Comprehensive Magnetic Study of Nanostructured Mesoporous Manganese Oxide Materials and Implications for Catalytic Behavior

Magnetic behavior of nanostructured mesoporous manganese oxide materials, designated UCT-1 and UCT-18, were studied using a combination of superconducting quantum interference device (SQUID) magnetometry and Mn-55 zero-field spin echo nuclear magnetic resonance (NMR). Curie Weiss fits to the magnetic susceptibility for the UCT-1 and UCT-18 samples calcined at 550 degrees C yielded paramagnetic moment values consistent with spin-only Mn3+ ions in the alpha-Mn2O3 phase (S = 2, 4.90 mu(B)). However, the magnetization and NMR results reported here clearly identify a small amount of the Mn3O4 second phase (ferrimagnetic with T-C approximate to 43 K) that does not appear in X-ray diffraction (XRD). The study resulted in the observation of fascinating magnetic behavior: (1) exchange bias, which occurs in cases where a ferromagnetic (or ferrimagnetic) phase forms a boundary with an antiferromagnetic phase and (2) a magnetic contribution attributed to uncompensated spins on the surfaces of the alpha-Mn2O3 nanoparticles. The presence of Mn3O4 and the interplay of Mn3+ and Mn2+ impact the catalytic properties.