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 homogeneous 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.