Construction Of Bismuth-based Heterojunctions For Visible-light Driven Photocatalytic Degradation Of Ammonium Dinitramide

Ammonium dinitramide（ADN）,as a new kind of high energy oxidant,has attracted extensive attention in the field of solid propellant.However,due to that ADN has certain toxicity and good water solubility,the wastewater generated in the industrial production process must undergo special treatment before being discharged.Photocatalytic oxidation technology can generate a variety of active species in situ,non-selectively attack the target pollutants and mineralize them into CO₂,H₂O or specific inorganic ions,which is a green method to eliminate environmental pollution.Semiconductor photocatalysis has been widely employed in the removal of 2,4,6 trinitrotoluene（TNT）,dinitrotoluene（DNTs）,cyclotrimethylene trinitramine（RDX）and cyclotetramethylene tetranitramine（HMX）from explosive wastewater,but little attention has been paid to wastewater containing ammonium dinitramide（ADN）.Therefore,the aim of this paper is construction of visible-light driven catalytic system and efficient removal of ADN.Detailed literature survey indicates that bismuth oxide semiconductors are the best candidate materials.The modification strategy of construction of two-dimensional/two-dimensional（2D/2D）heterojunctions was adopted to further improve their photocatalytic activity.The main research contents are as follows:（1）BiOX（X=Cl,Br,I）catalysts with different 2D morphology were prepared by changing the solvent of solvothermal method.Then crystal phase,morphology,BET surface area and photochemical properties of samples were characterized and analyzed.The photocatalytic activity of Bi OX on degradation of ADN was compared.The results indicate that photocatalytic activity of Bi OX with nanosphere morphology is better than that of Bi OX with large thick sheet and small thin sheet morphology,and photocatalytic activity of nanosphere Bi OBr is the highest,with degradation rate of ADN reaching 94.6%after 120min illumination.In order to improve photocatalytic performance of BiOCl and Bi OBr with small thin sheet morphology,2D/2D CN/Bi OBr and CN/BiOCl heterojunctions were constructed by coupling with CN,and the composite ratio was optimized.The results indicate that photocatalytic activity of 30%CN/Bi OBr and 30%CN/BiOCl is the highest,with degradation rate of ADN reaching 95.0%and 96.8%after180 min illumination,respectively.（2）BiOCl_xI_1-x（x=0.9,0.7,0.5,0.3,0.1）solid solutions were constructed by solvothermal method,and band gap and visible-light absorption range of BiOCl were regulated by I-doping.The successful preparation of solid solutions was proved by a series of characterization methods.The photocatalytic degradation experiments show that when x=0.7,the solid solutions have the best photocatalytic activity,but they are not ideal for photocatalytic degradation of ADN.Using BiOCl_0.7I_0.3-M（M refers to the mixed solvent in the preparation process）as catalyst,degradation rate of ADN is 87.2%after illumination for 180 min.When BiOCl_0.7I_0.3-W′（W′refers to the aqueous solvent）was used as catalyst,degradation rate of ADN is 59.9%after illumination for 60 min.The photocatalytic activity of BiOCl_0.7I_0.3-M and BiOCl_0.7I_0.3-W′catalysts was further improved by constructing heterojunctions with CN through in situ solvothermal method.The results show that photocatalytic activity of BiOCl_0.7I_0.3-M was improved insignificantly by combination with CN of different proportions.On the contrary,photocatalytic activity of BiOCl_0.7I_0.3-W′was improved significantly.When the composite ratios of CN are 20%and 30%,degradation rate of ADN can reach 99.9%after 60 min illumination,but when the composite ratio is 30%,degradation rate is faster.（3）Bi₂O₃/Bi₂WO₆and CN/Bi₂WO₆ heterogeneous catalysts were successfully prepared by in situ solvothermal method.The experimental results of visible-light driven photocatalytic degradation of ADN show that the optimal composite mass ratio is 5:5.The degradation rate of ADN is 97.9%after 180 min illumination over Bi₂O₃/Bi₂WO₆（5:5）,and degradation rate can reach 98.9%after 80 min illumination over CN/Bi₂WO₆（5:5）.Therefore,the photocatalytic activity of CN/Bi₂WO₆（5:5）is superior to that of Bi₂O₃/Bi₂WO₆（5:5）,which can be attributed to its excellent structural stability and efficient charge separation and transfer ability.（4）The kinetic study shows that photocatalytic degradation process of ADN basically conforms to the first-order kinetic model.The capture experiments indicate that·O₂^-and h⁺are the main active species.The structural stability of photocatalysts was evaluated by cyclic experiments and various characterization methods.It was found that other composite catalysts have excellent stability except for Bi₂O₃/Bi₂WO₆（5:5）.Besides,the band gap structure of semiconductors was obtained by means of VB-XPS and M-S,and the reasonable charge transfer mechanisms of heterojunctions were proposed combining with generation level of free radicals.In addition,the enhancement mechanism of photocatalytic activity of different bismuth-based composite catalysts was systematically analyzed,and main factors for the highest photocatalytic activity of 30%CN/BiOCl_0.7I_0.3-W′were the expansion of visible-light absorption range and the efficient separation and transfer ability of photogenerated carriers.（5）The reactive sites were predicted and possible degradation path of ADN was proposed by results of density functional theory（DFT）calculation.Furthermore,qualitative and quantitative analysis of liquid products taken out at different moments in the degradation process of ADN was carried out through characterization methods of UV-Vis,XPS and ion chromatography.The results of experimental analysis are consistent with those of DFT calculation,which show that NH₄⁺,NO₂^-and NO₃^-co-exist in the liquid products.With the prolonging of light irradiation time,the concentration of NH₄⁺is basically unchanged,the concentration of NO₂^-increases first and then decreases,while the concentration of NO₃^-increases rapidly,so NO₃^-is the main degradation product.