Photocatalytic Degradation Of Organic Pollutants By Polyoxometalates And N-doped Titanium Dioxide

Environmental pollution by chemicals has caused threatening to the existence human beings and to the long-term development of economics.Because of that,an efficient,low-cost and green way is urgently called for solve the environmental problem.Photocatalytic technique has great predominance over traditional catalytic method in treatment of chemical pollutions,since it can translate organic pollutants to CO₂ and inorganic ions at normal temperature and pressure only by using solar energy and molecular oxygen.However,due to some limitation,this technique has not been used in a large scale for practical application.Therefore,it is still very important to find the highly-efficient photocatalyst and to realize the reaction mechanism of the photocatalysis.In this thesis,recalcitrant textile dye X3B,phenol and dichromate are used as a model pollutant.Their redox reactions using polyoxometalates(POM)and N-doped TiO₂(N-TiO₂)as a photocatalyst are carried out.At the same time,the influencing factors are examined and related mechanism is discussed.Four chapters are included in this thesis.Chapter 1 is an introduction of semiconductor photocatalysis mainly about N-TiO₂ with visible light irradiation.Chapter 2 is an introduction of POM chemistry and the progress in POM photocatalysis.In chapter 3,N-TiO₂ was synthesized and sintered at different temperatures. Several techniques including XRD,XPS and ESR were used to study the structure properties and photocatalytic mechanism.Photodegradation of phenol was used to evaluate the catalyst photoactivity under UV and visible light irradiation,in the absence and the presence of NaF.Experiment result shows that the doped N exists in in the form of O-Ti-N and such doping enhances the stability of antase phase.For phenol degradation,the catalyst is active under either UV or visible light irradiation. The reaction is greatly affected by the sintering temperature,and involves the reactive species of·OH and O₂^·-.The presence of NaF in aqueous suspension results in great enhancement in the photoefficiency of TiO₂ and N-TiO₂ under UV irradiation.Chapter 4 is the main part of this thesis.Degradation of X3B and reduction of dichromate(Cr^â…¥)in an acidic aqueous solution were studied under 320 nm cut-off UV light irradiation in the presence of two polyoxometalates(POM),H₃PW₁₂O₄₀(PW) and H₄SiW₁₂O₄₀(SiW).The reactions in POM-X3B-Cr^â…¥system were faster than those in POM-X3B,POM-Cr^â…¥and X3B-Cr^â…¥systems.For all reactions,PW was more photoactive than SiW.The reaction rates were proportional to the initial concentration of each component.The effect of N₂,O₂ and air were small but regular, indicating Cr(â…¥)reduction by a reduced POM.Quenching experiments with H₂O₂ and ethanol revealed that X3B degradation mainly occurred through hydroxyl radicals (·OH).It is proposed that the production of·OH and a reduced POM by the reaction between H₂O and excited POM is the rate determining step,with which all evidence could be well interpreted.Different effect of POM concentration in a two and three-component system suggests that the reaction between H₂O and excited POM is reversible.