Molybdenum disulfide

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Molybdenum disulfide
Image:Molybdenite-3D-balls.png
IUPAC name	Molybdenum disulfide Molybdenum(IV) sulfide molybdenum sulfide
Other names	Molybdenite
Identifiers
CAS number	1317-33-5
RTECS number	QA4697000
Properties
Molecular formula	MoS2
Molar mass	160.07 g/mol
Appearance	black solid
Density	5.06 g/cm³, ?
Melting point	1185 °C decomp.
Structure
Crystal structure	see text
Coordination geometry	Trigonal prismatic at Mo, pyramidal at S
Hazards
EU classification	not listed
Flash point	n.a.
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 is the inorganic compound with the formula MoS₂. This black crystalline sulfide of molybdenum occurs as the mineral molybdenite. More so than other transition metal chalcogenides, MoS₂ is unreactive, being unaffected by dilute acids. In terms of its appearance and feel, molybdenum disulfide is similar to graphite and indeed it is widely used as a solid lubricant^[1][1] due to its low friction properties, sometimes to relatively high temperatures.

Contents 1 Production 2 Structure and basic properties 3 Use as lubricant 3.1 Specific uses 4 Use in petrochemistry 5 Future Developments 5.1 Lubrication 5.2 Photocatalyst 6 References 7 External links

Production

Molybdenite ore is processed by flotation to give relatively pure MoS₂, the main contaminant being carbon. MoS₂ also arises by the thermal treatment of virtually all molybdenum compounds with hydrogen sulfide.

Structure and basic properties

In MoS₂, each Mo(IV) center is trigonal prismatic, being bound to six sulfide ligands, each of which is pyramidal. The trigonal prisms are interconnected to give a layered structure, wherein molybdenum atoms are sandwiched between layers of sulfur atoms.^[1] Due to the weak van der Waals interactions between the sheets of sulfide atoms, MoS₂ has a low coefficient of friction, resulting in its lubricating properties. Other layered inorganic materials exhibit lubricating properties (collectively known as solid lubricants or dry lubricants) including graphite, which requires volatile additives, and hexagonal boron nitride.^[1]

MoS₂ is diamagnetic and a semiconductor.

Use as lubricant

MoS₂ with particle sizes in the range of 1-100 µm is a common dry lubricant. Few alternatives exist that can confer the high lubricity and stability up to 350 °C in oxidizing environments. Sliding friction tests of MoS₂ using a pin on disc tester at low loads (0.1-2 N) give friction coefficient values of <0.1.

Molybdenum disulfide is often a component of blends and composites where low friction is sought. A variety of oils and greases are used, since they retain their lubricity even in cases of almost complete oil loss, thus finding a use in critical applications such as aircraft engines. When added to plastics, MoS₂ forms a composite with improved strength as well as reduced friction. Polymers that have been filled with MoS₂ include nylon, with the trade name Nylatron, Teflon, and Vespel. Self-lubricating composite coatings for high-temperature applications have been developed consisting of molybdenum disulfide and titanium nitride by chemical vapor deposition. [1]

Specific uses

It is often used in two-stroke engines, e.g., motorcycle engines. MoS₂ is also used in CV and universal joints. During the Vietnam War, the molybdenum disulfide product "Dri-Slide" was used to lubricate weapons, although it was supplied from private sources, not the military.[2] MoS₂-coatings allow bullets easier passage through the rifle barrel with less deformation and better ballistic accuracy.

Use in petrochemistry

Synthetic MoS₂ is employed as a catalyst for desulfurization in petroleum refineries, e.g., hydrodesulfurization.^[1] 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.

Future Developments

Lubrication

There are currently no clear lubrication alternatives to molybdenum disulfide or the very similar tungsten disulfide that can resist temperatures higher than 350°C in oxidizing environments. Research has been conducted on compacted oxide layer glazes, which form during metallic surface sliding wear at several hundred degrees Celsius. However, because these oxide layers are physically-unstable, their use has currently not proven practical.

Photocatalyst

A research group headed by Prof. LI Can with the Dalian Institute of Chemical Physics has claimed that when combined with Cadmium sulfide it significantly increases the rate of photocatalytic Hydrogen production [3].

References

External links

• Molybdenum sulfide MSDSde:Molybdän(IV)-sulfid

nl:Molybdeensulfide ja:二硫化モリブデン