The six metals

Around 37% are used for catalytic converters, with the remaining approximately 23% being used in other industrial applications around the world.

Platinum and its relatives, iridium and osmium, are the most dense metals known to man (platinum is nearly twice as dense as lead and 11% more dense than gold).

Platinum has a high melting point and temperature stability, a great corrosion resistance, and it is a good oxidation catalyst, conductive and oxidation resistant. Most amazingly, platinum in certain compounds can inhibit the growth of cancerous cell growth, and because it is biologically compatible it is an important component in many medical applications.

Platinum contributes to environmental protection in a number of ways. As already mentioned its efficiency as a catalyst is employed in motor cars, greatly reducing air pollution and greenhouse gases. It also makes a major contribution to waste reduction as its durability and strength mean that goods containing platinum are more reliable and longer-lasting than those containing non-PGM metals. Furthermore, the high recyclability of PGMs means they can be reused many times, which minimizes their load on the environment.

Properties

Electrical conductivity* 0.0966 106 cm-1 Ohm-1
Density* 21.45 g/cc
Hardness (Brinell value)* 392 MN m-2
Melting point* 1772 ºC
Chemical element of Group VIII (Mendeleev)
Atomic number 78
Atomic weight 195.08
Thermal conductivity 73 watts/metre/°C
Tensile strength 14 (annealed condition kg/mm2)

History Pre-Columbian South American Indians are known to have found ways to use platinum for decorative purposes.

The first known example of platinum goes back to prehistoric times in South America - where a small head statue is known to have been shaped out of the metal. Awareness in Europe dates back to 1557 to a French-Italian scientist Julius Scaliger who was in Central America when large deposits of the metal were discovered in South America by the Conquistadors. It's from this discovery that platinum got its name, from platina - the Spanish word for "little silver".

However, it was not until samples began arriving in Europe in the middle of the 18th Century, that processes to melt and refine platinum were applied. In 1801, English physician William Hyde Wollaston obtained the first pure sample and his methods still form the basis of the techniques which produce platinum today.

Wollaston's discovery was an immediate commercial success and he and his collaborators went on to isolate other platinum group metals (PGMs). In 1824, the Ural Mountains were found to harbour significant platinum deposits and by the end of the 19th Century, a new jewellery style had developed combining the "new" metal with diamonds from recently discovered mines in South Africa.

In 1924 South Africa became a rich source of platinum itself when German geologist Hans Merensky discovered there the largest deposits ever found. The Second World War brought great restrictions on the use of platinum as it was needed in armaments and it wasn't until late in the 20th Century that its popularity as a choice for jewellery returned.

Palladium's remarkable qualities mean that it is no less crucial in a number of important applications.

Its melting point is still high compared with other popular metals (for example, over four and half times that of lead) and it has high temperature stability and corrosion resistance. Palladium is also a good oxidation catalyst, conductive, oxidation resistant and ductile when annealed.

But its most incredible property is the ability to absorb 900 times its own volume of hydrogen at room temperature. This makes palladium an efficient and safe hydrogen storage medium and purifier. It is also used in chemical processes that require hydrogen exchange between two reactants, such as that which produces butadiene and cyclohexane, the raw materials for synthetic rubber and nylon.

Palladium's catalytic qualities find it playing a key role in catalytic converters and air purification equipment. Its chemical stability and electrical conductivity make it a more effective and durable plating than gold in electronic components.

Properties

Electrical conductivity* 0.0965 106 cm-1 Ohm-1
Density* 12.02 g/cc
Hardness (Brinell value)* 37.3 MN m-2
Melting point* 1554 ºC
Chemical element of Group VIII (Mendeleev)
Atomic number 46 Atomic weight 106.42
Thermal conductivity 76 watts/metre/°C
Tensile strength 17 (annealed condition kg/mm2)

History

It took nearly two centuries for palladium's significance to be recognised - and the fight against global pollution owes a lot to this unique metal.

