Reaction Mechanism Of FeV^- And PtC_n^- Activated N₂ Based On Photoelectron Spectroscopy

Photoelectron spectroscopy is used to obtain the spectral characteristics of anions and neutral clusters by the photoelectric effect principle,so it is widely used in the microscopic fields of atoms,molecules and clusters.In this dissertation,the reaction mechanism between transition metal clusters Fe V^–and transition metal carbide clusters Pt C_n^–（n=4-6）and N₂ resource small molecules has been studied in detail by using a self-developed time of flight mass spectrometry coupled photoelectron spectrometer.Combined with quantum chemical calculation,the geometric configuration,reaction paths,electronic properties and chemical bonding have been discussed in depth.The main research of this dissertation includes:1.Study on reaction mechanism of transition metal heteronuclear dimer with nitrogen:Taking heteronuclear transition metal cluster Fe V^–as an example,the reaction mechanism of Fe V^–anion on N₂ activation was studied.The results show that at room temperature,Fe V^–can adsorb N₂ and fully activate it,resulting in cluster anion Fe VN₂^–medium N≡N triple bond completely break.Further potential energy surface analysis,bond length analysis and natural population analysis（NPA）showed that the activation of N₂ by Fe V^–was realized through the cooperative transfer of electrons from bimetallic atoms to nitrogen molecules and the return of electrons from nitrogen groups to bimetallic nuclei,which indicated that heteronuclear bimetallic anion clusters played an important role in the activation of nitrogen.2.Study on the reaction mechanism of transition metal carbide clusters with nitrogen:Taking the mononuclear precious metal carbide Pt C_n^–（n=4-6）as an example,the structural information of Pt C_n^–and Pt C_nN₂^–（n=4-6）was determined.The structure of Pt C_n^–and Pt C_nN₂^–（n=4-6）is mainly manifested as linear chain structure with C_∞vsymmetry and the characteristics of ²Σ⁺electronic states.For Pt C_nN₂^–（n=4-6）,molecular orbital（CMO）analysis,Wiberg bond orders analysis of N-N bonds,and natural population analysis（NPA）of Pt C_n^–and Pt C_nN₂^–（n=4-6）indicate that the charge transfer of Pt and C_n to N₂ promotes the fixation and activation of N₂.Simultaneously,the significance of theπback-donation of the 5d orbital of Pt to the antibondingπorbits of N₂ for dinitrogen fixation and activation is emphasized.In this process,we find that bothπback-donation from the lone electron of 5d orbital in Pt atom into theπantibonding orbits of N₂（referred to as nπ^*）and contribution from theπelectron of theπbond into the antibondingπorbits of N₂（referred to asππ^*）can weaken the triple bond≡N,and nπ^*is slightly more effective thanππ^*in weakening the N≡N bond in this system.In this study,Pt C_nN₂^–（n=4-6）species was identified as an indispensable intermediate in the nitrogen fixation process of mononuclear metal carbide Pt C_n^–（n=4-6）anion clusters,which provided a potential direction for inorganic nitrogen fixation.3.Structure and evolution of large size platinum carbon cluster Pt C_n^–/0（n=7-16）:The evolution process of the structure of platinum carbon clusters（anions,neutral molecules）from chain to ring was studied.Experimental and theoretical analysis indicate that the cluster size of the chain-to-ring structural evolution for the Pt C_n^–anions occurs at n=14,whereas that for the Pt C_n neutrals at n=9,revealing a significant effect of charge on the structures of metal carbides.The most importance of these building blocks is the strong preference of the Pt atom to expose in the outer side of the chain or ring,exhibiting the active sites for catalyzing potential reactions.These findings provide unique spectroscopic snapshots for the formation and growth of platinum carbide clusters and have important implications in the development of related single-atom catalysts with isolated metal atoms dispersed on supports.Therefore,using large-sized platinum carbon clusters to activate nitrogen is the focus of the next stage of research.In conclusion,the research in this dissertation provides an effective experimental and theoretical basis for seeking a reaction path for N₂ activation under appropriate conditions,which is very important for promoting the activation and fixation of nitrogen,and also has a certain guiding significance in the research and development of synthetic ammonia catalysts.