Redox electrochemistry of Keggin type iridium-substituted heteropolytungstates and their electrocatalytic activity toward the reduction of nitrite ion

Abstract

Abstract Redox electrochemistry of Keggin type iridium-substituted heteropolytungstate anions, [H 2 OIr IV XW 11 O 39 ] n − (X=B, n = 5; X=Si, Ge, n = 4;X=P, n = 3), have been investigated in acetonitrile and aqueous solutions using cyclic voltammetry. In acetonitrile solutions, these heteropolyanions exhibit two-step one-electron reduction processes. These redox waves are ascribed to theW(VI→V) processes of the heteropolytungstate framework. The slope of a plot of standard redox potentials against the ionic charge is evaluated. In acid buffer solutions (pH 1.0 ∼ 6.0), all these anions exhibit two pairs of reversible, one-electron redox peaks. One pair is assigned to the Ir IV center and another (at more negative potential) is attributed to electron addition and removal from the tungsten-oxo framework that comprises the structure of each anion. These iridium-substituted heteropolyanions are shown to be excellent catalysts for the electroreduction of nitrite ion. The unsubstituted parent anions show no catalytic activity. The catalytic mechanism involves the rapid formation of a nitrosyl complex with of the Ir III form the catalyst, which seems to depend on the electron configuration of Ir III and the inorganic structure of the iridium-substituted heteropolytungstates. The pH dependence of the rate constant for the catalytic reaction shows that nitrous acid is the reactive form of the nitrite ion atpH