Design Of Carbon Nitride Structure For Photocatalytic Molecular Oxygen Selective Reduction And Organic Pollutant Degradation

Solar photocatalysis technology has broad application prospects in the field of energy and environment,but the low efficiency of solar energy conversion is the bottleneck of its practical application.The design and development of efficient and stable photocatalytic materials has always been the core of photocatalytic technology research.Polycarbon nitride is a kind of non-metallic organic polymer semiconductor,which has attracted much attention due to its excellent chemical and physical stability and good visible light response.However,the low dielectric constant of carbon nitride frame materials severely inhibits the separation and transmission of photogenerated charge,resulting in low solar energy conversion efficiency.In this study,the aim of improving charge separation and transport of carbon nitride copolymerized materials was to improve the photophysical and chemical properties of photocatalytic materials by means of melting salt regulation of crystallinity,heterostructure and two-stage polymerization crystallization,including broadening the light response range,enhancing exciton separation,accelerating charge migration,and designing the central site of surface electron transfer.Scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction spectroscopy（XRD）,X-ray photoelectron spectroscopy（XPS）,N₂physical adsorption,UV-Vis diffuse reflection spectroscopy（UV-Vis DRS）,fluorescence spectroscopy（PL）,and photochemical properties test and other characterization techniques were used.The crystal phase,morphology,chemical composition,energy band structure and physical and chemical properties of the designed copolymerized carbon nitride photocatalytic materials were analyzed in detail,and the application of the materials in the photocatalytic production of hydrogen peroxide and its cooperation with ozone-activated pollutant degradation were discussed.The photocatalytic reaction mechanism of various copolymerized carbon nitride photocatalysts in each reaction system was systematically analyzed.The main research results are as follows:（1）Solar photocatalysis is a promising and sustainable way to produce hydrogen peroxide.The polymer carbon nitride framework is favorable for photocatalytic selective oxygen reduction（2e^-ORR）,and increasing its crystallinity greatly improves exciton separation and charge transport,thus advancing the 2e^-ORR surface process.In this study,ammonium thiocyanate（AT）,an inorganic ionic compound,was used as the precursor,and KCl/Li Cl eutectic salt was used as the reaction medium to synthesize the highly efficient copolymerized carbon nitride framework structure（AT-CN）.Compared with melamine as the precursor（m-CN）,the crystallinity of AT-CN was significantly improved.Spectral studies show that the exciton separation of AT-CN crystals is significantly enhanced,and AT-CN shows excellent performance in the process of photocatalytic hydrogen peroxide production and ozone activation in conjunction with pollutant degradation.This work demonstrates the polymerization process of an inorganic precursor in molten salt to obtain a polymerized carbon nitride framework structure with high crystallization properties to achieve efficient hydrogen peroxide production and pollutant degradation.（2）By designing oxygen reduction active sites and water oxidation active sites respectively,the heterojunction between K⁺doped highly crystalline carbon nitride frame structure and indium zinc sulfide（Zn In₂S₄）was constructed,and the highly selective ORR site on the surface structure of carbon nitride and the water oxidation active site on Zn In₂S₄were fully utilized.To achieve solar driven air and water as reactants to prepare hydrogen peroxide;This process of synthetic hydrogen peroxide by artificial light is of great significance for water disinfection and pollutant degradation by AOPs in some remote areas and dispersed application scenarios.In addition,a systematic study was carried out on the heterojunction between the constructed K⁺doped polymer carbon nitride and indium zinc sulfide（Zn In₂S₄）.The partial charge transfer of ZIS to K-CN was confirmed by spectrographic techniques,which resulted in significantly enhanced photoionization charge separation.The photocatalytic hydrogen peroxide production rate is up to 1729.9μmol g^-1h^-1and apparent quantum yield of 2.43%.Mechanistic investigations reveal the essential role of superoxide radical（·O₂^-）in O₂to H₂O₂conversion.The catalytic system showed higher photocatalytic hydrogen peroxide production performance in natural water,so the reaction system has a good application potential in actual water.（3）In order to further improve the crystallization properties and exciton separation efficiency of the carbon nitride framework structure,so as to achieve the performance of photocatalytic hydrogen peroxide production with higher efficiency,a two-stage controlled thermal polymerization process was adopted in this study to construct the highly crystalline carbon nitride framework structure doped with K⁺.Through the secondary polymerization of CN in Li Cl-KCl eutectic salt,the load of potassium ions in the carbon nitride framework was increased,and the structure order was improved.The effect of the first stage polymerization temperature on the physicochemical properties and photocatalytic activity of the prepared KCCNT was compared and analyzed.The optimal first stage polymerization temperature was450℃.After the second further polymerization in Li Cl/KCl,the crystallinity of the carbon nitride copolymerized was significantly improved.In the process of photocatalytic hydrogen peroxide production,the total performance of the outstanding performance,an hour yield of 9.63 m M and AQY of 51.8%.