Photocatalytic Properties Of Porphyrin-based And D-A Type Framework Materials

Photocatalysis has been proved to be a feasible way to address the climate change and energy crisis.Although inorganic semiconductors have made considerable progress in the field of photocatalysis,the development of new materials is still necessary to improve the catalytic performance.Frameworks such as metal organic frameworks(MOFs)and organic covalent frameworks(COFs)have attracted widespread attention due to their well-defined crystal structure and tunable properties.The energy level,electronic structure and surface dynamics characteristics can be easily regulated at the molecular level by chemical methods,thus providing a basis for clarifying the structure-property relationship and further improving photocatalytic performance.This thesis focuses on the structural design of framework materials(MOFs and COFs),constructing a series of porphyrin-based and D-A framework materials by regulating the active sites,energy bands,electronic structures and surface/interface effects to enhance the performance of photocatalytic H2 production and photocatalytic reduction of CO2.The main contents are as follows:(1)Porphyrin-based Metal-Organic Frameworks for Efficient Photocatalytic H2 Production under Visible-Light IrradiationA series of bifunctional porphyrin-based Zr-MOFs(PCN-H2/Ptx:y,x:y=4:1,3:2,2:3,0:1)containing different ratio of H2TCPP and Pt?TCPP(TCPP=tetrakis(4-carboxyphenyl)-porphyrinate)as isostructural ligands and Zr6 clusters as nodes were synthesized.Under visible light irradiation(??400 nm),PCN-H2/Pt0:1 shows higher average H2 production rate(HER,351.08?mol h-1 g-1 than PCN-H2/Pt2:3(308.54?mol h-1 g-1),PCN-H2/Pt3:2(24.44?mol h-1g-1)and PCN-H2/Pt4:1(6.40?mol h-1 g-1)at the same condition,which decrease along with lowing the ratio of Pt?TCPP in the PCN-H2/Ptx:y series.The differences in photocatalytic activity are attributed to more uniformly dispersed Pt2+ ions in PCN-H2/Pt0:1,which promotes the charge transfer from porphyrins(photosensitizers)to platinum(?)ions(catalytic centers).The PL measurements reveal that the photo-induced charge separation and transfer can be improved by increasing the ratio of PtTCPP in the PCN-H2/Ptx:y series,in turn leading to improved photoactivity for H2 generation.The bifunctional MOFs with photosensitizers and catalytic centers provide new insights for the design and application of porphyrin-based photocatalytic systems for the visible-light-driven H2 production.(2)The Synthesis and Visible Light Photocatalytic Hydrogen Production Performance of 2D D-A COFsA benzene-1,3,5-tricarbaldehyde(BT)and 4,4'-(benzo-1,2,5-thiadiazole-4,7-diyl)dianiline(BTDA)based two-dimensional(2D)donor-acceptor(D-A)COF,denoted as BT-COF,was fabricated and further modified by using an alternative electron-donating unit,2-hydroxybenzene-1,3,5-tricarbaldehyde(HBT),to the polycondensation reaction,yielding HBT-COF with enhanced internal D-A effect and hydrophilicity.Interestingly,the photocatalytic H2 production rate of HBT-COF reaches 19.00 ?mol h-1,which is 5 times higher than that of BT-COF(3.40 ?mol h-1)under visible light irradiation(?? 420 nm).The increase in photocatalytic activity of HBT-COF is rationally attributed to the finely tuned energy levels and the improved wettability,which in turn leading to the broadened visible light absorption,the efficient photo-induced charge separation and transfer,and the enhanced interactions between the COF catalyst and reaction substrates.The present results demonstrate that a subtle structural modification can significantly modulate the band structure and interfacial property,thus provide a feasible strategy for the design and optimization of COF-based photocatalytic hydrogen evolution systems.(3)Synthesis and Photocatalytic performance of porphyrin-based COFsPorphyrin-based COF materials Por-M1M2(M1M2=HH,HZn,ZnZn,CoH,CoZn,CoCo)were synthesized by Schiff-base condensation reaction with 5,10,15,20-tetra(4-aldehyde phenyl)porphyrin(M1TFPP,M1=H2,Co,Zn)and 5,10,15,20-tetra(4-aminophenyl)porphyrin(M2TAPP,M2=H2,Co,Zn)under solvent thermal conditions.The photocatalytic H2 production properties of Por-HH,Por-HZn and Por-ZnZn were studied under visible light irradiation(??420 nm).The photocatalytic H2 production rate of Por-ZnZn reaches to 23.71 mmol h-1 g-1,which is 12 and 3 times higher than that of Por-HZn(6.06 mmol h-1 g-1)and Por-HH(1.93 mmol h-1 g-1),respectively.Taking the electron absorption spectroscopy,steady-state and transient emission spectroscopy,and photoelectrochemical studies into account,the excellent photocatalytic H2 production activity of Por-ZnZn can be attributed to:1)zinc porphyrin exhibits lower fluorescence quantum yield than free-base porphyrin,thus possesses higher photo-induced charge separation and photosensitization ability;2)zinc porphyrin unit has smaller recombination energy than free-base porphyrin due to the rigid structure;3)the photogenerated electrons can transport between layers through the Zn-Zn interaction,and the holes can transport between Por rings due to the AA overlapped stacking mode of COFs,which is beneficial to improve the charge separation and transport efficiency.Por-CoH,Por-CoZn and Por-CoCo were used to photocatalytic reduction of CO2 with[Ru(bpy)3]Cl2 as photosensitizer.The three porphyrin-based COFs can reduce CO2 to CO under visible light irradiation(??420 nm).Por-CoCo(4983?mol h-1 g-1,80%)has the highest CO production rate than Por-CoZn(4005 ?mol h-1 g-1,83%)and Por-CoH(1566 ?mol h-1 g-1,79%).The reasons for the difference in photocatalytic efficiency are suggested to be:1)Por-CoCo can provide more active centers(Co2+)compared with Por-CoZn and Por-CoH;2)Por-CoZn can absorb more CO2 than Por-CoH due to the presence of Zn2+,which can increase the contact between the catalyst and the reaction substrate;3)Compared with free-base porphyrin,the metalloporphyrin unit is beneficial to improve the charge separation and transfer efficiency of COFs.