Co (II) porphyrin adsorbed on SiO2/SnO2/phosphate prepared by the sol–gel method: Application in electroreduction of dissolved dioxygen

This paper describes the immobilization procedure of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21-H,23-H-porphyrin ion on SiO2/SnO2/phosphate, obtained by the sol–gel processing method. P 2p X-ray photoelectron and the 31P MAS NMR spectra revealed that dihydrogen phosphate is the species present on the surface. The porphyrin was adsorbed on the surface of the modified material and furthermore metallized in situ with Co (II) ion. The porphyrin metallation process was followed with UV–vis spectroscopy by inspecting the Q bands of the free and metallated porphyrin. The free porphyrin presented four Q bands associated to a D2h local symmetry and the metallated one, two bands related to a D4h local symmetry. The amount of electroactive species adsorbed on the material was estimated by integrating the area under the peak of Co (II) → Co (I) reduction by using the pulse differential voltammetric technique. The amount of the metallated porphyrin was 2.3 × 10−10 mol cm−2. A carbon paste electrode of the modified material containing metallated porphyrin was used to study the electrocatalytic reduction of dissolved dioxygen by means of cyclic voltammetry, chronoamperometry and linear sweeping voltammetry. The modified electrode was very stable and exhibited the electrocatalytic reduction of dissolved dioxygen at −180 mV versus SCE by a two-electron mechanism, producing hydrogen peroxide at pH 5.4. The electroactive species was strongly retained on the material surface, presumably inside the pores of the material, since in a test of various oxidation–reduction cycles no significant decrease of the current densities was detected, indicating that it was not leached off during the experiment.