Preparation Of LDH-Based Porous Composites And Photocatalytic CO₂ Reduction Properties

With the rapid development of industrialization and the continuous overuse of fossil fuels,energy crisis and environmental problems have become two major challenges.The consumption of fossil fuels releases a large amount of CO₂ gas,resulting in global warming.Photocatalytic technology can effectively use abundant solar energy to turn CO₂into valuable fuels and other fine chemicals.However,CO₂ has a high bond energy(750k J·mol^-1),showing chemical inertia and thermal stability,so it is necessary to prepare suitable photocatalysts to promote the occurrence of CO₂ reduction reaction.Layered double hydroxide（LDH）is a two-dimensional（2D）layered inorganic multifunctional compound.It has attracted more and more attention in the field of photocatalysis because of its highly uniform distribution of metal cations in the laminate,interlayer anion exchangeability,visible light response,strong alkalinity and adsorption.However,the photocatalytic activity of single LDH is limited by the rapid recombination of photogenerated carriers and the poor ability of CO₂ adsorption and activation,so it is difficult to be directly applied to photocatalytic CO₂ reduction.Therefore,based on LDH,this paper designed and synthesized catalysts with high efficiency,high selectivity and high stability by means of morphology regulation,changing the surface oxygen defects of materials,carbon based materials and photosensitive material composite,so as to improve the photocatalytic CO₂ reduction efficiency.The main research contents of this paper are as follows:（1）CoAl-LDH ultra-thin hexagonal nanosheets were prepared by hydrothermal means using cobalt and aluminum metal salt solution as precursors,and CoAl-LDH/CdS composites were in situ synthesized.The experimental results show that the composite CoAl-LDH/CdS catalyst shows better performance than the single CoAl-LDH and CdS catalyst.Notably,the CoAl-LDH/CdS-1 showed a highest CO production rate of 7.80μmol·g^-1·h^-1,which was 4.2 and 2.5 times those of pure CoAl-LDH and bare CdS,respectively,and the production rate of CH₄ was 3.81μmol·g^-1·h^-1.After three cycle tests,CoAl-LDH/CdS-1 catalyst could maintain 85%of the original photocatalytic activity and showed a good photostability.The close contact between CoAl-LDH nanosheets and CdS nanoparticles forms a rich coupling interface,which accelerates the metastasized of photogenerated carriers,greatly reduce the recombination of photogenerated charges,enhance the utilization of visible light,and improve the photocatalytic efficiency.（2）Using nickel and aluminum metal salt solution as precursor,NH₄F as morphology control agent,urea as precipitator,two dimensional NiAl-LDH flower nanosheets were prepared by hydrothermal method,and NiAl-LDH/CeO₂ composite was in situ synthesized.The experimental results show that the composite NiAl-LDH/CeO₂catalyst has better performance than the single NiAl-LDH and CeO₂ catalyst.Notably,the NiAl-LDH/CeO₂-20 composite showed a highest CH₄ production rate of 16.33μmol·g^-1·h^-1,which was 3.8 times that of pure CeO₂.The NiAl-LDH/CeO₂ catalyst showed excellent stability and CH₄ selectivity.After four cycles,the NiAl-LDH/CeO₂-20catalyst retained 82%of the original photocatalytic activity and the CH₄ selectivity of the system can reached 90%.Fine CeO₂ nanoparticles are deposited and coated on NiAl-LDH flower like nanosheets,with a smoother spherical morphology.The highly dispersed metal cations in the NiAl-LDH structure served as CeO₂ charge separation center,which accelerates the separation of photogenerated electron-hole pairs and improves the photocatalytic efficiency.（3）C₃N₄ nanosheets were synthesized by heat shrinkage method,and NiAl-LDH/C₃N₄ ultra-compact sheet heterojunction were prepared by hydrothermal in-situ synthesis.The NiAl-LDH/C₃N₄/GA composites was prepared by introducing graphene aerogels（GA）into network frameworks.The results showed that the composite catalysts exhibited better performance than the single NiAl-LDH and C₃N₄ catalysts.With the increase of C₃N₄ content,the catalytic performance of NiAl-LDH/C₃N₄/GA composites first increased and then decreased.When the C₃N₄ loading was 20%,the photocatalytic activity of CO₂ reduction was the highest,and the CO production rate of 28.83μmol·g^-1·h^-1,which was 24 and 16 times of that of pure NiAl-LDH and pure C₃N₄,respectively.The composite NiAl-LDH/C₃N₄/GA-20 catalyst also showed strong photostability,and could maintain 91%of the original photocatalytic activity after four cycles.An ultra compact sheet heterojunction was formed between NiAl-LDH and C₃N₄,which reduces the photoinduced charge recombination.As an electronic medium and winding agent,GA can further promote the charge transfer and coupling interface adhesion of the heterojunction,so as to enhance the separation and transportation of electron-hole pairs and improve the photocatalytic efficiency.