Life cycle of single atom catalysts: a M?ssbauer study on degradation and reactivation of tetrapyrrolic Fe-N-C powders

The degradation of a single-site atomically dispersed, model Fe-N-C powder catalyst with high activity is investigated using cryo-Mossbauer spectroscopy. The results indicate a degradation initiated by an Fe2+ to Fe3+ oxidation due to coordination of oxygen to tetrapyrrolic Fe-N4 sites at atmospheric conditions (change between characteristic doublets) before iron(iii) oxide is formed (sextet). Thermal reactivation can be used to restore substantial catalytic activity of aged Fe-N-C powders. Degradation of single-site model Fe-N-C powder tracked using cryo-Mossbauer spectroscopy suggests a simple mechanism comprising the oxidation of Fe(ii) to Fe(iii) followed by precipitation of iron oxide. Curiously, the characteristic doublet D2 is converted into D1 upon exposure to air.