Influence of the Electron-Density of FeN[sub 4]-Centers Towards the Catalytic Activity of Pyrolyzed FeTMPPCl-Based ORR-Electrocatalysts

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 -centers in which iron is mesomerically bonded to four nitrogen atoms are catalyzing the oxygen reduction reaction (ORR). X-ray induced photoelectron spectroscopy 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 -center. Moreover, in accordance with these, higher kinetic current densities toward the oxygen reduction were observed. The above results suggest a relationship between the electron density of the -centers and the catalytic activity, where an increase in electron density on the iron centers enables an improvement in the turnover 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 centers during the ORR.