New Insight into the Nature of Catalytic Activity of Pyrolysed Iron Porphyrin Based Electro-Catalysts for the Oxygen Reduction Reaction (ORR) in Acidic Media

In this contribution, catalysts, prepared by an impregnation technique have been characterized structurally and chemically via 57Fe Mößbauer spectroscopy, X-ray photoelectron spectroscopy (XPS) and by elemental analysis, respectively. On the basis of the structural characterization it was concluded that those FeN4 centres in which iron is mesomericly bonded to four nitrogen atoms, are catalyzing the ORR. Furthermore, XPS as well as Mößbauer spectroscopy revealed that with increasing pyrolysis temperature the electron density at iron atoms was increased while that of the coordinating nitrogen atoms was reduced. This behaviour was connected with a decreased electric field gradient(EFG) as estimated by quadrupole splitting. EFG is interpreted in terms of a larger iron-nitrogen-bond distance and/or a slight movement of iron out of plane. We propose that this characteristic of the centres within the graphene planes, in which the centres are incorporated, enables the higher turn-over frequency during ORR of pyrolysed porphyrins.