Magnetic study of the Co-MCM-41 catalyst: Before and after reaction

A combined magnetization (both dc and ac techniques), NMR, and EPR study of the magnetic properties has been carried out on a 3 wt. % Co-loaded Co-MCM-41 catalyst, before and after reaction. Before reaction, the Co magnetization can be described by the Curie-Weiss law (50 K ≤ T ≤ 350 K) consistent with the vast majority of the Co existing in the MCM-41 structure as weakly-interacting Co2+ paramagnetic moments with an effective value μp = 4.7(2) μB. In addition, there appears to be a contribution from a small number of Co “clusters” exhibiting a magnetic transition at ≈15 K. After reaction, the Co magnetization is dominated by a magnetic contribution which saturates with a value 58% that for bulk Co, and is attributed to Co metal nanoparticles. In addition, the Co nanoparticles are characterized by a superparamagnetic blocking temperature at 10.5 K. Before reaction, Co-MCM-41 showed two Co EPR signals: (1) a strong signal at g = 1.93 and (2) a weak signal at g = 1.99. After reaction, Co59 zero-field spin-e...