Heavy metal removal from water/wastewater by nanosized metal oxides: a review.

Recent advances in transition metal catalyzed oxidation of organic substrates with molecular oxygen.

Manganese oxide minerals have been used for thousands of years—by the ancients for pigments and to clarify glass, and today as ores of Mn metal, catalysts, and battery material. More than 30 Mn oxide minerals occur in a wide variety of geological settings. They are major components of Mn nodules that pave huge areas of the ocean floor and bottoms of many fresh-water lakes. Mn oxide minerals are ubiquitous in soils and sediments and participate in a variety of chemical reactions that affect groundwater and bulk soil composition. Their typical occurrence as fine-grained mixtures makes it difficult to study their atomic structures and crystal chemistries. In recent years, however, investigations using transmission electron microscopy and powder x-ray and neutron diffraction methods have provided important new insights into the structures and properties of these materials. The crystal structures for todorokite and birnessite, two of the more common Mn oxide minerals in terrestrial deposits and ocean nodules, were determined by using powder x-ray diffraction data and the Rietveld refinement method. Because of the large tunnels in todorokite and related structures there is considerable interest in the use of these materials and synthetic analogues as catalysts and cation exchange agents. Birnessite-group minerals have layer structures and readily undergo oxidation reduction and cation-exchange reactions and play a major role in controlling groundwater chemistry.

https://www.pnas.org/content/pnas/96/7/3447.full.pdf

Manganese oxide minerals: Crystal structures and economic and environmental significance

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Recent Advances in Zn-Ion Batteries

MnO2 is currently under extensive investigations for its capacitance properties. MnO2 crystallizes into several crystallographic structures, namely, α, β, γ, δ, and λ structures. Because these structures differ in the way MnO6 octahedra are interlinked, they possess tunnels or interlayers with gaps of different magnitudes. Because capacitance properties are due to intercalation/deintercalation of protons or cations in MnO2, only some crystallographic structures, which possess sufficient gaps to accommodate these ions, are expected to be useful for capacitance studies. In order to examine the dependence of capacitance on crystal structure, the present study involves preparation of these various crystal phases of MnO2 in nanodimensions and to evaluate their capacitance properties. Results of α-MnO2 prepared by a microemulsion route (α-MnO2(m)) are also used for comparison. Spherical particles of about 50 nm, nanorods of 30−50 nm in diameter, or interlocked fibers of 10−20 nm in diameters are formed, which d...

Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties

A thermally stable 3 x 3 octahedral molecular sieve corresponding to natural todorokite (OMS-1) has been synthesized by autoclaving layer-structure manganese oxides, which are prepared by reactions of MnO(4)(-) and Mn(2+) under markedly alkaline conditions. The nature and thermal stability of products depend strongly on preparation parameters, such as the MnO(4)(-)/Mn(2+) ratio, pH, aging, and autoclave conditions. The purest and the most thermally stable todorokite is obtained at a ratio of 0.30 to 0.40. Autoclave treatments at about 150 degrees to 180 degrees C for more than 2 days yield OMS-1, which is as thermally stable (500 degrees C) as natural todorokite minerals. Adsorption data give a tunnel size of 6.9 angstroms and an increase of cyclohexane or carbon tetrachloride uptake with dehydration temperature up to 500 degrees C. At 600 degrees C, the tunnel structure collapses. Both Lewis and Bronsted acid sites have been observed in OMS-1. Particular applications of these materials include adsorption, electrochemical sensors, and oxidation catalysis.

https://science.sciencemag.org/content/sci/260/5107/511.full.pdf

Manganese Oxide Octahedral Molecular Sieves: Preparation, Characterization, and Applications

