Study On The Performance And Mechanism Of Boron And Carbon Enhanced Carbon Nitride Photocatalytic Production Of Hydrogen Peroxide

In recent years,China’s environmental situation has become severe with frequent pollution incidents,and the severe use of non-green chemicals has aggravated environmental pollution.Therefore,there is an urgent need to develop efficient,high-quality,and clean energy materials to meet people’s growing material and cultural needs.The use of clean energy can effectively prevent ecological environmental pollution,alleviate China’s resource shortages,and promote the sustainable development of resources in the country,which has important environmental,economic,and social significance.Hydrogen peroxide（H₂O₂）is an environmentally friendly oxidant because its decomposition products are only water and oxygen,and it does not cause secondary pollution to the environment.Traditional H₂O₂ production methods（such as anthraquinone method and direct synthesis of hydrogen and oxygen）are severely polluting,energy-consuming,and difficult to store and transport.To change this unfavorable development pattern,researchers have been committed to greening the production of H₂O₂.The oxygen reduction reaction based on photocatalytic materials can achieve efficient,high-quality,and clean production of H₂O₂.Graphitic carbon nitride（g-C₃N₄）has received widespread attention from researchers due to its appropriate band structure,safety,abundance of precursors,and chemical stability.However,g-C₃N₄ is still limited by low visible light utilization,low specific surface area,and easy electron-hole recombination,which restrict its practical applications.Studies have shown that surface modification can significantly improve the photocatalytic performance of g-C₃N₄.Therefore,this paper proposes the modification of g-C₃N₄ by boron（B）and carbon materials,explores the influence of the chemical composition,morphology,optical properties of the composite material on the photocatalytic production of H₂O₂,and the main research contents are as follows:（1）Study on the photocatalytic performance of B-doped g-C₃N₄ for H₂O₂ production:B-doped g-C₃N₄（BCN）was successfully prepared by thermal polymerization using B₂O₃ and melamine as raw materials under N₂ conditions.The research results show that the photocatalytic generation of H₂O₂ by B-doped g-C₃N₄ is influenced by factors such as the amount of B₂O₃ used,solution p H,gas conditions,and sacrificial agents.When the amount of B₂O₃ used is 2.5 g,p H=3,oxygen flow rate is 0.1 L/min,and 1 ml of isopropanol is present,the amount of H₂O₂ generated by photocatalysis with B-doped g-C₃N₄ is twice that of g-C₃N₄,with a maximum of 352.6 μmol/L,and the effect is optimal.UV-vis diffuse reflectance spectroscopy indicates that B-doping does not cause a significant red shift of the absorption edge,but significantly enhances the absorption capacity in the ultraviolet region,providing more energy for the photocatalytic production of H₂O₂.Nitrogen adsorption-desorption curves show that the specific surface area of B-doped g-C₃N₄ is significantly higher than that of gC₃N₄,being 1.58 times that of g-C₃N₄,reaching 24.47 m²/g,which is beneficial for providing more reaction sites for photocatalytic oxygen reduction to produce H₂O₂.XPS analysis indicates that B-doping helps g-C₃N₄ form nitrogen vacancies,effectively inhibiting carrier recombination and accelerating the separation and transfer of electrons and holes.The photocurrent of B-doped g-C₃N₄ is about 1.68 times that of g-C₃N₄.The charge transfer resistance（Rct）of B-doped g-C₃N₄ is smaller than that of g-C₃N₄,further indicating that the introduction of B can effectively promote charge separation and transfer to the catalyst surface,which is consistent with the XPS analysis results.Comparison of the EPR spectra of B-doped g-C₃N₄ and g-C₃N₄ under illumination shows that the higher separation efficiency of Binduced electron holes leads to a stronger superoxide radical（·O2—）signal,indicating that the main pathway for photocatalytic H₂O₂ production with B-doped g-C₃N₄ is a two-step singleelectron oxygen reduction.