Theoretical Studies On Multinuclear Vanadium Oxide Clusters: V₃O_n^-/0 And V₆^-/0（n=0-8,m=12-15）

Early transition-metal oxides have many technical applications in catalysis, materials, and surface sciences due to their diverse physical and chemical properties. Vanadium is one of the most important early transition metals. Vanadium and vanadium oxides are widely used as both catalysts and chemical agents for many important chemical reaction processes because they are cheap, high reaction activity and excellent stability, such as SO2â†'SO3oxidation process, photocatalytic decomposition of water and selective oxidation of hydrocarbons. In addition, vanadium and vanadium oxides have been found an increasingly wide utilization in steel industry and medical field and so forth. Therefore, it is very meaningful to understand materials’characters and functional designing and development. In order to develop a comprehensive understanding on the nature of bulky materials, it is necessary to make a full study of gas-phase clusters. As we all know that gas-phase clusters may be used as a bridge to connect macro-with micro-systems, because the size of clusters is between atoms and macroscopical systems. Metal-oxide clusters as molecular models can be used to mimic the geometric and electronic properties of metal oxide surfaces/defects, provide fundamental insights into the mechanisms of catalytic reactions, and give theoretical support in designing novel high-active catalysts and functional materials. In this dissertation, we report the theoretical studies on multinuclear vanadium oxide clusters:V3On-/0(n=0-8) and V6Om-/0(m=12-15). A summary of our work is given as following:1. Starting from V3-/0bare clusters, the electronic properties, structural evolution and chemical bonding of vanadium oxide clusters are investigated as a function of O content until stoichiometric V3O8-. DFT calculations are performed to search for the lowest energy structures and low-lying isomers for both the anionic and neutral clusters. A sequential oxidation is found in the process of structural evolution from V3-to V3O8-clusters.2. Due to the previous experimental reports about a novel planar V6O12, we focus on studying the structures and bonding characters of gas-phase V6O12-/0clusters. Our calculations suggest that both anionic and neutral V6O12clusters prefer cage-type structures rather than planar structures. Our extensive structural searches allows the assignment of the C1(2A) anionic structure as the dominant species in the V6O12-cluster beam, which is a tri-prism with three terminal oxygens attached at the same V3plane similar to the lowest-energy Cs (1A’) neutral state. Molecular orbital and spin density analyses of VeO12-clusters are investigated, and suggest that gas-phase V6O12-/0clusters can be viewed as molecular models for O-deficient defect sites.3. Density functional theory (DFT) calculations are carried out to investigate the electronic and structural properties of hexvanadium oxide clusters, V6Om-/0(m13-15). Generalized Koopmans’ theorem is applied to simulate the photoelectron spectra (PES) and predict the vertical detachment energies. A sequential oxidation is observed in the Y6Om-(m=13-15) clusters as a function of m by DFT calculations. All of these large clusters are found to be three-dimentional cage-type clusters. Molecular orbital analyses are carried out to elucidate the chemical bonding in the Y6Om-(m=13-15) clusters.