Kramm, U.I.; Herrmann-Geppert, I.; Bogdanoff, P.; Fiechter, S.: Effect of an ammonia treatment on structure, composition, and oxygen reduction reaction activity of Fe-N-C catalysts. The Journal of Physical Chemistry C 115 (2011), p. 23417-23427
10.1021/jp207417y
Abstract:
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 FeTMPPCl. The ORR activity was determined by RRDE measurements in acidic solution. In order 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 catalyticly active centers thus, enabling an increase of the turn-over frequency.