Modification Of Graphite Phase Carbon Nitride And Research On The Properties And Mechanism Of Photocatalytic Preparation Of Hydrogen Peroxide

As a general cleaning oxidant,hydrogen peroxide（H₂O₂）is commonly used in pulp bleaching,disinfection,organic synthesis and chemical production because its products are only water and oxygen.However,the traditional way of H₂O₂production,transportation and storage is in great danger.Photocatalysis is an emerging H₂O₂preparation method,which can overcome the shortcomings of traditional production methods and realize the on-site preparation and use of H₂O₂.Therefore,it is expected to replace the traditional H₂O₂production methods.Carbon nitride（C₃N₄）is a photocatalyst commonly used in photocatalytic preparation of H₂O₂.It has the advantages of suitable band structure,high selectivity for the synthesis of H₂O₂,non-toxic and easy preparation.However,traditional C₃N₄has the disadvantages of rapid recombination of photogenerated electron-hole pairs,relatively low absorbance of visible light and low quantum yield.Therefore,efficient photocatalytic preparation of H₂O₂by modifying C₃N₄is a current research focus.Based on this,in this paper,the photocatalytic efficient preparation of H₂O₂by C₃N₄was achieved through three modification methods:non-metallic atomic doping,metal ion doping and introduction of defects.The main work of this paper is as follows:（1）Using melamine as the precursor of C₃N₄,alkali metal salt potassium chloride as the template and sodium thiosulfate as the oxygen source,the O-doped C₃N₄-based photocatalyst was prepared by one-step synthesis of non-metallic element O atom doped into the bulk phase of C₃N₄.The doping of nonmetallic element O atom reduces the charge transport barrier and enhances the separation and mobility of charge carriers in the in-plane structure.At the same time,the energy band structure is improved,the O₂reduction capacity of C₃N₄is enhanced,the band gap is reduced,the light absorption capacity is improved,and the activity of C₃N₄to prepare H₂O₂is improved.In the experiment of photocatalytic preparation of H₂O₂,the O-doped C₃N₄showed a high H₂O₂preparation efficiency(1787μmol·g^-1·h^-1),which was much higher than the bulk C₃N₄and 15.40 times of the latter.（2）Using urea as the precursor of C₃N₄,lithium chloride as the template and Li source,the Li⁺doped C₃N₄photocatalyst was prepared by means of secondary polymerization.The doping of metal Li⁺optimizes the polymerization process of C₃N₄,causes the stripping of the structure of C₃N₄,improves the crystallinity of C₃N₄,and promotes the migration and separation of photogenerated charge between the interface.At the same time,the band gap is reduced,the light absorption capacity of C₃N₄is improved,and the photocatalytic activity of C₃N₄to prepare H₂O₂is improved.The photocatalytic experiments showed that the Li⁺doped C₃N₄had a better H₂O₂preparation efficiency(2680μmol·g^-1·h^-1),which was 36.73 times that of the original C₃N₄.（3）Using urea as the precursor of C₃N₄,alkali metal salt potassium chloride as the template,potassium nitrate as the initiator of the N defect,the N defect was introduced into the C₃N₄phase by secondary polymerization,and the C₃N₄-based photocatalyst with the presence of the N defect was prepared.Among them,metal salt KNO₃plays the role of pruning heptazine ring in the secondary polymerization process,successfully opens heptazine ring and introduces C≡N group.As the active site,C≡N inhibits the photogenerated electron-hole pair recombination,enhances the electron transfer,improves the band structure of C₃N₄,makes it have a more negative conduction band,enhances the reduction ability of O₂,thus improving the activity of photocatalytic preparation of H₂O₂.In the experiment of photocatalytic synthesis of H₂O₂,the photocatalytic production efficiency of photocatalyst C₃N₄-NV(3480μmol·g^-1·h^-1)is 46.4 times that of bulk C₃N₄.