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

• Published in: Chemistry of materials 2018-02, Vol.30 (3), p.1164-1177
• Main Authors: Moharreri, Ehsan, Hines, William A, Biswas, Sourav, Perry, David M, He, Junkai, Murray-Simmons, Dustin, Suib, Steven L
• Format: Article
• Language: English
• Subjects: Chemistry; Materials Science
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.7b05280

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 55Mn 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 °C yielded paramagnetic moment values consistent with spin-only Mn3+ ions in the α-Mn2O3 phase (S = 2, 4.90 μB). However, the magnetization and NMR results reported here clearly identify a small amount of the Mn3O4 second phase (ferrimagnetic with T C ≈ 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 α-Mn2O3 nanoparticles. The presence of Mn3O4 and the interplay of Mn3+ and Mn2+ impact the catalytic properties.