Precise Design And Synthesis Of Carbon Nitride-Based Photocatalysts For Green Production Of Hydrogen Peroxide

Considering the pressing need to restore the water environment,it is increasingly urgent to comprehensively develop and widely apply advanced oxidation technology to address various water pollution problems effectively.Hydrogen peroxide(H2O2)is an environmentally friendly strong oxidizing agent that can be utilized in the Fenton or Fenton-like process.Additionally,it can be directly used to inactivate pathogenic microorganisms,and the demand for H2O2 is expected to increase significantly.Solardriven photocatalysis is a green and economical approach for H2O2 production compared to the energy-intensive anthraquinone process.The semiconductor material graphitic carbon nitride(gCN)has advantages such as good stability,non-toxicity,easy preparation,visible light response,two-dimensional layered structure,easy regulation of band structure,etc.However,the intrinsic activity of gCN for H2O2 production is low due to its small surface area,insufficient light absorption capacity,poor charge separation performance,and low selectivity of two-electron oxygen reduction.Moreover,adding sacrifice agents to deplete holes and improve oxygen reduction efficiency brings separation and purification problems,limiting its practical application.Therefore,it is necessary to construct a reasonable catalytic system using H2O as the electron and proton donor and avoid adding sacrifice agents to achieve on-site production of pure H2O2 aqueous solution.In this study,we conducted a precise structural design to optimize the electronic structure of gCN,improve the photoelectric properties of gCN,enhance the intrinsic activity,reveal the structure-activity relationship,and achieve the efficient photosynthesis of H2O2.The main research results of this paper are as follows:(1)To address the issue of insufficient light absorption of gCN,nitrogen vacancies were introduced into gCN nanosheets by multi-step annealing to adjust the band-gap of gCN.The rate of H2O2 production by N-vacancy gCN reached 4980 μmol g-1 h-1,18 times that of the pristine gCN.The results show that sp2 N in gCN formed the vacancy defect,which enhanced the light-harvesting ability,promoted charge transfer,and enhanced the O2 adsorption and two-electron oxygen reduction selectivity.These findings offer a practical approach for the design of polymer photocatalysts with reasonable defect configurations and band-gaps.(2)The disordered structure of amorphous gCN results in the poor interlayer and in-plane charge transfer.Density functional theory(DFT)calculations were aimed at this problem,which predicted that ion intercalation(K+ and I-)could optimize the electronic structure of gCN and improve its internal carrier migration.Subsequently,molten salt-assisted thermal polymerization prepared K+ and I-intercalated semicrystalline gCN.As a result,the photocatalytic H2O2 generation rate reached 13100μmol g-1 h-1,and the apparent quantum efficiency at 400 nm reached 23.6%.This study reveals the synergy of crystallinity regulation and intercalation engineering in enhancing charge transfer,and inspires that two-dimensional materials can achieve excellent carrier transport properties and high catalytic activity through ion intercalation.(3)Aiming at the weak oxidation ability of the valence band and the need for a sacrifice agent to promote oxygen reduction in the gCN system,DFT calculations demonstrated the feasibility of doped boron regulating the band structure of gCN,and then a series of B-doped gCN were synthesized.The experimental results are consistent well with the calculated results.The doping site of boron determined the regulation direction of the band gap,and the doping concentration affected the regulation intensity,resulting in the continuous controllable adjustment of the band structure of gCN.Moreover,B-doping improved the valence band oxidation ability of gCN,so H2O2 photosynthesis could be carried out without sacrificing agents.This study found that the doping site plays a decisive role in regulating material properties,and the precise control of doping sites should be emphasized in doping engineering.(4)To solve the problem of the slow kinetics of H2O2 photosynthesis without a sacrifice agent,inspired by the structure of photosynthetic pigment chlorophyll,an Mg single-atom active center was successfully constructed on ultrathin gCN nanosheets,which could directly produce H2O2 from H2O and O2 under visible light with a yield of 1078 μmol g-1 h-1.The results show that Mg centers could enrich the photogenerated holes to promote water splitting and provide protons for H2O2 formation.The evolution mechanism of H2O2 was investigated by in-situ spectroscopy.This work represents the first investigation of the photocatalytic properties of the Mg single-atom catalyst and the construction of an efficient system for on-site photosynthesis of H2O2.This study provides a feasible solution for producing oxidizing agents needed for environmental restoration and their coupling with other water purification technologies.