Molybdenum disulfide
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| Molybdenum disulfide | |
|---|---|
| Image:Molybdenum disulfide.jpg | |
| General | |
| Systematic name | Molybdenum disulfide Molybdenum(IV) sulfide |
| Other names | Molybdenite |
| Molecular formula | MoS2 |
| Molar mass | 160.07 g/mol |
| Appearance | black solid |
| CAS number | 1317-33-5 |
| Properties | |
| Density and phase | 5.06 g/cm³, ? |
| Solubility | insoluble in all solvents |
| Melting point | 1185 °C decomp. |
| Structure | |
| Coordination geometry |
Trigonal prismatic at Mo,
pyramidal at S |
| Crystal structure | see text |
| Hazards | |
| MSDS | External MSDS |
| EU classification | not listed |
| NFPA 704 | |
| Flash point | n.a. |
| RTECS number | QA4697000 |
| Supplementary data page | |
| Structure and properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| 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 |
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Molybdenum disulfide, also called molybdenum sulfide or molybdenum(IV) sulfide, with the formula MoS2, is a black crystalline sulfide of molybdenum and one of several compounds known as transition metal dichalcogenides. It occurs as the mineral molybdenite. It is insoluble in water and unreactive toward dilute acids. Its melting point is 1185 °C, but it starts oxidizing in air from 315 °C, limiting the range of its use as a lubricant in the presence of air between the temperatures of -185 and +350 °C; in nonoxidizing environments it is stable up to 1100 °C.[1]
Contents |
[edit] Use as Lubricant
The structure, appearance, and feel of molybdenum disulfide is similar to graphite - a sandwich of layers of molybdenum atoms between the layers of sulfur atoms. Due to the weak interactions between the sheets of sulfide atoms, MoS2 has a lubricating effect. 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, which allows the mechanisms lubricated by it to keep running for a while longer, even in cases of almost complete oil loss - finding an important use in aircraft engines. It is often used in motorcycle engines, especially in areas of two-stroke engines which are not otherwise well lubricated.
MoS2 grease is recommended for CV and universal joints.
It is also used as a lubricating additive to special plastics, notably nylon and teflon.
During the Vietnam war, a commercial molybdenum disulfide product, "Dri-Slide", was used for lubricating troops' weapons; the military refused to supply it, as it was "not in the manual", so it was sent to soldiers by their parents and friends privately.[2] Another application is for coating bullets, giving them easier passage through the rifle barrel with less deformation and better ballistic accuracy.
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. [3]
[edit] 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.
[edit] References
- Topsøe, H.; Clausen, B. S.; Massoth, F. E. "Hydrotreating Catalysis, Science and Technology"; Springer-Verlag: Berlin, 1996.
