Study On Photocatalytic Reduction Of CO₂ By G-C₃N₄ With Nitrogen Defects And Its Composite Catalyst

With the advancement of industrial globalization,the extensive use of fossil fuels such as coal and petroleum has caused excessive emissions of CO₂,which has led to global temperature rise,frequent extreme weather,and energy shortages.Researchers favor photocatalysis technology because of its green,cleanliness,and sustainable energy advantages.Therefore,it is expected that photocatalytic CO₂ technology can alleviate climate and environmental issues in the future.At present,the development of efficient and stable visible light-responsive catalysts is still the key to photocatalytic reduction of CO₂.Carbon nitride（g-C₃N₄）is gradually being used in the study of photocatalytic reduction of CO₂ because of its proper band gap,excellent thermal stability and chemical resistance.In this paper,carbon nitride was modified by constructing nitrogen defects,and it was further combined with indium sulfide（β-In₂S₃）and indium vanadate（In VO₄）to form high-efficiency heterostructures,thereby improving the performance of photocatalytic reduction of CO₂.In order to increase the active sites of carbon nitride,an appropriate amount of nitrogen defects can be introduced during the synthesis of carbon nitride.Firstly,dicyandiamide was mixed with sodium chloride and sodium hydroxide,heated and calcined in a nitrogen atmosphere,and finally sodium doped flaky carbon nitride with nitrogen defects was obtained.Under the conditions of visible light irradiation with triethanolamine as a sacrificial agent,the ability of carbon nitride catalyst to photocatalytically reduce CO₂ to produce CO and CH₄ was investigated.The experimental results exhibited that the CO yield of the optimal sample bmw-DCN-30 was 30.6μmol·g^-1·h^-1,and the CH₄ yield was 5.4μmol·g^-1·h^-1.The productivity was15 and 11 times as much as that of bulky g-C₃N₄（BCN）.A series of characterization analysis proved that the sodium-doped nitrogen-containing defect flaky carbon nitride had stronger visible light absorption capacity and CO₂ adsorption capacity,and its photo-generated carrier separation efficiency had been improved.In order to further broaden the visible light response range of carbon nitride with nitrogen defects,β-In₂S₃ and carbon nitride NDCN were combined to form a heterojunction catalyst.Firstly,amorphous In₂S₃ was synthesized by hydrothermal method,then In₂S₃ and NDCN were heated in ethanol solution to anchor each other,and finallyβ-In₂S₃/NDCN composite catalyst was obtained by calcined under nitrogen atmosphere.The photocatalytic reduction of CO₂ was carried out under the condition of no sacrificial agent and visible light irradiation.The experimental results showed that the photocatalytic active sites of the optimal composite sample20%β-In₂S₃/NDCN(CO:20.32μmol·g^-1·h^-1;CH₄:2.12μmol·g^-1·h^-1),and its total gas-phase yield was 3.7 times as much as that ofβ-In₂S₃ and 1.7 times higher compared with NDCN.A series of characterization indicated that a strong interaction force exists betweenβ-In₂S₃ and NDCN,which promoted the separation and transport of carriers.In order to construct a Z-type heterostructure to improve the photocatalytic performance,a one-step hydrothermal method was used to compound In VO₄ with carbon nitride HDCN.Under the conditions of visible light and gaseous water as hole quencher,the yields of the reduction products CO and CH₄ were detected.The experimental results showed that the photocatalytic active sites of the best composite sample 30%In VO₄/HDCN(CO:20.14μmol·g^-1·h^-1;CH₄:3.46μmol·g^-1·h^-1),and its total gas-phase yield was 2.8 times as much as that of In VO₄,and 1.8 times higher compared with HDCN.Related characterization proved that the heterojunction catalyst could accelerate the transfer of photogenerated electrons and holes,and enhance the material’s ability to absorb visible light.The XPS and EPR analysis both confirmed the rationality of the composite sample as a Z-type heterostructure.