Synthesis And Photocatalytic CO₂ Reduction Of Zr-based Metal-Organic Framework Containing Binuclear Co（Ⅱ） Units

Converting CO₂ into industrial chemicals can greatly alleviate the energy crisis and the greenhouse effect.The key is to develop highly efficient,selective and easily accessible catalysts.Metal-organic Frameworks（MOFs）are considered as an ideal research platform for the development of CO₂ reduction photocatalysts due to its large internal surface area,high porosity,excellent CO₂ capture and structural tunability.In this paper,two Zr-based MOFs containing binuclear Co（Ⅱ）sites were designed and synthesized by introducing a metal complex molecular catalyst into MOFs.Then the effects of functional groups and metal centers on their catalytic properties were explored.The specific research content mainly includes the following two parts:1.Under solvothermal reaction conditions,an amino functionalized Zr-based Co₂-MOF（-NH₂）was constructed with a binuclear Co（Ⅱ）complex（{Co₂（CH₃）₂C[CH₂N=CH（1-COOH-4-OH-3,5-C₆H₃）CH=NCH₂]₂C（CH₃）₂}（Cl O₄）₂）,2’-amino-[1,1’:4’,1"-triphenyl]-4,4"-dicarboxylic acid（H₂TPDC-NH₂）and Zr Cl₄.Meanwhile,Co₂-MOF without amino group and L-MOF（-NH₂）without Co（Ⅱ）site were synthesized.PXRD and SEM demonstrated that all of them are similar in structure to Ui O-68.In a photocatalytic CO₂ reduction system without additional photosensitizer,the CO yield catalyzed by Co₂-MOF（-NH₂）was up to 37.80μmol·g^-1·h^-1,which was 7.5 times that of Co₂-MOF(5.41μmol·g^-1·h^-1).In addition,L-MOF（-NH₂）has no catalytic capacity.Solid UV-Vis spectra and CO₂ adsorption experiments further elucidated that the presence of amino group can not only prolong the light absorption in the visible region,but also promote the adsorption of CO₂,so as to improve the catalytic performance.Mechanism studies proved that the excited[-TPDC-NH₂]*was quenched through oxidation quenching pathway by transferring electrons to Co（Ⅱ）active center,then Co（I）center delivered electrons to CO₂ and reduced it to CO.2.In order to investigate the possible synergistic catalytic effect of binuclear Co（Ⅱ）sites during photocatalytic CO₂ reduction,the mononuclear Co-MOF（-NH₂）with similar structure to Co₂-MOF（-NH₂）was prepared.The two MOFs were employed as catalysts to test and compare the properties on photo-reducing CO₂ to CO with results as follows:Co₂-MOF（-NH₂）exhibited excellent CO₂ photoreduction activity,the CO yield(37.80μmol·g^-1·h^-1)of which was 18.4 times as high as Co-MOF（-NH₂）(2.05μmol·g^-1·h^-1)under the same reaction conditions.Electrochemical tests manifested that the charge transfer efficiency of Co₂-MOF（-NH₂）is superior to Co-MOF（-NH₂）.Density functional theory（DFT）calculations represented that there was synergistic effect between the two Co（Ⅱ）sites in Co₂-MOF（-NH₂）:one Co（Ⅱ）serves as the catalytic center and the other Co（Ⅱ）serves as the co-catalytic site,which jointly promote the split of C-OH bond in*COOH intermediates and the production of*CO intermediates,thus significantly improved catalytic performance.