Electrochemical reduction of CO2 mediated by poly-M-aminophthalocyanines (M = Co, Ni, Fe):: poly-Co-tetraaminophthalocyanine, a selective catalyst

The electrochemical reduction of carbon dioxide was studied on a glassy carbon electrode modified with either polymeric M-tetrakis aminophthalocyanines (M = Co, Ni, Fe) or with the polymeric free ligand, in aqueous electrolyte. The reaction products are dependent on the central ion: for Co-polymer the only reaction product found was formic acid; for Ni polymer, formic acid and formaldehyde were found, whereas formaldehyde and H, were detected for Fe polymer. For the free ligand polymer only H-2 was detected. Spectroelectrochemical experiments show that in the case of Co-polymer, Co(l) is the active site of the electrocatalysis but the reduced cobalt center and the reduced ligand are not enough to promote the reduction of the carbon dioxide and an extra overpotential is necessary. In the case of the Ni polymer, the reaction takes place at the same potential where the complex is double reduced and it is not necessary to apply more potential. On the other hand, there are important differences between the morphologies of both polymers as demonstrated by electrochemical impedance spectroscopy. The experiments show that the metallic center affects the kinetics of polymerization and the polymer morphology. On the other hand, the chemical nature of the metal center of the catalyst is the most important factor in the electrochemical reduction of CO, and the products involved. (C) 2004 Elsevier B.V. All rights reserved.