Oxidation of magnetite

Maghemite formed by oxidation of magnetite is non porous, whereas that obtained by dehydration of lepidocrocite is meso porous. 

Magnetite is obtained in aqueous, alkaline systems by precipitation from a mixed Fe /Fe solution, by oxidation of Fe solution via green rust or Fe(OH)2, or by interaction of Fe with ferrihydrite. Another pathway involves high temperature reduction of Fe oxides (e. g. with H2). Maghemite forms topotactically by wet or dry oxidation of magnetite or by heating lepidocrocite and by thermal decomposition of various organic Fe-salts (cf chap. 20). 

Controlled oxidation of magnetite at 500 °C gives maghemite, i.e. 2 Fe304 + /2 O2 3 Y-FC203... 

David, 1. Welch, A.J.E. (1956) The oxidation of magnetite and related spinels. Trans. Faraday Soc. 52 1642-1650... 

Gallagher, KJ. Feifknecht,W. Marmweiler, U. (1968) Mechanism of oxidation of magnetite to Y-Fe20j. Nature 217 1118-1121 Gallias, J.L. (1998) Microstructure of the interfacial transition zone around corroded reinforcements. In Katz, A. Benier, M. Alexander, M. Arliguie, G. (eds.) The interfadal transition zone in cementitions composites. E.F.N. Spon, London, 171-178 Galvez, N. Barron,V. Torrent, J. (1999) Preparation and properties of hematite with structural phosphorus. Clays Clay Miner. 47 375-385... 

Oxidation of magnetite to hematite is also a heterogeneous reaction ... 

Since the spinel phase must be prepared at low temperatures (by hydrothermal synthesis or by careful oxidation of magnetite at a temperature T < 300 C, for example) it has been widely suspected that some incorporation of hydrogen is needed to stabilize it. However, Schrader and Buttner have shown that pure y-Fe203 does exist, and Coey et al. have been able to prepare Fe3 j04 in the compositional range 0 < x < 0.08 by quenching non-stoichiometric magnetite prepared at 1450 °C. There is no evidence that hydrogen is needed to stabilize the system. 

A detailed investigation about the chemical and structural composition of maghemite (diameters in the range of 10-20 nm) based MF samples, prepared through the oxidation of magnetite nanoparticle, the latter synthesized by the coprecipitation route, was reported [53]. The precipitate containing the... 

In the synthesis of y-Fe203, careful oxidation of magnetite in a spinel lattice to a superstructure of crystallizing ferrimagnetic y-Fe203 (maghemite) under well-defined conditions is necessary ... 

In the course of the spontaneous oxidation of magnetite to y — Fe-jOa, the magnetic moment of the particles changes linearly with the Fe +/Fe3+ ratio between that of pure magnetite and pure y — Fe203. 

Maghemite is rare as a direct precipitate, and usually forms by the oxidation of magnetite (Fe304) or the dehydration of lepidocrocite. Hematite and goethite are probably the most stable ferric oxyhydroxides under earth-surface conditions. Hematite is rare as a direct precipitate... 

Surface passivating layers are further discussed by White and Peterson (55) in the oxidation of magnetite by Cr and by Fendorf (29) in the reduction of pyrolusite by Co -EDTA complexes. In both cases, surface passivation removes the bulk mineral from further reaction with the solution and significantly reduces the redox capacity of the mineral. 

Coupled Half Cell Reactions. The oxidation of magnetite under anoxic conditions without the presence of reducible aqueous metal species consists of coupled anodic and cathodic half cell reactions (16)... 

Equation 5 describes the oxidation of magnetite to form maghemite and equation 6 describes the reductive dissolution of magnetite (22). The decoupled half cell reactions describing ilmenite oxidation (equation 3) are... 

In a solid state reaction at low temperatures a solid reactant may be entirely converted to a solid product that keeps the original structure of the reactant. In such a thermal process new nonequilibrium solids can be made that cannot form when the system would be close to equilibrium. The temperature cannot be too low so that the reaction remains possible (sufficient mobility), and it should not be so high as to prevent crystallization to equilibrium structures. This procedure is called chimie douce or soft chemistry. An example of chimie douce from industrial chemistry is the oxidation of magnetite (a mixed valence form of iron oxide, FCjOJ to the ferromagnetic y-Fe203 (maghemite) in a microstructure that renders it suitable for... 

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