Synthesis Of Metal Cluster Complexes And Their Photocatalytic CO₂ Conversion Performance

Metal-organic cluster-based coordination polymer,constructed from multi-metal ions and organic ligands through coordination bonds,have been widely utilized as catalysts in many catalytic fields.Compared with single-atom catalysts,metal-organic cluster-based coordination polymer has the advantages of providing multi-electron/multi-proton transfer and synergistic catalytic activity,which is conducive to achieve complex multi-step catalytic reactions.Compared with traditional nanoparticle catalysts,they have well-defined structures,which is beneficial to explore the catalytic reaction mechanism from the atomic level and provide accurate molecular model to establish a clear structure-property relationship.In addition,due to the high adjustability of their structures,metal-organic cluster-based coordination polymer can effectively improve the poor stability and low surface area of metal cluster catalysts.In recent years,metal-organic cluster-based coordination polymer has been widely used in the catalytic conversion of CO2.But there are still some difficulties and challenges,such as:It is hard to effectively couple CO2RR and WOR to realize the artificial photosynthetic overall reaction;It is difficult to convert CO2 into high value-added C2+products with high selectivity and durability;It is tough work to accurately control the hierarchical assembly of high nuclear clusters;It is a challenge to give consideration to both high catalytic activity and high stability.Based on this,we designed and constructed a series of metal-organic cluster-based coordination polymer and utilized them as catalysts into catalytic conversion of CO2.This paper mainly contains four parts as follows:(1)We used Nis,constructed from Ni(II)ions and benzotriazole(bzt-),as precursor directly bond with polyoxometalate PMo12 through a bridging-O atom to synthesize a novel reduction-oxidation(RO)hetero-metal cluster assembly.Benefiting from the reducibility of Nis and oxidability PMo12,RO can effectively couple photocatalytic CO2RR and WOR to accomplish artificial photosynthesis without additional photosensitizer and sacrificial agent.Furthermore,we synthesize other two assemblies RORO and OR-RO to explore the effect of connection mode between Ni5 and PMo12 on the artificial photosynthesis performance.The test results showed that OR-RO has the best efficiency of CO2-to-CO conversion along with H2O-to-O2 reaction.The characterization results indicated the connection mode of OR-RO can restrain the recombination of photogenerated electro-hole pairs and achieve higher performance.(2)We used Ni(Ⅱ)and bzt" to synthesize a solvent coordinated cluster Ni9-DMA,and further carbonylated at high temperature to obtained a Ni(Ⅰ)-based cluster Ni9.Due to Ni(Ⅰ)active sites with low valence state and low coordination numbers,Ni9 revealed an outstanding performance for photocatalytic converting CO2 to CH3COOH(selectivity 86%)without any additional photosensitizer,sacrificial agent or co-catalyst.Furthermore,we developed a congeneric Ni32 molecular complex including sixteen equivalent active Ni(Ⅰ)sites that are expected to further promote the photocatalytic performance.Ni32 showed a higher selectivity(90.2%)and durability(at least 720 h,a month)of CO2-to-CH3COOH conversion.This is the first case to utilized crystalline metal cluster-based coordination polymer to achieve photocatalytic CO2 into C2 product.(3)During we constructed the carbonylated clusters Ni9 and Ni32,we found that the raw materials and reaction temperature determined the nuclear number of products and oxidant state of catalytic sites.Compared to directly add metal salt and organic ligands,using soluble low nuclear cluster as raw material can be easier to obtain higher nuclear clusters with higher purity and yield.It provided the feasibility for constructing high nuclear cluster-based framework.In this regard,we introduced 1,2,4-triazole and successfully synthesized a framework(Ni32-CO-MOF)assembling from Ni32 as secondary building unit(SBU)and a linear cluster as linker.This is the biggest SBU of reported metal-organic framework up to now.Because of the board light absorption and active Lewis acid sites,Ni32-CO-MOF showed high efficiency for photocatalytic CO2 cycloaddition at room temperature and pressure.(4)We using a triazole-based rigid ligand(H2bptb)and Cu(I)to construct an eight nuclear copper cluster NNU-32,and a layered compound based on NNU-32 as SBU,NNU-33(S).We observe the in-situ dynamic transition of catalyst structure from NNU33(S)to NNU-33(H)in 1 M KOH.The resultant enhanced cuprophilic interactions endow NNU-33(H)with an outstanding Faradic efficiency of 82%at-0.9 V(vs.reversible hydrogen electrode,j=391 mA cm-2)and durability(at least 5 hours of continuous electroconversion)for CO2-to-CH4 conversion.Importantly,NNU-33(H)is the best crystalline catalyst for electrochemical CO2-to-CH4 conversion to date and we first uncover the significant impacts of the intrinsic cuprophilic interactions in Cu(Ⅰ)based catalyst on the selectivity and activity of electrocatalytic CO2-to-CH4 conversion.