeneous physicochemical properties of nano-sized metallic clusters make them easy to disperse well in liquid phases and solutions, which increase the activated efficiency of catalyst. Therefore, the nano-sized metallic cluster is a species of catalyst with high selectivity and activity, and exhibits extensively potential applications in catalysis field. However, this kind of ultra-fine particle catalyst also shows a vital disadvantage of weak thermal stability. Under extremely restricted conditions in catalysis reactions, nano-sized metallic clusters tend to aggregate together growing as large-sized particles, which causes the deactivation of catalysis of metallic clusters. Accordingly, it is a very important project to investigate the structure and thermal stability of metallic clusters aiming at their further applications in catalysis.At present, there exist many experimental methods to study the properties of metallic clusters. However, most of them cannot provide the precise relationship between structures and properties at an atomic level. Also, most of the experiments, which are performed under high temperature and high pressure, require strict means for preparation and treatment, increasing the difficulty of experimental study. Here we used molecular simulation to study catalyst properties of the metallic clusters, which is free of the restriction of extreme conditions in practical experiments. Furthermore, the relationship between structures and properties can be provided at atomic level.In earlier calculations, the initial configurations of the metallic clusters were often assumed as the fragments of bulk solids such as face centered cubic (FCC) or body centered cubic (BCC). Recently, both experiments and theories confirmed that the metallic clusters exhibit polyhedron structures made up of geometric shells of atoms. At present, few reports are focused on melting behavior of the metallic clusters, especially for the core-shell structured bimetallic clusters. In this work, on the basis of Sutton-Chen (SC) many body potential, we used molecular dynamics simulation to investigate the melting behavior of mono-metallic and bimetallic clusters with shell-symmetric structures. The main contents and findings are summarized as following: 1．First, the applicability of MD method and SC many-body potential were verified by simulating the melting behavior of bulk Pd. The simulated melting point and static structure factor were in agreement well with experimental results. Then, the melting behavior of 309-atom, Cubooctahedral (Cub), Dectahedral (Dec) and Icosahedral shell-symmetric structured Pd clusters was simulated. The effect of shell-symmetry on the melting behavior of Pd clusters was discussed. Finally, the melting behavior of eight kinds of Ag and Rh bimetallic clusters with Dec or Ico shell-symmetric structures, consisting of 309 or 55 atoms, was studied. The melting points of the eight mono-metallic clusters were found, and the effects of atom number and shell-symmetry on the melting points were discussed. Furthermore, structure transition from Dec to Ico was observed both for the Ag and Rh clusters consisting of 55-atom at very low temperatures.2．Four typical core-shell structured Ag-Rh bimetallic clusters consisting of 309 atoms in total, with Dec and Ico shell-symmetric structure were simulated. It was found that the Ag atoms segregated on the surface of clusters exhibit strong activity before melting of the clusters. Furthermore, the Ag atoms still segregated on the surface of the cluster after melting transition. 3．Six kinds of Dec shell-symmetric structured Ag-Rh bimetallic clusters consisting of 309 atoms in total, with different compositions, were studied. The effects of composition and surface positions occupied by Ag atoms on the melting points were discussed. It was observed that the melting points of Ag-Rh bimetallic clusters were mainly related to the number of Ag atoms, which was that the melting points decreased while the number of Ag atoms increases. Meanwhile, it was found that the melting point of typical core-shell structured Dec_Ag12-Rh297 bimetallic cluster was lower than those of non-typical core-shell structured Dec_Ag23-Rh286 and Dec_Ag32-Rh277 bimetallic clusters, which indicated that the melting points of Ag-Rh bimetallic clusters were also related to the positions occupied by Ag atoms.