Hollandite and cryptomelane materials have been prepared using two different methods. Octahedral molecular siev (OMS) having the 2x2 hollandite structure with a one-dimensional pore diameter of 4.6 {Angstrom} Synthetic cryptomelane or OMS-2 can be formed by refluxing or autoclaving an acidic solution of KMnO{sub 4} and Mn{sup 2+}. Temperature, pH, and countercation are important synthetic parameters. The hollandite formed shows thermal stability up to 600 {degrees}C. OMS-2 formed by oxidation of Mn{sup 2+} under basic conditions and calcined at higher temperature (200-800 {degrees}C) is thermally stable up to 800 {degrees}C. X-ray powder diffraction and electron diffraction patterns have been simulated that show good agreement with experimental data supporting a tetragonal crystal system in the I4/m space group. Hollandites were also prepared in the presence of other transition-metal ions such as Cu{sup 2+} and Fe{sup 3+}. Electron paramagnetic resonance (EPR) data show that OMS-2 materials synthesized in the presence of Cu{sup 2+} and Fe{sup 3+} contain nonexchangeable Mn{sup 2+}. EPR data for Cu-OMS-2 showed a characteristic six-line pattern with a g value of 2.0 and A value of 85 G indicative of octahedral Mn{sup 2+} coordination. The Mn{sup 2+} EPR peaks in Fe-OMS-2 showed similar g and A values. EPR spectra,more » ion-exchange data, X-ray diffraction patterns, and the theoretical simulations of diffraction data suggest that Cu{sup 2+} and Fe{sup 3+} are located in the tunnels of OMS-2. 29 refs., 8 figs., 3 tabs.« less

Synthesis and characterization of octahedral molecular sieves (OMS-2) having the hollandite structure

Five hydrated inorganic divalent cations, Mg[sup 2+], Co[sup 2+], Ni[sup 2+], Cu[sup 2+], and Zn[sup 2+], have successfully been used as templates for the synthesis of manganese oxide octahedral molecular sieves (OMS-1) having the todorokite structure. The OMS-1 samples have been well characterized by X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, scanning electron microscopy/energy dispersive X-ray studies, inductively coupled plasma analysis, electron paramagnetic resonance, Fourier transform infrared spectroscopy, thiosulfate titration, and cyclohexane sorption. Catalytic CO oxidation and 2-propanol decomposition were carried out. Results show that these OMS-1 samples with a tunnel size of about 6.9 angstroms are crystalline and chemically pure. They have the following formulas: Mg[sub 3.17]Mn[sub 5.05]O[sub 12] [center dot] 4.52H[sub 2]O, Co[sub 1.84]Mn[sub 5.59]O[sub 12] [center dot] 3.45H[sub 2]O, Ni[sub 1.64]Mn[sub 5.75]O[sub 12] [center dot] 4.19H[sub 2]O, Cu[sub 3.50]Mn[sub 4.45]O[sub 12] [center dot] 5.33H[sub 2]O, and Zn[sub 3.55]Mn[sub 4.47]O[sub 12] [center dot] 2.59H[sub 2]O. Their thermal stability largely depends on the nature of the cations. 47 refs., 16 figs., 3 tabs.

Effects of Inorganic Cation Templates on Octahedral Molecular Sieves of Manganese Oxide

Reflux methods were used to prepare different metal cation (M:  Cu2+, Zn2+, Ni2+, Co2+, Al3+, or Mg2+) doped manganese oxide octahedral molecular sieve (M−OMS-2) materials with cryptomelane structure. Thorough characterization has been carried out for these M−OMS-2 materials. SEM micrographs of M−OMS-2 materials reveal their typical fibrous morphology. The results of ICP−AES elemental analyses for M−OMS-2 materials shed light on the distribution of doped metal cations in the cryptomelane structure. TGA and XRD studies indicate that the prepared M−OMS-2 materials are more thermally stable in O2 (up to 600 °C) than in inert (e.g., N2 or He) atmospheres (up to 500 °C). Phase transformations from cryptomelane to hausmannite or bixbyite were observed when M−OMS-2 materials were heated above these stable temperatures. Comprehensive N2 sorption analyses confirmed that M−OMS-2 materials are porous materials, which contain micropores and mesopores, with total BET surface areas in the range of 70−110 m2/g. Basicity...

Yan-Fei Shen

Papers

Manganese Oxide Octahedral Molecular Sieves: Preparation, Characterization, and Applications

Synthesis and characterization of octahedral molecular sieves (OMS-2) having the hollandite structure

Effects of Inorganic Cation Templates on Octahedral Molecular Sieves of Manganese Oxide

Characterization of Manganese Oxide Octahedral Molecular Sieve (M−OMS-2) Materials with Different Metal Cation Dopants

Studies of Decomposition of H2O2over Manganese Oxide Octahedral Molecular Sieve Materials