Following the perfection of his technique to obtain pure samples of platinum in 1801, William Hyde Wollaston went on to isolate palladium two years later by dissolving platinum ore in aqua regia (a mixture of hydrochloric and nitric acid). He named it after Pallas, the ancient Greek goddess of wisdom whose name had also been lent to the second asteroid ever discovered.

In an attempt to keep his techniques a secret, Wollaston offered samples of palladium for sale anonymously and his peers were cynical about the new metal's provenance, suspecting that it was an alloy of platinum. This forced him to publish details of his findings in 1805.

The use of palladium really took off in the 1970s when demand for catalytic converters - in which its remarkable properties play a key role - increased as automobile emission standards were introduced in the developed world. As these standards were tightened and applied globally in the 90s, demand for palladium expanded exponentially

Like its sister metals platinum and palladium, rhodium also has excellent catalytic activity. Many complex chemical compounds have been developed for use as catalysts, especially in the organic chemicals industry. Rhodium-platinum gauzes are used in the production of nitric acid.

In vehicle exhaust emission control, catalysts containing rhodium are of major importance on account of their exceptional activity and selectivity. Today's three-way catalyst for gasoline engines uses rhodium to catalyse the reduction of nitrogen oxides to nitrogen.

Rhodium's high melting point, high temperature stability and corrosion resistance makes it key to many industrial processes such as glass and glass fibre production. Rhodium's hardness makes it an excellent alloying agent to harden platinum.

Properties

Electrical conductivity* 0.211 106 cm-1 Ohm-1
Density* 12.41 g/cc
Hardness (Brinell value)* 1100 MN m-2
Melting point* 1960 ºC
Chemical element of Group VIII (Mendeleev)
Atomic number 45 Atomic weight 106.42
Thermal conductivity 150 watts/metre/°C
Tensile strength 71 (annealed condition kg/mm2)

History

The third platinum group metal to be discovered became a key factor in the fight against urban pollution. In 1804, William Hyde Wollaston took his discoveries further. From the chloride salts which remained after obtaining palladium from platinum, with aqua regia (a mixture of hydrochloric and nitric acid), he isolated a third distinct metal. By reduction with hydrogen gas, rhodium metal was obtained and named after the Greek for "rose" (rhodon) due to the pinkish hue of the salts.

Today, it is produced as a by-product of platinum mining and refining and of nickel mining in Ontario's copper-nickel sulfide region, Sudbury. Its catalytic qualities and strength were crucial to improving the converters' effectiveness.

Although brittle, it is extremely hard (over four times that of platinum itself) and with its high melting point, temperature stability and corrosion resistance, is used in high-temperature equipment such as the crucibles used to grow crystals for laser technology.

Its biological compatibility is what we owe most to iridium as this enables it to be used in a range of medical and surgical applications. Iridium can be found in health technology combating cancer, Parkinson's disease, heart conditions and even deafness and blindness.

A shiny, oxidation-resistant metal, iridium also adds to the brilliance and durability of jewellery. It also has industrial applications such as the production of chlorine and caustic soda.

Iridium in hydrogen production

Today, interest in iridium has particularly increased due to its application in Proton exchange membrane (PEM) electrolysis. PEM electrolysis is one of the technologies used to produce electrolytic hydrogen, using iridium and platinum, of which iridium will need to be actively managed as demand for PEM electrolysers grows. Iridium is the metal of choice because of the very harsh/acidic environment which only iridium can handle. Some other hydrogen production technologies also use PGMs.

The annual production of iridium amounts to around 7 to 8 tonnes and is closely related to the mining of platinum (PGMs occur together in the ore body and platinum is the main driver for production; iridium cannot be mined separately), hence, mining of iridium does not happen on its own.

Do we have enough iridium to scale up hydrogen production?

