Photocatalytic Removal Of Nitrogen Oxides By Modified Zinc Stannate

With the rapid development of the industrialized economy,the burning of a large amount of fossil fuels and the exhaust emissions of automobiles have produced serious air pollutants.Nitrogen oxides（NOx）,as a typical air pollutant,can cause atmospheric pollution such as smog,acid rain or photochemical smog.The traditional methods of controlling NOx pollution mainly include selective catalytic reduction,thermal catalysis and physical adsorption,etc.,which are not suitable for the removal of low-concentration ppb-level NOx,and are likely to cause secondary pollution.As an efficient green technology,photocatalysis has shown broad application potential in recent decades.As a typical ternary system n-type semiconductor photocatalytic material,zinc stannate has attracted much attention due to its high electron conductivity,electron transfer ability and outstanding thermodynamic stability.However,from the perspective of practical application,their photocatalytic activity still needs to be improved.In this paper,the surface（interface）design and energy band modulation are used to improve their photocatalytic NOx removal activity.The specific research contents are summarized as follows:（1）Modified Zn₂SnO₄ composites（ZSO/SnO₂-VOs）with synergistic effect of oxygen vacancies and SnO₂ quantum dots were prepared by a facile one-pot hydrothermal method and the removal rate of NO reaches 60%,which is three times that of pure Zn₂SnO₄.According to the analysis results of XRD,TEM and SEM,there are SnO₂ quantum dots on the surface of ZSO-1.5 nanomaterials.In addition,based on XPS and EPR measurement results,the existence of OVs was detected,and the modified sample（ZSO-1.5）had a much higher electron transfer rate than pure ZSO according to transient fluorescence spectroscopic analysis,and its electron transfer efficiency reached 99.99%.ESR and EPR tests combined with theoretical calculations,the results showed that the main active substances for the improvement of the catalytic performance of modified Zn₂SnO₄ were mainly DMPO-·O₂^-and DMPO-·OH.On this basis,the position of OVs,the electronic structure and charge transport properties of the interface between SnO₂ QDs and ZSO were revealed by DFT calculations,and it was found that SnO₂QDs acted as electron reservoirs and electron conductors,which were beneficial to current carrying separation of children.The reaction path of NO on the surface of Zn₂SnO₄ was analyzed.With the generation of SnO₂ quantum dots on the surface to form intermediate energy levels and the introduction of defects,the conversion path of polluted gas to the catalyst surface constructed a new charge transfer path at the interface.The reaction intermediate NO⁺was observed by in situ DRIFTS,which promotes NO activation by forming NO⁺to facilitate the oxidation of NO to the final product.ZSO-1.5 can generate more·OH and·O₂^-active radicals than ZSO,which is beneficial to the removal of NO,indicating that the special electronic structure of ZSO-1.5 inhibits the production of NO₂ toxic products.Our study provides a research idea in the construction of quantum dots and oxygen vacancies to co-modify the catalyst surface structure.Combined with experimental and theoretical calculations,the mechanistic insights into the selective oxidation of NO are fully discussed.（2）The microwave heating method has considerable application prospects in the synthesis of high-efficiency photocatalytic nanoparticles.At present,microwave heating has been applied to the synthesis of single catalytic materials,doped materials with different elements,and the construction of nanocomposites.The Zn₂SnO₄@Zn O composites were prepared by microwave method,and the removal of NO in air was studied by the synergistic effect of the construction of heterojunctions and oxygen vacancies.Compared with pristine ZSO,the ZSO-Z3（Zn₂SnO₄@Zn O）sample has the highest NO removal rate of 55%.According to the results of EPR,the signal of OVs was detected.In order to further intuitively and dynamically study the adsorption process and oxidation process of NO removal on the photocatalyst surface,in-situ diffuse reflectance infrared Fourier transform spectroscopy was used to test and study.The introduction of OVs not only reduces the band gap caused by the intermediate gap,but also acts as an active center to facilitate the adsorption of small molecules such as NO,O₂,and H₂O.Using DFT simulation calculation analysis,it can be seen that the heterojunction formed by the composite material promotes the generation of active species.ZSO-Z3（Zn₂SnO₄@Zn O）can generate more·OH and·O₂^-active radicals than ZSO,which is beneficial to the removal of NO,and the special electronic structure of ZSO-Z3（Zn₂SnO₄@Zn O）inhibits NO₂ toxicity production of products.By constructing a Zn₂SnO₄@Zn O heterojunction,we provide ideas for constructing the electronic structure of nanomaterials and provide insights into the selective oxidation of NO at low concentrations in the atmosphere.