Syntheses And Photocatalytic Properties Of Viologen-Polyoxometalate Hybrid Materials

Polyoxometalates have attracted widespread attention due to their various structures,excellent redox activities and their wide applications in catalysis,drugs,conductance,magnets and photochemistry.Recently,the combination of polyoxometalate and metal-organic framework to form an inorganic-organic hybrid system can combine the respective advantages of inorganic components and organic components,and pave an effective way to optimize the catalytic performance of polyoxometalates.Metal-viologen frameworks constructed with positively charged viologen compound as ligands,while the organic component can be used to extend the absorption range of the composite system,the electron deficient property of the main framework can also be used to adjust the oxidation properties of polyacid guest molecules.In order to construct a host-guest composite photocatalytic molecular system,hydrothermal method was used to introduce different types of polyacid anions into the electron-deficient metal-viologen framework to study on the structural design and synthesis and photocatalytic performance of metal viologen-polyoxometalate based inorganic-organic hybrid materials.The specific research contents are as follows:1.The Keggin-type or Dawson-type phosphotungstate is used as template guest molecules,the viologen compound N-?3-carboxyphenyl?-4,4?-bipyridyl salt?HCPBPY?as bridging ligand,and the transition metal ion Cu²⁺for the coordination metal center,two kinds of host-guest compounds were synthesized by self-assembly under hydrothermal conditions.[Cu₂?CPBPY?₄?H₂O?₂][PW₁₂O₄₀][OH]·6H₂O?1?H₂K₂[Cu?CPBPY??H₂O?₃][P₂W₁₈O₆₂]?8H₂O?2?,and reference compound 3[HCPBPY]₃[PW₁₂O₄₀]₂ was synthesized though solution method.Studies on the photocatalytic activity of those compounds found that 1 showed good photocurrent response characteristics under both visible and near-infrared light,and had the highest full-spectrum photodegradation efficiency under the same conditions compared with the similar compounds.The physicochemical characterization showed that 1 was a good semiconductor material.The catalytic reaction mechanism was characterized and discussed by electrochemical and energy level determination methods.Compared with the performance of Compound 2 containing Dawson-type phosphotungstate and Compound 3 with the absence of copper-viologen frameworks,it was confirmed that the electron-deficient absence of copper-viologen frameworks was a necessary condition for near-infrared photocatalytic activity.In addition,the photocatalytic cycling and stability experiments showed that the solid photocatalytic material 1 has good stability and can be recycled repeatedly.2.With the isopolymolybdate as the a template guest molecule,the viologen compound N-?3-carboxyphenyl?-4,4?-bipyridyl salt?HCPBPY?as bridging ligand,and the transition metal ion Cu²⁺for the coordination metal center.The host-guest compound[NH₄]₂[CuCl?CPBPY??H₂O?]₂[Mo₈O₂₆]·2H₂O?4?and the reference compound[H₂CPBPY]₂[Mo₈O₂₆]?5?were prepared by hydrothermal synthesis.Photoelectrochemical characterization showed that Compound 4 achieved effective electron-hole separation under visible and near-infrared light irradiation,while reference compound 5 had only a weak visible light response.The experiments of photocatalytic degradation of dyes showed that the electron-deficient copper-viologen frameworks can effectively improve the light absorption capacity and catalytic efficiency of the hybrid material.Cyclicity and stability experiments also showed that the solid photocatalytic material 4 has good stability and can be recycled repeatedly.3.The Keggin-type phosphotungstate was used as a template guest molecule,viologen compound N-?3-carboxyphenyl?-4,4?-bipyridyl salt?HCPBPY?as bridging ligand,and the transition metal ion Cu²⁺for the coordination metal center.Compound H₇[Cu₂O₃?CPBPY?₂][(CuPW₁₁O₃₉)?H₂O?]?7H₂O?6?was synthesized by hydrothermal method.Structural characterization shows that copper ions are inserted in the Keggin phosphotungstate anion by in-situ reaction to form a copper-substituted phosphotungstate.Compared with the photocatalytic performance of Compound 1,it was found that Compound 6 has a better light absorption range and carrier generation ability.The results of photocatalytic degradation of pollutants showed that the heterogeneous phosphotungstate photocatalyst compound 6 has high efficiency of degradation and good recycleability.4.The Keggin-type phosphotungstate was used as a template guest molecule,the viologen compound N-?3-carboxyphenyl?-4,4?-bipyridyl salt?HCPBPY?as bridging ligand,and the transition metal ions Ni²⁺and Co²⁺for the coordination metal center respectively.Two isomorphous compounds 7,[Ni₂?H₂O?₆?CPBPY?₂][?CPBPY?(PW₁₂O₄₀)]?2[OH]?3H₂O?abbreviated as Ni-L-PW?and compound 13[Co₂?H₂O?₆?CPBPY?₂][?CPBPY?(PW₁₂O₄₀)]?2[OH]?3H₂O?abbreviated as Co-L-PW?were synthesized by hydrothermal method.The physicochemical characterization showed that compound 7 has excellent light absorption in the near-infrared region,while compound 13 has better light absorption ability in the visible region.A series of nickel-cobalt doped compounds(8,Ni_0.9Co_0.1-L-PW),(9,Ni_0.7Co_0.3-L-PW),(10,Ni_0.5Co_0.5-L-PW),(11,Ni_0.3Co_0.7-L-PW),(12,Ni_0.1Co_0.9-L-PW)were synthesized in situ by adjusting the ratio of Ni²⁺and Co²⁺metal ions.The physicochemical characterization showed that the photocatalytic capacity of the compound 7¹3 changes with the proportion of metal ions,achieving the purpose of the regulation of the apparent photocatalytic properties of materials by adjusting the proportion of metal ions.