Study On The Electronic Structure Regulation Of Bismuth-based Photocatalytic Materials And The Reaction Mechanism Of NO_x Purification

Nitrogen oxides（NO_x）are typical air pollutants,which can not only induce photochemical smog but also are the common precursor of ozone and secondary PM_2.5generationleading to the formation of haze.In recent years,semiconductor photocatalytic technology exhibit excellent performance in environmental treatment,becoming a potential NO_x purification technology.The development and design of new photocatalysts is the key to realizing the application of photocatalytic technology.Bismuth-based photocatalytic materials exhibit unique optoelectronic properties due to their alternate layered ultra-thin structure,and have received extensive attention and research in photocatalytic oxidation reactions.However,some bismuth-based photocatalytic materials such as bismuth oxycarbonate（Bi₂O₂CO₃）and bismuth titanate(Bi₄Ti₃O₁₂)have a wide band gap,which limits their light utilization and photo-generated carrier separation efficiency.Therefore,developing effective modification strategies to control the electronic structure of bismuth photocatalytic materials with wide band gap and enhance light absorption is one of the research directions to improve the photocatalytic performance.In this study,we used a method of constructing point defects（atomic substitutions,vacancies）to achieve a reasonable modulation defect level,which canpromote the photo-generated electron-hole efficient separation,expand the range of light response andenhance absorption of visible light,thereby realizing a visible light catalytic performance improvement.In addition,we further discussed the instability problems and the unclear reaction mechanism of point defect structure in the photocatalytic oxidation process from the following aspects.（1）B doped Bi₂O₂CO₃hierarchical microspheres:enhanced photocatalytic performance and reaction mechanism for NO removal.Hydrothermal method is used to replace C atoms in bismuth oxycarbonate with B atoms to construct point defects.The results of experiments and density functional theory calculations confirm that the successful construction of the intermediate energy level,which plays a role in accelerating electron transfer.The formation of point defects（replaced atoms）promotes the separation of photo-generated electrons and holes,which causes an effective red shift of the light absorption band edge,thereby expanding the light response range.In addition,point defects also enhance the adsorption and activation of O₂,H₂O,and NO small molecules,thereby promoting the production of corresponding superoxide radicals,hydroxyl radicals,and NO₂⁺intermediate products,thereby enhancing the visible light performance of Bi₂O₂CO₃to remove NO.In addition,a new intermediate product（NO₂⁺）was discovered through In situ diffuse reflectance infrared fourier transform spectroscopy technology,and a more clear reaction path NO→NO₂→NO₂⁺→NO₃^-was proposed by analyzing the changes in the surface species of the catalyst during the reaction.Because the surface of Bi₂O₂CO₃with doping B atoms has more NO₂⁺,the reaction is promoted.The results of comprehensive analysis experiments and calculations confirm that the defect-modified Bi₂O₂CO₃photocatalyst can efficiently convert NO to non-toxic and harmless end products.（2）O_xygen deficient Bi₄Ti₃O₁₂microspheres:enhanced photocatalytic performance and reaction mechanism of removing NO.The Bi₄Ti₃O₁₂precursor is prepared by the hydrothermal method and then mixed with Na BH₄deoxidizer by stirring to produce a photocatalyst containing oxygen defects for efficient NO removal.By changing the concentration of Na BH₄to control the relative generation of oxygen defects.It is proved that moderate defects are beneficial to the photocatalytic purification effect but too many oxygen defects will backfire.The reason is that the presence of an appropriate amount of oxygen defects can be used as active sites for surface reactions,which can improve the material’s physical and chemical properties（electronic structure,geometric structure,material light absorption）.Excessive oxygen defects will cause the recombination of photogenerated electrons-holes at the empty spot.The existence of oxygen defects can significantly promote the separation of photogenerated electrons and holes,improve the absorption of visible light,and enhance the adsorption and activation of small molecules at the vacancies,thus generating more free radicals to participate in the subsequent photocatalytic oxidation reaction.The removal of NO by Bi₄Ti₃O₁₂containing oxygen defects is significantly improved,up to 66.1%.Besides,In situ diffuse reflectance infrared fourier transform spectroscopy technology is used to explain the reaction path of photocatalytic oxidation of NO.After modification,Bi₄Ti₃O₁₂will inhibit the formation of toxic products（N₂O₃）and will produce NO^-intermediate products.According to the theoretical caculations,The NO^-product is more likely to transform into cis-N₂O₂^2-and then to form the final product.The method of constructing point defects on bismuth-based materials proposed in this paper can be extended to other Bi-based photocatalysts,which can provide a new method for the horizontal development of photocatalysis.The photocatalytic oxidation reaction path analyzed in the two systems can also provide some theoretical support for future research on photocatalytic mechanism.