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Molybdenum(VI) oxide



Gas properties
IUPAC name Molybdenum trioxide
Molybdenum(VI) oxide
Other names Molybdic anhydride
Molybdite
Molybdenum oxide
Molybdic trioxide
Identifiers
CAS number 1313-27-5
Properties
Molecular formula MoO3
Molar mass 143.94 g/mol
Appearance yellow solid
Density 4.7 g/cm3, solid
Melting point

795 °C

Boiling point

1155 °C

Solubility in water 0.5 g/L (20 °C)
Structure
Crystal structure orthorhombic
Coordination
geometry
distorted octahedral
Thermochemistry
Std enthalpy of
formation
ΔfHo298
−745.17 kJ/mol
Standard molar
entropy
So298
77.78 J.K–1.mol–1
Hazards
EU classification Harmful (Xn)
Irritant (Xi)
R-phrases R36/37, R48/20/22
S-phrases (S2), S22, S25
Flash point not applicable
Related Compounds
Other anions Molybdenum disulfide
Other cations Chromium trioxide
Molybdenum dioxide
Tungsten trioxide
Related compounds Molybdic acid
Sodium molybdate
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Molybdenum(VI) oxide is chemical compound with the formula MoO3. This compound is produced on the largest scale of any molybdenum compound. It occurs as the rare mineral molybdite. Its chief application is as an oxidation catalyst and as a raw material for the production of molybdenum metal.

Additional recommended knowledge

Contents

Structure

In the gas phase, three oxygen atoms are double bonded to the central molybdenum atom. In the solid state, anhydrous MoO3 is composed of layers of distorted MO6 octahedra in an orthorhombic crystal. The octahedra share edges and form chains which are cross-linked by oxygen atoms to form layers. The octahedra have one short molydenum-oxygen bond to a non-bridging oxygen.[1]
The image shows a section of the chain made up from edge sharing distorted octahedra. The oxygen atoms above and below the chain link to other chains to build the layer.

Preparation and principal reactions

MoO3 is produced industrially by burning molybdenum disulfide, the chief ore of molybdenum:

2 MoS2 + 7 O2 → 2 MoO3 + 4 SO2

The laboratory synthesis entails the acidification of aqueous solutions of sodium molybdate:[2]

Na2MoO4 + H2O + 2 HClO4 → MoO3(H2O)2 + 2 NaClO4

The dihydrate loses water readily to give the monohydrate. Both are bright yellow in color.

Molybdenum(VI) oxide dissolves slightly in water to give "molybdic acid." In base, it dissolves to afford the molybdate anion.

Uses

Molybdenum(VI) oxide is used to manufacture molybdenum metal, which serves as an additive to steel and corrosion-resistant alloys. The relevant conversion entails treatment of MoO3 with hydrogen at elevated temperatures:

MoO3 + 3 H2 → Mo + 3 H2O

It is also a component of the co-catalyst used in the industrial production of acrylonitrile by the oxidation of propene and ammonia.

Because of its layered structure and the ease of the Mo(VI)/Mo(V) couple, MoO3 is of interest in electrochemical devices and displays.[3]

References

  1. ^ Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
  2. ^ Heynes, J. B. B.; Cruywagen, J. J. "Yellow Molybdenum(VI) Oxide Dihydrate" Inorganic Syntheses, 1986, volume 24, pp. 191. ISBN 0-471-83441-6.
  3. ^ Ferreira, F. F.; Souza Cruz, T. G.; Fantini, M. C. A.; Tabacniks, M. H.; de Castro, S. C.; Morais, J.; de Siervo, A.; Landers, R.; Gorenstein, A. Solid State Ionics. 2000, 136-137, 357.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Molybdenum(VI)_oxide". A list of authors is available in Wikipedia.
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