System Construction And Enhancement Mechanism Of Efficient Bi₂WO₆-based Visible Light Photocatalytic Cr(?) Reduction

As a common water pollutant,Cr(?) is featured by strong toxicity,easy dissolution and migration,as well as a huge annual emission amount.If failed to handling properly,it is easy to threaten and damage the ecological environment and human health.Therefore,finding an economical and efficient Cr(?)treatment method is an urgent issue.The technology of visible light photocatalysis has the features of economy and high efficiency,which has a good application prospect.Meanwhile,Bi₂WO₆-based visible light photocatalyst shows great potential in removing Cr(?)for its unique crystal structure and excellent photoelectric properties.However,influenced by the high recombination rate of photon-generated carrier,low reduction ability of photo-induced electron and narrow absorption range of visible light,the single Bi₂WO₆ limits the activity of photocatalytic reduction of Cr(?).In addition,complex water conditions,including the p H and organic matters will also influence the photocatalytic treatment effect of Cr(?).Regarding the above problems,standing at the views of accelerating the separation and transmission speed of photon-generated carrier,improving the reduction capacity of the catalyst,reducing the energy band gap and enhancing the utilization of visible light,the thesis constructed the reduction systems of Cr(?)with Bi₂WO₆-based visible light photocatalyst by adjusting the electron transmission path,energy band structure and morphology structure by means of semiconductor recombination and ion doping.Furthermore,combining with·CO₂^-produced in the reaction system,the goal of strengthening the reduction of Cr(?)with Bi₂WO₆-based visible light photocatalyst is finally achieved.In addition,a preliminary prediction method for the reduction rate of Cr(?)in the presence of typical organic compounds in the photocatalytic system was established through quantitative structure-activity relationship(QSAR)analysis,which theoretically guides the practical application of photocatalytic technology.The main research results of the thesis are as follows:(1)The construction and mechanism research of the system of photocatalytic reduction of Cr(?)with Sn O₂/Bi₂WO₆with electron transport energy level platform.Sn O₂/Bi₂WO₆photocatalytic materials with electron transport energy level platform were successfully synthesized by simple route of solvothermal-calcination.Ball-flower structure assembled by nano sheets were formed by the catalyst combined by Sn O₂and Bi₂WO_6,which increased the specific surface area and the number of surface adsorption sites.The mechanism research showed that the effective energy level platform between Sn O₂and Bi₂WO₆accelerated the separation and transfer of photon-generated carrier.The narrowing of band gap will enhance the response to visible light,which effectively enhanced the performance of the catalytic system for the reduction of Cr(?).The experiment results showed that 7Sn O₂/Bi₂WO₆ had the best photocatalytic performance,showing a high efficiency of 98.21%of Cr(?)photocatalytic reduction within 50 min,and its kinetic constant was 4.1 times that of pure Bi₂WO₆,and the catalytic reduction capacity of Cr(?)was 0.39 mg/(g·min).Finally,Cr(?)was reduced to Cr(?)and formed a stable and easily bioabsorbable complex with citric acid,the sacrificial agent of the system.(2)The construction and mechanism analysis of photocatalytic Cr(?)reduction system with direct Z-scheme PPy/Bi₂WO₆.The conductive polymer material PPy was deposited on the surface of Bi₂WO₆ by method of in-situ polymerization deposition,which constructed a direct Z-scheme PPy/Bi₂WO₆ heterostructure photocatalytic system.The results of work function and Fermi energy(E_f)showed that the internal electric field(IEF)was generated at the interface between PPy and Bi₂WO₆,which can drive the transmission of photon-generated electrons in Z-scheme,so as to maximize the reduction capacity of the catalyst while effectively separating carriers.In addition,the excellent physical and chemical properties of PPy enhanced the absorption capacity of the catalyst to visible light,increased the specific surface area and accelerated the carrier transmission speed.The experiment results showed that2PPy/Bi₂WO₆ had the best Cr(?)reduction performance,the reduction efficiency of Cr(?)can reach 99.70%within 15 min,and the kinetic constant was as high as 0.221min^-1,which was 19.2 times that of pure Bi₂WO₆ The photocatalytic reduction capacity of Cr(?)by 2PPy/Bi₂WO₆ was 4.43 mg/(g·min).In addition,based on the complexity of the actual water body,the reduction of Cr(?)in the presence of several common drugs and individual protection organic pollutants was investigated.The method for predicting the reduction rate of Cr(?)in the presence of typical organic compounds in this system was proposed by establishing the regression equation by QSAR analysis.(3)The construction and mechanism analysis of photocatalytic Cr(?)reduction system with direct Z-scheme Fe³⁺-Bi₂WO₆/PPy.The composite photocatalyst is prepared by the method of solvothermal-calcination-in-situ deposition,where Fe³⁺replaced Bi³⁺in the lattice,thus introducing the impurity energy band on the top of the valence band,which effectively shortened the band gap,significantly increased the width of the valence band tail,and expanded the absorption range and response ability to visible light.In addition,Fe³⁺changed the growth morphology of Bi₂WO₆and formed a hollow ball-flower structure,increasing the specific surface area,and improving the visible light utilization efficiency.The results of work function and E_f showed that the surface deposition of PPy produced IEF at the contact interface,induced the transmission of photo-generated electrons in Z-scheme,which made the carriers separated effectively and the reduction capacity of the catalyst enhanced.In addition,under illumination,Fe³⁺in the catalyst lattice will undergo a ligand-to-metal electron transfer process with citric acid,the sacrificial agent in the system,resulting in·CO₂^-with strong reduction ability,which strengthened the reduction performance of system.Under the optimum condition,the photocatalytic reduction efficiency for Cr(?)can reach 98.24%and the kinetic constant was 0.275 min^-1 within 10 min,which was 27.5 times that of pure Bi₂WO₆.The photocatalytic reduction capacity of the photocatalyst was 6.55 mg/(g·min),which was significantly higher than the reported Bi₂WO₆-based photocatalyst.In addition,the results of QSAR analysis showed that the method had a perfect applicability in predicting the reduction rate of Cr(?)in the presence of typical organic compounds.By controlling the electron transportation path,morphology and energy band structure of Bi₂WO₆-based visible photocatalyst,the thesis constructed the Cr(?)reduction system with Bi₂WO₆-based visible photocatalyst,and made the system produce·CO₂^-with strong reduction ability,which significantly strengthened the ability of photocatalytic reduction of Cr(?).The mechanism of the system to strengthen the reduction of Cr(?)was also explains.The results provide a new strategy and research basis for the design and construction of the system of reduction Cr(?)with Bi₂WO₆-based photocatalytic with higher activity,which provide a feasible method for the prediction of photocatalytic reduction rate of Cr(?)in the presence of typical organic compounds.