Molybdenum disulfide
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2008-07-13T05:32:52Z
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{{Chembox new
| Name = Molybdenum disulfide
| ImageFile = Molybdenite-3D-balls.png
| ImageSize = 200px
| ImageName = Molybdenum disulfide
| IUPACName = Molybdenum disulfide<br/>Molybdenum(IV) sulfide<br/>molybdenum sulfide
| OtherNames = [[Molybdenite]]
| Section1 = {{Chembox Identifiers
| CASNo = 1317-33-5
| RTECS = QA4697000
}}
| Section2 = {{Chembox Properties
| Formula = MoS<sub>2</sub>
| MolarMass = 160.07 g/mol
| Appearance = black solid
| Density = 5.06 g/cm³, ?
| MeltingPt = 1185 °C ''decomp.''
}}
| Section3 = {{Chembox Structure
| Coordination = [[Trigonal prism]]atic at Mo,
pyramidal at S
| CrystalStruct = ''see text''
}}
| Section7 = {{Chembox Hazards
| EUClass = not listed
| FlashPt = n.a.
}}
| Section8 = {{Chembox Related
| OtherAnions = [[Molybdenum(IV) oxide]]<br />[[Molybdenum trioxide]]
| OtherCations = [[Tungsten disulfide]]
| Function = [[lubricant]]s
| OtherFunctn = [[Graphite]]
}}
}}
'''Molybdenum disulfide''' is the [[inorganic chemistry|inorganic compound]] with the [[chemical formula|formula]] MoS<sub>2</sub>. This black crystalline [[sulfide]] of [[molybdenum]] occurs as the mineral [[molybdenite]]. More so than other [[transition metal]] [[chalcogenide]]s, MoS<sub>2</sub> is unreactive, being unaffected by dilute [[acid]]s. In terms of its appearance and feel, molybdenum disulfide is similar to [[graphite]] and indeed it is widely used as a solid lubricant<ref>G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6.</ref><ref>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.</ref> due to its low friction properties, sometimes to relatively high temperatures.
==Production==
Molybdenite ore is processed by [[Froth flotation|flotation]] to give relatively pure MoS<sub>2</sub>, the main contaminant being carbon. MoS<sub>2</sub> also arises by the thermal treatment of virtually all molybdenum compounds with [[hydrogen sulfide]].
==Structure and basic properties==
In MoS<sub>2</sub>, 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.<ref>Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.</ref> Due to the weak [[van der Waals]] interactions between the sheets of sulfide atoms, MoS<sub>2</sub> has a low [[coefficient of friction]], resulting in its lubricating properties. Other layered inorganic materials exhibit lubricating properties (collectively known as [[solid lubricant]]s or [[dry lubricant]]s) including graphite, which requires volatile additives, and hexagonal [[boron nitride]].<ref>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.</ref>
MoS<sub>2</sub> is diamagnetic and a [[semiconductor]].
==Use as lubricant==
MoS<sub>2</sub> with particle sizes in the range of 1-100 µ[[metre|m]] 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 MoS<sub>2</sub> 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 [[oil]]s and [[grease (lubricant)|grease]]s 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 engine]]s. When added to [[plastic]]s, MoS<sub>2</sub> forms a [[composite material|composite]] with improved strength as well as reduced friction. Polymers that have been filled with MoS<sub>2</sub> 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]]. [http://www.ornl.gov/info/press_releases/get_press_release.cfm?ReleaseNumber=mr19950329-01]<!--vanity citation?-->
====Specific uses====
It is often used in [[two-stroke engine]]s, e.g., motorcycle engines. MoS<sub>2</sub> is also used in [[Constant-velocity joint|CV]] and [[universal joint]]s. 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.<!---Dri-Slide was not moly, it was PTFE. Moly is dark-like graphite, and the Dri-Slide is off-white. --->[http://mmd.foxtail.com/Archives/Digests/199809/1998.09.05.09.html] MoS<sub>2</sub>-coatings allow [[bullet]]s easier passage through the rifle barrel with less deformation and better ballistic accuracy.{{Fact|date=July 2008}}
==Use in petrochemistry==
Synthetic MoS<sub>2</sub> is employed as a [[catalyst]] for desulfurization in [[Petrochemistry|petroleum refineries]], e.g., [[hydrodesulfurization]].<ref>Topsøe, H.; Clausen, B. S.; Massoth, F. E. "Hydrotreating Catalysis, Science and Technology"; Springer-Verlag: Berlin, 1996.</ref> The effectiveness of the MoS<sub>2</sub> catalysts is enhanced by [[Doping (semiconductor)|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 H<sub>2</sub>S 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 glaze]]s, 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 [http://english.cas.ac.cn/eng2003/news/detailnewsb.asp?InfoNo=27192].
==References==
<references/>
==External links==
* [http://www.gfschemicals.com/Search/MSDS/1468MSDS.PDF Molybdenum sulfide MSDS]
[[Category:Molybdenum compounds]]
[[Category:Sulfides]]
[[Category:Lubricants]]
[[Category:Semiconductor materials]]
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