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 Co 2+ paramagnetic moments with an effective value μ p =4.7(2) μ B . 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, Co 59 zero-field spin-echo NMR on Co-MCM-41 yielded a broad asymmetric spectrum (200 MHz≤ν≤240 MHz) with a peak at 220 MHz, characteristic of magnetic Co particles which are nanometer in size and single domain. Although there were no discernable spectral features which could be used to identify the short-range atomic order, the peak frequency indicated a face centered cubic (fcc) structure. This is consistent with XRD scans which revealed a broad (111) fcc peak and enabled an estimate of 2 nm for the particle size.