Microwave Synthesis Of G-C₃N₄ And Its Application In Photocatalytic Carbon Peroxide Production

Graphitic phase carbon nitride（g-C₃N₄,CN）,a nonmetallic conjugated polymer semiconductor,has received wide attention in fields such as photocatalysis because of its good chemical stability,suitable band gap and unique electronic structure.However,bulk CN（BCN）has limited its wide application due to problems such as small specific surface area,low quantum efficiency and high carrier complexation rate.In addition,the conventional method of preparing CN is energy-consuming and long-period,which is not in line with the concept of green production.In this thesis,a certain proportion of melamine-cyanuric acid（MCA）supramolecular aggregates were used as precursors to prepare CN by microwave heating synthesis instead of the traditional thermal polymerization method,reducing energy consumption,achieving efficient preparation and modification of CN,and the preparation and modification mechanism of CN were systematically studied.Further,the morphological modulation,alkali metal and non-metal doping and synergistic modification of carbon vacancies of CN and its mechanism were systematically investigated,and a stable and optimized CN microwave method preparation and modification process was established.Moreover,considering H₂O₂ as an important environment-friendly oxidant,it has a wide range of applications in bleaching,disinfection,organic synthesis,and wastewater treatment.However,the industrial production of H₂O₂ still relies highly on the anthraquinone method,which is energy-intensive and environmentally unfriendly.Therefore,a green and environmentally friendly method for H₂O₂production is urgently needed.In this study,the prepared CN was applied to photocatalytic H₂O₂ production,and its efficiency of photocatalytic H₂O₂ production was substantially improved compared to BCN.The main research contents and results are as follows.（1）By dissolving a certain proportion of melamine and cyanuric acid in dimethyl sulfoxide solution,the self-assembled melamine-cyanuric acid（MCA）supramolecular aggregates were synthesized.MCA aggregates were used as precursors to synthesize structurally and morphologically stable spherical CN（SCN）by microwave heating,and the effect of microwave heating time on the morphology and physical phase structure of SCN was investigated.SCN has a porous spherical morphology with 15 times more specific surface area than bulk CN（BCN）,and its light absorption capacity is greatly enhanced.Compared with BCN,its photocatalytic hydrogen peroxide production performance was improved by 1.77 times and it also effectively reduces the energy consumption and cost of preparation..（2）The K-doped CN（K-CN）was obtained by doping the SCN with the alkali metal element K,while maintaining the porous spherical structure of the SCN.The results show that the K-doping helps to improve the energy band structure of the photocatalyst,thus reducing the bandgap and improving the light absorption ability of the sample,and also helps to inhibit the recombination of photogenerated carriers and promote carrier separation and migration.Applying K-CN to photocatalytic H₂O₂ production,the H₂O₂ production rate reached 2006μmol h^-1 g^-1 under simulated solar radiation,which is 4.4 times higher than the H₂O₂ production rate of BCN;the mechanism of the enhanced photocatalytic performance of K-CN can be attributed to the synergistic effect of the high specific surface area of the mesoporous structure and the change of the energy band structure caused by the K ion doping.（3）Based on the CN obtained by morphology modulation,the oxygen-doped and carbon vacancy synergistically modified CN（O-CNC）was successfully synthesized by microwave heating of a mixture of MCA supramolecules precursor and oxalic acid.The obtained O-CNC has a mesoporous structure,suitable energy band structure,high carrier separation rate and high visible light absorption efficiency,which significantly improves the H₂O₂ production capacity of CN two-step single-electron reduction,and the H₂O₂ production rate reaches more than 4times that of BCN under simulated solar radiation.The mechanism of the enhanced photocatalytic performance of O-CNC can be attributed to the synergistic effect of high specific surface area,oxygen element doping and carbon vacancies brought by the mesoporous structure.