Performance Of Phosphorus-doped And Carbon Quantum Dot-loaded Modified Porous Graphitic Phase Carbon Nitride For Hydrogen Peroxide Production Under Visible Light

It is well known that hydrogen peroxide（H₂O₂）is widely used as a green oxidant in medical,textile,and paper industries,etc.Currently,the anthraquinone method is used to prepare H₂O₂ in industry,and the energy consumed by this method is not in line with the contemporary green theory because it requires multi-step hydrogen oxidation.Compared with anthraquinone method,photocatalytic technology has the advantages of clean environment,mild conditions and low power consumption.Therefore,it has high research value.Among many photocatalysts,graphite-phase carbon nitride（g-C₃N₄）is a promising photocatalyst for H₂O₂ production using water and oxygen.However,the amount of H₂O₂ production from pure g-C₃N₄ is far from satisfactory due to its limited light absorption,fast photogenerated electron-hole complexation and poor surface electron migration.To solve the above problems,we designed to modify porous g-C₃N₄（p-CN）by doping with phosphorus（P）and loading with carbon quantum dots（CQDs）.Porous g-C₃N₄ was chosen because the results of our previous studies showed that it is more favorable for the deposition of CQDs.The details of the study are as follows.（1）p-CN was prepared using melamine hydrochloride as the precursor,and the P-doped photocatalyst p-P_xCN（x=0.5,1,1.5）was obtained by thermal polymerization using hexachlorotriphosphonitrile as the phosphorus source.The structure,morphology,optical and electrochemical properties were systematically investigated by various characterization methods such as XRD,XPS,FT-IR,TEM,SEM,UV-Vis,Mott-Schottky,PL and EIS.The results show that the doping of P can effectively enhance the light absorption ability and photogenerated electron-hole separation of p-CN,and also enhance the adsorption of p-CN to oxygen.In the performance test of H₂O₂ production,p-P₁CN showed the highest photocatalytic H₂O₂ production activity,which could reach 0.036 mmol/L in 5h.The H₂O₂ production capacity was enhanced by 1.5 times compared with that of p-CN.The H₂O₂ production rate constants（Kf）and H₂O₂ decomposition rate constants（Kd）of p-P₁CN were calculated to be 10.69μMh^-1 for Kf and 0.22 h^-1 for Kd,which were 1.6 and 1.2 times higher than those of p-CN,respectively.5 cycles of H₂O₂ production experiments of p-P₁CN showed its good cycling stability.Finally,the pathways and mechanisms of H₂O₂ production by p-P₁CN were explored and analyzed by radical trapping experiments,quantitative superoxide radical trapping experiments,O₂ chemisorption tests and control experiments.（2）To further enhance the performance of p-P₁CN photocatalytic H₂O₂ production,carbon quantum dots（CQDs）prepared by alkali-assisted sonication were loaded on the surface of p-P₁CN by hydrothermal method to prepare the composite photocatalyst p-P₁CN/CQDs₂₅.The structural morphology was characterized by XRD,XPS,FT-IR,TEM,SEM,etc.to demonstrate the successful loading of CQDs.The optical and electrochemical properties such as UV-Vis,Mott-Schottky,PL,EIS and DFT theoretical calculations demonstrated that the loading of CQDs greatly enhanced the photogenerated electron-hole separation and photoresponsiveness of the composite photocatalyst,while the band gap of the composite photocatalyst was shortened and the valence and conduction bands were positively shifted.The results of photocatalytic H₂O₂ production showed that p-P₁CN/CQDs₂₅ exhibited excellent H₂O₂production activity under visible light,and the H₂O₂ concentration produced in 5 h reached 494μM/L（21 and 14 times that of p-CN and p-P₁CN,respectively）,with a Kf value of 238μM h^-1,34 times that of p-CN,and a Kd value of 0.43 h^-1,only 2 times that of p-CN,demonstrating that the loading of CQDs further enhanced the performance of p-P₁CN in H₂O₂ generation.In addition,the results of theoretical model construction and calculation,electrochemical tests and H₂O₂ production experiments demonstrate that P acts as an electron transport bridge between p-CN and CQDs,promoting the photogenerated electron-hole separation and migration of the composite photocatalyst.Finally,the pathways and mechanisms of H₂O₂ production by p-P₁CN/CQDs₂₅ were explored and analyzed by radical capture experiments,quantitative superoxide radical capture experiments,O₂ chemisorption tests,and control experiments.In summary,in this paper,p-P_xCN photocatalysts were prepared by P-doping modification of p-CN,followed by loading CQDs on its surface to successfully prepare the composite photocatalyst p-P₁CN/CQDs₂₅,which greatly enhanced the efficiency of H₂O₂ production from p-CN.Through a series of test characterization and comparative study of H₂O₂ production experiments,the pathways and mechanisms of H₂O₂ production by p-P₁CN and p-P₁CN/CQDs₂₅ were explored and analyzed.This study provides a new idea for the modification of H₂O₂ production by g-C₃N₄-based photocatalytic materials and provides a certain theoretical basis for the realization of H₂O₂ production under visible light.