Rhodium is found in platinum or nickel ores with the other members of the platinum group metals. It was discovered in 1803 by William Hyde Wollaston in one such ore, and named for the rose color of one of its chlorine compounds.
The element's major use (consuming about 80% of world rhodium production) is as one of the catalysts in the three-way catalytic converters in automobiles. Because rhodium metal is inert against corrosion and most aggressive chemicals, and because of its rarity, rhodium is usually alloyed with platinumorpalladium and applied in high-temperature and corrosion-resistive coatings. White gold is often plated with a thin rhodium layer to improve its appearance, while sterling silver is often rhodium-plated to resist tarnishing. Rhodium is sometimes used to cure silicones: a two-part silicone in which one part containing a silicon hydride and the other containing a vinyl-terminated silicone are mixed; one of these liquids contains a rhodium complex.[9]
Rhodium detectors are used in nuclear reactors to measure the neutron flux level. Other uses of rhodium include asymmetric hydrogenation used to form drug precursors and the processes for the production of acetic acid.
For decades, the rare element had only minor applications; for example, by the turn of the century, rhodium-containing thermocouples were used to measure temperatures up to 1800 °C.[17][18] They have exceptionally good stability in the temperature range of 1300 to 1800 °C.[19]
The first major application was electroplating for decorative uses and as corrosion-resistant coating.[20] The introduction of the three-way catalytic converterbyVolvo in 1976 increased the demand for rhodium. The previous catalytic converters used platinum or palladium, while the three-way catalytic converter used rhodium to reduce the amount of NOx in the exhaust.[21][22][23]
Rhodium is a hard, silvery, durable metal that has a high reflectance. Rhodium metal does not normally form an oxide, even when heated.[24]Oxygen is absorbed from the atmosphere only at the melting point of rhodium, but is released on solidification.[25] Rhodium has both a higher melting point and lower density than platinum. It is not attacked by most acids: it is completely insoluble in nitric acid and dissolves slightly in aqua regia.
Naturally occurring rhodium is composed of only one isotope, 103Rh. The most stable radioisotopes are 101Rh with a half-life of 3.3 years, 102Rh with a half-life of 207 days, 102mRh with a half-life of 2.9 years, and 99Rh with a half-life of 16.1 days. Twenty other radioisotopes have been characterized with atomic weights ranging from 92.926 u (93Rh) to 116.925 u (117Rh). Most of these have half-lives shorter than an hour, except 100Rh (20.8 hours) and 105Rh (35.36 hours). Rhodium has numerous meta states, the most stable being 102mRh (0.141 MeV) with a half-life of about 2.9 years and 101mRh (0.157 MeV) with a half-life of 4.34 days (see isotopes of rhodium).[31]
The industrial extraction of rhodium is complex because the ores are mixed with other metals such as palladium, silver, platinum, and gold and there are very few rhodium-bearing minerals. It is found in platinum ores and extracted as a white inert metal that is difficult to fuse. Principal sources are located in South Africa; in river sands of the Ural Mountains in Russia; and in North America, including the copper-nickel sulfide mining area of the Sudbury, Ontario, region. Although the rhodium abundance at Sudbury is very small, the large amount of processed nickel ore makes rhodium recovery cost-effective.
The main exporter of rhodium is South Africa (approximately 80% in 2010) followed by Russia.[36] The annual world production is 30 tonnes. The price of rhodium is highly variable.
