There seem to be many different forms of manganese oxide that they consider. MnO is mentioned and MnO2 is also mentioned.
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Supplementary Information 6 - Proposed Combustion Mechanism
We propose that the mechanism in facilitating combustion involves the low temperature decomposition of
manganese dioxide, stimulated by the reactive gases derived from wood pyrolysis and the consequent release of oxygen that both reduces the critical temperature for ignition and increases the rate of char combustion. Pure
manganese dioxide with a
β-MnO2 structure decomposes on heating, releasing oxygen in several stages, to
Mn2O3 at 631oC, to
Mn3O4 (
hausmannite) at 963oC and finally to
MnO at temperatures above 1300oC (21,22). The TGAs of the manganese dioxides without wood (Supplementary Information 5) show a first decomposition temperature of 555oC for MD4 (96% MnO2) and approximately 640oC for the Pech-de-l'Azé I material MD3 and no further transition below 900oC. However, in contrast to the very high temperatures required to produce hausmannite, XRD confirmed that the manganese dioxides and Pech-de-l'Azé I blocs transformed to hausmannite during combustion at the comparatively low temperatures in the combustion experiments (Figure 2 and Supplementary Information 4). The presence of reactive gases has a strong effect on manganese dioxide decomposition, lowering the temperature of transformation to MnO from over 1300oC to around 450oC (23,24,25). The pyrolysis of biomass materials has a similar effect, allowing manganese dioxide to decompose to MnO at temperatures between 300oC and 500oC (26). In a similar manner, wood produces reactive, chemically-reducing volatile compounds as it undergoes thermal decomposition of the hemi-cellulose to volatile organic compounds followed at higher temperatures by the decomposition of the cellulose and lignin (27). The beech wood used in the combustion experiments releases volatile organic compounds above 220oC with a peak rate at 300oC (Figure 4a).
We suggest that the reducing gases and volatile organic compounds released in the initial decomposition of wood provide a reactive environment at the manganese dioxide surface that lowers the decomposition temperature to approximately 290oC to 300oC. The decomposition of manganese dioxide to hausmannite releases oxygen that reacts with the char, producing the glowing combustion.
The overall effects of the manganese dioxide are a substantial reduction in the auto-ignition temperature of the wood and a substantially increased rate of combustion (Figure 4). Continuing combustion is supported by oxygen from the air. The initial thermal input for initiating wood pyrolysis, manganese dioxide decomposition and the modest temperatures required for auto-ignition can be produced with spark-lit tinder; under identical conditions wood alone does not ignite. The
manganite (Υ-MnOOH) found in most of the blocs transforms to manganese dioxide at 300oC (18) and probably contributes to the combustion process
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Support for the mechanism is provided by the lack of effect shown by the thermally stable oxides, zinc oxide, aluminium oxide and titanium dioxide that do not release oxygen when heated to temperatures normally experienced in a wood fire. Whereas manganese dioxide facilitates wood ignition, no such effect was found with wood and romanèchite (
hydrated barium manganese oxide) mixtures. The lack of effect of romanèchite on wood ignition is perhaps explained by the significantly higher temperatures required for the decomposition of romanèchite, the lower amounts of oxygen released (22) and a reduced effect of wood decomposition volatiles on the decomposition process for romanèchite (see the DTGs for MD7 and MD7 and wood mixtures in Supplementary Information 5).
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http://www.nature.com/article-assets/npg/srep/2016/160229/srep22159/extref/srep22159-s1.pdfPlease forgive my ignorance, but does the electronegativity difference method ((O) 3.44 - (Mn) 1.55 = 1.89) of determining bond type work for compounds that have multiple types? Would MnO, MnO2 and Mn2O3 all share the same bond types based on the same difference calculation?
The following webpage lists MnO2 as containing 1 covalently bonded unit:
My chemistry is very rusty, and my physical chemistry is non-existent. How would one characterize chemical bond type in terms of quantum mechanics? Is there an expectation value of some kind that can be calculated and associated with a bond type?
At any rate, thanks for any suggestions or corrections that you may offer.
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