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For the first time, the effect on structure and composition by a second heat treatment in ammonia was studied in detail for Fe–N–C catalysts prepared by oxalate-supported pyrolysis of chloroirontetrametoxyphenylporphyrin (Fe(TMPP)Cl). The oxygen reduction reaction (ORR) activity was determined by rotating ring-disk electrode (RRDE) measurements in acidic solution. To evaluate the origin of the observed increase in ORR activity, bulk elemental analysis 57Fe Mössbauer spectroscopy, N2-sorption measurements, and X-ray diffraction were performed. A second heat treatment in ammonia was found to affect the ORR activity of Fe–N–C catalysts significantly; induced by NH3 treatment, all catalysts contained iron nitride; as more nitride was formed, the effect on ORR activity was more pronounced. A complete removal of the nitride by a subsequent acid leaching was possible but decreased the ORR activity slightly. On basis of these results, we conclude that the nitride itself cannot be the site responsible for the main catalytic increase. However, its formation positively affects either the carbon environment or the constitution of catalytically active centers, thus enabling an increase of the turnover frequency.
Kramm et al. (Fri,) studied this question.