Design And Synthesis Of Carbon Nitride-based Visible Light Photocatalysts For Photocatalytic Synthesis Of Hydrogen Peroxide

The sustainable development of human society is being affected by the global energy crisis and environmental issues,which motivate researchers to develop green and sustainable technologies to prepare various high-value-added chemicals.Among various chemicals,hydrogen peroxide（H₂O₂）is considered as one of the most important 100 chemical raw materials due to its wide application in environmental treatment and industrial synthesis.However,the current industrial a nthraquinone method to synthesize H₂O₂ not only consumes high energy,but also easily generates organic waste,which does not meet the requirements of green chemistry.In contrast,photocatalytic synthesis of H₂O₂ is a promising green alternative because it only requires H₂O and O₂ as starting materials and produces no b y-products under illumination.In this regard,various photocatalysts have been developed for photocatalytic synthesis of H₂O₂,such as Ti O₂,Cd S,and Bi VO₄.Although some progress has been made,the disadvantages of low quantum efficiency,limited light utilization,and complex fabrication process limit its large-scale practical application.Therefore,there is an urgent need to explore low-cost,efficient,and easy-to-prepare visible-light-driven photocatalysts.Due to its suitable electronic band struct ure and facile preparation method,graphitic carbon nitride（g-C₃N₄）shows promising application in photocatalytic synthesis of H ₂O₂.However,the ability of pristine g-C₃N₄ to synthesize H₂O₂ is still limited by the unsatisfactory light utilization and low specific surface area.Therefore,it is very important to design,synthesize and modify g-C₃N₄ for these deficiencies.In this thesis,the visible light type g-C₃N₄catalyst was designed and synthesized from the morphology structure,sample composition,electronic structure and defect type.The performance of the catalyst sample for photocatalytic synthesis of H₂O₂ was deeply studied,and the mechanism of performance enhancement was revealed.The specific research contents and results are as follows:Firstly,two different types of nitrogen vacancies were successfully introduced into the g-C₃N₄ structural framework by pyrolysis of melamine under argon and ammonia atmospheres,respectively,combined with the subsequent HNO ₃ oxidation treatment.It is found that the pyrolysis atmosphere has a significant effect on the type of nitrogen vacancies introduced,with tertiary nitrogen groups(N _3C)and sp²-hybridized nitrogen atoms(N_2C)being the preferred sites for the formation of nitrogen vacancies under ammonia and argon pyrolysis,respectively.In addition,the nitrogen vacancies formed by N_3C have been experimentally and theoretically confirmed to be beneficial for the reduction of the band gap and the enhancement of the oxygen adsorption capacity.As expected,the optimal catalyst exhibited high H₂O₂yield of 451.8μM within 4 h,which was 3.8 times that of pristine g-C₃N₄（119.0μM）,and showed excellent stability after 10 photocatalytic runs.Secondly,the simultaneous introduction of nitrogen vacancies,cyano groups,and sodium doping into the structural framework of g-C₃N₄ can be achieved ingeniously by pyrolyzing the uniformly ground mixture of melamine and Na Cl O.It was found that the introduction of Na Cl O played two roles:（1）Na⁺intercalation and Cl O^-oxidation simultaneously induced sodium doping and nitrogen vacancies and formed cyano groups;（2）The Na Cl formed during the pyrolysis process was easily removed by water washing,thus increasing the specific surface area and expos ing more active sites.Benefiting from the above advantages,the optimal catalyst showed a high H₂O₂ yield of 415.5μM within 4 h,a quantum yield of 16%at 420 nm wavelength,and a solar energy conversion efficiency of 0.3%under simulated sunlight,which is 4.2 times that of the pristine Ar-CN catalyst,and exhibited an excellent performance cycling stability and structure cycling stability.The improved photocatalytic efficiency is mainly attributed to the enhance d light absorption capacity and the enhanced photo-generated electrons and holes separation efficiency.Thirdly,the introduction of carbon vacancies into the g-C₃N₄ structural framework was cleverly achieved by using water vapor-induced melamine pyrolysis.It was found that the induction of water vapor can play three roles:（1）promoting the low-temperature polymerization of melamine to form g-C₃N₄ with high crystallinity;（2）promoting the formation of mesoporous structure,thereby increasing the specific surface area and exposing catalytic active sites;（3）increasing the light absorption capacity of the ultraviolet region.Benefiting from the above advantages,the optimal catalyst showed a high H₂O₂ yield of 180μM within 4 h,which is approximately 2times that of the pristine CN catalyst,and exhibits excellent cycling stability.The enhanced photocatalytic efficiency is mainly attributed to the enhanced light absorption capacity and the enhanced specific surface area.