Platinum Group Metals

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Platinum Group Metals are commonly found in asteroids and may possibly be found in lunar impact craters.

The platinum-group metals (PGM) comprise six closely related metals: platinum, palladium, rhodium, ruthenium, iridium, and osmium, which commonly occur together in nature and are among the scarcest of the metallic elements. Along with gold and silver, they are known as precious or noble metals. They occur as native alloys in placer deposits or, more commonly, in lode deposits associated with nickel and copper. Nearly all of the world's supply of these metals are extracted from lode deposits in four countries--the Republic of South Africa, the U.S.S.R., Canada, and the United States. The Republic of South Africa is the only country that produces all six PGM in substantial quantities. - USGS Platinum-Group Metals Statistical Compendium[1]

Applications

The catalytic properties of the six platinum group metals (PGM)– iridium, osmium, palladium, platinum, rhodium, and ruthenium – are outstanding. Platinum's wear and tarnish resistance characteristics are well suited for making fine jewelry. Other distinctive properties include resistance to chemical attack, excellent high-temperature characteristics, and stable electrical properties. All these properties have been exploited for industrial applications. Platinum, platinum alloys, and iridium are used as crucible materials for the growth of single crystals, especially oxides. The chemical industry uses a significant amount of either platinum or a platinum-rhodium alloy catalyst in the form of gauze to catalyze the partial oxidation of ammonia to yield nitric oxide, which is the raw material for fertilizers, explosives, and nitric acid. In recent years, a number of PGM have become important as catalysts in synthetic organic chemistry. Ruthenium dioxide is used as coatings on dimensionally stable titanium anodes used in the production of chlorine and caustic. Platinum supported catalysts are used in the refining of crude oil, reforming, and other processes used in the production of high-octane gasoline and aromatic compounds for the petrochemical industry. Since 1979, the automotive industry has emerged as the principal consumer of PGM. Palladium, platinum, and rhodium have been used as oxidation catalyst in catalytic converters to treat automobile exhaust emissions. A wide range of PGM alloy compositions is used in low-voltage and low-energy contacts, thick- and thin-film circuits, thermocouples and furnace components, and electrodes. - USGS Platinum-Group Metals Statistics and Information[2]

Iridium

Atomic symbol:	Ir
Atomic number:	77
Group:	9
Period:	6
Series:	Transition Metals

Natural Isotopes

Iridium 191
Iridium 193

General Information
Iridium metal is silvery-white, similar to platinum, but has a slight yellow tinge. Iridium is harder and more difficult to work than any other face centered cubic metal and this has been attributed to trace impurities. Due to its high tensile strength and high melting point it has been increasingly used for crucibles for crystal growing. These can only be satisfactorily deep drawn to form seamless crucibles if the work is carried out above the recrystallisation temperature (1000°C).
Iridium possesses quite remarkable properties. It is the most resistant of all metals to corrosion, it is insoluble in mineral acids including aqua regia and is unattacked by many molten metals or silicates at high temperatures, but it is attacked by molten salts, such as NaCl and NaCN, and to a lesser extent, by alkali metal hydroxides under oxidising conditions.
Iridium occurs uncombined in nature, and in combination with platinum and other metals of this family in alluvial deposits. It also occurs in sulphide mineralisation associated with ultramafic ore bodies.
The earliest uses were for making durable tips for fountain pen nibs and compass bearings. In 1908 Sir William Crookes first used iridium for crucibles and high temperature apparatus, and more recently for electrical contacts.
Present day uses include parts for radioisotope thermoelectric motors for space missions. The fuel consists of plutonium-238 dioxide in the form of spheres which are encapsulated in iridium to provide a secure shielding of exceptionally high melting point and strength. A large proportion of iridium production is consumed in alloys with the other platinum group metals, which then take on something of the strength and good corrosion resistance of iridium itself.

Iridium only occurs as a very small percentage of the platinum metals content of an ore so that production tends to fall short of industrial requirements.
PGM Database[3]

Osmium

Atomic symbol:	Os
Atomic number:	76
Group:	8
Period:	6
Series:	Transition Metals

Natural Isotopes

Osmium 184
Osmium 186
Osmium 187
Osmium 188
Osmium 189
Osmium 190
Osmium 192

General Information
The metal is lustrous, bluish white, extremely hard and brittle even at high temperatures. It has the highest melting point and lowest vapour pressure of the platinum group metals. The metal is very difficult to fabricate but can be sintered in a hydrogen atmosphere at a temperature around 2000°C.
The solid metal is not affected by air at room temperature but the powdered or spongy metal slowly gives off osmium tetroxide, which is a powerful oxidising agent, and has a strong smell. The tetroxide is highly toxic and boils at 130°C.
The metal has relatively few industrial uses, but has been used to produce very hard alloys with other platinum group metals, for fountain pen tips, for instrument pivots, for gramophone needles and electrical contacts.
It is used in some catalytic applications and has an interesting chemistry based on cluster compounds.
PGM Database[4]

Palladium

Atomic symbol:	Pd
Atomic number:	46
Group:	10
Period:	5
Series:	Transition Metals

Natural Isotopes

Palladium 102
Palladium 104
Palladium 105
Palladium 106
Palladium 108
Palladium 110

