Iron(II,III) oxide

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Iron(II,III) oxide
IUPAC name iron(II) diiron(III) oxide
Other names ferrous ferric oxide, ferroso ferric oxide, iron(II,III) oxide, magnetite, black iron oxide, lodestone
Identifiers
CAS number [1317-61-9
1309-38-2]
Properties
Molecular formula Fe3O4
Molar mass 231.533 g/mol
Appearance black powder
Density 5.17 g/cm3
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Infobox disclaimer and references

Iron(II,III) oxide is the chemical compound with formula Fe3O4. It is one of possible 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.[1] 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.[2]

Contents

[edit] 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[2] :
C6H5NO2 + 9Fe + 2H2O → C6H5NH2 + Fe3O4
  • Oxidation of FeII 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.[2]

Reduction of Fe2O3 with hydrogen[3][4]:

3Fe2O3 + H2 → 2Fe3O4 +H2O

Reduction of Fe2O3 with CO[5]:

3Fe2O3 + CO → 2Fe3O4 + CO2

Production of nano-particles can be performed chemically by taking for example mixtures of FeII and FeIII salts and mixing them with alkali to precipitate colloidal Fe3O4. The reaction conditions are critical to the process and determine the particle size.[6]

[edit] Reactions

Reduction of magnetite ore by CO in a blast furnace is used to produce iron as part of steel production process[1]:

Fe3O4 + 4CO → 3Fe + 4CO2

Controlled oxidation of Fe3O4 is used to produce brown pigment quality γ-Fe2O3 (maghemite)[7]:

2Fe3O4 + ½ O2 → 3(γ-Fe2O3)

More vigorous calcining, (roasting in air), gives red pigment quality α-Fe2O3 (hematite)[7]:

2Fe3O4 + ½ O2 → 3(α-Fe2O3)

[edit] Structure

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.[1] Fe3O4 samples can be non-stoichiometric.[1]
The ferrimagnetism of Fe3O4 arises because the electron spins of the FeII and FeIII ions in the octahedral sites are coupled and the spins of the FeIII 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.[1]

[edit] 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.[8] This effect has been extensively investigated and whilst various explanations have been proposed, it does not appear to be fully understood.[9]
Fe3O4 is a an electrical conductor with a conductivity is significantly higher (X 106) than Fe2O3, and this is ascribed to electron exchange between the FeII and FeIII centres.[1]

[edit] Uses

Fe3O4 is used as a black pigment and is known as C.I pigment black 11 (C.I. No.77499).[7]
Fe3O4 is used as a catalyst in the Haber process and in the water gas shift reaction.[10] The latter uses an HTS (high temperature shift catalyst) of iron oxide stabilised by chromium oxide.[10] This iron-chrome catalyst is reduced at reactor start up to generate Fe3O4 from α-Fe2O3 and Cr2O3 to CrO3.[10]
Nano particles of Fe3O4 are used as contrast agents in MRI scanning [11]

[edit] Biological Occurrence

Magnetite has been found as nano-crystals in magnetotactic bacteria (42-45 μm)[2] and in homing pigeon beak tissue[12]

[edit] References

  1. ^ a b c d e f Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
  2. ^ a b c d Rochelle M. Cornell, Udo Schwertmann 2007 The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses Wiley-VCH ISBN 3527606440
  3. ^ US patent 2596954, 1947, Process for reduction of iron ore to magnetiteHeath T.D.
  4. ^ Kinetics of reduction of iron oxides by H2 Part I: Low temperature reduction of hematite, A. Pineau, N. Kanari, I. Gaballah, Thermochimica Acta, 447, 1, 1 (2006), 89-100 doi:10.1016/j.tca.2005.10.004
  5. ^ The effects of nucleation and growth on the reduction of Fe2O3 to Fe3O4 Hayes P. C., Grieveson P Metallurgical and Materials Transactions B (1981), 12, 2, 319-326,doi:10.1007/BF02654465 10.1007/BF02654465
  6. ^ Arthur T. Hubbard (2002) Encyclopedia of Surface and Colloid Science CRC Press, ISBN 0824707966
  7. ^ a b c Gunter Buxbaum, Gerhard Pfaff (2005) Industrial Inorganic Pigments 3d edition Wiley-VCH ISBN 3527303634
  8. ^ Electronic Conduction of Magnetite (Fe3O4) and its Transition Point at Low Temperatures, Verwey E. J. W., nature 144, 327-328 (1939) doi:10.1038/144327b0
  9. ^ The Verwey transition - a topical review Walz F. J. Phys.:Condens. Matter (2002) 14, 285-340 doi:10.1088/0953-8984/14/12/203
  10. ^ a b c Sunggyu Lee (2006) Encyclopedia of Chemical Processing CRC Press ISBN 0824755634
  11. ^ Synthesis of Iron Oxide Nanoparticles Used as MRI Contrast Agents: A Parametric Study, Babes L, Denizot B, Tanguy G, Le Jeune J.J., Jallet P. Journal of Colloid and Interface Science, 212,2, (1999), 474-482, doi:10.1006/jcis.1998.6053
  12. ^ Superparamagnetic Magnetite in the Upper Beak Tissue of Homing Pigeons Hanzlik M., Heunemann C., Holtkamp-Rötzler E., Winklhofer M., Petersen N., Fleissner G. BioMetals, (2000), 13, 4, 325-331 {doi|10.1023/A:1009214526685}}
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