Keggin-Type Polyoxometalates-based Electrochemistry Detection Catalyst For Synthesis And Performance

Polyoxometalates(POMs)is the nano-sized polyoxometalates with diverse compositions and particular structures,which have important applications in catalysis,medicinal chemistry,materials,magnetism and so on.Keggin-type polyoxometalates,a classical structures,exhibit excellent catalytic activity in electrochemical redox reactions due to their stable structure,easy preparation,and fast reversible multi-electron redox conversion properties.It is an ideal and controllable electrocatalyst material.Wells-Dawson polyoxometalates as another classical structures,which have excellent redox properties due to the unique pore structure and high negative charge of anion clusters.In this work,eight POMs were synthesized by hydrothermal method and solvent evaporation method:including seven Keggin polyoxometalate-based complexes and one Dawson phosphotungstate complex.The structure and electrochemical redox activity were characterized by single crystal X-ray diffraction,FTIR,XRD,TG and CV.Electrochemical sensing small molecules and ionic pollutants such as H2O2,Cr(VI)and NO2-in water was studied.The research are as follows:1.By introducing H8[Si(W2O7)6]·xH2O as a precursor,Cd(NO3)2·4H2O as a source of transition metal Cd element,using rigid nitrogen-containing organic ligands Imidazole and 1,2,4-1H-Triazole,respectively,three new Keggin-type polyoxometalate-based silicotungstate supramolecular compounds were obtained:[Cd3(H2O)2(Htrz)6][HSiW12O40]2·17H2O(1)[Cd(imi)6]2[SiW12O40]·24H2O(2)(Himi)4·[SiW12O40]·H2O(3)The structural units formed by the coordination of the trinuclear cadmium cluster[Cd3(H2O)2(Htrz)6]6+and the anion cluster in compound 1 are finally extended to the three-dimensional supramolecular network structure via hydrogen bonding.The silicotungstate anion cluster of compound 2 is surrounded by six small windmill-like metal-organic fragments[Cd(imi)6]2+,which are connected by hydrogen bonds and electrostatic interactions to form layered structure.In compound 3,the polyoxoanion clusters and partially protonated imidazole ligands form a porous "host-guest"supramolecular framework.Three compounds were used as electro-detection catalysts for water environmental pollutants such as Cr(VI),NO2-and H2O2,and all showed excellent electrocatalytic performance.The synergistic effect between metal organic fragments and anionic clusters is the main factor affecting electrocatalytic performance.2.A novel semi-rigid and semi-flexible carboxylate ligand pmic was introduced to regulate the precursors H7[P(Mo2O7)6]·xH2O and H7[P(W2O7)6]·xH2O.Three Keggin-type polyoxometalate-based supramolecular complex were obtained by hydrothermal synthesis.The molecular formula is as follows:(H2pmic)1.5·[PMo12O40]·4H2O(4)(H2pmic)1.5·[PW12O40]·4H2O(5)(Hspmic)·[PW12O40]·6H2O(6)(pmic=1,1-(1,4-phenylene bis(methylene))bis(1-H-imidazole-4-carboxylic acid))The arrangement of protonated organic ligands between polyoxoanion clusters of compound 4 and compound 5 are similar to the "Z" type,while the arrangement of protonated organic ligands between polyoxoanion clusters in compound 6 is similar to the "S" type.Three compounds modified working electrodes showed good electrocatalytic reduction activity toward Cr(VI)ions,NO2-ions and H2O2.The detection results of these compounds all met the standard of WHO for Cr(VI)and H2O2 in drinking water system and the standard concentration of NO2-in drinking water specified by European Community.Experiments show that the Keggin-type polyoxometalate cluster with Mo as the coordination atom has greater conductivity and faster electron transport than the Keggin-type polyoxometalate cluster with W as the coordination atom,so it has better electrocatalytic performance.The detection performance of compound 4 can be comparable to some POM-based catalysts or noble metal electrocatalysts.This work has reference significance for the design of efficient Keggin-type POMs based electrocatalysts.3.According to these synthesis,reaction temperature is more important.Semi-rigid and semi-flexible ligand enhances the coordination flexibility,and the presence of N/O atoms in the ligand greatly increases the stability of the structure.At the same time,the self-assembly strategy also shows great advantages in the preparation of POMs-based inorganic-organic supramolecular compounds.Therefore,in this part,Keggin-type polyoxometalate-based complex 7 and Dawson-type phosphotungstic acid-based complex 8 were successfully synthesized by using pmic ligand and increasing the reaction temperature.The formula is as follows:(H2pmi)l.5·[PW12O40]·3H2O(7)(H2pmi)3·[P2W18O62]·8H2O(8)(pmi=1,1-(1,4-phenylene bis(methylene))bis(1-H-imidazole))Interestingly,the organic ligand pmic has undergone in-situ transformation into pmi,and the protonated organic ligand and inorganic cluster in compound 7 are finally extended to 3-D supramolecular network structure through electrostatic interaction and rich C-H...O,N-H...O hydrogen bond interactions.There are three different configurations of protonated organic components in compound 8,which are arranged as guest molecules between the 3-D supramolecular host framework formed by polyoxoanion clusters.In addition,the electrochemical detection toward small molecules and ionic pollutants in water of two compounds was studied.Wells-Dawson type phosphotungstate with unique pore structure has relatively excellent electrocatalytic reduction performance for pollutants,which provides an experimental basis for the subsequent design of efficient and multifunctional Dawson type polyoxometalate-based electrocatalysts.