Preparation And Photocatalytic Properties Of Three-dimensional Porous Al₂O₃@SiC/g-C₃N₄ Composite Materials

Semiconductor photocatalysts have broad application in the fields of energy and environment.Silicon carbide（SiC）is a non-metal semiconductor photocatalyst.It has good chemical stability and thermal stability.In particular,SiC nanoparticles are beneficial to improve photocatalytic activity due to their high specific surface area.However,SiC nanoparticles as photocatalysts still face the following problems:First,they are easy to agglomerate in liquid phase reaction due to their high surface energy from small size effect,resulting in the reduction of active sites,and they are not conducive to sedimentation for separation and recycling;second,SiC nanoparticles are highly hydrophobic,which is not conducive to contact with the reactants in the liquid;third,like all single-component photocatalyst,photogenerated electron-hole are easy to recombine during photocatalytic reactions.In this thesis,three-dimensional（3D）porous Al₂O₃@SiC/g-C₃N₄ composites were prepared using polymer-assisted vacuum freeze-drying,calcination,and gas-solid reaction.The high specific surface area of the 3D porous structure is used to increase reactive sites,and the multi-level pore structure enhances the mass transfer characteristics of the reaction,thereby improving the catalytic reaction activity;at the same time,the three-dimensional structure characteristics are used to improve the separation and reuse performance.Combining hydrophobic SiC powder with hydrophilic g-C₃N₄ and Al₂O₃ improves the hydrophilicity of the composite catalyst.Constructing heterojunctions of SiC and g-C₃N₄further improves the separation efficiency of photogenerated electron and hole pairs.The main content is as follows:（1）Preparation of 3D porous Al₂O₃@SiC composites.First,3D porous polyvinyl alcohol（PVA）/aluminum isopropoxide(C₉H₂₁Al O₃)/SiC nanoparticle composites were obtained by freeze-drying the precursors of PVA/C₉H₂₁Al O₃/SiC aqueous solution.Then,3D porous Al₂O₃@SiC composites were obtained by calcination 3D porous PVA/C₉H₂₁Al O₃/SiC.The loading content of SiC is adjusted by varying the content of SiC nanoparticles in the precursor solution.The specific surface area of Al₂O₃@SiC composite could reach 361.76 m²?g^-1,which is greater than that of SiC nanoparticles(30.76 m²?g^-1).As the loading of SiC increases,the composite’s specific surface area and hydrophilicity are decreased.As the calcination temperatures increase,they still maintain 3D porous structures,but their specific surface area gradually decreases.Photocatalytic performance of 3D porous Al₂O₃@SiC for the degradation of rhodamine B（Rh B）under ultraviolet light.The study has found that as the loading of SiC nanoparticles increases,the degradation rate of Al₂O₃@SiC composites first increases and then decreases,possibly because that its specific surface area and hydrophilicity decrease as the loading of SiC nanoparticles increases.As the calcination temperature increases,the degradation rate of the Al₂O₃@SiC composite is also decreased due to its reduced specific surface area and reactive sites.At the same time,the 3D porous structure of Al₂O₃@SiC composite also prevents the agglomeration of SiC nanoparticles in a liquid phase reaction.The composite can be easily separated and recovered by natural precipitation.After four cycles,the samples still maintains a degradation efficiency of 86%.（2）Preparation of 3D porous Al₂O₃@SiC/g-C₃N₄ composite material.The 3D porous Al₂O₃@SiC/g-C₃N₄ composite was obtained by in-situ growth of g-C₃N₄ nanosheets on the3D porous Al₂O₃@SiC composite using urea as the reaction source through a gas-solid reaction method.The study has found that SiC nanoparticles in Al₂O₃@SiC/g-C₃N₄composites have almost no effect on the loading content of g-C₃N₄.The obtained Al₂O₃@SiC/g-C₃N₄ composites all show good hydrophilic properties,and their hydrophilicity decreases slightly when the loading content of SiC nanoparticles increases.The photocatalytic performance of 3D porous Al₂O₃@SiC/g-C₃N₄ for the degradation of Rh B.The study has found that the reaction rate constants of Al₂O₃@SiC-20/g-C₃N₄ composites are 22 times and3.5 times that of Al₂O₃@SiC-20 and Al₂O₃@g-C₃N₄ composite nanomaterials under AM 1.5light irradiation.The studies on photocurrent,photoluminescence,and transient photoluminescence lifetime further prove that the heterojunction system has higher charge separation performance.The capture experiments and the electron spin resonance tests show that superoxide radicals and photogenerated holes play a major role in the degradation process.Due to its 3D porous structure,3D porous Al₂O₃@SiC/g-C₃N₄ can be easily separated and recovered by natural precipitation.They also show stable photocatalytic performance after four cycles.