The Study On Enhanced Photocatalytic Performance Of G-C₃N₄ Modified By Lithium And Potassium Ions

Hydrogen as a clean,renewable and environmentally friendly energy,can replace the traditional fossil energy in the future to meet the growing energy needs of mankind.Converting solar energy into hydrogen energy is one of the trends of hydrogen energy development in the future.Photocatalytic hydrogen production is an effective method of hydrogen source.The preparation of highly efficient and stable photocatalyst is an urgent problem in the development of photocatalytic technology.At present,non-metallic carbon-based materials are playing an increasingly important role in the field of photocatalysis.The most promising organic photocatalyst material,carbon nitride?g-C₃N₄?,has attracted extensive attention and research at home and abroad due to its advantages of good stability,low-cost preparation and visible light response.However,due to the narrow light absorption range and high photogenic carrier recombination rate,the photocatalytic performance of massive carbon nitride is poor,which limits its development and application in hydrogen production.At present,it is urgent to improve the crystallinity of g-C₃N₄,inhibit the recombination of photogenic carriers,increase its specific surface area and improve its utilization of visible light.In this paper,g-C₃N₄ with excellent photocatalytic performance was synthesized by molten salt method.After a series of experimental explorations,the structure,performance and structure-activity relationship of the synthesized g-C₃N₄ were deeply studied by combining characterization analysis and performance test,and the possible photocatalytic mechanism was explained.The main research contents are as follows:?1?In view of the poor crystallinity of the bulk g-C₃N₄ and the high carrier recombination rate,the metal ions embedding modification of g-C₃N₄ was studied in this paper.The porous g-C₃N₄ was prepared with urea as the precursor,and then the Li+ and K+modified g-C₃N₄ photocatalytic material?Li+and K+embedded simultaneously?was prepared with KCl/LiCl as the low melting point mixed eutectic molten salt as the reaction medium.Experimental studies have proved that Li and K elements are successfully embedded into g-C₃N₄ structure,which increases the crystallinity of g-C₃N₄,expands the absorption range of visible light and reduces the recombination rate of photogenerated carriers,effectively promoting the separation of photogenerated carriers.Theoretical simulation shows that the adsorption of Li and K ions to cavity in g-C₃N₄ structure can reduce the overpotential of HER,facilitate carrier migration,and enhance photocatalytic hydrogen production performance.?2?Given the results of?1?.On the one hand,this chapter studies the regulation of nitrogen vacancy defect structure of carbon nitride.With urea as the precursor,g-C₃N₄ photocatalysts with different nitrogen vacancies?ring nitrogen vacanies and pendent nitrogen vacanies?were successfully prepared in different atmospheres.On the other hand,this chapter adopted an effective way to modify the bulk g-C₃N₄ with N vacanies by molten salt method,and prepared Li+and K+modified g-C₃N₄ photocatalytic materials?Li+and K+embedded simultaneously?by molten salt method.It was found that the photocatalytic activity of g-C₃N₄ photocatalyst under visible light irradiation was significantly enhanced?about 19.5 times?after molten salt treatment.Especially g-C₃N₄ containing pendent nitrogen vacanies modified by metal ions,the photocatalytic hydrogen production rate upto 771.35 mol h-1,which is the highest value in carbon nitride photocatalytic materials.The results showed that the g-C₃N₄ structure containing pendent nitrogen vacanies is beneficial for Li+and K+embedding,while more Li and K metal ions increase the electron delocalization,promote the charge transfer rate and carrier separation rate of the internal structure of g-C₃N₄ and reduce HER overpotential,thus enhance photocatalytic activity.