Study On Alkali Metal Salt Regulation Of Carbon Nitride And Their Photocatalytic Production Of Hydrogen Peroxide

In recent years,energy crisis and environmental problems have attracted much attention.Hydrogen peroxide（H₂O₂）,as a green oxidant,has wide applications in chemical synthesis,wastewater treatment,pulp bleaching,textile industry and energy technology.Facing the problems of high energy consumption and pollution in traditional industrial production of H₂O₂,it is especially important to study new H₂O₂ production technologies.Photocatalytic oxygen reduction reaction（ORR）for H₂O₂ production（O₂+2H₂O→2H₂O₂）is gaining more and more attention due to the advantages of mild reaction conditions,low cost,green and non-polluting.The development of efficient photocatalysts is the key to whether the photocatalytic reaction can be applied in practice.Among many photocatalysts,graphitic phase carbon nitride（GCN）is widely used in photocatalytic water decomposition,CO₂ reduction,organic degradation and H₂O₂ synthesis due to its visible light response,abundant and easy to obtain raw materials,stable chemical properties and easy to regulate structure.However,unlike inorganic semiconductors,GCN as a polymer semiconductor forms Frenkel excitons due to strong Coulomb forces,which limits the separation of photogenerated electron-hole pairs,which in turn inhibits the photocatalytic activity.Although researchers have modulated GCN by defect modification,heteroatom doping,and surface functional group introduction in recent years,the corresponding performance is still not very satisfactory.Therefore,there is an urgent need for a more in-depth modification and modulation study of GCN.Based on the property that alkali metal salts can easily form low melting point molten salts,they show good potential for GCN modification.Based on that,this thesis focuses on the synthesis of potassium thiocyanate（KSCN）,rubidium chloride（RbCl）,cesium chloride（CsCl）and other alkali metal salt-modulated GCN,and the performance of modified GCN samples for photocatalytic production of H₂O₂ was investigated.The main studies are as follows:1.Polymer graphitic phase carbon nitride KNCN with K element doping and N defects were synthesized by CCl₄-assisted calcination using melamine,KSCN and LiCl.the structure and morphology of KNCN were characterized by scanning electron microscopy（SEM）,Fourier transform infrared spectroscopy（FT-IR）,X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）.The structure and morphology of KNCN were characterized by FTIR,XRD and XPS.The results show that the method successfully introduces edge nitrogen vacancies and heteroatomic K dopants into the carbon nitride structure.Compared with the K-doped KCN,the introduced nitrogen vacancies exhibit novel electron storage properties,successfully extending the electron lifetime from the conventional ns-us order of magnitude to tens of days.This property significantly enhances the activation and conversion efficiency of O₂ under visible light irradiation,and the H₂O₂ release rate under visible light irradiation reaches 10.02 mmol L^-1 h^-1 with an apparent quantum yield of 22.80%,which is 176 and 76 times higher than that of pristine GCN and KCN,respectively.Meanwhile,the long-term storage of electrons enabled the photocatalyst to exhibit delayed photocatalytic performance,decoupling the light reaction process from the dark reaction process and overcoming the real-time dependence of the photocatalytic reaction on light.2.Poly（heptazine-imine）photocatalysts（Rb-PHI）with Rb dopants and N defects were designed and synthesized for the photocatalytic synthesis of H₂O₂ by a one-step calcination method with RbCl molten salt modulation,and the successful introduction of Rb dopants and N defects into PHI was demonstrated by a series of experiments and characterization.Compared with poly（triazine-imide）（PTI）,PHI has a stronger π-conjugated structure that can effectively promote carrier separation and thus improve photocatalytic activity.Meanwhile,the introduction of Rb and N defects can adjust the energy band structure of carbon nitride and act as active sites for ethanol and oxygen adsorption,respectively,resulting in higher photocatalytic activity of Rb-PHI.Under visible light irradiation,the photocatalytic synthesis of H₂O₂ by Rb-PHI reached a yield of 11.38 mmol L^-1 h^-1,which was 307 times higher than that of ordinary GCN under the same preparation conditions.3.The mixture of melamine,CsCl and LiCl was calcined in CCl₄ atmosphere to prepare a highly polymerized carbon nitride photocatalyst CsNCN with Cs doping and N defect co-regulation for photocatalytic oxygen reduction synthesis of H₂O₂.After a series of experiments and characterization,it is proved that CsNCN with higher degree of polymerization can effectively improve the separation efficiency of photogenerated carriers,inhibit the separation of photogenerated electron-hole pairs,and improve the synthesis efficiency of photocatalytic oxygen reduction synthesis of H₂O₂.Under visible light irradiation,the H₂O₂ production efficiency of CsNCN reached 10.13 mmol L^-1 h^-1,which was 211 times that of the original carbon nitride.At the same time,CsNCN exhibits extremely high H₂O₂ selectivity,and the selectivity is as high as 98.7%.