Structural Properties And Thermodynamics Of Free And Supported Bimetallic Clusters By Molecular Simulation

As is well known, metal clusters present unusual catalytic properties, and have been used extensively in chemical industries. Due to its novel catalytic properties, metal clusters are recognized as the fourth generation of catalysts. More interestingly, bimetallic cluster catalysts show greater activity and selectivity than pure metal ones. It is found that it can be used in petrochemical, energy and environmental protection fields, including the reforming of petrochemicals, automobile catalytic converters, and electrocatalysis, etc. The major interest in bimetallic clusters is that their properties depend not only on size but also on composition and atomic ordering. In addition, bimetallic clusters may display structures and properties which are distinct from those of the pure elemental clusters.However, structural properties and thermodynamics of bimetallic clusters cannot be understood purely from experimental data for their small scales and complex structures. Therefore, it is necessary to simultaneously use molecular simulation or other theoretical methods. Molecular simulation method, based on the lower computational expense of empirical potentials, is suitable to study the structural properties and thermodynamics of bimetallic clusters with big size. The methods of molecular simulation include mainly Monte Carlo (MC) and molecular dynamics (MD). However, MC method is faster in sampling the configuration space than MD method, and has been applied to the structures and meltinglike transitions in single component metals and bimetallic clusters.In this work, structural information and thermal properties for free and supported bimetallic clusters were investigated by using MC method, based on the second-moment approximation of the tight-binding potentials (TB-SMA). Structural information and thermal properties of bimetallic clusters, and metal-filled carbon nanotubes and supported bimetallic clusters have been addressed in a systematic here as follows.1. Development of a molecular simulation platform "MCBC Code Package". A simulation platform "MCBC Code Package" so-called "A Monte Carlo Method to Study Bimetallic Clusters" was developed by C language.2. Molecular simulation studies of structures of free bimetallic clusters and nanowires. The structures of Ag-Cu, Ag-Ni, Ag-Pd, Ag-Au, and Pd-Pt bimetallic clusters and nanowires, and Ag-Ni bimetallic clusters with different size, composition, and temperature were investigated by using MC method. The simulation results show that the Ag-Cu and Ag-Ni bimetallic nanomaterials, including clusters and nanowires, possess core-shell structures at different compositions, in which the Ag atoms lie on the surface, while the Cu or Ni atoms occupy the cores of the clusters and nanowires. In addition, a new kind of multishell onion-ring structures of bimetallic nanowires and clusters for three systems (A/B = Pd/Pt, Ag/Au, and Ag/Pd) was firstly reported. In the structures of multishell onion-ring bimetallic nanowires and clusters, A atoms and B atoms occupy alternately the layers of the nanowires and clusters, thus forming the onion-ring morphology. It is found that the Ag-Rh bimetallic clusters also possess core-shell structures at different compositions, in which the Ag atoms lie on the surface, while the Rh atoms occupy the cores of the clusters. It is also found that 55-atom Pd-Pt bimetallic clusters possess three-shell onion-like structures, in which a single Pd atom is located in the center, and the Pt atoms are in the middle shell, while the Pd atoms are enriched on the surface. In addition, the pentagonal multi-shell-type structure can be transformed into cylindrical multi-shell-type structures for Ag-Cu, Ag-Ni, Ag-Au, and Ag-Pd bimetallic nanowires at 100, 300, and 500 K.3. Melting of free bimetallic and trimetallic clusters. The melting of bimetallic and Ag-Cu-Au trimetallic clusters with different composition and structures were investigated by canonical Monte Carlo simulations. 1) It is found that structure has a significant effect on the melting properties of bimetallic clusters. 2) Simulation results reveal that the dependence of melting point on the composition is different. For the 55-atom Pd-Pt bimetallic clusters, it is not a monotonic change, but experiences three different stages. The melting temperatures of the Ag-Pd bimetallic clusters increase monotonically with the concentration of Ag atoms first. Then, it reaches a plateau with almost a constant value. Finally, it decreases sharply at a specific composition. In addition, the simulation results show that the melting point increases with the concentration of Cu in the 55-atom Cu-Au bimetallic cluster. The melting temperatures of the Pd-Pt bimetallic clusters show two peaks with the Pd-concentration. 3) The substitution of a single Cu or Ag impurity doped in the icosahedral monometallic clusters can significantly increase their melting temperatures. Doping of Au₅₅ with a single Cu atom can sharply raise the melting point of the cluster from 380 K to 530 K. Also, doping of Ag₅₅ with a single Cu atom can sharply raise the melting point of the cluster from 570 K to 630 K. It is also found that doping of Au₅₅ with a single Ag atom can sharply raise the melting point of the cluster from 380 K to 420 K. The substitution of a single Cu impurity doped in the icosahedral bimetallic clusters can also greatly increase their melting points. 4) The surface segregation of Ag atoms in the Ag-Cu, Ag-Au bimetallic clusters, and the Ag-Cu-Au trimetallic clusters occurs even after melting.4. Molecular simulation studies of metal clusters encapsulated in carbon nanotubes. Thermal evolution of an icosahedral Pt₅₅ cluster encapsulated in the (15, 15) and (20, 20) single wall carbon nanotubes (SWNTs) was investigated by a MC method. The melting-like structural transformation is found for the icosahedral clusters encapsulated in SWNTs. The melting-like transformation temperatures of the icosahedral clusters encapsulated in SWNTs are 280 and 320 K, respectively. The simulations indicate that the melting-like transformation temperature for the encapsulated icosahedral clusters increases with the pore size of SWNTs. At higher temperatures, a stacked structure in layers is found for the encapsulated icosahedral Pt₅₅ clusters. Simulation results reveal that SWNTs have a significant effect on the structures of the encapsulated icosahedral Pt clusters.5. Molecular simulation studies of metal-oxide supported metal clusters. Thermal evolution of icosahedral Pd₅₅, Pd₄₃Pt₁₂, and Pd₁₃Pt₄₂ clusters supported on the MgO(100) surface were studied by a Monte Carlo simulation. The solid-solid structural transformation from the icosahedral structure to the layered fcc structure is found for the Pd₅₅, Pd₄₃Pt₁₂, and Pd₁₃Pt₄₂ clusters supported on the MgO(100) surface, determined by the changes of the total potential energies and variations of the deformation parameters. It is found that the supported Pd₅₅, Pd₄₃Pt₁₂, and Pd₁₃Pt₄₂ clusters possess the layered epitaxial fcc structure at higher temperatures after structural transformation. In addition, the composition effect on the transition temperatures of free and MgO(100)-supported Pd-Pt bimetallic clusters is discussed.