Molybdenum disulfide

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]

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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.