Visible Light Driven Removal Of Pollutants In Water And The Mechanism On The Photocatalysis Under COF TzDa Based Composite Materials

With the development of industry,the problem of water pollution has become more and more serious.As a kind of advanced oxidation technology,photocatalysis has the advantages of safety,economy,and no secondary pollution compared with the traditional water treatment methods such as adsorption,biological and chemical methods.As a new type of semiconductor material,covalent organic frameworks(COFs)have the characteristics of good stability,low density,high specific surface area and easy modification compared with other porous materials such as molecular sieves and metal organic frameworks(MOFs).Among them,the π-π conjugated stack in the 2D COFs connected by imine bonds can improve the charge transfer efficiency and its stability in acidic and basice conditions is considered to be one of the most promising photocatalysts.In this thesis,a 2D triazine covalent organic framework of COF TzDa was prepared,and then two organic-inorganic composite materials TzDa/BiOBr and TzDa/AgBr were prepared by hydrothermal synthesis and co-precipitation.The performance and mechanism of the two photocatalysts in degrading pollutants in water are respectively explored.The main contents include:(1)4,4’,4’’-(1,3,5-triazine-2,4,6-triyl)trianiline(Tz)and 2,5-dihydroxyterephthalald-ehyde(Da)were used as the monomers to synthesize COF TzDa with high-crystallinity from the solvents of o-dichlorobenzene and N,N-dimethylacetamide.(2)The COF TzDa/BiOBr composite was prepared by solvothermal method.Various properties of the material were characterized by XRD,SEM,TEM,BET,DRS,XPS,etc.The dye rhodamine(Rh B),antibiotic ciprofloxacin(CIP)and heavy metal ion Cr(Ⅵ)were used as target pollutants,and the Xenon lamp source was used as a simulated visible light source to study the photocatalytic performance and mechanism of COF TzDa/BiOBr.The experimental results show that Rh B could be completely degraded within 35 minutes in the range of p H=2.1-6.1;at p H= 4.0-8.0,the degradation rate of CIP reaches 85% after 40 minutes;and under the condition of p H=2,96% of Cr(Ⅵ)is reduced to low toxicity Cr(Ⅲ)after irradiating for 90 min.The photocatalytic removal experiment of the mixed pollutant solution of "Rh B+Cr(Ⅵ)" shows that the photocatalytic oxidative degradation of Rh B and the photocatalytic reduction of Cr(Ⅵ)are both higher than the single-component solution.Among them,Rh B can be completely degraded within 20 minutes,while the reduction rate of Cr(Ⅵ)can reach 98% within 40 minutes.The improvement of COF TzDa/BiOBr photocatalytic performance mainly comes from the expansion of the optical absorption range,and the construction of heterojunction realizes the effective separation of photogenerated electrons and holes.According to Mott-Schottky and DRS tests,free radical trapping and electron spin resonance(ESR)analysis,it is proposed that COF TzDa/BiOBr is a direct Z-scheme heterojunction and the photocatalytic degradation mechanism of Rh B or Cr(Ⅵ)and their mixture is analyzed.This work uses 2D COFs materials and BiOBr to construct a Z-scheme heterojunction to improve and enhance the photocatalytic activity of traditional inorganic photocatalysts.It provides a new method for the elimination of organic-inorganic composite wastewater pollutants,and provides a new strategy for the application of COFs in the field of photocatalysis and the preparation of high-efficiency photocatalysts.(3)Using water as solvent,COF TzDa/AgBr composite photocatalyst was prepared by co-precipitation method.Various properties of the material were characterized by XRD,SEM,TEM,BET,DRS,XRS,etc.The tetracycline(TC)and heavy metal ion Cr(Ⅵ)were used as target pollutants,and the Xenon lamp source was used as a simulated visible light source to study the photocatalytic performance and mechanism of COF TzDa/AgBr.The results show that under visible light irradiation,COF TzDa/AgBr exhibits the strongest catalytic activity compared with the single components.In the p H value of 4.4-10.4,the degradation rate of TC reached 79.8% within 30 minutes;the photocatalytic reduction rate of Cr(Ⅵ)under the condition of p H=2.5,reaches 95.5% after irradiating for 90 min,The COF TzDa and AgBr comparison experiment shows no photocatalytic effect.At the same time,the performance of COF TzDa/AgBr for the photocatalytic removal of "TC+Cr(Ⅵ)" mixed pollutant solution was studied.The degradation rate of TC was 85.5% after irradiating for 70 min,which was higher than the degradation rate of single TC(79.2%),indicating that the degradation efficiency of TC was improved in the presence of Cr(Ⅵ).Similarly,Cr(Ⅵ)in mixed solution can be completely removed after lighting for 70 min,while the degradation rate of single Cr(Ⅵ)solution within the same reaction time is 84.3%.This is attributed to the reaction of Cr(Ⅵ)with photogenerated electrons and TC with photogenerated holes,which can make full use of the hole-electron pairs produced by COF TzDa/AgBr and inhibit their recombination,thereby improving the redox capacity of COF TzDa/AgBr.The research results of the mechanism show that the improvement of the catalytic performance comes from the synergistic effect of the formation of silver nanoparticles(Ag NPs)and the charge transfer mechanism of the Z-scheme heterojunction.This work provides a new method for the elimination of organic-inorganic composite wastewater pollutants and the design of efficient COF-based Z-scheme heterojunctions.