Catalytic water oxidation based on Ag(i)-substituted Keggin polyoxotungstophosphate†
Abstract
A 1D chain-like Ag(I)-substituted Keggin polyoxotungstophosphate, K3[H3AgIPW11O39]·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 [H3AgI(H2O)PW11O39]3− is formed and is stable in a solution of pH 3.5–7.0. The oxidation of [H3AgI(H2O)PW11O39]3− by S2O82− has been studied by ESR, UV-Visible spectroscopy, 31P NMR and UV-Raman spectroscopy. It was found that [H3AgI(H2O)PW11O39]3− can be oxidized to dominantly generate a dark green Ag(II) anion-complex [H3AgII(H2O)PW11O39]2− and a small amount of Ag(III) complex [H3AgIIIOPW11O39]3−, simultaneously evolving O2. Compared with [AgI(2,2′-bpy)NO3] and AgNO3, [H3AgI(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 [H3AgI(H2O)PW11O39]3− and S2O82−, respectively. A possible catalytic water oxidation mechanism of [H3AgI(H2O)PW11O39]3− is proposed, in which the [H3AgII(H2O)PW11O39]2− and [H3AgIIIOPW11O39]3− intermediates are determined and the rate-determining step is [H3AgIIIOPW11O39]3− oxidizing water into H2O2.