Material of the month – Platinum

Materials World magazine
,
1 Jan 2015

This month, Anna Ploszajski explores the properties of platinum. 

The name evolved from the Spanish ‘platina’, meaning ‘little silver’, but in no way is platinum subordinate to its lustrous older sister. Its widespread applications, from sparkling on the heads of royalty to sustainable energy, mean platinum’s value is far greater than its price tag alone.

Platinum is young compared to precious metals such as gold and silver. Two individuals are credited with the discovery of platinum. The first was Spaniard Antonio de Ulloa, a highly regarded explorer, author and astronomer. His reputation earned him a place on the French Geodesic Mission, which measured one degree of latitude at the equator. This project brought Ulloa to Ecuador in 1736, where he first encountered platinum. On his return journey eight years later, British forces intercepted his ship and he became a political prisoner. Fortunately, his winning charm and scientific prowess quickly endeared him to his captors, and gained him a position within The Royal Society. Returning to Spain in 1748, Ulloa published a report, that brought platinum research into the mainstream.

Around the same time in 1741, British metallurgist Charles Wood, encountered examples of Colombian platinum on a visit to Jamaica. He sent these samples to his colleague William Brownrigg, who, in 1750, presented a detailed account of his research to The Royal Society, which precipitated investigation of this exciting new metal across Europe.

By 1786, platinum’s popularity prompted King Charles III of Spain to award scientist Pierre-François Chabaneau with a library and laboratory to aid his research into the material. Chabaneau was able to produce 23kg of pure, malleable platinum by hammering and compressing white-hot platinum sponge. Having realised the value of his work, he went on to found a business with Joaquín Cabezas, producing ingots and utensils out of the precious metal.


The process

Platinum’s relatively late discovery on the metallurgical timeline can be explained by its rarity. It is one of the most rare elements on Earth, and even today only about 120t are mined each year. In fact, we’d have better luck extracting platinum from the moon, where it occurs at a concentration of more than 50,000 times that of the Earth’s crust. Platinum’s inertness means that it is most commonly found chemically uncombined, with 77% of its global production coming from native deposits in South Africa. It can also be found in copper and nickel ores, such as in the platinum arsenides in the Sudbury Basin deposit, in Canada. These are thought to be present due to a bolide impact resulting in volcanic action.

There are three ways to obtain pure platinum after it is dug out of the ground. One way is to recycle it from second-hand catalytic converters. The second is to obtain it as a by-product from industrial nickel processing and electrolytic copper refining. The third way is to extract it from fresh platinum ore, which requires 4.5t to produce a single gramme of pure platinum, in a series of processes that can take up to six months to complete.

In the first of these processes, the platinum ore must be crushed and milled, and then mixed with water and special reagents. Air is passed through this mixture, isolating the platinum-rich particles. This froth floatation process is repeated a number of times for maximum yield. After drying the platinum-rich particles, they are melted in an electric furnace at 1,500°C. Iron and sulphur impurities are then removed by blowing air through converters. The next step of electrolytic refining separates out the nickel, copper and cobalt impurities. Finally, a combination of solvent extraction, distillation and ion exchange techniques removes traces of gold and silver to yield pure platinum.


Properties and uses

Platinum’s catalytic competence was first used in William Grove’s 1839 design for an electrochemical cell, which used platinum and zinc electrodes. Grove went on to lay the foundations for the modern hydrogen fuel cell in 1842. Contemporary designs feature platinum as the catalyst component of the electrode, which is mounted on a porous carbon support. Hydrogen fuel cells combine hydrogen gas with oxygen from the air to make electricity, and the only by-product is water. This is achieved with the help of a platinum catalyst on the anode to aid the splitting of hydrogen molecules into protons and electrons. These protons are able to travel through a polymer membrane which is insulating to electrons. The electrons therefore are forced to travel around an external circuit, and harnessed as electrical power in an external device. To complete the circuit, platinum at the cathode catalyses the reduction of oxygen from the air, combining with the protons and electrons to form water vapour. Provided that the hydrogen input is produced in sustainable processes, such as the electrosplitting of water powered by solar or wind energy, hydrogen fuel cells provide completely carbon-neutral electrical power.

The use of expensive platinum catalysts limits hydrogen fuel technologies from breaking into the mainstream hydrogen fuel cell technologies, so finding ways to optimise the catalyst’s performance have become a key focus. Reducing the size of the platinum particles to the nanoscale provides a significantly higher surface area for the reactions to take place. Furthermore, engineering nano-particles with high index facets gives a greater density of reactive sites per unit area of catalyst surface. This means certain facets are less reactive with some impurities, providing inherent immunity from poisoning and subsequent reduction in overall fuel cell performance. Finally, alloying platinum with cheaper metals, such as nickel can reduce the amount of platinum required and enhance the catalytic activity.

Although hydrogen fuel cells will be essential for alleviating our dependence on fossil fuels, platinum also reduces the impact of today’s diesel-driven society. In 2010, just less than half (46%) of the platinum sold globally was used in catalysts in vehicle emissions-control devices for internal combustion engines. Modern three way catalytic converters turn toxic by-products, CO and unburned hydrocarbons, into less toxic pollutants, CO2 and H20 via oxidation reactions. Investigation into the molecular mechanisms behind platinum’s catalysis role in these reactions earned Gerhard Ertl the Nobel Prize in Chemistry in 2007. Even better, these devices also reduce toxic nitrogen oxides into harmless nitrogen and oxygen. The vulnerable catalysts are, however, very sensitive to poisoning if substances such as lead enter the exhaust gas stream, as it can clog up surfaces. This is the reason that vehicles equipped with catalytic converters must only be run on unleaded fuels.


Modern applications

After catalytic converters, the next highest use of platinum in 2010 was jewellery, accounting for nearly a third of global platinum produced (31%). Platinum’s attractive lustre and rarity as well as its hypo-allergenic properties, make it a desirable choice for jewellers and high-end consumers. Ornaments that have been fashioned from platinum include the setting of the Hope Diamond, the frame of the crown of Queen Elizabeth and the Queen Mother, and limited edition designs by Cartier, Faberge and Tiffany. Alloying elements such as copper, palladium, cobalt, gallium or indium can be added up to 10% to produce a harder product that is easier to cast and work with and has increased scratch resistance and shine.

Extraordinary corrosion resistance in aggressive, high-temperature environments and good biocompatibility give platinum extensive application in the medical field, such as in dental implants, surgical equipment, prostheses and cancer-fighting drugs. These drugs include platinum complexes, which react in the body to cause cross-linking of DNA to kill cancerous cells.

Further development in 1879 by the French Academy of Science requested that George Matthey (of Johnson and Matthey fame) produce 30 bars of platinum-iridium (90:10 alloy). Of these 30 bars, number six was found to be exactly the same as Mètre des Archives, the first prototype of a platinum bar standard in 1799. It became the international prototype metre bar, the benchmark for a one metre length. The remaining bars were calibrated to this length, and distributed among the signatory nations of the metre convention. The USA picked the short straw with bar number 27, which had been calibrated 1.6 micrometres shorter than the international prototype.

In popular culture, platinum has become synonymous with great achievements, out-stripping gold and silver discs as a symbol of high music record sales. The metal itself has already achieved greatness, providing feasible renewable energy solutions, cancer treatments and emissions abatement. However, with its powerful properties continually growing, platinum’s full potential is likely yet to be realised.