Molybdenum(VI) oxide

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.

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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.