The Study On The Construction Of Defective Semiconductor Photocatalyst And Its Mechanism Towards NO Photocatalytic Oxidation

Nowadays,in the context of the era of economic globalization,with the acceleration of industrial processes and the rapid growth of population,the demand for fossil fuels is growing exponentially.According to the latest report on“Energy Outlook（2020 Edition）”,the global demand for non-renewable energy will continue to grow before 2050,and these fuels will release a large number of toxic substances during the combustion process.With the continuous innovation of monitoring technology and the enhancement of environmental protection awareness,the improved living environment is urgently needed for all human beings.Therefore,it is of great importance to develop green,energy-saving,and safe countermeasures to solve current and future environmental pollution issues.Photocatalytic technology is a new type of advanced oxidation technology.Due to no energy consumption,harmful substances free,it can be excited by sunlight to remove low concentration pollutants in water and atmosphere,so it is a potential technology in terms of environmental pollution treatment.For example,as represented by TiO₂,it has the advantages of fast reaction speed,simple processing,low energy consumption,complete degradation and so on.Therefore,it has become an efficient and clean pollutant treatment method,which has been widely concerned by domestic and foreign researchers.However,the common semiconductor catalysts have low utilization rate of light and fast recombination rate of photogenerated carriers,and even produce poisonous by-products in the catalytic oxidation process,resulting in poor catalyst activity or stability.Hence,it is necessary to design semiconductor material with strong absorption in the visible region,efficient transfer and separation efficiencies of photogenerated carriers,good photocatalytic behavior and robust stability.This dissertation mainly focused on the elemental Bi loading and defect engineering to modify the photocatalysts,so as to improve the removal rate of air pollutants NO_x over those photocatalytic materials.Based on the existing characterization technologies and density functional theory（DFT）,the deeply photocatalytic mechanism was proposed accordingly.（1）Using XRD,XPS,SEM,TEM and other characterization techniques,it was found that the introduction of metallic Bi would not destroy the chemical composition and crystal structure of Zn₂SnO₄,but change its morphology and particle size.This is due to the addition of the precursor which can form metallic Bi nanoparticles,and thus alter the micro-environment of synthesis of octahedral Zn₂SnO₄,thereby lead to the formation of oxygen vacancies（OVs）.The co-introduction of metallic Bi and OVs in Zn₂SnO₄ would on the one hand enhance the light harvesting ability and thus improve the utilization of visible light due to the surface plasmon effect（SPR）of Bi element.In addition,the Mott-Schottky effect on the interface between metallic Bi and Zn₂SnO₄ is conducive to changing the carrier transport path and shortening the migration distance of Zn₂SnO₄,thereby effectively improving the separation efficiency of electron-hole pairs;On the other hand,the presence of OV_S can promote the surface electron reconstruction,which is conducive to providing active sites for the adsorption of small molecules.DFT calculation was used to simulate the adsorption behavior of small molecules on the photocatalyst surface,which provided theoretical support for the production of active free radicals.At the same time,combing with in-situ infrared spectroscopy,the oxidation process of NO_x was deeply explored,and the reaction path of direct conversion of NO to nitrate was proposed.（2）A simple and environmentally friendly one-step calcination method was designed to prepare C₃N₄ containing three-coordinate nitrogen vacancies(N_3C).Characterization methods such as XPS,FTIR and EPR were used to study the microstructure changes on the surface of the as-prepared C₃N₄ during the preparation process and determine the N_3C vacancies and their formation mechanism.DFT simulation calculations show that the presence of N_3C vacancies on the surface is conducive to the adsorption and activation of small molecules of O₂ and NO,thereby promoting the generation of singlet oxygen（¹O₂）.The existence of these active species can significantly inhibit the formation of toxic by-product NO₂,which is further verified by in-situ spectroscopy.Therefore,compared with bulk C₃N₄ materials,C₃N₄ modified with N_3C vacancies has higher oxidation performance and excellent stability.This study provides a potential and sustainable reaction path for stable and effective NO oxidation.（3）Tubular g-C₃N₄ containing carbon vacancies（Cvs）was prepared by pyrolysis of melamine-urea mixture in N₂ atmosphere after pretreatment.Based on common characterization and analysis techniques,the existence of Cvs in the prepared samples was determined.Combing DFT calculation with active species capture techniques,special tubular hollow microstructures and abundant surface defect sites are discovered,which accelerates the separation and transfer of photogenerated carriers.The activity test and in-situ infrared spectroscopy analysis showed that the Cvs modified C₃N₄hollow microtubes have higher selectivity,that is,the selective oxidation of NO to the final product NO₃^-.This work provides a new understanding of the improvement of photocatalytic performance by constructing defective semiconductors.Based on the experimental results and characterization tests as well as DFT calculations,different modification strategies are used to effectively steer the electronic structure and performance of Zn₂SnO₄ and C₃N₄ semiconductor photocatalysts,and provide new insights on the reaction mechanism of pollutants removal and the formation of toxic by-products.At the same time,this study is expected to expand the application range of semiconductor materials in photocatalytic pollutant removal,and offer theoretical support for the practical application of semiconductor materials such as Zn₂SnO₄ and C₃N₄.