Molecular Simulation Study Of Carbon Nanofiber:Microstructures And Interaction With Metal Nanoparticles

As a new category of carbonaceous material, carbon nanofiber (CNF) has been attracted much attention due to their unique structure and properties. CNF is classied into three categories according to the orientation of graphene sheet:platelet-type (p-CNF), tubular-type (t-CNF), and fishbone-type (f-CNF). The microstructure of f-CNF is rather complicated, which has both the basal and the edge plane exposed on the surface. The basal and the edge plane played different roles in catalysis. In the thesis, a substantial theoretical effort using several methods has been devoted to a better undertand of the microstructure of carbon nanofiber and the interaction with metal nanoparticles. A practical approach to the construction of a cone-helix model for f-CNF is proposed, and then molecular mechanics and X-ray diffraction simulations are performed to evaluate the reliability of the proposed models. The relationship between the ratio of edge carbon and the diameter and apex angle of f-CNF is examined. The mechanism of loop formation at the f-CNF edge by thermal annealing is revealed. The effects of f-CNF microstructure and Pt particle size on the interaction between f-CNF and metal nanoparticles are studied using molecular dyanmics simulations. First-principle calculations based on density functional theory are used to investigate the adsorption of a single Pt atom on graphene. The main results of the work are summarized as follows:1. Proposing a plausible model for f-CNF proves to be a challenge. A practical approach to the construction of a cone-helix model is suggested on the basis of early experimental observations and from a geometric perspective. With the introduction of disclination angle and overlap angle into graphene sheets, the resultant helical cones have variable morphologies which rationalize the broad distribution of the f-CNF apex angles. A fraction of overlap angles can produce energetically favorable cone-helix models with high densities of coincident lattice points. Once the nearest coincident points to the apex is identified, both the overlap angle and the degree of graphitic alignment can be obtained. Periodic boundary conditions are imposed on the cone-helix models to depict the f-CNF morphologies along the principal axes. The lattice strain induced by the multilayered model is found to have a negative effect on the structural stability. After the central parts of f-CNFs are removed, the lattice strain around the cone tips is eliminated, and the cone-helix model of more graphite layers is energetically more favorable. 2. X-ray diffraction simulations are conducted to study the crystal structure of f-CNF as well as to evaluate the reliability of the proposed models and identify the reflection at the diffraction angle of44.5°. The positions of the simulated diffraction lines and their intensities fit the experimental data well. The rotation of graphene sheets in cone-helix model is believed to be the primary reason for the diminished (10L) diffraction lines in the XRD pattern. The experimentally obtained reflections at the diffraction angle of44.5°are produced by the residual catalytic metals rather than by f-CNFs.3. The ralationship between the ratio of edge to overall carbon and the diameter and apex angle of f-CNF is derived on the basis of molecular mechanics simulation and geometric perspective. The solutions agree well with simulated results and are in the same order of magnitude with the experimental results, indicating the reliability of the formula.4. Molecular dynamics simulations of CNF at different temperatures (from1000K to2500K) indicate that the interaction between the CNF edges becomes higher with the increase of temperature, leading to more C-C bonds formation at the edge site and the relative fixation of graphene sheets. It is very easy to form new C-C bonds near the existed C-C bonds, resulting in the carbon ring formation. With the temperature increasing to2500K, more and more carbon rings are formed, leading to the loop formation.5. Molecular dynamics simulations based on a reactive force field (ReaxFF) are performed to examine the effects of the variable morphologies of f-CNFs on the microstructures of supported Ptioo clusters. Four f-CNF cone-helix models with different basal-to-edge surface area ratios and edge plane terminations are employed. The calculated results indicate upon the adsorption of Ptioo clusters a fraction of Pt atoms migrate from the metal particles onto the f-CNFs either to accumulate at the metal-support interface or to attain a single atom adsorption on the supports. With decreasing the apex angle or the introduction of H termination, the Pt atoms are more likely to be coordinated to the basal planes, and the binding energies of the Ptioo clusters to the f-CNFs are lowered, accompanied by a lower degree of the cluster reconstruction. On the contrary, if more f-CNF edge planes are exposed, a higher Pt dispersion, lower surface first-shell Pt-Pt coordination numbers, and longer Pt-Pt surface bonds are attained. Considering the interplay between the geometric and electronic structures of transition metal surfaces, the relationship among the support morphologies, the metal-support interactions, and the catalytic properties of the active Pt clusters is eventually elucidated.6. Molecular dynamics simulations based on ReaxFF are performed to examine the effects of particle size on the microstructures of supported Ptioo clusters. Upon adsorption, all Pt clusters move more closely to f-CNF and are formed Pt-C bonds with f-CNF. The Pt-C bond distribution depends mainly on the microstructure of f-CNF. The relationship between the cohesive enerngy and the atom number is studied. Small Pt cluster has low cohesive energy and therefore has large reconstruction upon adsorption. The degree of morphology reconstruction and property changes decrease with the increase of atom number. The Pt dispersion and the mean first-shell Pt-Pt coordination number increase with the particle size. Therefore, one can employ dispersion, first-shell Pt-Pt coordination number and morphology projection methods to identify metal particle size. Meanwhile, in order to design single-atoms catalysts, small metal nanoparticle and f-CNF with more edge planes exposed are recommended.7. Adsorption of a single Pt atom on poly aromatic hydrocarbons has been investigated systematically using density functional theory calculations. The bond length between the Pt and the nearest C atom increases with the coordination of the Pt atom and the Pt charge increases with shorter Pt-C bond distances, indicating that the catalytic activity of the Pt atom can be tuned by modifying its chemical bonding. A computational approach in line with the SSB-D functional, with the ZORA model for relativistic effects and basis sets like TZ2P or QZ4P is required for accurate results for the Pt/C interaction.