Water oxidation catalyzed by polyoxometalate containing iron cubane structure under visible light

Hydrogen is a kind of renewable and clean energy that can be obtained by photocatalytic water splitting, a promising technique to solve environmental problems. Water splitting by photocatalysis involves two key half-reactions: hydrogen evolution and oxygen production. The water oxidation reaction is the limiting step in photocatalytic water splitting due to its complicated transfer process involving four electrons and four protons. In this research, three iron containing polyoxometalates (POMs) catalysts Na₂₁[NaFe₁₅(OH)₁₂(PO₄)₄(A-α-SiW₉O₃₄)₄]·85H₂O (Fe₁₅POM), Cs_4.5K₁₄Na_1.5[Na₂Fe₁₄(OH)₁₂(PO₄)₄(A-α-SiW₉O₃₄)₄]·105H₂O·2Na₃PO₄(Fe₁₄POM), K₅[SiFe(H₂O)W₁₁O₃₉]<strong></strong>(FeSiW₁₁) were synthesized. Among them, Na₂₁[NaFe₁₅(OH)₁₂(PO₄)₄(A-α-SiW₉O₃₄)₄].85H₂O Fe₁₅POM contains Fe₄O₄ cubane structure. The photocatalytic water oxidations over these catalysts containing Fe in the dye sensitization system of [Ru(bpy)₃]²⁺ photosensitizer /S₂O₈^2- sacrificial reagent were investigated. Optimizing reaction conditions was explored by changing the photosensitizer concentration, sacrificial reagent, and polyoxometalates. Furthermore, the pH value of the reaction solution, reaction process and catalyst stability of the dye-sensitized photocatalytic reaction system were researched using a series of characterization methods. Fe₁₅POM gives the highest activity among all iron catalysts, the amount of O₂ reaches 5.40 μmol with a yield of 14.4%. The cubane structure not only enhances the stability of the iron-based POMs but also enhances efficient charge transfer during the water oxidation process. This research opens a new pathway in developing Iron-containing novel cubane POMs for water oxidation.