Study On The Modification Of Bi₂WO₆ And Its Photocatalytic Performance For Selective Oxidation Of Toluene

Benzaldehyde is an important chemical intermediate,and the photocatalytic selective oxidation of toluene to benzaldehyde has important research significance.The development of photocatalysts with excellent performance is the focus of this reaction.Bismuth tungstate（Bi₂WO₆）,which is a widely studied photocatalyst among scientists because of its unique properties such as easily available,good visible light absorption capacity,suitable band gap structure and appropriate oxidation properties.However,Bi₂WO₆ still has shortcomings such as severe photo-generated carrier recombination and poor stability.Modification of Bi₂WO₆-based photocatalysts by doping,compounding with other semiconductors and surface modification can further improve its photocatalytic performance.In this thesis,Bi₂WO₆ was modified by two different methods,namely,non-noble metal ion doping and compounding with other semiconductor materials.The obtained photocatalysts were used in the selective oxidation of toluene into benzaldehyde under visible light irradiation.The morphology,structure,optical and electrochemical properties of the modified Bi₂WO₆ were characterized by SEM,XPS,ESR,ICP,TEM,XRD,PL,photocurrent measurement and other technical methods were involved to study their physico-chemical properties and the structure-activity relationship between the photocatalyst and the reaction performance,the possible photogenerated carrier transfer path and the reaction mechanism were proposed.The main research content and innovative of this paper are as follows:1)Iron-doped bismuth tungstate（Fe-Bi₂WO₆）photocatalyst.A mild hydrothermal method was developed to synthesize Fe-Bi₂WO₆ which assembled from nanosheets.Paritially replacing W⁶⁺by Fe³⁺is beneficial for the formation of oxygen vacancie,which not only increase the separation and migration of photogenerated carriers,but also enhance the ability of the catalyst to activate oxygen.When the photocatalysts were used in the selective oxidation of toluene to benzaldehyde,the Fe（0.26）-BWO with 0.26%of Fe³⁺performed the best formation with a benzaldehyde formation rate up to 1303.8μmol·g^?1·h^?1 after 5 hours of visible light irradiation.The existence of oxygen vacancies was confirmed by characterizations such as XPS and ESR.Theoretical calculation results show that the formation energy of oxygen vacancies reduced after Fe³⁺doping,suggesting the formation of oxygen vacancies in Bi₂WO₆was promoted by Fe³⁺.The oxygen activation experiment results showed that the iron-doped Bi₂WO₆ has a stronger ability to activate oxygen to superoxide radicals.The doping of iron supports the flower-shaped bismuth tungstate to a certain extent and can transfer part of the active sites,so the stability of Bi₂WO₆ is improved.2)Oxygen vacancy and heterojunction rich Cs₃Bi₂Br₉-Fe-Bi₂WO₆ photocatalyst.Fe-Bi₂WO₆ nanoflowers were synthesized through a hydrothermal method according to our previous work and Cs₃Bi₂Br₉ was in-situ formed on Fe-Bi₂WO₆ according to an anti-solvent method.The obtained composite not only retains the advantages of oxygen vacancies,but also forms a tight heterogeneous interface,which is conducive to the rapid separation and transfer of photogenerated carriers.The composite material showed good activity for the photocatalytic selective oxidation of toluene to benzaldehyde.Under the synergistic effect of heterojunction and oxygen vacancies,the composite with 10%of Cs₃Bi₂Br₉（0.1Cs₃Bi₂Br₉-Fe-Bi₂WO₆）showed an excellent benzaldehyde formation rate of 3286.0μmol·g^?1·h^?1 after 3 hours of visible light irradiation.The formation rate is 2.3 and 2.5 times as high as pristine Cs₃Bi₂Br₉ and Fe-Bi₂WO₆ respectively.Moreover,the stability of the composite photocatalyst was improved to some extent because the recombination of Cs₃Bi₂Br₉ changes the chemical environment of Fe-Bi₂WO₆.In addition,The composite catalyst has good universality for the selective oxidation of toluene derivatives.Finally,through free radical capture experiments,the possible reaction process was revealed.