Cobalt porphyrins immobilized on niobium(V) oxide grafted on a silica gel surface: study of the catalytic reduction of dissolved dioxygen

Hematoporphyrin IX, H2HMP, 8,13-bis(1-hydroxyethyl)-3,7,12,17-tetramethyl-21H,23H-phorphine-2, 18-dipropionic acid and protoporphyrin IX, H2PP, 8,13-divinyl-3,7,12,17-tetramethyl-21H,23H-porphine-2,18- dipropionic acid were efficiently immobilized on niobium oxide grafted on a silica gel surface, SiO2/Nb2O5, by the -COO-Nb bond formed between the porphyrin carboxyl groups and the grafted Nb2O5 These immobilized porphyrins, SiO2/Nb2O5/H2HMP and SiO2/Nb2O5/H2PP, were further reacted with Co(II) in dimethylformamide, resulting in SiO2/Nb2O5/CoHMP and SiO2/Nb2O5/CoPP metallated complexes. The UV-vis spectra of the solid materials showed changes of the Q-bands (a(2u) --> e(g) transition) upon metallation, indicating that by incorporation of Co(II) in the porphyrin ring the local symmetry changed from D-2h to D-4h These materials, when incorporated in carbon paste electrodes, presented the property of electrocatalyzing O-2 reduction. Rotating disk experiments were performed in order to estimate the number of electrons involved in the process. It was observed that, for both modified electrodes, O-2 was reduced to water in a four-electron process. Amperometric studies showed the potentiality of both modified electrodes as sensors for the determination of dissolved dioxygen. The response time was less than 3 s. A Linear response for both systems was obtained between 2 and 12 ppm. Copyright (C) 2001 John Wiley & Sons, Ltd.5653754