Preparation And Photocatalytic Properties Of SiW₁₁Fe@δ-Bi₂O₃ Z-Scheme Heterojunction And Doped δ-Bi₂O₃

Polyoxometalates（POMs）have rich and diverse in structure and composition,and exhibit good redox activity and excellent electron and proton transport capabilities.HOMO and LUMO energy levels of POMs are similar to the valence and conduction bands of semiconductors,therefore POMs can be regarded as a class of quasi-semiconductors with certain photocatalytic properties.However,most POMs are easily soluble in water and difficult to recycle and reuse;their bandgaps are basically located in the ultraviolet region,and the absorption of light is poor.Loading POMs on appropriate insoluble active supports can not only solve the problem of POMs recovery,but also significantly improve the catalytic activity of the catalyst due to the synergistic effect.δ-Bi₂O₃ is one of bismuth-based semiconductors,cheap and easy to prepare.It has a narrow band gap,and is a good candidate to form Z-scheme heterojunctions with energy-level matched POMs.Doping is an effective means to improve the photocatalytic activity of semiconductors,which can adjust the energy level structure and introduce oxygen vacancies.The main works of this paper are as follows:1.Under solvothermal conditions,insoluble SiW₁₁Fe salts were supported onδ-Bi₂O₃ to prepare SiW₁₁Fe@δ-Bi₂O₃ composites with core-shell structure,and their photocatalytic nitrogen fixation properties were investigated.According to the energy band structure analysis ofδ-Bi₂O₃ and Si W₁₁Fe and electron paramagnetic resonance spin trapping experiments,it was proved that the insoluble Si W₁₁Fe salt andδ-Bi₂O₃ form a Z-scheme heterojunction.In the Si W₁₁Fe@δ-Bi₂O₃ composite,[Ru（bpy）₃]²⁺was introduced into the composite in the form of counter cation,which also acted as a photosensitizer and improves the utilization of light;Si W₁₁Fe also acted a role of electronic"reservoir"which provide sufficient electrons for photocatalytic nitrogen fixation.Through EPR and N₂-TPD experiments,it was proved that the introduction of insoluble Si W₁₁Fe salt caused the increase of oxygen vacancies on the surface ofδ-Bi₂O₃,and the chemisorption of nitrogen was significantly improved.All of these advantages make Ru_2.5Si W₁₁Fe@δ-Bi₂O₃ exhibit excellent photocatalytic performance.Under simulated sunlight irradiation,the formation rate of NH₃/NH₄⁺is 121μmol·g^-1_cat·h^-1 in pure nitrogen atmosphere,while the formation rate of NH₃/NH₄⁺is 89μmol·g^-1_cat·h^-1 in air atmosphere.The effect of the presence of oxygen on nitrogen reduction was investigated,and the yields of the products NH₃/NH₄⁺and H₂O₂ were determined when gas mixtures with different V（N₂）:V（O₂）ratios were used as feedstocks.The results show that oxygen reduction is more susceptible to photocatalyst reduction ability than nitrogen reduction.2.Different metals(Mn²⁺,Fe³⁺,Sm³⁺and Gd³⁺)dopedδ-Bi₂O₃ were prepared,and their catalytic performances in photocatalytic nitrogen reduction and degradation of water pollutants were investigated.The catalytic activity of the four metal doped catalysts is Gd³⁺>Sm³⁺>Fe³⁺>Mn²⁺.Among them,when the doping ratio is 0.05,it is the optimal doping ratio;δ-Bi_1.95Gd_0.05O₃ has the best catalytic activity.Under simulated solar irradiation,the NH₃/NH₄⁺yield ofδ-Bi_1.95Gd_0.05O₃ is 93μmol·g^-1_cat·h^-1.Under simulated solar irradiation for 3.5 h,the degradation efficiency of methyl orange（MO）is 95%,and the reduction rate of Cr（VI）is 53%.Mechanism exploration experiments show that metal doping can both adjust the conduction band position and introduce oxygen vacancies,thereby enhancing the catalytic activity of semiconductors.All of these catalysts exhibit good stability,and there is almost no loss of activity after being recycled for five times.