Characterization of the Iron Phase in CNx-Based Oxygen Reduction Reaction Catalysts

Catalysts for the oxygen reduction reaction were prepared from the pyrolysis of acetonitrile at 900 C over iron particles on various supports. The iron phases present in the active materials were characterized by XRD, TEM, and Mossbauer spectroscopy. Typically, the iron particles were encased in carbon after pyrolysis, explaining how they could survive subsequent acid washes. The Fe phases present included metallic gamma iron, cementite, and two oxidized phases. Although the relative abundance of the phases varied with different supports, with treatment time, and after washing, there was no apparent correlation between the presence or abundance of a phase and activity. The phases present are consistent with Fe particles used to catalyze the formation of carbon fibers by catalytic chemical vapor deposition. After being washed with acid, there was no evidence for the presence of nitrogen-stabilized Fe sites on the carbon surface. These results support the hypothesis that Fe catalyzes the formation of ORR-active carbon nanostructures during pyrolysis at 900 C in a carbon and nitrogen atmosphere and is not part of an active site itself.