It has been argued that the scarcity of iridium and its uniqueness in its applications creates an impossible challenge for scaling up PEM electrolysis to the capacities needed. However, this is not true. Research undertaken by industry players suggests that with appropriate management, notably through thrifting and recycling, there will be enough iridium and platinum available to allow PEM electrolysis and PEM fuel cells to scale up to the necessary levels to make a major contribution to the energy transition.

The Hydrogen Council estimates that to reach net-zero emissions by 2030, PEM capacity will need to increase from today’s level of <1 GW to potentially 80-100 GW by 2030 (assuming a 40% PEM market share). In 2021, the amount of iridium required for 1 GW of electrolyser capacity was 400 kg, leading some to argue that the 2030 target would require 32-40 tonnes of iridium. To ensure that electrolyser production can ramp up and the capacity of iridium is enough, iridium must be used much more efficiently by formulating much more efficient catalysts, membranes, and maximising performance, thereby decreasing the amount of iridium required for every GW.

At the same time, the recycling of iridium from the PEM sector must be ensured to become available for reuse in the same application.
Substantial quantities of iridium are currently circulating constantly in closed loops in existing applications, which is generally unseen by the market.
For some applications such as spark plugs, efficient recycling routes have not yet been established, although tonnes of iridium could be made accessible to the market. Here, the legislator (e.g., the EU) could step in to incentivize the recycling of material from scrap.

Learn more about the metal in the IPA White Paper on Iridium.

Properties

Electrical conductivity* 0.197 106 cm-1 Ohm-1
Density* 22.65 g/cc
Hardness (Brinell value)* 1670 MN m-2
Melting point* 2443 ºC
Chemical element of Group VIII (Mendeleev)
Atomic number 77
Atomic weight 192.22
Thermal conductivity 148 watts/metre/°C
Tensile strength 112 (annealed condition kg/mm2)

History

Having discovered platinum and palladium, William Hyde Wollaston handed over the remaining residues of ore to his commercial partner Smithson Tennant, a fellow Cambridge graduate with whom he had forged a partnership in 1800.

In 1804, Tennant isolated iridium (and osmium) from the residues and, due to its colourful compounds, named it after the Latin for rainbow, "iridis". Much of the credit for the discovery should also go to Frenchmen L.N. Vauquelin, A.F. de Fourcroy and H.V. Collet-Descotiles upon whose research Tennant also acted. Obtaining pure samples of iridium remained impossible, however, due to its high melting point, until 1842 when an American chemist called Hare used a hydrogen/oxygen flame to melt a small sample, allowing it to be separated from dross and other impurities.

It is still produced today from platinum ore and as a by-product of nickel mining. Iridium first found a use in the nibs of fountain pens, due to its extreme hardness.

In 1889, in Paris, a platinum-iridium alloy bar was cast as the standard unit length of the metre and remained as the definition for this distance until 1960 when more precise measurement methods replaced it. Many medical and surgical advances, such as pacemakers, have also relied upon iridium's unique qualities.

It is also worth noting that anomalous deposits of iridium can be found throughout the world at the 65 million year old interface between rocks of the cretaceous and tertiary eras. Such concentrations, thousands of times greater than that normally found in the Earth's crust, are believed to have arrived extra-terrestrially. Their presence is held up as evidence by supporters of the theory that a massive asteroid collision with our planet was the cause of the extinction of the dinosaurs at that same point in geological time.

It remains hard and brittle even at temperatures as high as 1500°C. Ruthenium is, however, a useful addition to platinum and palladium to impart hardness in certain jewellery alloys and to improve resistance to abrasion in electrical contact surfaces.

In the electronics and chemicals industries, ruthenium has some important applications on account of its electrical and electrochemical properties, good catalytic properties, good catalytic activity, resistance to corrosion and stability under varying operating conditions.

Its principal application in the electronics sector is for use in resistors. Increasingly, ruthenium is also being used in computer hard discs to increase the density at which data is stored.