（2）Study on the enhanced photocatalytic performance of B and carbon quantum dots comodified g-C₃N₄ for H₂O₂ production: Carbon quantum dots were prepared via a hydrothermal method using citric acid and thiourea as raw materials.They were then comodified with B-doped g-C₃N₄ under dark conditions by continuous stirring overnight,resulting in B and carbon quantum dots co-modified g-C₃N₄（CBCN）.The study showed that when the loading amount of carbon quantum dots was 4 ml and the p H was 3,B and carbon quantum dots co-modified g-C₃N₄ had the highest efficiency in photocatalytic H₂O₂ production,reaching 505.2 μmol/L,which was 2.8 times that of g-C₃N₄ and 1.4 times that of B-doped g-C₃N₄.After carbon quantum dots were loaded,the absorption edge of B and carbon quantum dots co-modified g-C₃N₄ shifted from 470 nm to 540 nm,significantly improving the utilization efficiency of visible light.Nitrogen adsorption and desorption curves showed that the specific surface area of B and carbon quantum dots co-modified g-C₃N₄ was 26.56m²/g,1.72 times that of g-C₃N₄,which is beneficial for photon absorption and providing more active sites for the oxygen reduction reaction.XPS analysis showed that sulfur or sulfur oxides were present in the structure of B and carbon quantum dots co-modified g-C₃N₄,inducing higher charge density and promoting higher charge separation efficiency.The photovoltaic current value of B and carbon quantum dots co-modified g-C₃N₄ was nearly 4 times higher than that of g-C₃N₄,and the charge transfer resistance（Rct）was lower than that of B-doped g-C₃N₄ and g-C₃N₄,further indicating that co-modification with B and carbon quantum dots can promote the faster migration of photogenerated charges in g-C₃N₄,and verifying the conclusion of XPS analysis.Using Rhodamine B as a probe,free radical capture experiments found that superoxide free radicals were the main active species during the reaction,indicating that photocatalytic H₂O₂ production was a continuous two-electron oxygen reduction process.The in-situ application of photocatalytic H₂O₂ production was investigated,and the results showed that the H₂O₂ produced by g-C₃N₄,B-doped g-C₃N₄,and B and carbon quantum dots co-modified g-C₃N₄ all had certain degradation effects on pollutants such as Rhodamine B and nitrophenol.Moreover,the degradation efficiency of B and carbon quantum dots comodified g-C₃N₄ towards the two pollutants was higher than that of the other two reaction systems due to the highest H₂O₂ yield.（3）Study on the enhanced photocatalytic performance of waste biomass resource utilization with co-modified g-C₃N₄ for H₂O₂ production: Using corn stalks as raw materials,carbon materials were prepared by hydrothermal method.To provide more loading sites for biomass carbon quantum dots,g-C₃N₄ nanosheets（n CN）with higher specific surface area were prepared by high-temperature calcination.Biomass carbon quantum dots were immersed in the dispersion of g-C₃N₄ nanosheets overnight to prepare biomass carbon quantum dots composite g-C₃N₄（Cn CN）.When the amount of carbon material added was 1 ml,p H = 3,oxygen flow rate was 0.1 L/min,and 1 ml isopropanol was present,biomass carbon quantum dots composite g-C₃N₄ had the highest photocatalytic activity for H₂O₂ production,reaching474.5 μmol/L,which was 1.3 times higher than that of g-C₃N₄ nanosheets.UV-vis diffuse reflectance spectroscopy revealed that the modification of biomass carbon materials did not cause significant absorption edge movement,indicating that solar light utilization efficiency was not the main factor affecting H₂O₂ generation.Nitrogen adsorption-desorption curves showed that the specific surface area of biomass carbon quantum dots composite g-C₃N₄ was85.53 m²/g,which was 0.68 times that of g-C₃N₄ nanosheets.Although the presence of biomass carbon covered some of the pores on the surface of g-C₃N₄ nanosheets,compared with pure g-C₃N₄ in chapter 3,the specific surface area was still increased by 5.5 times,which is beneficial for providing more reaction sites for oxygen reduction.XPS analysis found that there were carbon vacancies in the structure of biomass carbon quantum dots composite gC₃N₄,which inhibited the recombination of photogenerated charge carriers.Detection of photocurrent and impedance showed that the photocurrent of biomass carbon quantum dots composite g-C₃N₄ was 1.5 times that of g-C₃N₄ nanosheets,and the charge transfer resistance（Rct）was the smallest,effectively inhibiting the recombination of photogenerated charge carriers,proving that the modification of biomass carbon quantum dots can significantly promote the improvement of photocatalytic performance.Free radical capture experiments and electron paramagnetic resonance showed that the main active substance in the photocatalytic process of biomass carbon quantum dots composite g-C₃N₄ was superoxide free radicals and the reaction is a two-step single-electron oxygen reduction.