Rhodium is found in platinum or nickel ores with the other members of theplatinum group metals. It wasdiscovered in 1803 byWilliam Hyde Wollaston in one such ore, and named for the rose color of one of itschlorine compounds.
The element's major use (consuming about 80% of world rhodium production) is as one of thecatalysts in thethree-way catalytic converters in automobiles. Because rhodium metal is inert against corrosion and most aggressive chemicals, and because of its rarity, rhodium is usuallyalloyed withplatinum orpalladium and applied in high-temperature and corrosion-resistive coatings.White gold is often plated with a thin rhodium layer to improve its appearance, whilesterling silver is often rhodium-plated to resist tarnishing.
Rhodium detectors are used innuclear reactors to measure theneutron flux level. Other uses of rhodium include asymmetric hydrogenation used to form drug precursors and the processes for the production ofacetic 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.[18][19] They have exceptionally good stability in the temperature range of 1300 to 1800 °C.[20]
The first major application was electroplating for decorative uses and as corrosion-resistant coating.[21] The introduction of the three-waycatalytic converter byVolvo 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 ofNOx in the exhaust.[22][23][24]
Rhodium is a hard, silvery, durable metal that has a highreflectance. Rhodium metal does not normally form anoxide, even when heated.[25]Oxygen is absorbed from theatmosphere only at themelting point of rhodium, but is released on solidification.[26] Rhodium has both a higher melting point and lowerdensity thanplatinum. It is not attacked by mostacids: it is completely insoluble innitric acid and dissolves slightly inaqua regia.
The commonoxidation states of rhodium are +3 and +1. Oxidation states 0, +2, and +4 are also well known.[27] A few complexes at still higher oxidation states are known.[28]
The rhodium oxides includeRh 2O 3,RhO 2,RhO 2·xH 2O,Na 2RhO 3,Sr 3LiRhO 6 andSr 3NaRhO 6.[29] None are of technological significance.
All the Rh(III) halides are known but the hydrated trichloride is most frequently encountered. It is also available in an anhydrous form, which is somewhat refractory. Other rhodium(III) chlorides include sodium hexachlororhodate,Na3RhCl6, andpentaamminechlororhodium dichloride,[Rh(NH3)5Cl]Cl2. They are used in the recycling and purification of this very expensive metal. Heating a methanolic solution of hydrated rhodium trichloride withsodium acetate give the blue-greenrhodium(II) acetate,Rh2(O2CCH3)4, which features a Rh-Rh bond. This complex and relatedrhodium(II) trifluoroacetate have attracted attention as catalysts forcyclopropanation reactions. Hydrated rhodium trichloride is reduced bycarbon monoxide,ethylene, andtrifluorophosphine to give rhodium(I) complexesRh2Cl2L4 (L = CO,C2H4, PF3). When treated withtriphenylphosphine, hydrated rhodium trichloride converts to the maroon-coloredRhCl(P(C6H5)3)3, which is known asWilkinson's catalyst. Reduction ofrhodium carbonyl chloride giveshexarhodium hexadecacarbonyl,Rh6(CO)16, andtetrarhodium dodecacarbonyl,Rh4(CO)12, the two most common Rh(0) complexes.
Naturally occurring rhodium is composed of only oneisotope,103Rh. The most stableradioisotopes are101Rh with ahalf-life of 3.3 years,102Rh with a half-life of 207 days,102mRh with a half-life of 2.9 years, and99Rh with a half-life of 16.1 days. Twenty other radioisotopes have been characterized withatomic weights ranging from 92.926u (93Rh) to 116.925 u (117Rh). Most of these have half-lives shorter than an hour, except100Rh (20.8 hours) and105Rh (35.36 hours). Rhodium has numerousmeta states, the most stable being102mRh (0.141 MeV) with a half-life of about 2.9 years and101mRh (0.157 MeV) with a half-life of 4.34 days (seeisotopes of rhodium).[31]
Rhodium ores are a mixture with other metals such aspalladium,silver,platinum, andgold. Few rhodiumminerals are known. The separation of rhodium from the other metals poses significant challenges. Principal sources are located in South Africa, river sands of theUral Mountains in Russia, and in North America, especially thecopper-nickel sulfide mining area of theSudbury, 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 30tonnes. The price of rhodium is highly variable.
Rhodium is a fission product ofuranium-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 ahalf-life of 3.3 years, and102mRh with ahalf-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 acatalytic 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][40]
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]
Rh-based hydroformylation underpins the industrial production of products as diverse as detergents, fragrances, and some drugs. Originally hydroformylation relied on much cheaper cobalt carbonyl-based catalysts, but that technology has largely been eclipsed by rhodium-based catalysts despite the cost differential.
Rhodium is also known to catalyze many reactions involving hydrogen gas andhydrosilanes. These include hydrogenations and hydrosilylations of alkenes.[46] Rhodium metal, but not rhodium complexes, catalyzes the hydrogenation ofbenzene tocyclohexane.[47]
Rhodium finds use injewelry and for decorations. It iselectroplated onwhite gold and platinum to give it a reflective white surface 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 coatingsterling silver to protect against tarnish (silver sulfide, Ag2S, produced from atmospherichydrogen 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 anelectroplate. 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 theGuinness Book of World Records gavePaul 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[25] ofplatinum andpalladium. These alloys are used in furnace windings, bushings for glass fiber production,thermocouple elements,electrodes for aircraftspark plugs, and laboratory crucibles.[50] Other uses include:
Rhodium plated by eitherelectroplating or evaporation is extremely hard and useful for optical instruments.[52]
Filters inmammography 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 threePalo 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 anoble metal, pure rhodium is inert and harmless in elemental form.[57] However, chemical complexes of rhodium can be reactive. For rhodium chloride, themedian 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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^Usselman, Melvyn (1978). "The Wollaston/Chenevix controversy over the elemental nature of palladium: A curious episode in the history of chemistry".Annals of Science.35 (6):551–579.doi:10.1080/00033797800200431.
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^Reisner, B. A.; Stacy, A. M. (1998). "Sr 3ARhO 6 (A = Li, Na): Crystallization of a Rhodium(V) Oxide from Molten Hydroxide".Journal of the American Chemical Society.120 (37):9682–9989.doi:10.1021/ja974231q.
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^David R. Lide (ed.), Norman E. Holden inCRC Handbook of Chemistry and Physics, 85th Edition CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes.
^Heidingsfeldova, M. & Capka, M. (2003). "Rhodium complexes as catalysts for hydrosilylation crosslinking of silicone rubber".Journal of Applied Polymer Science.30 (5):1837–1846.doi:10.1002/app.1985.070300505.
^Halligudi, S. B.; et al. (1992). "Hydrogenation of benzene to cyclohexane catalyzed by rhodium(I) complex supported on montmorillonite clay".Reaction Kinetics and Catalysis Letters.48 (2):547–552.Bibcode:1992RKCL...48..505T.doi:10.1007/BF02162706.S2CID97802315.
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