Cobalt porphyrins immobilized on niobium(V) oxide grafted on a silica gel surface: study of the catalytic reduction of dissolved dioxygen
Abstract
Hematoporphyrin IX, H2HMP, 8,13-bis(1-hydroxyethyl)-3,7,12,17-tetramethyl-21H,23H-porphine-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 (a2u → eg transition) upon metallation, indicating that by incorporation of Co(II) in the porphyrin ring the local symmetry changed from D2h to D4h. These materials, when incorporated in carbon paste electrodes, presented the property of electrocatalyzing O2 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, O2 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.
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