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 solid lubricant due to its low friction properties, sometimes to relatively high temperatures.

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. 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.

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. 

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. 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. 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.

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.