Molybdenum disulfide

Molybdenum disulfide
Molybdenum disulfide
General
Systematic name	Molybdenum disulfide Molybdenum(IV) sulfide
Other names	Molybdenite
Molecular formula	MoS2
Molar mass	160.07 g/mol
Appearance	black solid
CAS number	1317-33-5
Properties
Density and phase	5.06 g/cm³, ?
Solubility	insoluble in all solvents
Melting point	1185 °C decomp.
Structure
Coordination geometry	Trigonal prismatic at Mo, pyramidal at S
Crystal structure	see text
Hazards
MSDS	External MSDS
EU classification	not listed
NFPA 704
Flash point	n.a.
RTECS number	QA4697000
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Molybdenum(IV) oxide Molybdenum trioxide
Other cations	Tungsten disulfide
Related lubricants	Graphite
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Molybdenum disulfide, also called molybdenum sulfide or molybdenum(IV) sulfide, with the formula MoS₂, is a black crystalline sulfide of molybdenum and one of several compounds known as transition metal dichalcogenides. It occurs as the mineral molybdenite. It is insoluble in water and unreactive toward dilute acids. Its melting point is 1185 °C, but it starts oxidizing in air from 315 °C, limiting the range of its use as a lubricant in the presence of air between the temperatures of -185 and +350 °C; in nonoxidizing environments it is stable up to 1100 °C.[1]

Use as Lubricant

The structure, appearance, and feel of molybdenum disulfide is similar to graphite - a sandwich of layers of molybdenum atoms between the layers of sulfur atoms. Due to the weak interactions between the sheets of sulfide atoms, MoS₂ has a lubricating effect. Finely powdered MoS₂ with particle sizes in the range of 1-100 µm is a common dry lubricant. It is also often mixed into various oils and greases, which allows the mechanisms lubricated by it to keep running for a while longer, even in cases of almost complete oil loss - finding an important use in aircraft engines. It is often used in motorcycle engines, especially in areas of two-stroke engines which are not otherwise well lubricated.

MoS₂ grease is recommended for CV and universal joints.

It is also used as a lubricating additive to special plastics, notably nylon and Teflon.

During the Vietnam war, a commercial molybdenum disulfide product, "Dri-Slide", was used for lubricating troops' weapons; the military refused to supply it, as it was "not in the manual", so it was sent to soldiers by their parents and friends privately.[2] Another application is for coating bullets, giving them easier passage through the rifle barrel with less deformation and better ballistic accuracy.

Self-lubricating composite coatings for high temperature applications were developed at the Oak Ridge National Laboratory. A composite coating of molybdenum disulfide and titanium nitride was created on the surface of parts by chemical vapor deposition. [3]

Use in Petrochemistry

Synthetic MoS₂ is employed as a catalyst for desulfurization in petroleum refineries, e.g. hydrodesulfurization. The effectiveness of the MoS₂ catalysts is enhanced by doping with small amounts of cobalt and the intimate mixture is supported on alumina. Such catalysts are generated in situ by treating molybdate/cobalt-impregnated alumina with H₂S or an equivalent reagent.

References

• Topsøe, H.; Clausen, B. S.; Massoth, F. E. "Hydrotreating Catalysis, Science and Technology"; Springer-Verlag: Berlin, 1996.

External links

• Molybdenum sulfide MSDS