Design And Synthesis Of Polyoxometalate-based Metal-organic Framework Composites And Their Photoelectrocatalytic Performance Study

Environmental crises and energy shortages are the focus of attention all over the world.The world’s heavy dependence on fossil fuels and excessive CO₂ emissions have led to serious climate problems such as global warming,acid rain,and rising sea levels.The storage and conversion of CO₂ is a potential method to reduce the amount of CO₂ in the atmosphere.Compared with CO₂ capture,CO₂ conversion is more promising because it can convert CO₂into low-carbon fuels or other useful chemicals,such as CO,CH₄,HCOOH,and C₂H₅OH.For industrial applications,electro-catalytic reduction of CO₂（CO₂RR）is considered to be a practical and potentially valuable clean energy production method due to its characteristics such as production at room temperature and pressure.Hydrogen energy is considered to be the most promising energy in the 21st century,because of its unique advantages such as pollution-free,high efficiency and recycling.Photocatalytic hydrogen production is to directly use sunlight to decompose water.Compared with electric energy for electrocatalytic hydrogen production,the photocatalytic system is simpler and more economically competitive.Metal-organic Frameworks（MOFs）are crystalline porous materials composed of metal ions or clusters connected with organic ligands by coordination bonds.Due to its ordered porous structure,large specific surface area and structural diversity of metal ions/clusters and organic ligands,it has great application potential in gas adsorption and separation,drug carrier,catalysis and other fields.As a kind of Zr-based porphyrin MOFs with semiconductor behavior,PCN-222 has a broad application prospect in photocatalysis and electrocatalysis.In this paper,polyoxometalates（POM_S）or quantum dots（QD_S）were supported on MOFs using PCN-222 as support by post modification method.The synergistic catalysis between host frame and guest molecule was explored by comparing the performance of the catalysts.It provides a strong theoretical support for the further application and development of MOFs,and broadens the application range of MOFs in the field of catalysis.The main work contents of this paper are as follows:（1）Study on the performance of H-POM@PCN-222（Co）composite for electrocatalytic reduction of CO₂.Combining the superiority of polyoxometalates（POMs）and catalytic single-metal site Co of metalloporphyrin,we successfully designed and synthesized a series of mixed-valence POM-based MOFs composites POM@PCN-222（M）(POM=[Co^IIICo^II（H₂O）W₁₁O₃₉]^7-,M=Co,Fe,Mn,Ni)by post-modification method.Among them,benefiting from the directional electron transfer from the POM to the single-metal site Co in PCN-222（Co）which promotes the multi-electron transfer process,significant enhancement of electrocatalytic activity was observed for POM@PCN-222（Co）.In particular,H-POM@PCN-222（Co）can selectively convert CO₂ to CO with a superior Faradaic efficiency of 96.2%and long stability exceeding 10 h.The finding of this work offers a platform for designing electron transfer channels at a molecular level,and we believe that the diverse structures of POMs will provide numerous opportunities to develop various CO₂RR catalysts.（2）Study on the photocatalytic hydrogen production performance of7.0%Mo S2@POM@PCN-222（Co）composite material.We have synthesized a photocatalyst that integrates light capture,catalysis,and recyclability.We used porphyrin-based MOF PCN-222 as a carrier to encapsulate Mo S₂ QDs and POM into MOF cavities through the immersion method to construct a ternary composite material Mo S₂@POM@PCN-222（Co）.The introduction of functional substances such as POM and Mo S₂ QDs into the ordered pores of PCN-222（Co）can not only reduce aggregation and increase active sites,but also establish a good interaction system between the host framework and the guest.The photocatalytic hydrogen evolution measurement results under visible light irradiation confirmed that the optimized 7.0%Mo S₂@POM@PCN-222（Co）composite has a hydrogen evolution rate of 565.85μmol·g^-1·h^-1 without any noble metal in the catalytic system.This work provides a way to further design and synthesize photocatalyst without precious metals for efficient hydrogen evolution.