Molybdenum disulfide
Encyclopedia
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. The natural amorphous form is known as the rarer mineral jordisite. MoS2 is less reactive than other transition metal
chalcogenide
s, being unaffected by dilute acid
s. 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, sometimes to relatively high temperatures.
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
.
Molybdenite
is the principal ore from which molybdenum metal is extracted.
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
s) including graphite, which requires volatile additives, and hexagonal boron nitride
.
MoS2 is diamagnetic and a semiconductor
.
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.
. 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 oil
s and grease
s 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 engine
s. When added to plastic
s, 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
.
and universal joint
s. MoS2-coatings allow bullet
s easier passage through the rifle barrel causing less barrel fouling allowing the barrel to retain ballistic accuracy much longer. 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.
MoS2 is also used in ski glide wax. Many ski wax manufacturers use it now to prevent static buildup in dry snow conditions and to add glide when sliding in dirty snow.
; e.g., hydrodesulfurization
. 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.
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.
, MoS2 increases the rate of photocatalytic hydrogen production
.
Inorganic chemistry
Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
MoS2. This black crystalline sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
of molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
occurs as the mineral molybdenite
Molybdenite
Molybdenite is a mineral of molybdenum disulfide, MoS2. Similar in appearance and feel to graphite, molybdenite has a lubricating effect that is a consequence of its layered structure. The atomic structure consists of a sheet of molybdenum atoms sandwiched between sheets of sulfur atoms...
. It is the principal ore from which molybdenum metal is extracted. The natural amorphous form is known as the rarer mineral jordisite. MoS2 is less reactive than other transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
chalcogenide
Chalcogenide
A chalcogenide is a chemical compound consisting of at least one chalcogen ion and at least one more electropositive element. Although all group 16 elements of the periodic table are defined as chalcogens, the term is more commonly reserved for sulfides, selenides, and tellurides, rather than...
s, being unaffected by dilute acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s. In its appearance and feel, molybdenum disulfide is similar to graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
. Indeed, like graphite, it is widely used as a solid lubricant because of its low friction properties, sometimes to relatively high temperatures.
Production
Froth flotation
Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in several processing industries...
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
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
.
Molybdenite
Molybdenite
Molybdenite is a mineral of molybdenum disulfide, MoS2. Similar in appearance and feel to graphite, molybdenite has a lubricating effect that is a consequence of its layered structure. The atomic structure consists of a sheet of molybdenum atoms sandwiched between sheets of sulfur atoms...
is the principal ore from which molybdenum metal is extracted.
Structure and physical properties
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. Because of the weak van der WaalsVan der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
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
Dry lubricant
Dry lubricants or solid lubricants are materials which despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid media....
s) including graphite, which requires volatile additives, and hexagonal boron nitride
Boron nitride
Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms...
.
MoS2 is diamagnetic and a semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
.
Chemical properties
Molybdenum disulfide is stable in air or oxygen at normal conditions, but reacts with oxygen upon heating forming molybdenum trioxide:- 2 MoS2 + 9 O2 → 2 MoO3 + 4 SO3
Chlorine attacks molybdenum disulfide at elevated temperatures to form molybdenum pentachloride:
- 2 MoS2 + 7 Cl2 → 2 MoCl5 + 2 S2Cl2
Molybdenum disulfide reacts with alkyl lithium under controlled conditions to form intercalation compounds LixMoS2. With butyl lithium
N-Butyllithium
n-Butyllithium is an organolithium reagent. It is widely used as a polymerization initiator in the production of elastomers such as polybutadiene or styrene-butadiene-styrene...
, the product is LiMoS2.
Use as lubricant
MoS2 with particle sizes in the range of 1–100 µm is a common dry lubricantDry lubricant
Dry lubricants or solid lubricants are materials which despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid media....
. 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 oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
s and grease
Grease (lubricant)
The term grease is used to describe semisolid lubricants. Although the word grease is also used to describe rendered fat of animals, in the context of lubrication, grease typically applies to a material consisting of a soap emulsified with mineral or vegetable oil...
s 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 engine
Aircraft engine
An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. Aircraft engines are almost always either lightweight piston engines or gas turbines...
s. When added to plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
s, MoS2 forms a composite
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
with improved strength as well as reduced friction. Polymers that have been filled with MoS2 include nylon
Nylon
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...
(with the trade name
Trade name
A trade name, also known as a trading name or a business name, is the name which a business trades under for commercial purposes, although its registered, legal name, used for contracts and other formal situations, may be another....
Nylatron
Nylatron
Nylatron is a tradename for a family of nylon plastics, typically filled with molybdenum disulfide lubricant powder. It is used to cast plastic parts for machines, because of its mechanical properties and wear-resistance....
), Teflon, and Vespel. Self-lubricating composite coatings for high-temperature applications have been developed consisting of molybdenum disulfide and titanium nitride
Titanium nitride
Titanium nitride is an extremely hard ceramic material, often used as a coating on titanium alloys, steel, carbide, and aluminium components to improve the substrate's surface properties....
by chemical vapor deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
.
Specific uses
MoS2 is often used in two-stroke engines; e.g., motorcycle engines. It is also used in CVConstant-velocity joint
Constant-velocity joints allow a drive shaft to transmit power through a variable angle, at constant rotational speed, without an appreciable increase in friction or play. They are mainly used in front wheel drive and all wheel drive cars...
and universal joint
Universal joint
A universal joint, universal coupling, U joint, Cardan joint, Hardy-Spicer joint, or Hooke's joint is a joint or coupling in a rigid rod that allows the rod to 'bend' in any direction, and is commonly used in shafts that transmit rotary motion...
s. MoS2-coatings allow bullet
Bullet
A bullet is a projectile propelled by a firearm, sling, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration...
s easier passage through the rifle barrel causing less barrel fouling allowing the barrel to retain ballistic accuracy much longer. 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.
MoS2 is also used in ski glide wax. Many ski wax manufacturers use it now to prevent static buildup in dry snow conditions and to add glide when sliding in dirty snow.
Use in petrochemistry
Synthetic MoS2 is employed as a catalyst for desulfurization in petroleum refineriesPetrochemistry
Petrochemistry is a branch of chemistry that studies the transformation of crude oil and natural gas into useful products or raw materials. These petrochemicals have become an essential part of the chemical industry today.-Origin of Petroleum:...
; e.g., hydrodesulfurization
Hydrodesulfurization
Hydrodesulfurization is a catalytic chemical process widely used to remove sulfur from natural gas and from refined petroleum products such as gasoline or petrol, jet fuel, kerosene, diesel fuel, and fuel oils...
. The effectiveness of the MoS2 catalysts is enhanced by doping
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
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.
Lubrication
With the exception of hexagonal boron nitride, 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 glazeCompacted oxide layer glaze
Compacted oxide layer glaze describes the often shiny, wear-protective layer of oxide formed when two metals are slid against each other at high temperature in an oxygen-containing atmosphere...
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
When combined with cadmium sulfideCadmium sulfide
Cadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow solid. It occurs in nature with two different crystal structures as the rare minerals greenockite and hawleyite, but is more prevalent as an impurity substituent in the similarly structured zinc ores...
, MoS2 increases the rate of photocatalytic hydrogen production
Hydrogen production
Hydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis...
.