Manganese oxides, crystalline phases

ZR Tian. From microporous to mesoporous manganese oxide crystalline phases syntheses, characterization, and applications. PhD dissertation. Storrs.CT University of Connecticut, 1998. 

More than 20 manganese oxide phases have been described from continental manganese deposits, but very few of these minerals have been positively identified from marine environments (Table 3). Useful summaries of the nature and nomenclature of the marine manganese minerals include those of Cronan (1976b), R. G. Bums and V. M. Burns (1979), and Post (1999). Marine manganese oxides are hydrous phases usually with poor crystallinity. They are often intimately intergrown with other phases, particularly iron-oxy-hydroxides and hydrous iron oxides. Marine manganese-oxide minerals host a variety of minor elements of economic... 

Manganese oxides in natural systems are often complex minerals characterized by poor crystallinity and mixed oxidation states (McKenzie, 1989). In addition to being electron acceptors in the oxidation of As(III) to As(V), Mn oxides can also adsorb As. Direct evidence for adsorption of As by Mn oxides comes from laboratory experiments in which pure mineral phases have been used. 

Oxides of manganese and iron. Acidified hydroxylamine hydrochloride releases metals from the manganese oxide phase with minimal attack on the iron oxide phases. Amorphous and crystalline forms of iron oxides can be discriminated between by extracting with acid ammonium oxalate in the dark and under UV light, respectively. 

Facile methods have been developed to synthesize single-crystalline manganese oxide and oxyhydroxide nanowires/nanorods. Pure phase of single-crystalline y-MnOOH nanowires/nanorods have been synthesized by hydrothermal transformation of granular y- or p-MnOa in water at 180 - 200 °C, and pure P-MnOa and a-MnaOa nanowires/nanorods formed after subsequent calcination at... 

Environmental samples such as soils consist of multiple solid phases, which can be crystalline, poorly crystalline (or amorphous), or a mixture of the two. The solid phases may be coated in part by other crystalline or amorphous phases (e.g., hydrous aluminum, iron, or manganese oxides), natural organic matter, fungi, or microbial biofilms, and are always in contact with an aqueous solution and/or a gas, as discussed above. The contaminant or pollutant species in environmental samples are often at low concentrations (e.g., parts per million, ppm, or parts per billion, ppb), and are... 

Multigram quantities of manganese oxide (K-OMS-2) nanomaterials have been prepared using microwave heating and standard reflux glassware. The reactions were performed using aqueous/organic solvent mixtures. In the case of a water/DMSO mixture, the crystalline cryptomelane phase was formed within 10 min and the relative... 

Suib and coworkers demonstrated that a mesoporous mixed-valent manganese oxide can be successfully synthesized by the direct precipitation of manganese hydroxide precursors in basic conditions.The ordered mesoporous structure with crystalline pore walls was retained after calcination as confirmed by powder x-ray diffraction (XRD) and transmission electron microscope (TEM). Such mixed-valent mesoporous oxides are semiconducting and possess catalytic activity in the oxidation of alkanes. The mixed oxidation states of manganese (III/IV) in these materials were introduced by the direct oxidation of the metal precursor [Mn(II)j. Both hexagonal and cubic phases of such mesoporous materials can be synthesized. 

---