Preparation Of Bismuth Oxybromide-based Composites And Their Photocatalytic Conversion Of Nitrogen Oxides

The massive combustion of fossil energy and productive life of human beings have led to serious exceedances of atmospheric nitrogen oxides（NO_x）,one of the typical pollutants in the atmosphere,which not only cause environmental problems such as PM2.5 and photochemical smog,but also pose serious threats to human health.Therefore,taking effective measures to reduce NOx pollution has been a challenge in air purification.Photocatalytic technology has shown promising applications in the removal of low concentrations of NO_x from the atmosphere due to its environmental friendliness,sustainability,and mild reaction conditions.China is rich in bismuth ore resources,and bismuth oxybromide（BiOBr）,as a typical bismuth-based photocatalyst,has the advantages of low price,non-toxicity,corrosion resistance and good visible light response,etc.Inevitably,it also has problems such as poor light absorption ability and easy compounding of photogenerated carriers,which need further modification.Composite photocatalysts can often overcome the shortcomings of single-component photocatalysts and further enhance the photocatalytic performance of the materials.In this thesis,BiOBr photocatalyst was studied,and the composite modification of BiOBr was carried out by two methods:construction of heterojunction,introduction of plasma Biand oxygen vacancy defects.The microscopic phenomena and reaction mechanisms in the photocatalytic conversion of nitrogen oxides were intensively investigated,with the main work as follows.（1）Preparation of BiOBr/Bi₄Ti₃O₁₂ heterojunction and its photocatalytic conversion of nitrogen oxidesBiOBr/Bi₄Ti₃O₁₂ composite photocatalysts with different ratios were prepared by two-step solvothermal method,and the test results showed that the composite had the best photocatalytic performance when the molar ratio of Bi₄Ti₃O₁₂ to BiOBr was 0.6.The NO removal rate of BOB/BTO-0.6 photocatalyst reached42%in 20 min under visible light,which was about twice as high as that of BiOBr photocatalyst,and almost no NO₂ toxic by-products were produced.XPS analysis further revealed the electron transfer path of the composite photocatalyst,and combined with energy band analysis,established that the BOB/BTO-0.6photocatalyst is a typical type Ⅱ heterojunction.The effective construction of the heterojunction system promoted the directional transfer of photogenerated charges at the interface,suppressed photogenerated carrier complexation,further enhancing the photocatalytic activity of the catalyst.The results of active species capture experiments and EPR indicated that·O₂^-,e^-and h⁺were involved in the photocatalytic reaction.The results of in situ IR tests showed that NO was deeply oxidized to nitrite and nitrate species.This work provides a new idea for the design of heterojunction photocatalysts.（2）Preparation of defective Bi/BiOBr nanoflower and its photocatalytic conversion of nitrogen oxidesOxygen vacancy modified Bi/BiOBr nanoflowers were successfully prepared by a one-pot solvothermal method.Thanks to the synergistic effect of oxygen vacancy and plasma bismuth,the visible removal of NO by the catalyst increased from the initial 26%to 63%,and the nitrogen dioxide yield was 7%,which was significantly lower than that of 59%for BiOBr.In addition,the effect of relative humidity,an environmental factor,on the photocatalytic NO conversion process was investigated.The results showed that there was a weak competitive adsorption behavior between NO molecules and water molecules on the catalyst surface,and the effect of relative humidity on the photocatalytic NO removal rate was small.However,the production of NO₂ gradually decreased with increasing relative humidity,further indicating that high humidity can promote the conversion of the toxic intermediate NO₂ to nitrate.Finally,possible NO adsorption and photocatalytic conversion pathways on the catalyst surface were proposed based on the results of temperature programmed desorption and in-situ DRIFTS tests.This work provides insights into the deep photocatalytic oxidation of nitrogen oxides.