ELECTROCHEMISTRY AND REDOX CHEMISTRY OF H2OFEIIISIW11O39(5-) IN THE PRESENCE OF H2O2 AND OH

Heteropolytungstates in which one of the positions normally occupied by a tungsten cation is occupied instead by an iron cation are shown to be catalysts for the electroreduction of H2O2. The rate constant governing the reduction of H2O2 by H2OFe11SiW11O396− was measured by stopped flow as 9 × 102 M−1 s−1. A catalytic mechanism involving an Fe(IV) intermediate generated from the reaction between the Fe(III) form of the heteropolytungstate anion and hydroxyl radicals is presented. The Fe(IV) intermediate can react with itself to produce H2O2 and the Fe(III) form of the heteropolytungstate. Alternatively, the Fe(IV) intermediate can consume additional H2O2 by oxidizing it to O2. Competition between these two reaction pathways accounts for the nonintegral stoichiometry observed under some experimental conditions during the electroreduction of H2O2 catalyzed by H2OFe11SiW11O396−. Pulse-radiolysis experiments were employed to detect the Fe(IV) intermediate, to evaluate rate constants for the reactions in which it is formed and decomposed, and to measure the rate of reduction of the Fe(III) form of the heteropolytungstate by O2−. © 1990, American Chemical Society. All rights reserved.