Preparation Of Crystalline Porous Framework Composite Materials And Application In Photocatalytic CO₂ Reduction

With the massive consumption of fossil fuels,an increasing CO₂ concentration gathered in atmosphere,as well as the convertion and utilization of CO₂ becomes one of the urgent issues for current society.Among various CO₂ conversion methods,photocatalysis is considered to be the most reasonable routes due to its no need for secondary energy.Besides,the investigation of photocatalysts with highly efficient and stable is the critical technique for photocatalysis CO₂ conversion.Currently,COFs have shown great application value in the field of photocatalytic CO₂ reduction due to their excellent light absorption capacities,large specific surface areas and adjustable pore structures.However,the extremely high photoelectron-hole recombination rate and following low electron utilization existed in crystalline porous frameworks seriously limits their photocatalyst performances.To solve above problems,two kinds of crystalline porous framework were combined to form the type II heterojunction with increasing electron utilization and CO₂ reduction activity in this research.The main research contents are as follows:（1）In order to improve the utilization of photo-generated electrons,the type II heterojunction was constructed and applied to photocatalysis CO₂ reduction.The COF-316NSs-1 and THFB-COF-Zn NSs were synthesized via ultrasonic method and combined by hybrid rotary evaporation method to form the nanosheets-like type II heterojunction.The structural characterizations confirm the successful combination of composite materials with unbroken crystal forms and functional groups.The morphology characterizations confirm the nanosheet morphology of the material.The energy level characterization suggest that the band gap of above materials satisfies the conditions for type II heterojunction constructing.The gas-liquid phase photocatalytic CO production of COF-316/THFB-COF-Zn（5:5）is 95.9μmol·g^-1·h^-1,approximately2.8 and 8.46 times higher than that of COF-316NSs-1 and THFB-COF-Zn NSs,respectively.The mechanism characterizations confirm that the construction of COF-316/THFB-COF-Zn nanosheet heterojunction can ensure both electrons and holes migrate according to the designed pathway and avoid the recombination.Base on above characterization and test results,we can sure that the construction of the nanosheets-like type II heterojunction is beneficial to the improvement of the electron-hole separation rate and the catalytic activity,but the electron transport resistance between the interface is expected to be further reduced.（2）In order to further improve the electron transport efficiency,the nanosheets-like type II heterojunction with coordination bonds was constructed and applied to the photo-thermal catalytic CO₂ reduction.The COF-316NSs-2 were synthesized by metal coordination method and modified by nickel ions.Subsequently the as-prepared nanosheets were added into the synthetic system of CAT-1 to form COF-316/CAT-1epitaxial composite material.The structural and morphology characterizations confirm the successfully combination between two components by coordination bonds and nanosheets epitaxial structure,respectively.The energy level characterization suggest that composite materials meet the thermodynamic conditions of CO₂ reduction.The gas-solid phase photo-thermal catalytic CO production of COF-316/CAT-1（12:1）material is 261.9μmol·g^-1·h^-1,which is 30μmol·g^-1·h^-1 higher than that of COF-316+CAT-1（12:1）without coordination bonds.The mechanism characterizations show that the coordination connection can serve as a bridge for transporting electrons,and effectively reducing the transmission resistance of the heterogeneous interface.Base on above characterization and test results,we can sure that the coordination bonds can effectively promote the electrons transmissing,reduce the transmission resistance between interfaces,and further enhance the catalytic activity of nanosheets-like type II heterojunction.