An electrochemical and spectroelectrochemical study of pentacoordinated CoTPP on glassy carbon electrode

A glassy carbon electrode (GCE) modified with a monolayer of 4-aminothiophenol (4-ATP) and 5,10,15,20-meso-tetraphenylporphyrin cobalt (CoTPP) was prepared and used to examine the electroreduction of oxygen. Cyclic voltammetry showed that CoTPP chemisorbed on carbon via a 4-ATP monolayer (GCE/4-ATP–CoTPP) displayed a catalytic effect similar to that of physisorbed CoTPP on a GCE (GCE/CoTPP). UV–visible spectra recorded in solution and on chemisorbed layers showed that pentacoordination of the cobalt and a cobalt oxidation state +II were required to facilitate the complexation with oxygen. A stable oxygenated intermediate (4-ATP–Co(II)TPP–O2) was identified both in solution and on a carbon substrate. Finally, UV–visible reflectance (UVDRS) spectra recorded at different potentials revealed the relation between this oxygenated intermediate and the production of peroxide. Based on these data a catalytic mechanism is proposed.