| Molybdenum disulfide | |
|---|---|
|
Molybdenum disulfide |
|
|
Other names
|
|
| Identifiers | |
| CAS number | 1317-33-5 |
| ChemSpider | 14138 |
| ChEBI | CHEBI:30704 |
| RTECS number | QA4697000 |
| Jmol-3D images | Image 1 |
|
|
|
|
| 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 | Hexagonal, hP6, space group P63/mmc, No 194 |
| Coordination geometry |
Trigonal prismatic (MoIV) Pyramidal (S2−) |
| Hazards | |
| MSDS | External MSDS |
| EU Index | not listed |
| Related compounds | |
| Other anions | Molybdenum(IV) oxide |
| Other cations | Tungsten disulfide |
| Related lubricants | Graphite |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
|
| Infobox references | |
Molybdenum disulfide is the inorganic compound with the formula MoS2.
This black crystalline sulfide of molybdenum occurs as the mineral molybdenite. It is the principal ore from which molybdenum metal is extracted. MoS2 is relatively unreactive, being unaffected by dilute acids and oxygen. In its appearance and feel, molybdenum disulfide is similar to graphite. Indeed, like graphite, it is widely used as a solid lubricant because of its low friction properties and robustness.
Contents |
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. The natural amorphous form is known as the rarer mineral jordisite.
Molybdenite is the principal ore from which molybdenum metal is extracted.[1]
In MoS2, each Mo(IV) center occupies a trigonal prismatic coordination sphere, being bound to six sulfide ligands. Each sulfur centre is pyramidal, being connected to three Mo centres. In this way, the trigonal prisms are interconnected to give a layered structure, wherein molybdenum atoms are sandwiched between layers of sulfur atoms.[2] Because of 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 lubricant) including graphite, which requires volatile additives, and hexagonal boron nitride.[3]
MoS2 is diamagnetic. As a semiconductor it has a electronic bandgap of about 1.7 eV. Because of its anisotropic structure, it exhibits anisotropic conductivity. It has been often investigated as a component of photoelectrochemical (e.g. for photocatalytic hydrogen production) and microelectronics applications.[4]
Molybdenum disulfide is stable in air or oxygen at normal conditions, but reacts with oxygen upon heating forming molybdenum trioxide:
Chlorine attacks molybdenum disulfide at elevated temperatures to form molybdenum pentachloride:
Molybdenum disulfide reacts with alkyl lithium under controlled conditions to form intercalation compounds LixMoS2. With butyl lithium, the product is LiMoS2.[1]
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.[5][6]
Molybdenum disulfide is often a component of blends and composites where low friction is sought. A variety of oils and greases are used, because 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.[7]
MoS2 is often used in two-stroke engines; e.g., motorcycle engines. It is also used in CV and universal joints. MoS2-coatings allow bullets easier passage through the rifle barrel causing less barrel fouling allowing the barrel to retain ballistic accuracy much longer.[8] This resistance to barrel fouling comes at a cost of lower muzzle velocity with the same load due to a decreased chamber pressure. MoS2 is applied to bearings in ultra-high vacuum applications up to 10−9 torr (at −226 to 399 °C). The lubricant is applied by burnishing and the excess is wiped from the bearing surface.[9]
MoS2 is also used in ski wax to prevent static buildup in dry snow conditions and to add glide when sliding in dirty snow.[10][11]
Synthetic MoS2 is employed as a catalyst for desulfurization in petroleum refineries; e.g., hydrodesulfurization.[12] The effectiveness of the MoS2 catalysts is enhanced by doping with small amounts of cobalt or nickel and the intimate mixture is supported on alumina. Such catalysts are generated in situ by treating molybdate/cobalt or nickel-impregnated alumina with H2S or an equivalent reagent.
|
|||||