Fabrication Of Porphyrin/g-C₃N₄/POMs Composite For Photocatalytic Hydrogen Production

With the continuous development of economy and society,the consumption of fossil energy,such as coal and oil,increases rapidly.On the one hand,it causes serious environmental pollution problems,on the other hand,it also causes energy shortage and exhaustion.The fundamental way to solve these problems is to develop new renewable non-fossil energy.Among the new renewable energy sources,hydrogen energy has attracted wide attention due to its green products,high combustion value,non-toxicity and renewability.The direct conversion of solar energy into hydrogen energy by photocatalytic water decomposition is a clean production mode,which will not cause secondary pollution.But at present,the efficiency of photocatalytic water decomposition hydrogen production system is still relatively low,so the development of stable and efficient catalysts is the main challenge in the field of photocatalytic hydrogen production.As one of the main photocatalyst of photosynthesis system in nature,porphyrin has attracted much attention because of its large absorption coefficient,wide absorption wavelength range and diverse molecular structure fabricated by chemical modification.At present,the research of porphyrin-based photocatalyst mainly focus on improving its catalytic activity by self-assembly and hybriding with other inorganic materials,but there are still some problems such as low catalytic efficiency,complex synthesis steps and using precious metal platinum as co-catalyst.Therefore,it is necessary to construct new and efficient non-precious porphyrin-based photocatalytic system.In view of the above problems,we design the following two parts of the research contents:1.Preparation of ZnTPP/g-C₃N₄ composite and its application in photocatalytic hydrogen production.By simulating the efficient electron transfer chain in photosynthesis of plants,the molecular-level heterojunction photocatalyst of ZnTPP/g-C₃N₄ was synthesized through a simple one-step hydrothermal method by co-assembling of ZnTPP and g-C₃N₄ via pi-pi interactions and Zn-N coordination.By adjusting the mass ratio of ZnTPP to g-C₃N₄,it was found that g-C₃N₄ excess at 1:1.After that,the increase of g-C₃N₄ would lead to the excess and self-reorganization of g-C₃N₄ to form rod-like structure.The morphology and element distribution of ZnTPP/g-C₃N₄ composites were characterized by SEM and TEM mapping.The results showed that the size of ZnTPP/g-C₃N₄ composites was mainly distributed between0.2 and 1μm,and the dispersibility of C,N and Zn was good.Ultraviolet-visible absorption spectra,fluorescence spectra and XPS characterization show that ZnTPP and g-C₃N₄ are combined by weak pi-pi conjugate interaction and strong Zn-N coordination.This is mainly because g-C₃N₄ is a supramolecular polymer formed by hydrogen bond and pi-pi interaction.Hydrogen bond dissociation occurs during hydrothermal process,resulting in g-C₃N₄ oligomer fragments co-assembled with ZnTPP to form molecular-level heterojunctions,which plays an important role in electron transfer and carrier lifetime extension.The results of photocatalytic hydrogen production show that the photocatalytic efficiency of ZnTPP/g-C₃N₄ under visible light irradiation is significantly higher than that of pure ZnTPP.The highest photocatalytic hydrogen production efficiency is 305.05 mmol/g/h,which is one of the most efficient photocatalytic hydrogen production catalysts reported at present.This study shows that co-assembly is an effective way to construct molecular-level heterojunction structure to improve the efficiency of photocatalytic system.2.Preparation of ZnTPP/g-C₃N₄/POMs and ZnTPP/g-C₃N₄@POMs and their application in photocatalytic hydrogen production.On the basis of the above work,two photocatalysts with different structures,ZnTPP/g-C₃N₄/POMs and ZnTPP/g-C₃N₄@POMs,were synthesized by one-step solvothermal method and step-by-step encapsulation method,which is a new non-noble metal photocatalytic system.The morphology and size of ZnTPP/g-C₃N₄/POMs and ZnTPP/g-C₃N₄@POMs did not change much compared with that of ZnTPP/g-C₃N₄.The photocurrent density of ZnTPP/g-C₃N₄@POMs>ZnTPP/g-C₃N₄/POMs>ZnTPP/g-C₃N₄/POMs>ZnTPP/g-C₃N₄ also confirmed the results.ICP results showed that the content of polyacid in ZnTPP/g-C₃N₄@POMs was higher than that in ZnTPP/g-C₃N₄/POMs.That is to say,the structure of outer coated polyacids exposes more polyacids than that of doped polyacids,which is more efficient for electron separation and transfer,and also provides abundant active sites for photocatalytic reaction.The photocatalytic hydrogen production rate of ZnTPP/g-C₃N₄@POMs was nearly twice that of ZnTPP/g-C₃N₄/POMs under the same visible light source without Pt,reaching 1.28 mmol/g/h.