Full reoxidation of CuMn 2 O 4 spinel catalyst triggered by epitaxial Mn 3 O 4 surface nanocrystals

Transmission electron microscopy was performed on a model system of a novel CuMn 2 O 4 spinel catalyst in order to study the effects of reduction by carbon monoxide and subsequent reoxidation by oxygen both processed at 600°C. A phase transition from CuMn 2 O 4 spinel into separated MnO and Cu‐rich phases, which are CuO or Cu depending on the reduction period, was observed in the reduced samples. Furthermore, a surface coverage of the MnO grains by epitaxially grown, nanocrystalline Mn 3 O 4 was found in all reduced samples. These Mn 3 O 4 nanocrystals are assumed to act as seed crystals in the subsequent reoxidation step completely reversing the phase transitions and reconstituting single‐phase CuMn 2 O 4 spinel. In addition, superlattice reflections due to cation ordering occurred as a typical feature in the as‐prepared spinel. In the reoxidized spinel, the superlattice reflections were absent, hence indicating a disordered cation distribution.