Catalytic water oxidation based on Ag(i)-substituted Keggin polyoxotungstophosphate

Abstract

A 1D chain-like Ag(I)-substituted Keggin polyoxotungstophosphate, K₃[H₃Ag^IPW₁₁O₃₉]·12H₂O, has been synthesized in a high yield and characterized by single-crystal X-ray diffraction, XRD, IR, TG/DTA and elemental analysis. When the polyoxotungstophosphate is dissolved in aqueous solutions, ³¹P NMR, MS and conductivity analyses indicate that a Ag(I) anion-complex formulated as [H₃Ag^I(H₂O)PW₁₁O₃₉]³⁻ is formed and is stable in a solution of pH 3.5–7.0. The oxidation of [H₃Ag^I(H₂O)PW₁₁O₃₉]³⁻ by S₂O₈²⁻ has been studied by ESR, UV-Visible spectroscopy, ³¹P NMR and UV-Raman spectroscopy. It was found that [H₃Ag^I(H₂O)PW₁₁O₃₉]³⁻ can be oxidized to dominantly generate a dark green Ag(II) anion-complex [H₃Ag^II(H₂O)PW₁₁O₃₉]²⁻ and a small amount of Ag(III) complex [H₃Ag^IIIOPW₁₁O₃₉]³⁻, simultaneously evolving O₂. Compared with [Ag^I(2,2′-bpy)NO₃] and AgNO₃, [H₃Ag^I(H₂O)PW₁₁O₃₉]³⁻ has the higher activity in chemical water oxidation. This illustrates that the [PW₁₁O₃₉]⁷⁻ ligand plays important roles in both the transmission of electrons and protons, and in the improvement of the redox performance of silver ions. The rate of O₂ evolution is a first-order law with respect to the concentrations of [H₃Ag^I(H₂O)PW₁₁O₃₉]³⁻ and S₂O₈²⁻, respectively. A possible catalytic water oxidation mechanism of [H₃Ag^I(H₂O)PW₁₁O₃₉]³⁻ is proposed, in which the [H₃Ag^II(H₂O)PW₁₁O₃₉]²⁻ and [H₃Ag^IIIOPW₁₁O₃₉]³⁻ intermediates are determined and the rate-determining step is [H₃Ag^IIIOPW₁₁O₃₉]³⁻ oxidizing water into H₂O₂.