ATTEMPTS TO IMMOBILIZE CATALYTICALLY ACTIVE HETEROPOLYTUNGSTATES ON THE SURFACES OF ELECTRODES

The iron-substituted heteropolytungstate anion, FeSiW11O39(5-) has been attached to carbon electrode surfaces by three different means: anion exchange into coatings of protonated and/or quaternized polyvinylpyridine, incorporation into oxidized films of electropolymerized polypyrrole, and deposition of insoluble composite coatings containing the anion and the tetrapositive tetrakis(4-N-methylpyridyl)porphine (and its cobalt(II) derivative). Unfortunately, all of the electrode coatings obtained exhibited less stability and/or electrocatalytic activity than is available from homogenous solutions of FeSiW11O39(5-). The iron centers of the FeSiW11O39(5-) anions incorporated in the polyvinylpyridinium films or the insoluble composite coatings were gradually lost from the coatings, probably as a result of electrostatic interactions within the coatings. The FeSiW11O39(5-) anion is reasonably stable in polypyrrole coatings but when the coatings are used for catalytic reductions of H2O2 the polypyrrole appears to be degraded and the catalytic anion is lost. The feasibility of electronically conducting polymers and cation-anion multilayers as suitable vehicles for holding transition metal-substituted polyoxometallates on electrode surfaces was demonstrated but more stable derivatives will be required to achieve long-lived catalytic surfaces.