Manganese disproportionation

Reaction 10 produces Mn and could substitute for Reaction 2 in a closed cycle where reaction aflBnities A3 and Aio are both positive. Because some of the electron transfers that would have gone toward completing the manganese oxide disproportionation are diverted in Reaction 10, lead can probably be expected to slow the manganese disproportionation process. 

Modified spinels that contain excess lithium, and preferably an admetal (Li,+ n2 y04, M=AP+, Cr +, Ga ), offer improved storage stability in the discharged state as manganese disproportionation is inhibited when the Mn rMn" ratio is reduced. Materials with admetal content offer fade rates at 55°C of 0.05%/cycle (0.05 mAh/g per cycle), whereas current coated materials offer irreversible capacity loss on storage of less than 1% per week at 55°C in the discharged state, or 20% capacity loss after 500 cycles at 55T. ... 

As the oxidation state of manganese increases, the basicity declines, eg, from MnO to Mn20y. Oxyanions are more readily formed ia the higher valence states. Another characteristic of higher valence-state manganese chemistry is the abundance of disproportionation reactions. 

The Mn ion is so unstable that it scarcely exists in aqueous solution. In acidic aqueous solution, manganic compounds readily disproportionate to form Mn ions and hydrated manganese(IV) oxide, Mn02 2H20 in basic solution these compounds hydroly2e to hydrous manganese(III) oxide, MnO(OH). Sulfuric acid concentrations of about 400 450 g/L are required to stabilize the noncomplexed Mn ion in aqueous solutions. 

Oxidation of manganese dioxide to higher valence states takes place in the fusion process of Mn02 and KOH. A tetravalent manganese salt identified as K MnO [12142-27-7] (63) which disproportionates spontaneously is formed. 

The purple permanganate ion [14333-13-2], MnOu can be obtained from lower valent manganese compounds by a wide variety of reactions, eg, from manganese metal by anodic oxidation from Mn(II) solution by oxidants such as o2one, periodate, bismuthate, and persulfate (using Ag" as catalyst), lead peroxide in acid, or chlorine in base or from MnO by disproportionation, or chemical or electrochemical oxidation. 

There is a paucity of information on reactions of the strongly-oxidising manganese(lll) with inorganic substrates. The main reason for this neglect lies in the tendency of Mn(lII) to disproportionate, viz. 

The oxidation of tartaric and glycollic acid by chromic acid also induces the oxidation of manganous ions. In the presence of higher concentrations of manganese(II) the rate of oxidation of the acids is diminished to about one-third of that in the absence of manganous ions. The decrease of the rate has been attributed to manganese(II) catalysis of the disproportionation of the intermediate valence states of chromium probably chromium(IV). 

The second pathway includes a step in which the trivalent manganese ions formed as intermediates disproportionate. 

Bacterial SODs typically contain either nonheme iron (FeSODs) or manganese (MnSODs) at their active sites, although bacterial copper/zinc and nickel SODs are also known (Imlay and Imlay 1996 Chung et al. 1999). Catalases are usually heme-containing enzymes that catalyze disproportionation of hydrogen peroxide to water and molecular oxygen (Eq. 10.2) (Zamocky and Koller 1999 Loewen et al. 2000). 

Manganese(lll) is a powerful oxidant, with interesting mechanistic chemistry.It can be generated in situ from MnOl" and Mn + in acid solution. By using excess Mn ions and high acidity (3-5 M HCi04) the marked disproportionation and hydrolytic tendencies of Mn(IlI) are suppressed and such solutions are stable for days at room temperature. 

Visual detection of surface layers on cathodes using microscopy techniques such as SFM seems to be supportive of the existence of LiF as a particulate-type deposition.The current sensing atomic force microscope (CSAFM) technique was used by McLarnon and co-workers to observe the thin-film spinel cathode surface, and a thin, electronically insulating surface layer was detected when the electrode was exposed to either DMC or the mixture FC/DMC. The experiments were carried out at an elevated temperature (70 °C) to simulate the poor storage performance of manganese spinel-based cathodes, and degradation of the cathode in the form of disproportionation and Mn + dissolution was ob-served. °° This confirms the previous report by Taras-con and co-workers that the Mn + dissolution is acid-induced and the electrolyte solute (LiPFe) is mainly responsible. 

Manganese is used by nature to catalyze a number of important biological reactions that include the dismutation of superoxide radicals, the decomposition of hydrogen peroxide, and the oxidation of water to dioxygen. The dinuclear manganese centers that occur in Lactobacillus plantar-aum catalase and Thermus thermophilus catalase have attracted considerable attention and many model compounds have now been synthesized that attempt to mimic aspects of these biological systems.The catalases have at least four accessible oxidation states (Mn Mn , Mn°Mn , Mn" Mn", and Mn Mn ) it is believed that the Mn"Mn"/Mn"Mn" redox couple is effective in catalyzing the disproportionation of water. 

This process is catalyzed by a variety of catalase enzymes, the most common being the heme catalases, which accomplish the two-electron chemistry of Eq. (12) at a mononuclear heme center. Here, both the iron and its surrounding porphyrin ligand participate to the extent of one electron each in the redox process. Manganese catalases contain a binuclear Mn center and cycle between Mn2(II,II) and Mn2(III,III) oxidation states while carrying out the disproportionation of H2O2. The enzyme can... 

Fig. 14 Proposed mechanism for disproportionation of H2O2 by manganese catalase (reprinted with permission from Ref 107, Copyright 2004 American Chemical Society).

Manganese(III) is a strong oxidizing agent and is subject to disproportionation as well. Complexes of Mn(III) are also relatively unstable with the exception of [Mn(CN)6]3, which forms readily upon exposure of a solution of manganese(II) and cyanide to air. A few iron(IV) compounds are known. 

The first member of this family, manganese, exhibits One of the most interesting redox chemistries known thus it has already been discussed in detail above. Technetium exhibits the expected oxidation states, and associated with these are modest emf values. All of the isotopes of technetium are radioactive but "Tc has a relatively long half-life (2.14 k 10s years) and is found in nature in small amounts because of the radioactive decay of uranium. Oxidation slates of rhenium range from +7 to - 3, with some species ReOj and Re3+) unstable with respect to disproportionation. 

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