The Construction Of Transition Metal-Organic Frameworks And Their Photo/Electrochemical Studies

Metal-organic frameworks?MOFs?,also known as porous coordination compounds?PCPs?,are crystalline porous materials composed by organic ligands and metal ions/metal clusters.Since the discovery of MOFs in the 1990s,MOFs have been at the research hotspot and have achieved explosive development.Due to the crystalline nature,structural diversity,tailorability and ultra-high specific surface area of MOFs,MOFs have potential applications in gas adsorption separation,chemical sensing,proton conduction and catalysis,etc.Up to now,more than 20,000 MOF structures have been reported,but this number is still increasing.MOFs connect inorganic and institutional units through coordination bonds.The inorganic unit may be metal ions or metal clusters and the metal clusters are widely referred to as secondary building units?SBU?,although the connector may also be regarded as geometric SBU.The organic units?linker/bridged ligand?are usually organic compound containing carboxylic acid functional groups,or other organic anions such as phosphates,sulfonate and heterocyclic compounds.As researchers delve deeper into MOFs,people have moved from the initial discovery of new structures to the study of MOFs applications.Based on the synthesis and applicability of MOFs,the research work is divided into the following sections.In the first chapter,through the research of MOFs literature,we introduce the structural characteristics of metal organic framework materials,the synthesis methods and the progress of application of MOFs.In Chapter 2,we use three organic carboxylic acid ligands as main ligands.Introducing nitrogen-containing ligands such as 1,2-bis?4-pyridyl?ethene?dpe?and4,4'-bipyridyl?4,4'-bpy?with transition metals such as Cu???,Co???,Ni???,Cd???and Zn???ions successfully constructed 8 new metal-organic framework materials.{Cu₃?TCA?₂?dpe?₃?H₂O?₃}n 1{Co₃?TCA?₂?dpe?₃?H₂O?₆}n 2{Ni₃?TCA?₂?dpe?₃?H₂O?₆}n 3{Co₆?TCA?₆?dpe?₆}n 4{Ni₄?TCA?₂?dpe?_3.5?H₂O?₃}n 5{Zn₃?TCA?₂dpe}n 6{[Ni₄?HL?₂?4,4'-bpy?₃?H₂O?₂]·H₂O}n 7{[Cd?H₃L??H₂O?₂]·?4,4'-bpy?·H₂O}n 8{[Co₂?HL1??4,4'-bipy??H₂O?]·0.5?4,4'-bipy?·3H₂O}n 9{[Ni_1.5?H₂L1??4,4'-bipy?_1.5?H₂O?₃]·2H₂O}n 10In this chapter,we describe in detail the synthesis of these ten new crystals,the structural analysis of the crystals,and the physicochemical properties of the crystals.In the third chapter,we selected the 1-3 crystals described above.These three crystals are isomorphic in configuration.The metal centers are Cu^II?MOF-Cu?,Co^II?MOF-Co?,and Ni^II?MOF-Ni?.Through the ultraviolet spectroscopy test,these three isomorphic crystals have a wide range of visible light absorption between 450 and 800nm.In order to determine their semiconductor properties,we conducted a Mott-Schottky test.Since the conduction band positions of these three crystals are more negative than the potential required to reduce CO₂ to CO,theoretically these three crystalline materials can be used as catalysts for photocatalytic reduction of CO₂.To this end,we carried out a series of experiments on photocatalytic reduction of CO₂ in these three cases.The experimental results show that MOF-Ni exhibits extremely high catalytic activity and high selectivity in photocatalytic reduction of CO₂,and its photocatalytic reduction of CO₂ is CO.The selectivity is as high as 97%,exceeding the vast majority of MOF-based catalysts that have been reported.Through photocurrent and AC impedance experiments,we found that the photogenerated electron efficiency and charge transfer rate of MOF-Ni and MOF-Co are significantly higher than MOF-Cu.In addition,we further study the causes of the difference in the performance of these three crystalline materials in photocatalytic reduction of CO₂ by density functional theory?DFT?calculation.The calculations show that the photocatalytic performance is better than the other two crystals due to the high binding energy between MOF-Ni and CO₂ and the high energy barrier in the photocatalytic reduction of CO₂.This reaction system not only provides a good model for studying the effect of MOF in different metal centers on photocatalytic reduction of CO₂,but also provides us with more insights into the mechanism of photocatalytic reduction of CO₂ in molecular models.This achievement was successfully published on the internationally renowned academic journal ACS Catalysis?IF=11.384?.In the fourth chapter,we selected the crystalline materials 9 and 10 synthesized in the second chapter as catalysts for electrocatalytic oxygen evolution,and tested the electrocatalytic oxygen evolution properties of these two crystalline materials.The experimental results show that crystalline material 9 exhibits good oxygen evolution performance.The over-potential of crystalline material 9 at a current density of 10 mA cm^-2 is 395 mV,and its Tafel slope is 68 mV/dec.The research results were published in the internationally renowned journal Journal of Solid State Chemistry?IF=2.179?.