Structure Design Of Ni-Based MOF Catalyst And Study Of Synchrotron Radiation For Oxygen Electric Reduction

As one of the most important chemicals in the world,environmentally friendly hydrogen peroxide（H₂O₂）oxidant has been widely used in textile bleaching,disinfection and sterilization,and wastewater treatment.The demand for H₂O₂ shows a growing trend,and the global H₂O₂ market is expected to exceed 5.5 billion dollars in 2023.However,today’s H₂O₂ industrial production mainly uses traditional energyintensive anthraquinone preparation processes,which require a large and centralized infrastructure,large energy consumption,and expensive palladium catalysts,as well as producing large amounts of organic waste.Therefore,the search for an effective,environmentally friendly,low energy consumption and distributed production process of H₂O₂ has aroused extensive attention.In recent years,the use of electrocatalytic reduction of oxygen to generate hydrogen peroxide is considered to be a clean and sustainable way,in which only oxygen and water are used as raw materials without producing by-products.At the same time,H₂O₂ produced by electrocatalytic oxygen reduction can be coupled with advanced fuel cell technology,wind energy and other renewable energy to achieve green continuous production,which is also highly consistent with the "green and low-carbon" development goal proposed in the current"14th Five-Year Plan" for Industrial Green Development.In order to improve the energy conversion efficiency,it is necessary to design highly selective,active and stable hydrogen peroxide generating electrocatalyst,among which revealing its energy conversion mechanism is the key to realize its large-scale production and promote its commercial application.In this dissertation,the highly conductive nickel organic framework（Ni MOF）materials were prepared by solvothermal method,which is easy to operate.And the electronic structrue of metal sites in this material was regulated by structure engineering strategies,so as to optimize the activity and selectivity of electrocatalytic two-electron oxygen reduction reaction.To study the structure evolution of the active site and the reactant evolution of the catalytic materials during the working process were monitored in real time,we performed in-situ synchrotron radiation X-ray absorption fine structure（XAFS）and Fourier transform infrared spectroscopy（FTIR）,which also revealed the microscopic reaction mechanism of two-electron oxygen reduction and established the struction-activity relationship between structure and performance of this catalysts.It provides theoretical guidance for the synthesis of stable and efficient two-electron electrocatalysts.The specific research content of this dissertation are as follows:1.Crystallinity regulation and investigation of two-electron oxygen reduction selectivity of Ni-based MOF catalystThe design of electrocatalysts with high conductivity and exposure of large active sites plays an important role to optimize the activity and selectivity of hydrogen peroxide during electrocatalytic oxygen reduction.Here,we designed a series of twodimensional Ni-based MOF electrocatalysts with different precursor ratios by solvothermal method,which have high conductivity,large specific surface area,and different crystallinity of the surface.The 5-Ni3（HITP）2 catalytic materials with low crystallinity show the best selectivity for hydrogen peroxide production.Through the structural characterization of high-resolution electron microscopy,X-ray diffraction（XRD）and photoelectron spectroscopy（XPS）,it was found that there was a synergistic coupling reaction mechanism between the Ni active center of the Ni MOF catalyst with low crystallinity and the adjacent unsaturated N,which exhibited better selectivity of two electron oxygen reduction.80%selectivity of hydrogen peroxide production was achieved on this conductive two-dimensional Ni MOF,and mass activity of 292 A gNi1 was achieved.2.In situ synchrotron radiation study on two electron oxygen reduction properties of Nickel-based bimetallic MOFIn order to achieve higher two-electron oxygen reduction selectivity on the basis of the hydrogen peroxide production potential of two-dimensional conducting Ni MOF,the key is to study the dynamic evolution of metal active site structure and reaction intermediates during electrocatalytic reaction.Therefore,we have synthesized a series of bimetallic Ni-M（M=Co,Cu,Zn）MOF catalysts by a simple one-step wet chemical method,in which the 3d orbital of nickel generates a strong electronic interaction with the introduced second binary transition metal ions,endowing it a highly selective hydrogen peroxide production ability.Through in situ XAFS characterization technique,it was found that the introduction of Zn metal resulted in the dynamic optimization of the oxidation state of Ni into the high-valence state Ni^（2+δ）+（0<δ<1）.Meanwhile,the in situ FTIR experiment was used to trace that the high-valence state of Ni^（2+δ）+accelerated the rapid accumulation of*OOH over the active site,thus improving the activity and selectivity of the hydrogen peroxide produced by two-electron oxygen reduction.The selectivity was up to 90%.3.In situ technology study on the selectivity of two-electron oxygen reduction of halogen ion confinement Ni-based MOF.Nonmetallic element doping is an effective means to regulate the electronic structure of metal active sites,which is very important to improve the efficiency of hydrogen peroxide generation by two-electron oxygen reduction.In this work,based on the application of two-dimensional conducting Ni-based MOF in oxygen electroreduction,we adopted a series of halogen ions confinement strategies,by selfsealing the halogen ions in the nano cavity of Ni MOF,resulting an lattice shrinkage effect,effectively regulating the atomic and electronic structure of nickel-based catalysts.Real-time monitoring of the electrocatalytic oxygen reduction reaction process was conducted by in situ synchrotron radiation technologies.It was found that the Ni MOF catalysts after bromide ion confinement can effectively inhibit the dissociation of*OOH intermediates during reaction,which promotes the selectivity of the catalyst in two-electron oxygen electro-reduction.In Addtion,we measured the yield of hydrogen peroxide for the first time by combining chemical titration and ultraviolet spectrum method（UV-vis）.The cumulative hydrogen peroxide yield was 1534 mmol gcatalyst^-1 h^-1 in the flow cell.