Preparation Of Salen-COFs-Zn Heterojunction And Study On Photocatalytic CO₂ Reduction

The excessive dependence of human beings on fossil energy has caused the concentration of CO₂ in the atmosphere to exceed the warning line,causing serious greenhouse effect and environmental pollution problems.CO₂ conversion technology aiming at carbon reduction and carbon recycling is one of the ways to promote the realization of the“two-carbon”goal.Solar-driven CO₂ reduction（CO₂RR）technology is a green way of CO₂ conversion and utilization,the key of which lies in the research and development of efficient catalysts.Covalent organic frameworks（COFs）are a new type of long-range ordered crystalline porous materials,which are formed by the covalent bonding of organic molecules,and due to its structural porosity and adjustable energy band,high specific surface area,and excellent visible light absorption,it has become a new generation of photocatalytic CO₂ conversion active materials.However,the improvement of the photocatalytic activity of single-component COFs is still limited due to the weak electron-hole separation.In response to this scientific problem,in this paper,heterojunction catalysts were constructed with Salen-COFs-Zn（THFB-COF-1-Zn and THFB-COF-2-Zn）as the matrix,which effectively promoted the separation of charge carriers and improved the CO₂ reduction activity.The main research contents are as follows:The WO₃@Salen-COFs-Zn composite photocatalyst was prepared by in situ synthesis,and its Z-type carrier transport pathway was confirmed to promote the performance.Structural characterization proved that WO₃@THFB-COF-1-Zn and WO₃@THFB-COF-2-Zn heterojunction materials were successfully synthesized.Morphological characterization showed that WO₃ nanoparticles were uniformly wrapped by Salen-COFs-Zn.Photocatalytic CO₂RR experiments showed that WO₃@THFB-COF-1-Zn exhibited a maximum CO production of 273.0μmol·g^-1 in5 hours,which was 6.8 times that of THFB-COF-1-Zn,and WO₃@THFB-COF-2-Zn exhibited a maximum CO production of 373.3μmol·g^-1 in 5 hours,which was 5.4times that of THFB-COF-2-Zn.Optical and electrochemical characterization results show that the heterojunction can effectively promote electron-hole separation and migration.Electrons follow a Z-type transfer mechanism confirmed by X-ray photoelectron spectroscopy and in situ electron paramagnetic resonance analysis.Furthermore,a type II heterojunction photocatalyst of metal phthalocyanine and Salen-COFs-Zn was constructed by post-combination method,which combining the excellent visible light absorption of metal phthalocyanines and the advantages of planar metal active sites with Salen-COF-Zn,the CO₂ reduction activity was greatly improved.Structural characterizations demonstrate the successful synthesis of PcCo@THFB-COF-1-Zn and PcCo@THFB-COF-2-Zn heterojunction materials,morphological characterization shows that the heterojunction material is PcCo uniformly dispersed on the surface of COFs,avoiding phthalocyanine agglomeration and exposing the active sites of metal phthalocyanine to the greatest extent.Photocatalytic CO₂RR experiments showed that PcCo@THFB-COF-1-Zn exhibited a maximum CO production of 367.9μmol g^-1 in 5 hours,which was 49 times and11.7 times higher than that of pure PcCo and parent COF materials,respectively.PcCo@THFB-COF-2-Zn exhibited a maximum CO production of 692.2μmol g^-1 in5 hours,which was 92.3 times and 9.9 times higher than that of pure PcCo and parent COF materials,respectively.Optical and electrochemical characterizations demonstrate that the energy band structure of the heterojunction material meets the requirements for photocatalytic CO₂RR,and the electron-hole separation and electron transfer of the heterojunction material are more efficient.Using X-ray photoelectron spectroscopy and in situ electron paramagnetic resonance analysis,it was confirmed that electrons follow the type II transfer mechanism,and in situ infrared spectroscopy proved that Co²⁺of PcCo plays an important role in CO₂RR.