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

Abstract

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 55 Mn 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 Mn 3+ ions in the α-Mn 2 O 3 phase (S = 2, 4.90 μ B ). However, the magnetization and NMR results reported here clearly identify a small amount of the Mn 3 O 4 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-Mn 2 O 3 nanoparticles. The presence of Mn 3 O 4 and the interplay of Mn 3+ and Mn 2+ impact the catalytic properties.

Aerobic Self‐Esterification of Alcohols Assisted by Mesoporous Manganese and Cobalt Oxide

“…In our previous studies introducing small amounts of cations enhanced the catalytic activity by providing basic character, labile oxygen, and easy reducibility . Complete and general catalyst characterization has been provided in preceding studies . Here, representative characterization (XRD, N 2 adsorption, SEM, and TEM) results are provided to confirm the properties of the catalysts.…”

“…A proposed reaction mechanism is shown in Scheme . The mixed phase nature of the catalyst was previously elucidated with two different readily interchangeable Mn oxidation states . Decarboxylation steps and removal of CO 2 are demonstrated by TG‐MS analysis of used catalysts after reaction with isotope labeled carboxylic acid (Figure S5).…”

“…Mechanistic studies on meso MnOx suggest a redox reaction between Mn 3+ and Mn 2+ is essential for catalysis . Meso MnOx calcined at 350 °C is shown to be mainly amorphous while at 450 °C shows a clear presence of Mn 3 O 4 and Mn 2 O 3 . An amorphous appearance is evident in the unused catalyst with some local Mn 3 O 4 crystallinity.…”

“…Seeing the value in non‐precious metals catalysis for esterification of broad range of substrates, we sought a green chemistry approach by simplifying the reaction to aerobic conditions and mesoporous metal oxides. University of Connecticut synthesis (UCT) is a group of mesoporous materials synthesized by a surfactant assisted sol‐gel method of making mesoporous materials and has been shown to have broad applicability for catalysis . Here, we report the self‐esterification reaction enabled by aerobic heterogeneous UCT metal oxide catalysts mainly mesoporous manganese oxides (meso‐MnOx) and mesoporous cobalt oxides (meso‐CoOx).…”

Advanced Materials for Next‐Generation Spacecraft

“…Figure illustrates other possible applications of the advanced materials that could significantly enhance the thruster's service life, efficiency, and reliability. Among others, we will briefly emphasise here the possible cold cathodes that utilize electron emission via strong electric fields enhanced by the long nanostructures, such as carbon nanotubes and graphene flakes; advanced magnetic system based on nanostructured magnetically active materials; self‐healing design elements analogous to those described for the satellite level, metamaterials, and many other advanced material systems capable of bringing the future thruster to a new orbit of functionality …”