Preparation And Photocatalytic Hydrogen Evolution Performance Of Keggin Polyoxometalate-based Materials Modified By Copper-Azole

Polyoxometallates?POMs?have the characteristics of adjustable size,rich variety and excellent electron transfer ability,so as a potential photocatalyst.POMs have attracted more and more attention in the field of catalytic hydrogen production,showing a good momentum of development.At present,the preliminary research results show that there are three main technical problems of traditional POMs as photocatalysts.That is,?1?POMs are usually soluble in water and not easy to be recycled.?2?Photogenerated electrons and holes are easy to recombine,requiring the loading of noble metal Pt,etc;?3?POMs generally only have light response in the ultraviolet region,which limit their application in the field of photocatalytic hydrogen production.In this paper,a new method for the preparation of POM-based materials heterogeneous visible light catalytic hydrogen production materials is presented.Keggin POMs were modified by copper ions and two azole ligands,and four novel POM-based inorganic organic crystal hybrid materials were prepared.The structure of the crystal materials were systematically characterized by combination of single crystal X-ray diffraction,infrared spectroscopy?IR?,powder X-ray diffraction?PXRD?,elemental analysis?EA?and ultraviolet visible diffuse reflection spectroscopy?UV-vis-DRS?.The effects of triethylamine,POMs and ligands on the structure of the compounds were investigated.Furthermore,some compounds were choosed,and their photocatalytic hydrogen production activities were studied.The influences of molecular spatial structure and types of POMs on the photocatalytic hydrogen production performance were investigated.The main findings are as follows:1.Preparation and photocatalytic hydrogenation of Keggin POM inorganic organic crystal hybrid modified by copper-pyrazole derivatives.Compound 1 and 2were synthesized by hydrothermal synthesis with different POM anions[GeW₁₂O₄₀]^4-(GeW₁₂),[VW₁₂O₄₀]^3-(VW₁₂)in the same Cu+PPZ reaction system,the molecular formulas for compounds 1 and 2 are as follows:Cu₂^I?ppz?₄][HVW₁₂O₄₀]·8H₂O?1?[Cu₂^I?ppz?₄][H₂GeW₁₂O₄₀]·8H₂O?2?ppz=3-?pyrid-4-yl?pyrazoleThe results of single-crystal diffraction analysis showed that compounds 1 and2 were one-dimensional chain structures,and the photocatalytic rates of compounds1 and 2 reached 11.54 mol/g/h and 7.96 mol/g/h,respectively,after continuous illumination for 5 hours in the visible light range.2.Preparation and photocatalytic hydrogenation of Keggin type POM inorganic organic crystal hybrid modified by copper-tetrazolium derivatives.On the basis of synthesis of compounds 1 and 2,compound 3 was synthesized only by regulating ligand 2-ptza,a tetrazolium derivative,using the same reaction system.Furthermore,compound 4 was obtained by introducing appropriate amount of triethylamine to regulate the structure.The molecular formulas for compounds 1 and 2 are as follows:[Cu₅^II?2-ptza?₆?H₂O?₄(GeW₁₂O₄₀)]·4H₂O?3?[Cu₂^ICu₅^II?2-ptza?₆?OH?₂?4 H₂O?GeW₁₂O₄₀]·6H₂O?4?2-ptza=5-?2-pyridyl?tetrazoleThe results of single crystal diffraction analysis show that compounds 3 and 4are three-dimensional intercalation structures composed of GeW₁₂2 anion columnar support units and organic-metal cationic layers,but the composition and structure of organic-metal cationic layers are significantly different.In compound 3,the GeW₁₂anions pillar support the planar[Cu₅^II?2-ptza?₆?H₂O?₄]⁴ⁿ⁺cationic layer to form 3D structure;Incompound4,GeW₁₂anionspillarsupportthestepped[Cu₂^ICu₅^II?2-ptza?₆?OH?₂?4H₂O?]⁴ⁿ⁺cationic layer forming 3D structure.The photocatalytic hydrogen production performance of compound 3 is better than that of the typical similar materials reported previously,the photocatalytic rates of compounds 3 and 4 reached 213.82 mol/g/h and 104.39 mol/g/h,respectively,after continuous illumination for 5 hours in the 500W Xe light.At the same time,compounds 3 and 4 have excellent photocatalytic stability.Under the condition of xenon lamp,irradiate continuously for 10 hours,hydrogen production and XRD of the compounds after the reaction have not changed significantly.