Construction Of Highly Active Non-metallic G-C₃N₄ Composite And Its Photocatalytic Preparation Of Hydrogen Peroxide

Hydrogen peroxide（H₂O₂）,an important liquid phase chemical and fuel,its preparation and application have always been the concern of researchers.The traditional preparation methods of H₂O₂ include anthraquinone method and direct synthesis of hydrogen and oxygen.However,these methods generally have a series of disadvantages,such as low safety,high cost,toxic by-products and so on.Artificial photosynthesis is favored because of its advantages of green,energy saving,high efficiency and spontaneity.It is expected to become an ideal path in the field of new energy material development and environmental remediation.Following the photocatalysis technology of photosynthesis in nature,that is,semiconductor nanocomposites undergo redox reaction under sunlight,which can be effectively used for the preparation of new energy materials（such as hydrogen,methane,H₂O₂,etc.）and the treatment of environmental pollution（such as haze,main components of industrial wastewater,etc.）.Therefore,the research on photocatalytic reaction based on semiconductor materials has theoretical significance and important application value.At present,most catalysts contain precious metal elements.Although they have high efficiency,many of them have high cost and are easy to cause secondary pollution to the environment.These obvious problems hinder their commercial development.Therefore,the development of low-cost and easy to synthesize non noble metal catalysts has become the direction of our efforts.However,selectivity and efficiency of non-noble metal catalysts are not satisfactory.So far,titanium dioxide（Ti O₂）,graphite carbon nitride（g-C₃N₄）,bismuth based materials and transition metal complexes have been reported as the main catalysts for the production of hydrogen peroxide.Among them,g-C₃N₄ based semiconductor has become a research hotspot because of its visible light driven photocatalytic properties and obvious photoelectric properties.Among the semiconductor materials used for H₂O₂ preparation,g-C₃N₄ has two advantages over metal oxide catalysts:（1）g-C₃N₄ based catalysts have higher H₂O₂ yield under visible light irradiation;（2）g-C₃N₄ based catalyst can maintain the stability of H₂O₂formed.Therefore,g-C₃N₄ can be used as a photocatalytic catalyst to prepare H₂O₂.However,the rapid recombination rate of electron（e^-）hole（h⁺）pairs,poor solar efficiency and low specific surface area limit the application of g-C₃N₄.People are committed to improving the photocatalytic performance of g-C₃N₄ through a variety of strategies,such as the nanostructure regulation,element doping and the design of matrix composites.The main content of this paper,on the one hand,is to modify g-C₃N₄ based materials by means of heteroatom doping,molten salt assisted synthesis and material composite.For example,the specific surface area and crystallinity of g-C₃N₄ can be regulated by changing the synthesis mode of g-C₃N₄;By introducing organic molecules with high light responsiveness to regulate its light utilization;The optical absorptivity and carrier mobility are regulated by compounding with other semiconductor materials.On the other hand,as a green oxidant,we hope to apply materials to the field of environmental remediation.We expect to use the reactive oxygen species（ROS）produced by the decomposition of H₂O₂under light to degrade organic pollutants in situ,that is,the Fenton-like reaction in which iron-based species are replaced by non-metallic catalysts,and it can realize the in-situ consumption and utilization of H₂O₂ after forming a cascade reaction with H₂O₂ production.Therefore,we use the prepared materials for the preparation of photocatalytic H₂O₂,and in order to explore the cascade reaction mechanism of the reaction system in catalytic degradation,we also use them for the study of wastewater treatment.The main work contents are as follows:（1）A carbon nitride array（CNA）modified by perylene imide（PDI）was prepared.Firstly,the carbon nitride array was prepared by molten salt method,and then calcined with3,4,9,10-perylene tetracarboxylic anhydride（PTCDA）.The results show that the process selection of molten salt method and the introduction of PDI molecule greatly improve the photoelectric performance and carrier utilization of g-C₃N₄.The high degree of polymerization and regular micro morphology play a great role in the photocatalytic efficiency of H₂O₂ production.The highest H₂O₂ yield can reach 1405.32μmol h^-1 g^-1。This work demonstrates a new carbon nitride synthesis process and a high-efficiency photocatalytic scheme for small molecule polymerization.Through the design of molten salt mixed synthesis and small molecule mixed copolymerization,we prepared an efficient photocatalyst for H₂O₂ synthesis.It has regular micro morphology,higher polycondensation degree and excellent photoelectric properties.However,it does not show the corresponding ability in environmental protection.Therefore,we changed our thinking and tried to modify g-C₃N₄ synthesized by traditional calcination method,and on this basis,we explored the reaction mechanism of preparing cascade Fenton-like purified water with H₂O₂.（2）A metal free two-dimensional black phosphorus（BP）/porous oxygen enriched g-C₃N₄（OPCN）heterojunction was prepared.The porous g-C₃N₄（PCN）was prepared by calcining the precursor aqueous solution,and then a large number of oxygen functional groups were introduced by sulfuric acid acidification to obtain OPCN;In addition,in order to improve the absorption of visible light by g-C₃N₄,we compounded another new two-dimensional semiconductor material BP with the prepared OPCN.The results show that the introduction of an appropriate amount of BP material greatly improves the light absorption capacity and carrier mobility of g-C₃N₄.BP/OPCN showed excellent photocatalytic performance,and the highest H₂O₂ yield reached 3463μmol h^-1 g^-1.A large amount of H₂O₂produced in situ on BP/OPCN can then degrade organic pollutants in wastewater in one step through Fenton-like cascade reaction.This work demonstrated an efficient photocatalytic scheme for H₂O₂ preparation and wastewater treatment,and found the basic role of the surface chemical and physical structure of the photocatalyst.