Molybdenum(IV) oxide

Molybdenum(IV) oxide
IUPAC name Molybdenum(IV) oxide
Other names Molybdenum dioxide Tugarinovite
Identifiers
CAS number	18868-43-4 Y
Properties
Molecular formula	MoO2
Molar mass	127.94 g/mol
Appearance	brownish-violet solid
Density	6.47 g/cm3
Melting point	1100 °C decomp.
Solubility in water	insoluble
Solubility	insoluble in alkalies, HCl, HF slightly soluble in hot HSO4
Structure
Crystal structure	Distorted rutile (tetragonal)
Coordination geometry	Octahedral (MoIV); trigonal (O–II)
Hazards
EU Index	Not listed
Flash point	Non-flammable
Related compounds
Other anions	Molybdenum disulfide
Other cations	Chromium(IV) oxide Tungsten(IV) oxide
Related molybdenum oxides	"Molybdenum blue" Molybdenum trioxide
Y oxide (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Molybdenum dioxide is the chemical compound with the formula MoO₂. It is a violet-colored solid and is a metallic conductor. It crystallizes in a monoclinic cell, and has a distorted rutile, (TiO₂) crystal structure. In TiO₂ the oxide anions are close packed and titanium atoms occupy half of the octahedral interstices (holes). In MoO₂ the octahedra are distorted, the Mo atoms are off-centre, leading to alternating short and long Mo – Mo distances and Mo-Mo bonding. The short Mo – Mo distance is 251 pm which is less than the Mo – Mo distance in the metal, 272.5 pm. The bond length is shorter than would be expected for a single bond. The bonding is complex and involves a delocalisation of some of the Mo electrons in a conductance band accounting for the metallic conductivity^[1].
MoO₂ can be prepared :

• by reduction of MoO₃ with Mo over the course of 70 hours at 800 °C. The tungsten analogue, WO₂, is prepared similarly.

2 MoO₃ + Mo → 3 MoO₂

• by reducing MoO₃ with H₂ or NH₃ below 470°C ^[2]

Single crystals are obtained by chemical transport using iodine. Iodine reversibly converts MoO₂ into the volatile species MoO₂I₂^[3].

Molybdenum oxide is a constituent of "technical molybdenum oxide" produced during the industrial processing of MoS₂^[4]:

2 MoS₂ + 7O₂ → 2MoO₃ + 4SO₂

MoS₂ + 6MoO₃ → 7MoO₂ + 2SO₂

2 MoO₂ + O₂ → 2MoO₃

MoO₂ has been reported as catalysing the dehydrogenation of alcohols^[5] and the reformation of hydrocarbons^[6]. Molybdenum nano-wires have been produced by reducing MoO₂ deposited on graphite^[7]

Mineralogical form of this compound is called tugarinovite, only very rarely found.

References

1. ^ Oxides: Solid state chemistry McCarroll W.H. Encyclopedia of Inorganic Chemistry Ed R. Bruce King, (1994), John Wiley & sons ISBN 0-471-93620-0
2. ^ Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5 
3. ^ Conroy, L. E.; Ben-Dor, L. "Molybdenum(IV) Oxide and Tungsten(IV) Oxides Single-Crystals" Inorganic Syntheses 1995, volume 30, pp. 105–107. ISBN 0-471-30508-1
4. ^ Metallurgical furnaces Jorg Grzella, Peter Sturm, Joachim Kruger, Markus A. Reuter, Carina Kogler, Thomas Probst, Ullmans Encyclopedia of Industrial Chemistry
5. ^ A. A. Balandin and I. D. Rozhdestvenskaya, Russian Chemical Bulletin, 8, 11, (1959), 1573 doi:10.1007/BF00914749
6. ^ Molybdenum based catalysts. I. MoO₂ as the active species in the reforming of hydrocarbons A. Katrib, P. Leflaive, L. Hilaire and G. Maire Catalysis Letters, 38, 1–2, (1996) doi:10.1007/BF00806906
7. ^ Synthesis of Molybdenum Nanowires with Millimeter-Scale Lengths Using Electrochemical Step Edge Decoration M. P. Zach, K. Inazu, K. H. Ng, J. C. Hemminger, and R. M. Penner Chem. Mater. (2002),14, 3206 doi:10.1021/cm020249a