Theoretical Studies Of The Structural Evolution And Reaction Of Niobium Sulfide Clusters

Niobium sulfides have wide applications in optical devices,electrode materials,superconducting materials,semiconducting materials and industrial catalysts because of their unique physical and chemical properties.In particular,considerable activity and selectivity in catalytic hydrogenation have been found for niobium sulfides during petroleum refining.In order to understand the influence factors of its catalytic performance,we carry out a density functional theory?DFT?combined with coupled cluster theory?CCSD?T??study on gas phase niobium sulfide clusters system,and observe the effects of cluster size,sulfur content and charge states on their structures and chemical bonding.Besides,the reactive sites are studied by the means of gas phase cluster models,so as to provide theoretical basis for improving the performance of these functional materials.The work of this paper is summarized as follows:1.Density functional theory and coupled cluster theory are carried out to search for the ground states of a series of mono-nuclear niobium sulfide cdusters NbSn₂^-/-0?n = 3-5?and investigate their structural evolution,chemical bonding and reactivity.Generalized Koopmans' theorem is applied to predict the vertical detachment energies?VDEs?.and simulate the anionic photoelectron spectra?PES?of selected low-lying isomers.The following conclusions are obtained:Various polysulfide ligands?viz.,S₂ and S₃?emerge in the sulfur-rich systems,and the structures of NbS₅^2-/-/0 change obviously with the charge states.Additionally,similar structures and evolution rules are found between the NbSn₂^-/-?n = 3-5?and their corresponding valence isoelectronic MoS_n^-/0?n = 3-5?species.2.Theoretical calculations are carried out to search for the geometric configuration and chemical bonding of Nb₂S_n^-/0?n-3-7?ground states.We find that the ground states of Nb2S_n^-/0?n= 3-7?evolve from the Nb₂S₂ ring with the addition of sulfur atom.Besides,a behavior of sequential sulfidation as a function of sulfur content is observed.The various sulfide ligands appear successively as follows:Before Nb atom reaches its highest oxidation state of +5,the additional S atom preferred the bridging or terminal sites,and after Nb atom reaches its highest oxidation state the bridging S₂,terminal S₂ and bridging S₃ ligands will emerge successively.In addition,the Nb₂S₆ cluster may be used as a neutral model for the study of catalytic active sites on the surface of bulk NbS₃ catalysts.3.Theoretical calculations are carried out to search for the global minimums of the niobium sulfide clusters Nb₃S_n^-/0?n=6-8?.The anionic photoelectron spectra of selected low-lying isomers are simulated.Result shows that the ground states of Nb₃S_n^-/0?n = 6-8?evolve from the Nb₃ S₃ ring,and S₂ emerges in the sulfur-rich cluster Nb₃S₈.These structures prefer the lowest spin multiplicities and symmetry.The photoelectron spectra of sulfur-deficient clusters are more complicated than sulfur-rich clusters,which can be explained by means of molecular orbital analysis.With the increasing cluster size,the structural characters of?NbS₂?_n?n =1-3?clusters close to the bulk NbS₂ material gradually.