In the future, the use of ruthenium in alloys for aircraft turbine blades will help reduce the CO2 impact of air travel on the environment. If current prototypes are successful, their high melting points and high temperature stability will allow for higher temperatures and, therefore, a more efficient burning of aircraft fuel.

Ruthenium is also being used in certain catalytic applications in today's gas to liquids technology to generate various sulphur-free, high-quality fuels.

Properties

Electrical conductivity* 0.137 106 cm-1 Ohm-1
Density* 12.45 g/cc
Hardness (Brinell value)* 2160 MN m-2
Melting point* 2310 ºC
Chemical element of Group VIII (Mendeleev)
Atomic number 44
Atomic weight 101.07
Thermal conductivity 105 watts/metre/°C
Tensile strength 165 (annealed condition kg/mm2)

History

The last of the platinum group metals to be discovered has helped to improve the qualities and effectiveness of its relatives and other metals which improve our lives.

Named after the Latin for Russia, "Ruthenia", unsubstantiated reports of ruthenium's isolation had appeared in 1807 from the Polish chemist Andrzej Sniadecki who had christened it vestium.

It was given its final name in 1827 when Jöns Jacob Berzelius and Gottfried Osann examined the remains of platinum ore from the Ural mountains that was insoluble in aqua regia (a mixture of hydrochloric and nitric acid) and identified ruthenium oxide.

In 1844, Russian professor Karl Karlovich Klaus, obtained a pure sample from this oxide and ruthenium became the last of the platinum group metals (PGMs) to be isolated. The Urals are still a source of ruthenium and it is also found in North and South America and South Africa.

Today, ruthenium is used primarily as a hardener for platinum and palladium and has a remarkable effect on titanium whose corrosion resistance is boosted a hundred times by the addition of just 0.1 per cent of ruthenium.

It is ten times harder than platinum itself. Osmium also has a higher melting point than the other platinum group metals.

Osmium's extraordinary qualities allow for its use in a range of applications in which frictional wear must be avoided, including fountain pen nibs, styluses, and instrument pivots. It is often alloyed with other PGMs such as platinum and iridium.

Its conductivity means it can be used as a more effective and durable alternative to gold as plating in electronic products.

Like the other PGMs it is an extremely efficient oxidation catalyst and contributes to the environment through use in fuel cells. This quality is also uniquely applied in forensic science for staining fingerprints and DNA (as osmium tetroxide).

Properties

Electrical conductivity* 0.109 106 cm-1 Ohm-1
Density* 22.61 g/cc
Hardness (Brinell value)* 3920 MN m-2
Melting point* 3050 ºC
Chemical element of Group VIII (Mendeleev)
Atomic number 76
Atomic weight 190.23
Thermal conductivity 87 watts/metre/°C
Tensile strength - (annealed condition kg/mm2)

History

The densest substance known and the hardest of all platinum group metals, osmium has become a central material in many everyday and innovative items.

Having discovered platinum and palladium, William Hyde Wollaston handed over the remaining residues of ore to his commercial partner Smithson Tennant, a fellow Cambridge graduate with whom he had forged a partnership in 1800.

In 1804, Tennant isolated osmium (and iridium) from the residues and, due to the distinctive chlorine-like odour of its oxide, named it after the Greek for smell, "osme".

Originally, it was osmium's density and hardness that led to its widespread use - in everyday objects such as fountain pen nibs, styluses, electrical contacts and other tools where frictional erosion is likely to occur. By 1906, it was used in the filaments for incandescent lighting and is from where the company Osram derives it name.

In metal form, osmium's brittleness and hardness made it extremely difficult to work with and it is usually produced as a powder. This powder emits osmium tetroxide which is used in the detection of fingerprints and as a forensic stain for DNA samples.

Osmium occurs in a natural alloy with iridium called iridosule, in the platinum-bearing river sands of the Urals and North and South America, and as a by-product of nickel mining in Ontario's copper-nickel sulfide region, Sudbury.