A Molecular Water Oxidation Catalyst Based On Ag⁺-substituted Keggin Polyoxotungstophosphates:[H₃Ag^I（H₂O）PW₁₁O₃₉]^3-]

In the paper, a1D chain-like Ag+-substituted Keggin polyoxotungstophosphate,K3[H3AgIPW11O39]·12H2O, has been synthesized and characterized by single-crystal X-raydiffraction. When it is dissolved in aqueous solutions, the [H3AgI(H2O)PW11O39]3-anions areformed, which is determined by MS spectroscopy and conductivity.31P NMR ofK3[H3AgIPW11O39]·12H2O solutions indicates the [H3AgI(H2O)PW11O39]3-anions are stable inthe range of pH=3.5-7.0. It is found that [H3AgI(H2O)PW11O39]3-is oxidized by S2O82-,forming a dark green [H3AgII(H2O)PW11O39]2-species dominantly and a little of[H3AgIIIOPW11O39]3-, and giving off O2simultaneously. The cyclic voltammetry ofK3[H3AgIPW11O39]·12H2O solution indicates that [H3AgI(H2O)PW11O39]3-shows the catalyticactivity in electrochemical oxidation of water into O2. Compared with [AgI(2,2’-bpy)NO3]and AgNO3,[H3AgI(H2O)PW11O39]3-has the higher activity in chemical water oxidationinto O2, which illustrates that the [PW11O39]7–ligand plays important roles in both thetransmission of electrons and protons, and the improvement of redox performance ofAg2+/Ag+. The rate of O2evolution is a first-order law with respect to the concentrations of[H3AgI(H2O)PW11O39]3-and S2O82-, respectively. A possible catalytic water oxidationmechanism of [H3AgI(H2O)PW11O39]3-is proposed, in which it is deduced that the formationof [H3AgIIIOPW11O39]3-may be a rate-determining step.