Preparation and characterization of sol-gel derived carbon composite ceramic electrodes: electrochemical and XANES study of nitrite reduction

Sol-gel derived carbon composite ceramic electrodes (CCCE) containing iron phthalocyanine (FePc) have been used to investigate the electrochemical reduction of nitrite in an acidic solution. The electronic and structural aspects for the interaction between FePc and NO have also been studied by X-ray absorption near edge structure (XANES) spectroscopy. CCCEs were prepared by sol-gel methods where hydrolysis of methyltrimethoxy silane was carried out in a methanol + water mixture using acid catalyst. Before gelation took place, carbon powder preadsorbed with FePc was added to disperse carbon particles into the silica matrix. Thus prepared electrodes were very porous showing high electrical conductivity and hydrophobicity. Although voltammetry showed hardly discernable redox peaks of FePc, CCCEs still exhibited a linear response toward nitrite reduction over a concentration range of 0.05-3 mM. XAFS indicated the structure of FePc was maintained even in the silica matrix as a mu-oxo form as evidenced by the appearance of a strong 1s --> 3d transition. It was concluded that the catalytic activity of CCCEs toward nitrite reduction was attributed to the axial ligation of NO, which was produced by disproportionation of nitrite, to the metallic site through the porous structure of a silica matrix. In control experiments, the nitrosyl complex of FePc was synthesized and its electrochemical reduction was performed. (C) 1999 Elsevier Science S.A. All rights reserved.