ELECTROCHEMICAL AND ELECTROCATALYTIC PROPERTIES OF IRIDIUM(IV)-SUBSTITUTED DAWSON TYPE POLYOXOTUNGSTATE

The electrochemical properties of an iridium(IV)-substituted Dawson ty pe heteropolytungstate anion have been studied in organic and aqueous solutions using cyclic voltammetry. In acetonitrile solution, this spe cies exhibits four-step one-electron reduction processes, which are as cribed to the W(VI --> V) processes of the heteropolyanion framework. The iridium center plays an important role in these reduction processe s, though its redox waves are not observed in acetonitrile solution. I n acid buffer solution, the iridium-substituted derivative also exhibi ts four couples of reversible redox waves. The first one-electron coup le at more the positive potential is assigned to the redox process of the Ir-IV active center. The diffusion coefficient for the Ir-IV/Ir-II I couple in pH 3.0 buffer solution is evaluated, D-0 = 2.1 x 10(-6) cm (2) s(-1). The other one-electron and two two-electron couples are ass ociated with the redox processes of the tungsten-oxo framework. All th ese waves are pH dependent. The iridium center has an excellent cataly tic effect on the reduction of nitrite anion in acid solution. The lac unary polyanion, alpha(2)-[P2W17O61](10-), exhibits no catalytic effec ts under the same conditions. The maximum rate constant of the overall electrocatalytic reaction is measured to be 4.0 x 10(3) M-1 s(-1) at pH 3.0. The kinetic data point to HNO2 as the reactive form of nitrite in the catalytic reaction. The catalytic mechanism is also discussed. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.