Niobium oxide thin him grafted on porous silica gel nas prepared and used to immobilize protoporphyrin IX: and hematoporphyrin IX. The porphyrins adhered strongly to the surface probably by the -COO-Nb chemical bond formed between the porphyrin carboxyl groups and Nb2O5. The immobilized hematoporphyrin IX and protoporphyrin IX, SiO2/Nb2O5/H2HMP and SiO2 Nb2O5 H2PP. respectively. were further metallated by means of in situ reaction with CoCl2 giving rise to SiO2/Nb2O5/ CoHMP and SiO2 Nb2O5 CoPP. The effectiveness of the reactions in producing the metallated adsorbed complexes was investigated by electronic absorption spretroscopy. Graphite paste electrodes of the metallated adsorbed porphyrins were prepared In order to study the electrocatalytic reduction of dissolved dioxygen by the cyclic voltammetry technique. The dioxygen eletroreduction was observed at - 0.18 and - 0.16 V versus SCE in pH 6 for hematoporphyrin and protoporphyrin complexes, respectively. The cathodic peak potentials at different solution pH remained practically constant between pH 4 and 7 for both materials. The current intensities for a fixed potential plotted against dissolved dioxygen concentrations showed linear responses between 2 and 12 ppm. (C) 1999 Elsevier Science S.A. All rights reserved.4772158163