Ruthenium

• 1. Introduction

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  Ruthenium sits at atomic number 44 on the periodic table, carrying the symbol Ru. In 1844, Karl Ernst Claus, a Russian scientist of Baltic-German ancestry working at Kazan State University, held in his hand just 6 grams of a metal no one had properly isolated before. He extracted it from the platinum residues left over from minting ruble coins. When it came time to name his discovery, Claus was unambiguous: "I named the new body, in honour of my Motherland, ruthenium. I had every right to call it by this name because Mr. Osann relinquished his ruthenium and the word does not yet exist in chemistry." The name derives from Ruthenia, the Latin word for Russia. What makes ruthenium remarkable is not any single dramatic property but a quiet versatility. It resists almost everything thrown at it. It hardens other metals at tiny concentrations. It shows up in jet engine turbine blades, in the nibs of fountain pens, and as a capping layer in the most advanced semiconductor manufacturing on earth. Annual production has risen from about 19 tonnes in 2009 to 35.5 tonnes in 2017, and yet most people have never heard of it. The story of how this rare metal went from laboratory curiosity to industrial workhorse begins with a long and contentious history of competing claims.

• 2. A Disputed Discovery

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  Jons Berzelius and Gottfried Osann came close in 1827, working through residues left after dissolving crude platinum from the Ural Mountains in aqua regia. Berzelius found nothing unusual. Osann believed he had found three entirely new metals, which he named pluranium, ruthenium, and polinium. The disagreement between the two men became a long-standing controversy. Because Osann could not repeat his own isolation, he eventually withdrew his claims entirely. The name ruthenium, however, did not disappear with him. It was Claus who later showed that Osann's prepared compounds had contained small amounts of a genuine new element all along. Before either of them, the Polish chemist Jedrzej Sniadecki had published an announcement in 1808 claiming to have isolated element 44 from South American platinum ores in 1807. He proposed calling it vestium, after the asteroid Vesta, which had been discovered shortly before. His work was never confirmed, and he too withdrew his claim. The platinum group metals as a whole had a slow unfolding. Though pre-Columbian Americans had used naturally occurring platinum alloys for a long time, and European chemists knew of the material from the mid-16th century, platinum itself was not identified as a pure element until the mid-18th century. Palladium, rhodium, osmium, and iridium were not separated from it until the first decade of the 19th century. Ruthenium was the last holdout. Claus isolated it the same way its heavier relative osmium had been discovered four decades earlier, and in doing so he started a naming convention that persists to this day: calling an element after a country.

• 3. What the Metal Actually Does

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  Ruthenium is a hard white metal that belongs to group 8 of the periodic table. Its outermost electron shell holds only one electron rather than the two seen in the other group 8 elements, an anomaly it shares with most elements in the range of atomic number 41-45, though this quirk has no effect on its chemical properties. At room temperature it does not tarnish, and it resists attack from acids including aqua regia. Sodium hypochlorite at room temperature and halogens at high temperatures can attack it, and it oxidizes when heated to 800 degrees Celsius. One of its more unusual distinctions is that it is the only 4d transition metal capable of reaching the oxidation state of plus-eight. A ruthenium-molybdenum alloy becomes superconductive below 10.6 Kelvin. Small additions of ruthenium noticeably increase the hardness of platinum and palladium, and a mere 0.1 percent ruthenium markedly improves the corrosion resistance of titanium. The metal can be deposited as a film by electroplating, sputtering, or chemical vapor deposition. Its main oxide, ruthenium dioxide, shares comparable electrical resistivity with the metal itself, and copper can be electroplated directly onto ruthenium, which makes it attractive for transistor gates and interconnects in microelectronics. The most common chemical precursor in industrial and laboratory work is ruthenium trichloride, a red solid that is versatile enough synthetically to serve as a starting point for dozens of further compounds and complexes.

