Synthesis,Assemblies,and Photocatalytic CO₂ Reduction Properties Of Novel Heteropolymolybdates

With the rapid development of industrialization,the consumption of fossil fuels has caused the concentration of CO₂ to rise year by year,which makes the ecological environment more and more seriously damaged.So,CO₂ reduction has become one of the global hot researches.It is of great significance that the convertion of CO₂ into high-value additional chemicals with the sustainable energy sources?solar energy?.The key to CO₂ reduction is to develop semiconductor materials with visible light absorption energy bands and stable structures.Compared with traditional inorganic semiconductor materials,metal-organic frameworks?MOFs?and organic covalent frameworks?COFs?,polyoxometalates?POMs?are porous,structurally adjustable,functional,etc.It is also an electronic"storage"and has capabilities of storing and transferring electrons.This feature not only overcomes the shortcomings of traditional inorganic semiconductors with poor photocatalytic performance and unclear active sites,but also effectively overcomes poor conductivities and weak electronic supply capacities of MOFs and COFs materials.Therefore,POMs are excellent photocatalytic materials for CO₂ reduction.So far,most researches have been used with traditional POMs prototypes,such as Keggin and Dawson.Due to the rapid development of the synthesis chemistry of POMs over the past decade,the photochemical properties of many POMs clusters have not been studied.Therefore,the current challenge is not only the optimization of existing systems,but also the research and development of new POMs materials.Based on the above considerations,this work focuses on the design and synthesis of POMs materials with novel structures,stable solvents,reasonable band gaps,and excellent performance for photocatalytic CO₂ Reduction:In view of the narrow band gaps and wide visible light absorption ranges of Co-POMs as well as the redox properties of POMs,We synthesized a wheel-shaped phosphomolybdate compound 1 under hydrothermal conditions.We study the photocatalytic performance of the system with 1 and[Ru^II?bpy?₃]²⁺under visible light and pure CO₂ conditions.Within 1 h,the yield of syngas?CO+H₂?is 74.9 mmol g^-1 h^-1,which is 8.2 times higher than that of[Ru^II?bpy?₃]²⁺alone.Considering the high cost and high energy consumption of purifying CO₂,it is more practical to directly use the low-content CO₂ in the flue mixture such as?3-15%CO₂?.Therefore,we investigated the catalytic performance with CO₂ concentrations of 3%,10%and20%.The results show that when the content of CO₂ is 20%,the productivity of syngas can reach137.9 mmol g^-1 h^-1,and remain unchanged in 10%of CO₂.It can also reach 74.7 mmol g^-1 h^-1even in 3%CO₂.We also explored the excellent photocatalytic reaction mechanism of this system in detail by using steady-state fluorescence spectroscopy and theoretical calculations.This result reveals the electron transfer path and the important role of 1 in the photocatalytic reaction.At the same time,our work provides a new thought to rationally design heterogeneous photocatalysts for efficient reduction of low concentration CO₂.Inspired by the compound 1,we used pyridine and imidazole ligands as templates to construct a novel organic-inorganic hybrid high-nuclear cobalt molybdenum phosphate compound 2.The compound is composed of{Co₄},{Mo₄}and{Mo₂}basic building units,and combines the characteristics of the"wheel cluster"and"boat cluster"structural units reported in the literatures.The compound 2 has the highest number of cores which exceed the previously reported wheel structure compounds{Mo₁₆M_xP₂₆}?x=16,M=Co,Ni;x=14,M=Ni?.The band gap reveals that it has excellent semiconductor properties,and the properties of photocatalytic CO₂ reduction were also studied.The results show that after 1 h of light,CO and H₂ are the main gas products,the yields are 22.4?mol and 12.6?mol.After five-run cycling experiments,the total yields of CO and H₂ hardly decayed,indicating that the compound 2 has good cycle stability.The discovery of this compound expands the family members of the wheel-shaped phosphomolybdates and provides a new idea for the preparation of new wheel-shaped phosphomolybdates.The{P₄Mo₆}unit has the characteristics of high negative charge,small volume,and can be bonded with multiple transition metals,which is conducive to regulating the band gap structure of POMs.We synthesize two{P₄Mo₆}-based POMs that is compounds 3 and 4 with novel structures with the appropriate pH under hydrothermal conditions.As far as we know,the compound 3 is the first{P₄Mo₆}-based compound containing[Ru^II?bpy?₃]²⁺cation.Under visible light,there is no need to add other photosensitizers.After 3 hours of light irradiation,the yield of CO and H₂ are 8.85?mol and 6.25?mol,respectiviely.The compound 4 is a high-dimensional open porous structure consisting of a rare 18-core metal ring cluster that expands outward to connect eight{P₄Mo₆}units.This compound breaks the conventional configuration of multidimensional structures constructed with mononuclear or dinuclear metal-linked{P₄Mo₆}dimer units.The 18 metal cluster unit can be regarded as a kind of secondary building units?SBUs?,which can be connected to{P₄Mo₆}as a multidentate inorganic ligand.The synthesis of this compound is of great significance for novel{P₄Mo₆}-based POMs.The{Mn₄O₅Ca}cluster is a well-known cubic alkane transition metal cluster,which plays an important role in solar energy conversion.Inspired by this structure,most sandwich-type POMs that artificially modeled cubic or quasi-cubane were synthesized.To date,many cubic and quasi-cubane POMs containing metals such as Co,Mn,and Ni constructed from vacant germaniumtungstates have been widely reported and widely used in photocatalytic water oxidation reactions.However,due to the instability of germanomolybdates in aqueous solution,it is difficult to synthesize precursors of germanomolybdates in advance.At present,the number of sandwich HPMs is still limited and mainly concentrated in the trivacant?3:9?and monovacant?1:11?species.In view of this,we have successfully in situ synthesized two new types of sandwich-type germanomolybdates 5 and 6 based on[?-GeMo₁₀O₃₆]^8-building units using sodium molybdate and germanium dioxide as raw materials with Ni and Co under hydrothermal conditions.As far as we known,this new type of anion building unit has not been reported even in germanium tungstate analogues.We also explored the catalytic activities of compounds 5 and6 for photocatalytic reduction of CO₂.In the presence of[Ru^II?bpy?₃]²⁺and pure CO₂,and after 3hours of light irradiation,the selectivity of compounds 5 and 6 for CO was 82%and 62%,respectively.The CO selectivity of compound 5 is higher than that of most[Ru^II?bpy?₃]²⁺participating systems.At the same time,we also studied the electrocatalytic properties of compounds 5 and 6.The test results show that these compounds also have good properties for reducing H₂O₂.