Study On Controlled Preparation And Enhanced Fenton Performance Of Fe-MOFs@Porphyrin-Based COFs Type Photocatalysts

The rapid growth of globalisation and industrialisation has led to serious water pollution problems,but as the natural environment is not capable of remediating itself,there is an urgent need to find efficient and economical ways of treating water.The Fenton reaction is widely used in water pollution treatment because of its ability to produce hydroxyl radicals（·OH）with strong oxidising power,and the photo-Fenton technology has become a hot topic of research due to its economic and environmental sustainability.In this technology,the photocatalyst is the core of photoconversion,but the defects of the photocatalyst such as narrow absorption range,easy compounding of photogenerated carriers and poor stability seriously limit the performance of the reaction.Therefore,research on photo-Fenton catalysts that can improve photoconversion efficiency,photogenerated electron utilisation efficiency and catalytic stability has become the focus of photo-Fenton research.In recent years,new porous materials have been widely used in the field of photofenton to treat water pollution.Among them,Fe-based metal-organic framework materials（Fe-MOFs）are organic-inorganic hybrid porous crystalline materials formed by metal ions or metal clusters connected with organic ligands through coordination bonds,which have large specific surface area and rich pore structure and can accelerate the mass transfer process of the reaction.They are flexible and can be modified by doping or loading to inhibit the compounding of photogenerated carriers and increase the rate of the photo-Fenton reaction.Covalent organic skeletons（COFs）,another new porous material of great interest,are characterised by abundant building units,large surface area,long-range ordered pore structure and high light absorption capacity.Among them,COFs based on porphyrin ligands exhibit superior optical properties due to their layered stackedπ-πconjugated structures forming conduction pathways and accelerating electron transport.Therefore,they also have great potential for application in the field of photo-Fenton treatment of water pollution.In this thesis,we prepared Fe-MOFs@COFs,a core-shell material with NH₂-MIL-101（Fe）covalently bonded to porphyrin-based COFs with tetra-（p-aminophenyl）porphyrin（H₂TAPP）as one of the ligands,and used them for the photo-Fenton catalytic degradation of pollutants.Details of the study are as follows:（1）Preparation,properties and catalytic mechanism of Fe-MOFs@COFs（NH₂-MIL-101（Fe）@TAPP-TFP-COFs）composite photocatalysts.Fe-MOFs with octahedral structure were synthesized by hydrothermal method,and Fe-MOFs@COFs core-shell materials were prepared by encapsulating COFs in Fe-MOFs using solvothermal method.The structure,morphology and photoelectrochemical properties of the material were characterised using XRD,XPS,SEM,TEM,UV-vis DRS,and the photo-Fenton properties of the composites were verified by tetracycline hydrochloride degradation experiments.The results show that the Fe-MOFs@COFs core-shell material retains the octahedral structure of Fe-MOFs but possesses better photoelectrochemical properties and electron transfer efficiency due to the composite of COFs based on porphyrin ligands.The Fenton-catalyzed degradation of tetracycline hydrochloride by Fe-MOFs@COFs-2 was best achieved by modulating the core-shell thickness of the porphyrin-based COFs,reaching 90.8%degradation within 45 min.Fe-MOFs@COFs-2 still showed high catalytic activity and good photochemical stability after three cycles.The catalytic mechanism of the photocatalytic degradation of pollutants is investigated by XPS test and active radical detection.The results show that the COFs shell layer is effective as an electron transport shell layer to transport electrons to the catalytic centre for catalytic degradation of pollutants.The construction of Fe-MOFs@COFs core-shell material promotes electron transport and improves the efficiency of photo-Fenton catalysis.（2）Preparation,properties and catalytic mechanism of Fe-MOFs@COFs-Cu composite photocatalysts.The Fe-MOFs@COFs-Cu composites were obtained by introducing Cu metal sites in the centre of the porphyrin ring of the COFs core-shell using a solvothermal method.The structural,morphological and electrochemical properties of the materials were characterized by XRD,XPS,SEM,TEM,UV-vis DRS and electrochemical tests.The analysis of the results showed that the degradation efficiency of Fe-MOFs@COFs-Cu composites was 92.7%for tetracycline hydrochloride within 45 min.the degradation effect of Fe-MOFs@COFs-Cu composites did not decrease significantly after three cycles of stability tests and had good chemical stability.The analysis shows that the central metal-free Fe-MOFs@COFs core-shell material in The COFs shell layer simply acts as an electron transport channel for photogenerated electron transfer to Fe-MOFs.After the introduction of Cu metal sites,the metal ions in the porphyrin ring form a metal coordination.The metalization of Fe-MOFs@COFs core-shell material can significantly enhance the electron migration rate while maintaining the structure of the core-shell material.The metalization of Fe-MOFs@COFs core-shell materials can significantly enhance the electron migration rate of the materials and further improve the efficiency of photo-Fenton-catalyzed degradation of TC.（3）Preparation,properties and catalytic mechanism of M@C-200℃-xh（x=1,2,3,4）composite photocatalysts.The M@C-200℃-xh materials were obtained by activating Fe-MOFs@COFs composites at 200℃using vacuum thermal activation method.The structure and morphology of the materials were characterised using XRD,IR spectroscopy and transmission electron microscopy.The key chemical properties of the composites are characterised by spectroscopic tests as well as electrochemical tests.The results show that the H₂O molecules around the Fe metal skeleton in Fe-MOFs@COFs core-shell materials are eliminated in the vacuum thermal activation reaction,which exposes more active sites in the materials and enables more contact of H₂O₂with the Fe³⁺/Fe²⁺reaction centre,further improving the photo-Fenton catalytic degradation of TC efficiency.Among them,the M@C-200℃-1h composite can complete 91.2%of the degradation of pollutants within 45 min,which is more efficient than the other samples of activation time.M@C-200℃-1h shows a smaller charge transfer resistance and higher photoresponse current,demonstrating that vacuum thermal activation can improve the photo-Fenton performance of the M@C-200℃-1h composite.