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Molybdenum disulfide



Molybdenum disulfide
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 lubricant.[1][2]

Additional recommended knowledge

Contents

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.[3] 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.[4]

MoS2 is diamagnetic and a semiconducting.

Use as lubricant

With a high melting point (1185 °C) and good chemical stability, MoS2 is a versatile solid lubicant. In air, it degrades at oxidizing in air 350 °C, which limits the range of its use as a lubricant. Sliding friction tests of MoS2 using a pin on disc tester at low loads (0.1-2N) give friction coefficient values of <0.1. 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 in order to lend its lubrication properties even in cases of almost complete oil loss - finding an important use in aircraft engines. It is often used in two-stroke engines, e.g., motorcycle engines. MoS2 grease is recommended for CV and universal joints.

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, and Teflon. In addition, it can be combined with polymer resin to apply low friction coatings to engineering components.

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. [1]

Military and ballistic uses

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.

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.

Due to the low-friction properties of MoS2 even at several hundred degrees centigrade, research has been focusing on the development of compacted oxide layer glazes for use in metallic sliding systems. However, because of the physically-unstable nature of the compound at this temperature, use for it in the near-future has not proven practical.

Use in petrochemistry

Synthetic MoS2 is employed as a catalyst for desulfurization in petroleum refineries, e.g., hydrodesulfurization.[5] 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

  1. ^ G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6.
  2. ^ Shriver, D. F.; Atkins, P. W.; Overton, T. L.; Rourke, J. P.; Weller, M. T.; Armstrong, F. A. “Inorganic Chemistry” W. H. Freeman, New York, 2006. ISBN 0-7167-4878-9.
  3. ^ Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
  4. ^ Thorsten Bartels, Wolfgang Bock, Jürgen Braun, Christian Busch, Wolfgang Buss, Wilfried Dresel, Carmen Freiler, Manfred Harperscheid, Rolf-Peter Heckler, Dietrich Hörner, Franz Kubicki, Georg Lingg, Achim Losch, Rolf Luther, Theo Mang, Siegfried "Lubricants and Lubrication" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley VCH: Weinheim, 2002. DOI: 10.1002/14356007.a15_423.
  5. ^ Topsøe, H.; Clausen, B. S.; Massoth, F. E. "Hydrotreating Catalysis, Science and Technology"; Springer-Verlag: Berlin, 1996.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Molybdenum_disulfide". A list of authors is available in Wikipedia.
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