Study On The Construction Of Functionalized MOF-based Composites And The Performance Of Photocatalytic Nitrogen Fixation

Ammonia(NH₃)is an essential chemical substance in agricultural production,and it is also one of the important raw materials in the production of various chemical products(such as explosives,fertilizers,etc.).Ammonia is vital to society as demand for food increases around the world.N₂makes up about 70%of the atmosphere and is nature's main source of nitrogen.Immobilization of free nitrogen to produce ammonia is an effective means to utilize natural resources.Today,the way of nitrogen fixation is mainly divided into two kinds,one is biological nitrogen fixation,the other is artificial nitrogen fixation.At the beginning of the 20th century,scientist Haber used nitrogen in the air for the first time to synthesize ammonia,Haber reaction has become the main way of human nitrogen fixation synthesis ammonia.However,the reaction emits a large amount of greenhouse gases,which does not conform to the concept of green chemical industry,and it is urgent to improve it.Photocatalytic nitrogen fixation,as a new mode of nitrogen fixation,has more advantages in environmental protection and energy utilization.Ti O₂has a good effect of photocatalytic synthesis of ammonia,but pure Ti O₂has a series of shortcomings.In order to solve the above problems,we introduced Metal Organic frameworks(MOFs)material to modify it.The advantages of MOFs over ordinary semiconductor materials make them a new star in the field of photocatalysis.The large specific surface area of MOFs facilitates gas adsorption and has adjustable energy band position to improve the utilization of electrons.In this paper,MOFs material as the substrate,through the design and modification of the preparation of a new type of high efficiency photocatalytic synthesis of ammonia composite catalyst.The characterization and performance tests of the materials were carried out,and the mechanism of different catalysts in the process of photocatalytic synthesis of ammonia was discussed.Specific research contents are as follows:(1)Using HKUST-1 as the substrate of the composite photocatalyst,Ag@Ti O₂/HKUST-1 ternary composite photocatalyst was prepared by impregnation reduction method and hydrothermal method.The ammonia synthesis performance of the ternary composite photocatalyst was compared with that of the binary composite photocatalyst and the simple catalyst.The results showed that the ternary composite photocatalyst had the best nitrogen fixation effect.As a substrate,HKUST-1 can effectively disperse Ti O₂and Ag nanoparticles while adsorbing N₂,which can provide more active sites for the reaction.The photoelectric properties of the composite catalyst were investigated by UV-vis and EIS.The results show that the local surface plasmon resonance effect of Ag nanoparticles enhances the absorption range of visible light.The morphology of ternary composite catalyst was characterized and the structure could improve the electron utilization rate effectively.In the experiment,we investigated the performance of Ag@Ti O₂/HKUST-1 photocatalytic synthesis of ammonia.The average photocatalytic performance of ammonia synthesis is 0.62?mol/g/h,which is 2.5 times that of pure Ti O₂.Ag@Ti O₂/HKUST-1 is a good photocatalytic catalyst for ammonia synthesis.(2)In order to further solve the problem of low solubility of N₂in the photocatalytic system,we prepared Ti O₂/HKUST-1/CM photocatalytic film and applied it in the improved gas-liquid-solid three-phase photocatalytic nitrogen fixation system.The three-phase photocatalytic system can effectively solve the problem of low solubility of N₂in water,and can effectively prevent the generated NH₃from being oxidized by O₂.HKUST-1/CM thin films were successfully prepared by hydrothermal method.Copper mesh was used as the copper source in the process,which saved the raw material and avoided the introduction of foreign impurities.Ti O₂/HKUST-1/CM composite photocatalytic films were prepared by impregnation and calcination method.We investigated the performance of ammonia synthesis by supporting Ti O₂photocatalytic films with different contents,and concluded that the best impregnation time was one minute.The microstructure of the Ti O₂/HKUST-1/CM film was characterized,and the photocatalytic film had good morphology.In the reaction process,the composite film is located at the gas-liquid interface of the three-phase reaction system,which strengthens the absorption of N₂by MOFs and effectively prevents the oxidation of ammonia.The photoelectric properties of the films were tested by UV,EIS,IT,etc.The photocatalytic films have good photoelectric properties.The photocatalytic mechanism of Ti O₂/HKUST-1/CM was discussed.The experimental test shows that in the three-phase photocatalytic system,the rate of synthesis of ammonia by Ti O₂/HKUST-1/CM is 0.048 umol/h/cm²,which is 2.8 times that of pure Ti O₂.In this thesis,the application of MOFs materials in the photocatalytic synthesis of ammonia was studied.By adjusting the system and modifying the materials,the photocatalytic ammonia synthesis performance of MOFs-based materials is improved.The morphology and performance of the catalyst were explored through characterization and experiments.The mechanism of photocatalytic synthesis of ammonia by composite catalyst was further explored.It provides a new idea for the preparation of an efficient MOFs-based composite photocatalytic ammonia synthesis system.