• 4. Where Ruthenium Hides

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  Ruthenium occurs at about 100 parts per trillion in the Earth's crust, placing it at 78th in the ranking of elemental abundance. Ores carrying the platinum group metals in the Ural Mountains and in North and South America are its primary home. Commercially important quantities also turn up in pentlandite extracted from Sudbury, Ontario, Canada, and in pyroxenite deposits in South Africa. The proportion of ruthenium in those ores varies considerably depending on where they formed: platinum group metal mixtures mined in South Africa contain on average 11 percent ruthenium, while those from the former USSR contain only about 2 percent as of 1992. The metal arrives at refineries as a by-product of processing nickel, copper, and platinum. During electrorefining, noble metals including silver, gold, and the platinum group metals settle out as anode mud. Ruthenium, osmium, rhodium, and iridium are all insoluble in aqua regia and precipitate together. Separating ruthenium from its neighbors requires successive chemical treatments, eventually producing ammonium ruthenium chloride, which hydrogen reduces to a fine metal powder. That powder can then be worked using powder metallurgy techniques or argon-arc welding. Roughly 30 tonnes are mined globally each year, and world reserves are estimated at 5,000 tonnes. A secondary potential source is spent nuclear fuel, where ruthenium appears as a direct fission product. After the unstable isotopes decay away, chemical extraction could yield ruthenium for standard industrial applications.

• 5. Electronics and Catalysis

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  Approximately 30.9 tonnes of ruthenium were consumed in 2016. Of that, 13.8 tonnes went into electrical applications and 7.7 tonnes into catalysis. Electronics is the largest single use. Ruthenium dioxide combined with lead and bismuth ruthenates forms the basis of thick-film chip resistors, and together with wear-resistant electrical contacts these two electronic applications account for 50 percent of total ruthenium consumption. A newer application places ruthenium as the capping layer for extreme ultraviolet photomasks used in semiconductor lithography, where its low volatility is a key advantage. On the catalysis side, the Grubbs catalysts based on ruthenium carbene complexes have been used in the preparation of drugs and advanced materials. Ruthenium-promoted cobalt catalysts play a role in Fischer-Tropsch synthesis. Chiral ruthenium complexes introduced by Ryoji Noyori are used for the enantioselective hydrogenation of ketones, aldehydes, and imines. Noyori received the Nobel Prize in Chemistry in 2001 for his contributions to asymmetric hydrogenation. Solutions containing ruthenium trichloride are also highly active for olefin metathesis, used commercially in producing polynorbornene. Ruthenium-based compounds appear in dye-sensitized solar cells proposed as a lower-cost photovoltaic option. Ruthenium red, a complex with six positive charges and three ruthenium atoms bridged by oxygen, serves as a biological stain in both light microscopy and electron microscopy for polyanionic molecules such as pectin and nucleic acids.

• 6. Niche Uses and Isotopes

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  From 1944 onward, the Parker 51 fountain pen carried a nib tipped with an alloy that was 96.2 percent ruthenium and 3.8 percent iridium, marketed simply as the "RU" nib on a 14-karat gold base. Jet engine turbine blades made from high-temperature nickel-based superalloys can contain ruthenium in proportions ranging from 3 percent in EPM-102 to 6 percent in TMS-162 and related compositions. Ruthenium tetroxide reacts with fatty oils or fats to produce a brown or black ruthenium dioxide pigment, making it useful for exposing latent fingerprints at crime scenes. The beta-decaying isotope 106Ru, with a half-life of 371.8 days, is used in radiotherapy of eye tumors, particularly melanomas of the uvea. Naturally occurring ruthenium consists of seven stable isotopes, and 34 synthetic radioactive isotopes have been produced. Beyond 106Ru, the most stable radioisotopes include 103Ru with a half-life of 39.245 days and 97Ru with a half-life of 2.837 days. In 2017, high concentrations of 106Ru were detected in the atmosphere over Europe, associated at the time with an alleged undeclared nuclear accident in Russia. Ruthenium-centered complexes are also under investigation as potential anticancer agents; in particular, so-called piano-stool compounds in the Ru(II) oxidation state show promise as candidates to replace platinum-based anti-tumor drugs currently in clinical use.