| Polyoxometalates(POMs)are a class of molecular inorganic oxide materials.They have many unique physicochemical properties,such as nanoscale size,abundant elemental composition and structure,strong redox and acidity,and have great potential in catalysis,magnetism,biomedicine and materials science.Titanium(Ti)-containing POMs combined the advantages of POMs and titanium,are a kind of new functional material developed rapidly in recent years.Therefore,the design and synthesis of Ti-containing POMs have become an important topic in the field of POMs synthesis chemistry.However,lacunary heteropolymolybdate anions are unstable and can be easily converted into saturated structures as building units.Ti-containing raw materials are easily hydrolyzed in the reaction system,which hinders the coordination between titanium ions and heteropolymolybdate anions.Therefore,the development of Ti-containing heteropolymolybdate is greatly challenged.In this article,a series of Ti-containing heteropolymolybdate compounds were synthesized by buffer solution degradation and in situ self-assembly using conventional aqueous solution methods,and the applications of these compounds in catalytic organic reactions,photoelectrochemical sensing and electrocatalysis were explored.The results are as follows:1.The saturated keggin heteropolymolybdate H3PMo12O40(PMo12)was selected as the precursor and degraded into three vacancy[PMo9O34]9-(PMo9)in the acetic acid/sodium acetate buffer solution(pH=5.28).Using degraded PMo9 as building units and water-soluble titanium oxide cluster K7H[Ti6O9(ida)6]Cl2(Ti6)as titanium source,a novel water-soluble Ti-containing heteropolymolybdate(compound 1)was synthesized:K6(H3O)8[{PMo9O34TiO}2]·32H2O(1)Compound 1 is the first water-soluble Ti-containing heteropolymolybdate,in which the titanium atom does not coordinate with the terminal oxygen atom on the PMo9 vacancy site,but coordinates with the bridge oxygen connecting the molybdenum atom,showing a unique?3-O coordination mode.The UV-vis diffuse reflectance spectra reveal that the optical bandgap of compound 1 was 3.24 eV,indicating it was an excellent wide semiconductor material.The photocatalytic oxidation of benzaldehyde shows that compound 1 can be used as an effective photocatalyst for photocatalytic oxidation of organic reactions.2.Four novel water soluble heteropolymolybdate(compounds 2-5)were synthesized by using(NH4)6Mo7O24,TiCl4,GeO2/Na2SiO3 and morpholine as raw materials in conventional aqueous solution:(C4H10ON)4(H3O)6[(GeMo11TiO39)2O(H2O)10]·20H2O(2)(C4H10ON)4(H3O)12[(SiMo11TiO39)2O][SiMo11TiO40)]·12H2O(3)(C4H10ON)4[(GeMo12O40)(H2O)4]·5H2O(4)(C4H10ON)4[(SiMo12O40)(H2O)4]·H2O(5)Compounds 2 and 3 are dimers of Ti-substituted Keggin type POMs[GeMo11TiO39]4- and[SiMo11TiO39]4-,respectively,which are connected by Ti-O-Ti bonds.These two compounds have the same structure but different heteroatoms,and would change to two new compounds 4 and 5 using the same reaction material under lower pH solution conditions.The compounds 4 and 5 are the morpholine salts of[GeMo12O40]4- and[SiMO12O40]4-.The UV-Vis diffuse reflectance spectra reveal that the optical bandgap of compound 2-5 were 2.48,2.4,2.47 and 2.5 eV,respectively.Mott-schottky curve results further confirm that the four compounds belong to n-type semiconductors.Photoelectrocatalytic of hydrogen peroxide test results show that the four compounds can be used as photoelectrochemical sensor materials,and titanium functionalized compounds 2 and 3 have better photoelectric sensing properties than compounds 4 and 5.3.A highly nuclear water-soluble polyperoxotitanmolybdate(compound 6)was synthesized by in-situ self-assembly method using TiCl4,H2O2,(NH4)6MO7O24 as raw materials.When using Na2WO4 instead of(NH4)6MO7O24 as raw material,another novel water-soluble compound with titanium as heteroatom(compound 7)can be also synthesized under the same conditions:[K8(H2O)8][Ti12(O2)6(OH)12Mo42O124(O2)18(H2O)17]·31H2O(6)[K5Na6(H3O)9][{TiW9O30(O2)4(H2O)15}2](7)Compound 6 was the first and largest polyperoxotitanmolybdate found to date.It is a nano-triangular cluster with a size of 1.8×1.6×1.8 nm3.24 peroxy groups are distributed on the {Ti12Mo42} cluster surface.The UV-Vis diffuse reflectance spectra reveal that the optical bandgap of compound 6 were 2.4 eV.Using the benzoic alcohol and benzaldehyde as the models,the compound 6 exhibit excellent oxidation ability and catalytic activity,in which compound 6 exhibits the catalytic activity with conversion rate of 99%and selectivity of 96.4%for benzoic alcohol,as well as photocatalytic activity with conversion rate of 92.6%and selectivity of 99%for benzaldehyde.4.Four novel sandwich heteropolymolybdates(compounds 8-11)were successfully synthesized by introducing transition metals Cu,Mn,Co and Ni into the Strandberg anion framework[P2Mo5O23]6- in conventional aqueous solution:(NH4)10[Cu(H2O)4(P2Mo5O23)2]·10H2O(8)(NH4)10[Mn(H2O)4(P2Mo5O23)2]·11H2O(9)(NH4)10[Co(H2O)4(P2Mo5O23)2]·6H2O(10)(NH4)10[Ni(H2O)4(P2Mo5023)2]·4H2O(11)In the compounds 8-11,the transition metal atoms coordinate with the phosphate eterminal oxygen atom in the anion[P2Mo5O23]6-,forming the first kind of pure inorganic Strandberg type anions with sandwich structure.The UV-Vis diffuse reflectance spectra reveal that the optical bandgaps of compound 8-11 were 3.48,3.52,3.71 and 3.70 eV,respectively,exhibiting the unique properties similar to inorganic semiconductors.Furthermore,electrochemical measurements show that compounds 8-11 could be used as effective electrocatalysts to catalyze nitrite and hydrogen peroxide. |
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