General Information
It is a steel white metal, does not tarnish in air and has the lowest density and melting point of the platinum group metals. When annealed it is soft and ductile. Cold working greatly increases its strength and hardness. Palladium is attacked by nitric and sulphuric acids. At room temperature the metal has the unique property of absorbing up to 900 times its own volume of hydrogen. Hydrogen readily diffuses through heated palladium and development of this property provides a means of puifying the gas.
Finely divided palladium is a good catalyst and is used for industrial hydrogenation and de-hydrogenation reactions.
Its alloys are used in the jewellery trade. White gold is often produced by addition of palladium. Like gold, palladium can be beaten into leaf as thin as 1/250,000 in. The metal and its alloys are used in dentistry, watchmaking, in making surgical instruments, and in electrical contacts.
PHYSICAL PROPERTIES.
Palladium is a monomorphic metal, with a face-centered cubic structure and extremely close atomic packing. The melting point of palladium is 1552°C and its boiling point is 3980°C. Like all platinum group metals, palladium has comparatively low electrical resistivity and a high temperature coefficient of resistance. It is paramagnetic and has the highest magnetic susceptibility of all the platinum group metals.
MECHANICAL PROPERTIES.
The Brinell hardness of commercial grade palladium is 31.5 Kgf/mm². Cold working palladium metal causes a sharp increase in its hardness and tensile strength. With a 10% deformation, the hardness of unalloyed palladium reaches 61 Kgf/mm². The increase in hardness becomes proportionally less as the deformation is further increased and only reaches 79 Kgf/mm² at 90% deformation.
Palladium has the lowest elastic characteristics of the platinum metals, having comparatively low strength, and high reduction in area at high percentage of elongation.
PGM Database[5]

Platinum

Atomic symbol:	Pt
Atomic number:	78
Group:	10
Period:	6
Series:	Transition Metals

Natural Isotopes

Platinum 190
Platinum 192
Platinum 194
Platinum 195
Platinum 196
Platinum 198

General Information
Platinum is a beautiful silvery-white metal, when pure, and is malleable and ductile. It has a coefficient of expansion almost equal to that of soda-lime-silica glass, and has been used to make sealed-in electrodes in glass apparatus. The metal does not oxidize appreciably in air at any temperature, but is corroded by halogens, cyanides, sulphur, and caustic alkalis. It is insoluble in hydrochloric and nitric acids, but dissolves when they are mixed to form aqua regia, forming chloroplatinic acid, an important compound.
The metal is extensively used in jewellery, wire, and vessels for laboratory use, and in many valuable industrial products including thermocouples. It is also used for electrical contacts, corrosion-resistant apparatus, and in dentistry. Platinum-cobalt alloys have powerful magnetic properties. One such alloy made of 76.7% Pt and 23.3% Co, by weight, offers a B-H (max) almost twice that of Alnico V. Platinum resistance wires are used for constructing high-temperature electric furnaces. The metal is used for coating missile nose cones, jet engine fuel nozzles, gas-turbine blades, etc., which must perform reliably for long periods of time at high temperatures under oxidising conditions. The metal, somewhat like palladium, absorbs hydrogen, retaining it at ordinary temperatures but giving it up at red heat.
In the finely divided state, platinum is an excellent catalyst, having long been used in the contact process for producing sulphuric acid. It is also used as a catalyst in cracking petroleum products. There is much current interest in the use of platinum as a catalyst in fuel cells and in antipollution devices for automobiles. Platinum clad anodes are extensively used in cathodic anti-corrosion protection systems for large ships and ocean-going vessels, pipelines, steel piers, etc.
The price of platinum has varied widely; more than a century ago it was cheap enough to be used to adulterate gold. It was nearly eight times as valuable as gold in 1920. The price in December 1987 was about $520/troy oz.
PGM Database[6]

Rhodium

Atomic symbol:	Rh
Atomic number:	45
Group:	9
Period:	5
Series:	Transition Metals

Natural Isotopes

Rhodium 103

General Information
The metal is silvery white and at red heat is slowly oxidised to the sesquioxide in air. At higher temperatures the oxide decomposes to the metal again.
Rhodium has a higher melting point than platinum, and lower density. It has a high reflectance and is hard and durable.
As the bulk metal it is mostly used as an alloying agent to harden platinum and palladium. Such alloys are used as furnace windings, thermocouple elements and to make bushings for glass fibre production. It is a useful electrical contact material due to its low electrical resistance, low and stable contact resistance, and it is highly resistant to corrosion. Electroplating or vacuum evaporation produces a highly reflective and exceptionally hard and durable surface suitable for use in optical instruments. Rhodium is also used for a range of catalyst applications and as alloys and coatings for jewellery.
PGM Database[7]

Ruthenium

Atomic symbol:	Ru
Atomic number:	44
Group:	8
Period:	5
Series:	Transition Metals

Natural Isotopes

Ruthenium 96
Ruthenium 98
Ruthenium 99
Ruthenium 100
Ruthenium 101
Ruthenium 102
Ruthenium 104

General Information
The metal is produced as powder by the hydrogen reduction of ammonium ruthenium chloride. Powder metallurgical techniques or argon arc melting are used to consolidate this powder.
Ruthenium is a hard, white metal with four crystal modifications. It does not tarnish at room temperature, but will oxidise in air at about 800°C. The metal is not attacked by hot or cold acids or by aqua regia. However, it oxidises explosively when potassium chlorate is added to the solution. It is also attacked by halogens and alkali hydroxides, etc.
The addition of 0.1% ruthenium to titanium immensely improves the corrosion resistance. It is a versatile catalyst and can be used to promote the splitting of hydrogen sulphide by light using an aqueous suspension of cadmium sulphide loaded with ruthenium oxide. This is thought to have application to removal of hydrogen sulphide during oil refining and other industrial applications.
PGM Database[8]