Design And Synthesis Of Metal-oxygen Cluster Complexes And Their CO₂ Reduction Properties

The excessive emission of CO₂ accompanied by the consumption of fossil energy has caused global warming and severely affected the ecological environment.Converting excess CO₂ into energy products can not only maintain ecological balance,but also develop chemical alternatives to traditional energy sources,alleviating the energy crisis.photo/electrocatalytic reduction is an important means to convert CO₂into energy products.The development of efficient catalysts for CO₂ reduction is the key to achieving the carbon recycle.At present,most materials with high activity for photo/electrocatalytic reduction of CO₂ belong to nanomaterials.However,they lack of clear structural information,resulting in great challenges to the study of reaction mechanisms.In contrast,molecular catalysts can provide a platform to clearly investigate the reaction mechanism at the molecular level,and the development of stable and efficient heterogeneous molecular catalysts can greatly promote the development of the CO₂ reduction process.Metal-organic frameworks（MOFs）are a kind of metal complexes self-assembled from metal/cluster and organic ligands,and are a periodically arranged porous framework crystalline materials.Porous MOFs can capture CO₂ and promote photo/electrocatalytic CO₂ reduction.Secondly,MOFs can be designed by selecting photo/electrosensitive ligands,such as porphyrins,which have strong photosensitivity,redox activity,and electron transfer capabilities,promoting photo/electroreduction of CO₂.Moreover,its determined molecular structure is conducive to studying the reaction mechanism of the reduction process.MOFs can be regulated and functionalized as efficient catalysts for photo/electric reduction of CO₂.Polyoxometalates（POM）have electron-rich structure and excellent redox capabilities,but are easily soluble in water and difficult to be used as heterogeneous catalysts.Utilizing the POM as node to build MOF,the soluble POM can be easily fixed and still maintain the MOF structure.This strategy can make full use of the redox properties of POM and the advantages of MOFs structure,which is more conducive to catalyzing CO₂ reduction.These MOFs（a kind of coordination compounds）composed of metal or clusters as nodes are expected to be excellent photo/electrocatalysts for CO₂reduction.Some unresolved problems and challenges in CO₂ reduction will be better illustratedand improved based on the molecular level as follows:the influence of anions has not been paid much attention to in the research on catalytic reduction of CO₂;it is difficult to clarify the mechanism of adsorption and activation of CO₂ molecules;how to improve the electron-donating and electron-transferring capacity of MOFs catalysts;and how to design catalysts to produce high value products.Based on the above elaboration,this paper focuses on the design and synthesis of metal/polyoxometalates-basedcoordinationcompoundsandtheir photo/electrocatalytic performance for CO₂ reduction:（1）Two stable MOFs（NNU-17 and NNU-18）are prepared by replacing different halogen ions which directly coordinated with active metal center in MOF structure,further to study the influence of different electronegative coordination anions on ECR performance.Cl and Br ions are coordinated on the metal center of NNU-17 and NNU-18 respectively.The greater the electronegativity,the stronger the electron-withdrawing ability.Therefore,the charge density of the active metal sites of NNU-17 is lower than that of NNU-18 and conducive to the attack of CO₂ intermediates,resulting in higher ECR performance of NNU-17 than NNU-18.NNU-17 exhibits the faradaic efficiency(FE_CO)reach to 90.3%at-1.0 V,while the maximum FE_CO（82.1%）of NNU-18 appears at-1.1 V and is lower than NNU-17.Utilizing different anions to control the charge density of the metal center,ECR performance of MOFs can be improved based on the catalytic activity center.（2）The OH^–-coordinated MOF（NNU-15）was designed and synthesized to simulate the surface state of catalyst in alkaline electrolyte,which is to further find out the activation mechanism of CO₂in ECR reduction.NNU-15 exhibits high faradaic efficiency for CO(FE_CO)about 99.2%at-0.6 V vs.RHE and over 96%in a wide potential（-0.6 to-0.9 V）,as well as a long-term stability over 110 h.Furthermore,single-crystal to single-crystal（SCSC）transformation between NNU-15 and NNU-15-CO₂demonstrates that the OH^–ions coordinating with the metal center can grip CO₂molecules not only in electrolyte but also in air.As a result,the linear CO₂ molecule is activated into HCO₃^–that will chelate to metal active center in ECR process,and the intermediate crystal model（NNU-15-CO₂）with HCO₃^–is obtained.Density functional theory（DFT）calculations validate that both O-adsorbed and C-adsorbed initial geometries are inclined to result in the formation of HCO₃^–during ECR process.（3）A series of polyoxometalate-metalloporphyrin organic frameworks（PMOFs）with different metal active centers were synthesized by in-situ high-temperature hydrothermal method.These PMOFs are constructed by TCPP ligand modified with different metals（Co,Fe,Ni,Zn）and Zn-ε-Keggin polyoxometalate clusters.The synergistic combination of Zn-ε-Keggin and M-TCPP in these PMOFs contributes to enhance electron enrichment,electron donation,electron migration and electrocatalytic active ingredients in CO₂ reduction reactions.POM as an electron concentrator cooperates with TCPP to form an electron transmission channel from POM to the metal porphyrin active center under the action of an electric field,improving the performance of ECR.Especially for Co-PMOFs,it shows excellent FE_CO approximately 99%and high turnover frequency（TOF）about 1656 h^-1 at-0.8 V.Moreover,the FE_CO can maintain over 94%in a wide potential ranging from-0.8 V to-1.0 V and the stability is more than 36 h.（4）Photocatalytic CO₂reduction into energy products is of utmost importance to alleviate the risks associated with accumulated CO₂ concentration and resource scarcity,but highly selective generation of desirable hydrocarbon fuel,such as methane（CH₄）,is still an extremely challenging work.Two stable polyoxometalate-porphyrin metal-organic frameworks,constructed with Zn-ε-keggin clusters and photosensitive TCPP ligands,having identical host networks but with different spatial symmetry.Because of the synergistic cooperation of advantages of polyoxometalate and visible-light responsive porphyrin group,these catalysts uniformly exhibit fairly high selectivity（>96%）for CH₄ in photocatalytic CO₂ reduction system,which has far surpassed many MOF photocatalysts.Significantly,the introduction of Zn-ε-keggin cluster with strong reducibility is the important origin for catalysts to obtain high photocatalytic selectivity for CH₄,considering that eight Mo^V atoms can theoretically donate eight electrons to fulfill the multi-electrons reduction process of CO₂ to CH₄.