The Preparation Of Surface-functionalized Carbon Nitride And Its Basic Study In Photocatalytic Production Of Hydrogen Peroxide

Solar energy is a green,sustainable energy source.Converting and storing solar energy into chemical energy is one of the important ways to deal with the global environmental crisis and energy challenges.The photosynthesis of plants in nature provides inspiration and direction for sustainable green chemical manufacturing for human beings.In the current study of typical artificial photosynthesis,various light-driven redox reactions produce chemicals including H₂O₂,H₂,CO,NH₃,hydrocarbons,etc.Among them,the photocatalytic preparation of hydrogen peroxide（H₂O₂）has received extensive attention from researchers.In the energy field,H₂O₂,as a carbon-neutral fuel,can realize direct power generation in single-component fuel cells,and is easy to store and transport as a liquid energy carrier;Then,H₂O₂ is widely used as a mild,green,and environmentally friendly oxidant in chemical production;Finally,H₂O₂,as one of the important Fenton reagents,play an irreplaceable role in the field of environmental remediation.Therefore,the photocatalytic production of H₂O₂ by using water and gaseous oxygen molecules as raw materials is a sustainable production process to realize the rational utilization of solar energy.Currently,one of the key challenges for the photocatalytic production of H₂O₂ is the selection of suitable semiconductor photocatalysts,which require excellent photocatalytic activity,stable physicochemical properties,and low cost.Among many photocatalytically driven catalysts for H₂O₂ production,graphitic carbon nitride（g-C₃N₄）has been extensively studied due to its distinct physicochemical,electronic,and optical structures.However,for pristine carbon nitride,the photocatalytic activity of them for H₂O₂ production is still limited by several unfavorable factors,including inferior active surface area,insufficient visible light collection,and tight recombination of photogenerated electrons.Therefore,a series of modification strategies for g-C₃N₄such as morphology tuning,defect control,doping with other elements,loading of noble metals,constructing heterojunction structures,etc.have been proposed to improve the photocatalytic production of H₂O₂.On the other hand,most modified carbon nitrides photocatalysts still have very low apparent quantum efficiency,and sacrificial agents need to be added in the process of H₂O₂ production,resulting in impurity problems and increased cost in the H₂O₂ production.Meanwhile,the interrelated mechanism among the active sites,charge separation kinetics,and photocatalytic activity of modified carbon nitride catalysts is still lacking in-depth research.Therefore,the performance improvement of g-C₃N₄ photocatalysts and the development of other low-bandgap metal-free semiconductor photocatalysts will promote the progress and practical application of photocatalytic production of H₂O₂.In response to the above^-mentioned series of problems,the following work has been carried out in this study,including（1）carbon nitride with enhanced tri-coordinate nitrogen（N3_C）vacancies,（2）controllable Na doping and nitrogen vacancies co-modification engineering,（3）organic semiconductor surface modification of carbon nitride and（4）novel metal-free hydrothermal carbon photocatalysts with stacked donor-acceptor pairs as the main photoactive structure of polyfuran resins.The above four reaction systems had been systematically studied.The performance and reaction mechanism of these prepared catalysts for photocatalytic production of H₂O₂ were studied.Firstly,a facile method for preparing g-C₃N₄ rich in three coordinate nitrogen（N3_C）vacancies by sodium persulfate co-crystal polymerization was investigated.Currently,the role of N3_C vacancies on g-C₃N₄ in the photocatalytic generation of H₂O₂ has not been investigated.The experiment results show that N3_C vacancies can be efficiently introduced into g-C₃N₄ by a persulfate^-assisted melt polymerization process,and the formation mechanism of N3_C vacancies and other defects during the copolymerization process is investigated in detail.The introduction of N3_C vacancies extends the light absorption range,restrains the recombination of photo-excited charges,enhances the adsorption of oxygen,and promotes the activation of oxygen.The results show that g-C₃N₄ with a large number of N3_C vacancies exhibits excellent photocatalytic kinetic performance with high formation rate（k_f,3.05μM min-1）and low decomposition rate（k_d,0.0043 min-1）,and the H₂O₂ yield(161.8μmol L^-1 h^-1)was rised by 4.5 times compared with the unmodified g-C₃N₄.This research provides a new strategy for developing surface^-modified g-C₃N₄ catalysts for photocatalytic precipitation of H₂O₂.Subsequently,sodium dicyandiamide with similar structure was selected as the alkali metal salt modifier for eutectic polymerization of