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 is formed and is stable in a solution of pH 3.5-7.0. The oxidation of H3Ag(I)(H2O)PW11O39 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 can be oxidized to dominantly generate a dark green Ag(II) anion-complex H3Ag(II)(H2O)PW11O39 and a small amount of Ag(III) complex H3Ag(III)OPW11O39, simultaneously evolving O2. Compared with [Ag(I)(2,2'-bpy)NO3] and AgNO3, H3Ag(I)(H2O)PW11O39 has the higher activity in chemical water oxidation. This illustrates that the PW11O39 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 and S2O8(2-), respectively. A possible catalytic water oxidation mechanism of H3Ag(I)(H2O)PW11O39 is proposed, in which the H3Ag(II)(H2O)PW11O39 and H3Ag(III)OPW11O39 intermediates are determined and the rate-determining step is H3Ag(III)OPW11O39 oxidizing water into H2O2.