Rhodium is a fission product of uranium-235: each kilogram of fission product contains a significant amount of the lighter platinum group metals. Used nuclear fuel is therefore a potential source of rhodium, but the extraction is complex and expensive, and the presence of rhodium radioisotopes requires a period of cooling storage for multiple half-lives of the longest-lived isotope (101Rh with a half-life of 3.3 years, and 102mRh with a half-life of 2.9 years), or about 10 years. These factors make the source unattractive and no large-scale extraction has been attempted.[37][38][39]
The primary use of this element is in automobiles as a catalytic converter, changing harmful unburned hydrocarbons, carbon monoxide, and nitrogen oxide exhaust emissions into less noxious gases. Of 30,000 kg of rhodium consumed worldwide in 2012, 81% (24,300 kg) went into this application, and 8,060 kg was recovered from old converters. About 964 kg of rhodium was used in the glass industry, mostly for production of fiberglass and flat-panel glass, and 2,520 kg was used in the chemical industry.[36]
In 2008, net demand (with the recycling accounted for) of rhodium for automotive converters made up 84% of the world usage,[41] with the number fluctuating around 80% in 2015−2021.[42]
Rhodium catalysts are used in a number of industrial processes, notably in catalytic carbonylation of methanol to produce acetic acid by the Monsanto process.[43] It is also used to catalyze addition of hydrosilanes to molecular double bonds, a process important in manufacture of certain silicone rubbers.[44] Rhodium catalysts are also used to reduce benzenetocyclohexane.[45]
The complex of a rhodium ion with BINAP is a widely used chiral catalyst for chiral synthesis, as in the synthesis of menthol.[46]
Rhodium finds use in jewelry and for decorations. It is electroplatedonwhite gold and platinum to give it a reflective white surface[47] at time of sale, after which the thin layer wears away with use. This is known as rhodium flashing in the jewelry business. It may also be used in coating sterling silver to protect against tarnish (silver sulfide, Ag2S, produced from atmospheric hydrogen sulfide, H2S). Solid (pure) rhodium jewelry is very rare, more because of the difficulty of fabrication (high melting point and poor malleability) than because of the high price.[48] The high cost ensures that rhodium is applied only as an electroplate.
Rhodium has also been used for honors or to signify elite status, when more commonly used metals such as silver, gold or platinum were deemed insufficient. In 1979 the Guinness Book of World Records gave Paul McCartney a rhodium-plated disc for being history's all-time best-selling songwriter and recording artist.[49]
Rhodium is used as an alloying agent for hardening and improving the corrosion resistance[24]ofplatinum and palladium. These alloys are used in furnace windings, bushings for glass fiber production, thermocouple elements, electrodes for aircraft spark plugs, and laboratory crucibles.[50] Other uses include:
Rhodium plated by either electroplating or evaporation is extremely hard and useful for optical instruments.[52]
Filters in mammography systems for the characteristic X-rays it produces.[53]
Rhodium neutron detectors are used in nuclear reactors to measure neutron flux levels—this method requires a digital filter to determine the current neutron flux level, generating three separate signals: immediate, a few seconds delay, and a minute delay, each with its own signal level; all three are combined in the rhodium detector signal. The three Palo Verde nuclear reactors each have 305 rhodium neutron detectors, 61 detectors on each of five vertical levels, providing an accurate 3D "picture" of reactivity and allowing fine tuning to consume the nuclear fuel most economically.[54]
In automobile manufacturing, rhodium is also used in the construction of headlight reflectors.[55]
Being a noble metal, pure rhodium is inert and harmless in elemental form.[57] However, chemical complexes of rhodium can be reactive. For rhodium chloride, the median lethal dose (LD50) for rats is 198 mg (RhCl 3) per kilogram of body weight.[58] Like the other noble metals, rhodium has not been found to serve any biological function.
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^J.V. Pearce, F. Edler, C.J. Elliott, A. Greenen, P.M. Harris, C.G. Izquierdo, Y.G. Kim, M.J. Martin, I.M. Smith, D. Tucker and R.I. Veitcheva, A systematic investigation of the thermoelectric stability of Pt-Rh thermocouples between 1300 °C and 1500 °C, METROLOGIA, 2018, Volume: 55 Issue: 4 Pages: 558-567
^Kushner, Joseph B. (1940). "Modern rhodium plating". Metals and Alloys. 11: 137–140.
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^Mayara da Silva Santos, Tony Stüker, Max Flach, Olesya S. Ablyasova, Martin Timm, Bernd von Issendorff, Konstantin Hirsch, Vicente Zamudio‐Bayer, Sebastian Riedel, J. Tobias Lau. The Highest Oxidation State of Rhodium: Rhodium(VII) in [RhO 3 ] +. Angewandte Chemie International Edition, 2022; 61 (38)
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