dicyandiamide.Modified carbon nitride（x%SD-CN）materials with different concentrations of Na+doping and nitrogen defects were successfully synthesized by adjusting the doping amount of sodium dicyandiamide（5%,10%,20%and 40%）.The optimized sample of 10SD-CN photocatalyst has a suitable band gap,light absorption and excellent carrier migration performance.10SD-CN shows excellent performance in the photocatalytic H₂O₂production(297.2μmol L^-1 h^-1),which is 9.8 times that of the unmodified g-C₃N₄.The relative contents of Na,N2_C vacancies,N3_C vacancies,NHx defects and cyano groups on the surface of modified g-C₃N₄ were quantitatively obtained by X-ray photoelectron spectroscopy characterization.By combining these apparent characterization data of these catalysts with the experimentally obtained photocatalytic activity data to establish a linear regression model,the potential correlation between active sites,optoelectronic properties and catalytic apparent activity was systematically analyzed.Then,the modified g-C₃N₄ photocatalyst（CN-DAMP-GLU）with cross-linked copolymerization of 4,6-diamino-2-mercaptopyrimidine（DAMP）small molecules and glucose on the surface was synthesized by an easy hydrothermal strategy.The morphology,nano-structure and photoelectrochemical properties of the obtained samples were characterized and compared in detail.The CN-DAMP-GLU exhibits excellent photocatalytic H₂O₂ production performance,which is 21 times that of unmodified g-C₃N₄.The characterization results showed that the modification of carbon nitride by DAMP mainly provided more reactive sites and enhanced electron transfer properties,while the polyfuran structure mainly enhanced the specific surface area and photoresponse of the modified samples.Furthermore,by establishing the correlation coefficient between the photocatalytic H₂O₂ production activity of each sample and various structural,electronic,and optoelectronic properties,the main reasons for the enhancement of photocatalytic activity of CN-DAMP-GLU from original g-C₃N₄ were evaluated.This work provides insight into the molecular charge dynamics of g-C₃N₄surface and a promising approach for surface molecular engineering modification for artificial photosynthesis of g-C₃N₄ to produce H₂O₂.However,the carbon nitride^-based photocatalysts prepared above still face some problems such as complex synthesis,low efficiency,and high cost.Therefore,a photocatalytic catalyst own visible light response was synthesized by hydrothermal method using glucose,sucrose and 5-hydroxymethylfurfural as precursors.Hydrothermal carbon（HTC）,a widely used low-cost,sustainable material,exhibits high mass-specific production rates(453μmol h^-1 g^-1)without sacrificial agents and p H adjustment under visible light.Moreover,the energy conversion efficiency of the system to generate H₂O₂ reached 0.051%under simulated sunlight irradiation.The low band gap（～2.0 e V）leads toπ-conjugated and interchainπ-stacked D-A units on furan polymer chains,which is favorable for photoelectrons to complete O₂ reduction under irradiation.Quenching experiments reveal that the photoreaction of H₂O₂ on HTC undergoes an indirect sequential two-step one^-electron and one^-step two-electron reduction reactions.This work is the first to report the hydrothermal preparation of low-bandgap furan resins containing conjugated quinone and aromatic furan units,in which theπstacking of the donor-acceptor（D-A）furan pair significantly enhances the photoelectron charge transfer efficiency.The reported low-cost and widely available HTC photocatalysts exhibit remarkable activity,showing great potential for artificial photosynthesis applications.In summary,this research focuses on the development of surface^-functionalized g-C₃N₄ for photocatalytic production of H₂O₂.The main study includes the next points:Firstly,the introduction method of peroxodisulfate eutectic polymerization to achieve customized N3_C defects on the surface of g-C₃N₄ was proposed;secondly,the eutectic polymerization method of sodium dicyandiamide and dicyandiamide realizes the Na⁺ion doping and surface defect structure of the synthesized modified g-C₃N₄;thirdly,to resolve the ambiguous structure^-activity relationship of active sites,physicochemical properties and photocatalytic activity performance,two scientific analysis methods of linear regression model and heat map was established;fourthly,a simple hydrothermal method was used to achieve the cross-linking copolymerization of DAMP small molecules and glucose on the surface of g-C₃N₄,and at the same time break through the bottleneck of sacrificing agent in the photocatalytic H₂O₂ production system of modified carbon nitride.fifthly,the hydrothermal carbon with efficient photocatalytic H₂O₂ production performance was synthesized through 12 precursors,expanding the types of non-metallic conductor photocatalysts other than carbon nitride^-based photocatalysts.