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

A 1D chain-like Ag(I)-substituted Keggin polyoxotungstophosphate, K3[H3Ag(I)PW11O39]·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, (31)P 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 S2O8(2-) has been studied by ESR, UV-Visible spectroscopy, (31)P 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(III)OPW11O39](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 S2O8(2-), 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(III)OPW11O39](3-) intermediates are determined and the rate-determining step is [H3Ag(III)OPW11O39](3-) oxidizing water into H2O2.