Steering The Electronic Structure In Semiconductor Photocatalysts:Photocatalytic NO Oxidation Reaction Mechnism

More and more attention has been paid to the global environmental and energy problems brought about by the rapid development of economy and the accelerated industrialization process.Among them,low concentration of NO_x in atmospheric environment has great damage to the environment.It is not only one of the main substances to form acid rain,but also an important factor to form atmospheric photochemical smog and ozone（O₃） depletion.Various kinds of atmospheric pollution will seriously damage the ecosystem and endanger human health.Solar photocatalytic technology as a new economic,environmental and efficient technology can convert light energy into chemical energy or electric energy,achieve high efficiency and stable removal of low concentration of NO_x under mild conditions,and transform it into nitrate as the end product.The resulting nitrate can be removed by water washing or rainwater scouring and transferred to the liquid phase.Therefore,photocatalytic technology has great potential and prospects for the treatment of low concentration air pollution.Semiconductor photocatalysts exhibit good prospects in environmental remediation and energy conversion.However,from the current application situation,the photocatalytic activity of many semiconductor photocatalysts is still unsatisfactory.One of the key factors restricting the photocatalytic performance is their electronic structure.The typical π-conjugated planar structure of graphite-like carbon nitride photocatalyst of non-metallic organic semiconductors makes the delocalized photogenerated carriers transfer randomly in-plane.Meanwhile,the weak van der Waals force between layers is not conducive to the interlayer transfer of photogenerated carriers,resulting in low separation rate of photogenerated carriers and unsatisfactory photocatalytic activity.The typical bismuth-based semiconductor photocatalyst bismuth oxycarbonate has relatively wide band gap,which hinders the generation,separation and migration of photogenerated carriers,thus making it show inadequate photocatalytic activity in the practical application of photocatalytic purification of environmental pollutants.Therefore,it is urgent to explore and study effective strategies to control the internal electronic structure of Semiconductor Photocatalysts to enhance the efficient separation and transmission of photogenerated carriers to promote photocatalytic performance.In addition,the accumulation of toxic by-products（e.g.NO₂） in the process of photocatalytic oxidation of NO not only inhibits the photocatalytic activity of semiconductor photocatalysts,but also causes secondary pollution when released into the air.However,the transformation pathway of intermediate toxic by-products is still unclear.Therefore,it is of great significance to explore the reaction mechanism of the conversion between intermediate products and end products in NO oxidation process.In this study,we use different strategies to regulate the electronic structure of semiconductor photocatalysts:（Ⅰ） Photothermal catalysts with special carrier cycle mechanism were prepared by coupling different types of catalysts to realize the regulation of electronic structure.MnO_x/g-C₃N₄ with different molar ratios was synthesized directly by room temperature precipitation method.MnO_x/g-C₃N₄ exhibited relatively stable and synergistic photothermal catalytic activity towards NO purification under UV-Vis light irradiation.Photothermal synergistic catalysis of MnO_x/g-C₃N₄ had a positive effect on NO and formed an important catalytic cycle mechanism.The specific process was the transfer of photogenerated electrons（e^-） to MnO_x to participate in photothermal synergistic reduction cycle(Mn⁴⁺→Mn³⁺→Mn²⁺),and low-valent Mn ions were easily given electrons（e^-） to combine with photogenerated holes（h⁺） inducing reverse cycling(Mn²⁺→Mn³⁺→Mn⁴⁺) to regenerate active oxygen vacancies.The intermediate products（NOH and N₂O₂^-） can be oxidized to final products（NO₂^- and NO₃^-） by reactive oxygen（O^-） generated by the valence change of MnO_x(Mn⁴⁺/Mn³⁺/Mn²⁺).（Ⅱ） An efficient photocatalyst with directional electron transfer channels was constructed by in situ co-doping of O/La elements to regulate the electronic structure of the catalyst.O/La co-functionalized amorphous carbon nitride was prepared by co-pyrolysis of urea and La₂（CO₃）₃.A directional electron transfer pathway（L2→La→L1→O） is proposed.Electron localization can directly activate O₂ and NO molecules,and significantly promote the production of active substances such as O₂^- and OH radicals,thus improving photocatalytic efficiency.With the optimized electronic structure,the photocatalytic efficiency of g-C₃N₄ can be significantly increased from 35.8% to 50.4%,and it can be recycled.（Ⅲ） By adjusting the exposure of crystal facets to realize the regulation of electronic structure,and photocatalysts were prepared with the direction of electron transfer and the mode of adsorption activation determined by the exposure of crystal facets.Flower-like Bi₂O₂CO₃ and uniformly dispersed Bi₂O₂CO₃ nanosheets with {110} and {001} exposed surfaces were synthesized by a simple hydrothermal method.Owing to the difference of atomic arrangement on {110} and {001} facets,001-BOC converts NO to NO^-or cis-N₂O₂^2-,while 110-BOC induces NO adsorption to NO⁺or N₂O₃.The{001}facet of Bi₂O₂CO₃ promotes the oxidation of intermediates to final products（NO₃^-） and enhances the desorption of NO₃^-.These different adsorption activation modes on {110} and {001} surfaces basically determine that the photocatalytic oxidation of NO depends on the exposure of different crystal planes.According to the experimental results,it can be proved that the above three methods can achieve the electronic structure regulation of photocatalyst and improve the removal efficiency of NO by photocatalyst.By comparing the three methods,it can be clearly found that in situ co-doping O/La elements to regulate the electronic structure of carbon nitride can obtain the best visible light catalytic purification performance of NO.In addition,by regulating the electronic structure in different ways,the adsorption and activation of the target pollutant（NO） can be directly affected,so as to induce different intermediate products and determine the way to transform them into end products.In addition,the accumulation of intermediate and final products in the reaction process was clarified by experimental characterization,in-situ infrared technology and density functional theory calculation.The generation pathway of active free radical species was explored,and the mechanism of photocatalytic oxidation of NO was revealed.Finally,a new strategy to improve photocatalytic activity and effectively inhibit intermediate toxic by-products was proposed.This study provides a theoretical support for further understanding the reaction mechanism of photocatalytic purification of pollutants and the regulation of toxic by-products,and finally realizes the safe and efficient purification of photocatalytic air pollution.