Kramm, U.I.; Abs-Wurmbach, I.; Herrmann-Geppert, I.; Radnik, J.; Fiechter, S.; Bogdanoff, P.: Influence of the electron-density of FeN4-centers towards the catalytic activity of pyrolyzed FeTMPPCl-based ORR-electrocatalysts. Journal of the Electrochemical Society 158 (2011), p. B69-B78
10.1149/1.3499621
Abstract:
FeTMPPCl impregnated on a carbon black was heat-treated to different temperatures. The obtained catalysts were characterized before and after acid-leaching by structural and chemical analyses. On the basis of the structural characterization it was concluded that those FeN4-centres in which iron is mesomerically bonded to four nitrogen atoms, are catalyzing the oxygen reduction reaction (ORR). XPS as well as Mößbauer spectroscopy revealed that higher pyrolysis temperatures cause a partial shift of electron density from the coordinating nitrogen atoms to the iron atom of the active FeN4-centre. Moreover, in accordance to these higher kinetic current densities towards the oxygen reduction were observed. The above results suggest a relationship between the electron density of the FeN4-centres and the catalytic activity, where an increase in electron density on the iron centres enable an improvement in the turn-over frequency during ORR, thus, compensating the lower concentration of active sites. This finding makes it most likely that on heat-treated porphyrin-based Fe-N-C-catalysts the oxygen molecules coordinate to these iron centres during the ORR.