Preparation And Photocatalytic Properties Of Polyoxometalate And Bi-Based Semiconductor Heterojunctions

Polyoxometalates（POMs）are widely used in materials chemistry and catalysis due to their acidic properties and excellent redox properties.However,most POMs are easily soluble in polar solvents,which makes it difficult to separate,recycle and reuse.And there are also other problems to be solved,such as p H limits for the reaction system and the catalytic activity should be improved.Rational design and synthesis of high activity,high stability and recyclable composite polyoxometalate catalyst is an effective way to solve these problems,a challenging task for polyoxometalate chemists.Ammonia is an important raw material for synthesizing fertilizers,explosives and pharmaceuticals.It is of great academic significance and application prospect to study the photocatalytic reduction of N₂ to NH₃ under mild conditions.In this paper,insoluble RuSiMo₁₂ are combined with Bi-based semiconductors（Bi₂MoO₆,δ-Bi₂O₃）with suitable band gap structure to prepare POMs/Bi-based semiconductor composite photocatalysts.Their photocatalytic properties were investigated by photocatalytic nitrogen reduction reaction.The main works are as follows:1.A new heterojunction composite photocatalyst[Ru（bpy）₃]₂SiMo₁₂O₄₀/Bi₂MoO₆(RuSiMo₁₂/Bi₂MoO₆)was prepared by a solvothermal method.Relevant experiments confirmed that the as-prepared binary composites formed a Z-scheme heterojunction.The construction of the Z-scheme heterojunction not only effectively promoted the separation of photogenerated electron-hole pairs,but also retained the strong redox properties of the two semiconductors.The introduction of[Ru（bpy）₃]²⁺units effectively improved the absorption of visible light of the catalyst;the introduction of POMs increased the oxygen vacancies of Bi₂MoO₆.The photocatalytic activity of the catalyst was the highest when the RuSiMo₁₂ loading was 15 wt.%.Under simulated sunlight for 2 h,the production of NH₃/NH₄⁺can reach 62.7μmol·g^-1_cat,which is about twice that of the single component Bi₂MoO₆(33.1μmol·g^-1_cat).The activity of the photocatalyst was almost not lost after 5cycles of use.These results indicate that the composite has good photocatalytic activity and stability.2.A new Z-scheme heterojunction[Ru（bpy）₃]₂SiMo₁₂O₄₀@δ-Bi₂O₃(RuSiMo₁₂@δ-Bi₂O₃)composite photocatalyst was prepared by a hydrothermal method.The prepared binary composite exhibits excellent charge separation property,enhanced photocurrent response,and significantly reduced charge transfer resistance.The experimental results show that the RuSiMo₁₂ catalyst loaded with 10 wt.%has excellent photocatalytic performance.Under simulated sunlight for 2 h,the production of NH₃/NH₄⁺reach 257.6μmol·g^-1_cat,which is twice that ofδ-Bi₂O₃(122.9μmol·g^-1_cat).The results of five cycle experiments show that the RuSiMo₁₂@δ-Bi₂O₃ composites also exhibit good stability and activity.The photocatalytic activity of the catalyst for the redox reaction of NO₃^-and NO₂^-was preliminarily explored.In the absence of sacrificial agent,33.1%NO₃^-is converted,and nitrite conversion is up to 98%after simulate sunlight exposure for 2 h.