Polyoxometalates With Visible-light Activity For Photocatalytic H₂Evolution Researches

Polyoxometalates can exhibit well elaborated substitution chemistry in which one ormore of the addenda atoms can be replaced with other transition metal ions. Transition metalsubstituted polyoxometalates （TMSPs） with diverse characteristics have attracted intensiveresearch attention as they can be rationally modified on the molecular level including shape,size, charge density as well as redox states, thus having various applications. Recently, almostall reported articles describe its catalytic oxidation and environmental improvement, while thephotocatalysis of TMSPs has not been reported.In this paper, The visible light photocatalytic H2evolution activities of a series oftransition-metal substituted Keggin-structure heteropoly blues （HPB absorb visible light400–800nm） K₆SiW₁₁O₃₉M（H₂O）·nH₂O （M=Co, Ni, Cu and Zn） were systematicallystudied with Pt nanoparticles as co-catalyst. The H2evolution rates were150,98,65and48μmol h^-1g^-1for SiW₁₁Cu, SiW₁₁Ni, SiW₁₁Co and SiW₁₁Zn, respectively, meaning that the orderof photocatalytic activity of these heteropoly blues from transition-metal substitutedpolyoxometalates was: SiW₁₁Cu> SiW₁₁Ni> SiW₁₁Co> SiW₁₁Zn. The catalysts could beefficiently reused for at least5cycles.On the other hand, Niobates possess the combination of unique characteristics thatinclude chemical and electrochemical stability, photoelectronic activity and heterogeneouscatalysis, these properties have attracted consideration for its potential applications asphotocatalysts for water splitting and pollutant decomposing. Especially, great progress hasbeen achieved in the development of photocatalysis for H2production. In this article, we triedto sensitize [Nb6O19]8that absorbs extend to the visible region by using the ligand of copper（Ⅱ）–ethylenediamine, we report the synthesis of a novel trimer polyoxoniobate compound1:K2[Cu（en）2]15[Nb24O72H8]2·38H2O, and systematically charactertize by all kinds of techniques.Visible light photocatalytic H2evolution activity was researched with1as visible-lightphotosensitizer and catalyst, cobaloximes [CoIII（dmgH）2pyCl] as co-catalysts, andtriethylamine（TEA） as sacrificial electron donor.The H2evolution rates in five runs were14.0,14.8,13.6,13.9and13.2μmol h^-1g^-1respectively. The total evolved H2during50h was79.5μmol. There was no significant change in the UV-Vis spectra of the photocatalyst duringphotocatalytic reaction, which shows that the catalyst is stable and can be regenerated forrepeated use. The photocatalytic activity of compound1under UV irradiation was alsostudied. The H2evolution rates in runs1,23,4and5were987,938982,940and1000μmolh^-1g^-1, respectively. Our results reveal the promising application of organic ligand modifiedpolyoxoniobates in visible-light photocatalysis which may stimulate further development inthis area.