Iron(II,III) oxide

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IUPACName = iron(II) diiron(III) oxide
OtherNames = ferrous ferric oxide, ferroso ferric oxide, iron(II,III) oxide, magnetite, black iron oxide, lodestone, rust
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CASOther = [1317-61-9 [
[1309-38-2]
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Formula = Fe₃O₄ FeO.Fe₂O₃
MolarMass = 231.533 g/mol
Appearance = black powder
Density = 5.17 g/cm3
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Iron(II,III) oxide is the chemical compound with formula Fe₃O₄. It is one of a number of iron oxides. It is found in nature as the mineral magnetite. It contains both Fe²⁺ and Fe³⁺ ions and is sometimes formulated as FeO.Fe₂O₃. It is encountered in the laboratory as a black powder. It exhibits permanent magnetism and is ferrimagnetic, but is sometimes incorrectly described as ferromagnetic.Greenwood&Earnshaw] Its most extensive use is as a black pigment which is synthesised rather than being extracted from the naturally occurring mineral as the particle size and shape can be varied by the method of production.Rochelle M. Cornell, Udo Schwertmann 2007 "The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses" Wiley-VCH ISBN 3527606440]

Preparation

Pigment quality Fe₃O₄, so called synthetic magnetite, can be prepared using processes that utilise industrial wastes, scrap iron or solutions containing iron salts (e.g. those produced as by-products in industrial processes such as the acid treatment (pickling) of steel):

*Oxidation of Fe metal in the Laux process where nitrobenzene is reacted with iron metal using FeCl₂ as a catalyst to produce aniline ::C₆H₅NO₂ + 9Fe + 2H₂O → C₆H₅NH₂ + Fe₃O₄
*Oxidation of Fe^II compounds, e.g. the precipitation of iron(II) salts as hydroxides followed by oxidation by aeration where careful control of the pH determines the oxide produced.

Reduction of Fe₂O₃ with hydrogen: [US patent 2596954, 1947, Process for reduction of iron ore to magnetiteHeath T.D.] [cite journal | title = Kinetics of reduction of iron oxides by H2 Part I: Low temperature reduction of hematite | author = A. Pineau, N. Kanari, I. Gaballah | journal = Thermochimica Acta | volume = 447 | issue = 1 | pages = 89–100 | year = 2006 | doi=10.1016/j.tca.2005.10.004] :3Fe₂O₃ + H₂ → 2Fe₃O₄ +H₂OReduction of Fe₂O₃ with CO: [cite journal | title = The effects of nucleation and growth on the reduction of Fe₂O₃ to Fe₃O₄ | author = Hayes P. C., Grieveson P. | journal = Metallurgical and Materials Transactions B | year = 1981 | volume = 12 | issue = 2 | pages = 319–326 | doi = 10.1007/BF02654465] :3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂

Production of nano-particles can be performed chemically by taking for example mixtures of Fe^II and Fe^III salts and mixing them with alkali to precipitate colloidal Fe₃O₄. The reaction conditions are critical to the process and determine the particle size. [Arthur T. Hubbard (2002) "Encyclopedia of Surface and Colloid Science" CRC Press, ISBN 0824707966]

Reactions

Reduction of magnetite ore by CO in a blast furnace is used to produce iron as part of steel production process::Fe₃O₄ + 4CO → 3Fe + 4CO₂

Controlled oxidation of Fe₃O₄ is used to produce brown pigment quality γ-Fe₂O₃ (maghemite):Gunter Buxbaum, Gerhard Pfaff (2005) "Industrial Inorganic Pigments" 3d edition Wiley-VCH ISBN 3527303634]

:2Fe₃O₄ + ½ O₂ → 3(γ-Fe₂O₃)More vigorous calcining, (roasting in air), gives red pigment quality α-Fe₂O₃ (hematite)::2Fe₃O₄ + ½ O₂ → 3(α-Fe₂O₃)

tructure

Fe₃O₄ has a cubic inverse spinel structure which consists of a cubic close packed array of oxide ions where all of the Fe²⁺ ions occupy half of the octahedral sites and the Fe³⁺ are split evenly across the remaining octahedral sites and the tetrahedral sites.

Both FeO and γ-Fe₂O₃ have a similar cubic close packed array of oxide ions and this accounts for the ready interchangeability between the three compounds on oxidation and reduction as these reactions entail a relatively small change to the overall structure. Fe₃O₄ samples can be non-stoichiometric.

The ferrimagnetism of Fe₃O₄ arises because the electron spins of the Fe^II and Fe^III ions in the octahedral sites are coupled and the spins of the Fe^III ions in the tetrahedral sites are coupled but anti-parallel to the former. The net effect is that the magnetic contributions of both sets are not balanced and there is a permanent magnetism.

Properties

Fe₃O₄ is ferrimagnetic with a Curie temperature of 858 K. There is a phase transition at 120K, the so-called Verwey transition where there is a discontinuity in the structure, conductivity and magnetic properties. [cite journal | title = Electronic Conduction of Magnetite (Fe₃O₄) and its Transition Point at Low Temperatures | author = Verwey E. J. W. | journal = Nature | volume = 144 | pages = 327–328 (1939) | doi = 10.1038/144327b0 | year = 1939] This effect has been extensively investigated and whilst various explanations have been proposed, it does not appear to be fully understood. [cite journal | title = The Verwey transition - a topical review | author = Walz F. | journal = Condens. Matter | year = 2002 | volume = 14 | pages = 285–340 | doi = 10.1088/0953-8984/14/12/203]

Fe₃O₄ is an electrical conductor with a conductivity is significantly higher (X 10⁶) than Fe₂O₃, and this is ascribed to electron exchange between the Fe^II and Fe^III centres.

Uses

Fe₃O₄ is used as a black pigment and is known as C.I pigment black 11 (C.I. No.77499).

Fe₃O₄ is used as a catalyst in the Haber process and in the water gas shift reaction.Sunggyu Lee (2006) Encyclopedia of Chemical Processing CRC Press ISBN 0824755634] The latter uses an HTS (high temperature shift catalyst) of iron oxide stabilised by chromium oxide. This iron-chrome catalyst is reduced at reactor start up to generate Fe₃O₄ from α-Fe₂O₃ and Cr₂O₃ to CrO₃.

Nano particles of Fe₃O₄ are used as contrast agents in MRI scanning [cite journal | title = Synthesis of Iron Oxide Nanoparticles Used as MRI Contrast Agents: A Parametric Study | author = Babes L, Denizot B, Tanguy G, Le Jeune J.J., Jallet P. | journal = Journal of Colloid and Interface Science | volume = 212 | issue = 2 | year = 1999 | pages = 474–482 | doi = 10.1006/jcis.1998.6053]

Biological Occurrence

Magnetite has been found as nano-crystals in magnetotactic bacteria (42-45 μm) and in homing pigeon beak tissue [cite journal | author = Hanzlik M., Heunemann C., Holtkamp-Rötzler E., Winklhofer M., Petersen N., Fleissner G | journal = BioMetals | year = 2000 | volume = 13 | issue = 4 | pages = 325–331 | doi = 10.1023/A:1009214526685 | title =Superparamagnetic Magnetite in the Upper Beak Tissue of Homing Pigeons ]

References