Preparation Of Bismuth-based Semiconductor Photocatalyst And Their Performance In Photocatalytic Organic Synthesis

How to solve the problems of energy shortage and environmental pollution are two important issues in today's society.The photocatalytic technology of semiconductor is new that converts solar energy into chemical energy.This technology is mild,environmental friendly,and sustainable.It is an effective way to solve current energy and environmental problems.However,the photocatalytic activity of conventional semiconductor photocatalysts is low due to their poor utilization of visible light and the difficulty of separating photo-generated electrons and holes.Therefore,it is very necessary to find and construct photocatalysts with excellent catalytic performance.In this paper,bismuth based semiconductor photocatalysts with good catalytic potential are used.The catalytic activity of bismuth based semiconductor photocatalysts in photocatalytic organic synthesis reaction is improved by adopting morphology control,metal doping and heterostructure construction.The following aspects are included:?1?BiVO₄ photocatalysts with different morphologies and crystal structures are prepared.With Bi?NO₃?₃·5H₂O and NH₄VO₃ as precursors,three preparation methods?including sol-gel method,nitric acid method,N,N-dimethylformamide method?are selected to prepare BiVO₄ with leaf-like,microsphere and blocklike structure.The effects of pH value,reaction temperature and reaction time of the reaction solution on the BiVO₄ catalyst are compared during the preparation process.Various characterization methods are used to analyze the morphology,crystal structure,visible light absorption capacity and specific surface area of the prepared BiVO₄ catalysts.The catalytic properties of different catalysts for selective oxidation of benzyl alcohol are measured and the relationship between the morphology,crystal structure and photocatalytic properties of the catalysts are explored.The experimental results show that the specific surface area of monoclinic scheelite BiVO₄?ms-Bi VO₄? prepared by the N,N-dimethylformamide method is the largest,and the conversion rate of ms-BiVO₄ prepared by this method is the best in the benzyl alcohol selective oxidation to benzaldehyde reaction.The ideal supports of photocatalyst are prepared in this result.?2?Ag/Pd/ms-BiVO₄?2 wt%,2:1? nano-microsphere photocatalyst is prepared.Using the ms-BiVO₄ prepared by the nitric acid method as the support,a series of nano-microsphere composite photocatalysts are obtained by the NaBH₄ reduction method.The catalytic properties of Ag-Pd bimetallic composite materials in different loading sequences are investigated,including Ag/Pd/ms-BiVO₄ with Ag loaded on the support after Pd,Pd/Ag/ms-BiVO₄ with Pd loaded on the support after Ag,and Ag-Pd/ms-Bi VO₄ with Ag and Pd loaded on the support at the same time.The effects of different metal mass ratios and different loading amounts on reaction performance are investigated.The photocatalytic performance of Ag/Pd/ms-BiVO₄?2 wt%,2:1? is the best in the selective oxidation reaction of benzyl alcohol,which is much higher than that of pure ms-Bi VO₄.The catalytic performance of the photocatalyst Ag/Pd/ms-BiVO₄?2 wt%,2:1? in the selective oxidation experiment of 13 alcohol derivatives is explored.The improvement of the catalytic activity is based on the strong hydrogen absorption capacity of Pd and the surface plasmon resonance effect of Ag.In addition,Ag is more electronegative than Pd.When Ag is loaded on the surface of Pd,the transfer of electrons from Pd to Ag reduces the recombination rate of photogenerated electrons and holes in the catalyst.Combined with the capture experiment of the active species,the possible mechanism of the reaction is proposed.?3?The x-Bi₂WO₆/g-C₃N₄ photocatalyst with Z-scheme heterostructure are prepared.Flower-like microspheres Bi₂WO₆ and bulk g-C₃N₄ are prepared by the hydrothermal method and high-temperature calcining method,respectively.A method based on the combination of ultrasonic stripping and mechanical stirring is used to obtain uniform composite x-Bi₂WO₆/g-C₃N₄?x is the Bi₂WO₆ mass ratio? photocatalysts with a Z-scheme heterojunction.In the reaction of benzyl alcohol and aniline to form imine,the catalytic effect is the best when 30%-Bi₂WO₆/g-C₃N₄ is used as catalyst,and the conversion rate reached 87.6%,which is much higher than that of pure Bi₂WO₆ and g-C₃N₄.Characterization by SEM and TEM show that the ultrasonically stripped g-C₃N₄significantly reduced the radial dimension compared to bulk g-C₃N₄.The composite photocatalysts are dispersed uniformly and maintained the structure of Bi₂WO₆ flower-like microsphere.The characterization result of BET shows that the specific surface areas of the g-C₃N₄ nanosheets increase approximately ten times after ultrasonic stripping.The photostability of the catalyst is verified by cyclic experiments.The activation energy of the coupling reaction between benzyl alcohol and aniline decreases by 29.8 kJ·mol^-1 under visible light irradiation than that in the dark.A capture experiment is used to verify the active species in the photocatalytic system,and the reaction is mainly completed through the synergistic action of h⁺,e^-and ·O₂^-.?4?The bismuth-based binary heterostructure x-Bi₂O₂CO₃/Bi₂WO₆ photocatalyst are prepared.Two bismuth-based oxide semiconductor photocatalysts Bi₂O₂CO₃ and Bi₂WO₆ are prepared by hydrothermal method.The x-Bi₂O₂CO₃/Bi₂WO₆?x is the Bi₂O₂CO₃ mass ratio?photocatalysts with different composite ratios are obtained by the mechanical stirring composite method.The characterization results show that Bi₂O₂CO₃ and Bi₂WO₆ have a similar two-dimensional sheet structure,which ensures the possibility of forming a heterogeneous structure between the two materials.At the same time,the formation of the heterostructure improves the light absorption ability of the Bi₂O₂CO₃ semiconductor photocatalyst in the visible light region.In the selective oxidation reaction of benzyl alcohol,the photocatalytic performance of bismuth-based binary heterostructure x-Bi₂O₂CO₃/Bi₂WO₆photocatalyst is excellent,in which the conversion rate of 30%-Bi₂O₂CO₃/Bi₂WO₆ is the best.The capture experiment results show that the reaction is mainly completed by the synergy between h⁺,e^- and ·O₂^-.