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

Hematoporphyrin IX, H 2 HMP , 8,13-bis(1-hydroxyethyl)-3,7,12,17-tetramethyl-21 H ,23 H -porphine-2,18-dipropionic acid and protoporphyrin IX, H 2 PP , 8,13-divinyl-3,7,12,17-tetramethyl-21 H ,23 H -porphine-2,18-dipropionic acid were efficiently immobilized on niobium oxide grafted on a silica gel surface, SiO 2/ Nb 2 O 5, by the - COO – Nb bond formed between the porphyrin carboxyl groups and the grafted Nb 2 O 5. These immobilized porphyrins, SiO 2/ Nb 2 O 5/ H 2 HMP and SiO 2/ Nb 2 O 5/ H 2 PP , were further reacted with Co (II) in dimethylformamide, resulting in SiO 2/ Nb 2 O 5/ CoHMP and SiO 2/ Nb 2 O 5/ CoPP metallated complexes. The UV-vis spectra of the solid materials showed changes of the Q-bands ( a 2 u → e g transition) upon metallation, indicating that by incorporation of Co (II) in the porphyrin ring the local symmetry changed from D2 h to D4 h . 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.