Theoretical Studies On The Structural Evolution And Chemical Bonding Of Multi-rhenium Oxide Clusters

Due to the changeable chemical valences and diverse coordination patterns of rhenium, transition-metal rhenium oxides show various physical and chemical properties. As catalysts, catalyst promoters and catalyst supports, the rhenium oxides have wide applications in organic synthesis, petrochemical industry and so forth. In order to develop a further understanding on the catalytic properties of rhenium oxides, it is necessary to make a comprehensive understanding of their structures. Gas-phase clusters can be used as molecular models to mimic the geometric and electronic properties of metal oxide surface defects and provide fundamental insights into the mechanisms of catalytic reactions, which will help the design of novel catalysts with tailored catalytic performances. In this thesis, we report the theoretical studies on the structural evolution and chemical bonding of multi-rhenium oxide clusters. A summary of the work is given as following:1. Taking Re3O-/0 clusters as the starting point, theoretical calculations are carried out to identify the electronic and structural properties of tri-rhenium oxide clusters Re3On-/0(n= 1-6). With increasing O content in Re3On-/0(n= 1-6), the structural evolution, electronic properties, and chemical bonding are also investigated. Generalized Koopmans theorem is applied to predict the vertical detachment energies (VDEs) and simulate the anionic photoelectron spectra (PES). The Re3 triangle is found to be maintained in the structures of Re30n-/0(n= 1-6) clusters and the sequential oxidation is clearly observed in the Re3On-/0(n= 1-6) series.2. Density functional theory (DFT) calculations are employed to investigate the electronic and structural properties and chemical bonding of the Re3On-/0(n= 7-10) anion and neutral clusters. The TGMin software, which is based on the basin hopping algorithm, is used to explore the potential energy surfaces of Re3On-/0(n= 7-10) clusters. The candidates within-3.0 eV are then re-optimized at the B3LYP level of theory. The calculation results show that the Re3 triangle frame is still maintained in Re3On-/0(n= 7,8) clusters, while Re3On-/0(n= 9,10) clusters possess significant ReO4-/0 motifs. Combining the study of Re3On-/0(n= 1-6) in the previous chapter, we can draw a conclusion that tri-nuclear rhenium oxide clusters tend to be formed by the integration of low-nuclear rhenium oxide clusters along with the increasing of oxygen content.3. The stoichiometric rhenium clusters ReO3+, ReO4-, Re2O7, Re3010+, Re3O11- and are investigated systematically by DFT calculations. The theoretical results show that the global minimum for Re3O11- is formed by weak-interaction of ReO4- and Re2O7 units. The higher-nuclear rhenium oxide clusters tend to adopt the structures which can be viewed as the combination of mono- or di- nuclear rhenium oxide clusters ReO3+、ReO4- and Re2O7. This phenomenon is qualitatively explained through electronic and structural properties, chemical bonding, frontier molecular orbital, electrostatic potential and thermodynamic properties analyses. This work provides a theoretical guidance for the study of rhenium oxide catalysts.