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Rhodium is a chemical element; it has symbol Rh and atomic number 45. It is a very rare, dark silvery-white, hard, corrosion transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring isotope, which is 103Rh. Naturally occurring rhodium is usually found as a free metal or as an alloy with similar metals and rarely as a chemical compound in minerals such as bowieite and rhodplumsite. It is one of the rarest and most valuable . Rhodium is a group 9 element (cobalt group). It is used for the enhancement of jewelry.
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 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 with platinum or palladium and applied in high-temperature and corrosion-resistant 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 detectors are used in 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. (1993). 9780803114661, ASTM International. ISBN 9780803114661 They have exceptionally good stability in the temperature range of 1300 to 1800 °C.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
The first major application was electroplating for decorative uses and as corrosion-resistant coating. The introduction of the three-way catalytic converter by Volvo 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.
Rhodium belongs to group 9 of the periodic table, but exhibits an atypical ground state valence electron configuration for that group, having only one electron in its outermost s orbital. This is also observed in the elements niobium (41) and ruthenium (44).
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The common of rhodium are +3 and +1. Complexes with rhodium in oxidation states 0, +2, and +4 are also well characterized. The few compounds at still higher oxidation states include rhodium pentafluoride, a tetrameric complex with the true formula ), and rhodium hexafluoride.
Three rhodium oxides are (a paramagnetic black powder), (black when anhydrous but green as a hydrate), and (only stable in the gas phase). A rhodium sulfide, , occurs naturally as a rare mineral miassite. Synthetic are used as catalysts in for example - fuel cells.
All the Rh(III) halides are known. Synthesis of many rhodium complexes begin with , the hydrated trichloride, but , the anhydrous form, is largely inert. Other rhodium(III) chlorides include , , and pentaamminechlororhodium dichloride, . They are used in the recycling and purification of this very expensive metal. Heating a methanolic solution of hydrated rhodium trichloride with sodium acetate gives the blue-green rhodium(II) acetate, , which features a Rh–Rh bond. This complex and related rhodium(II) trifluoroacetate have attracted attention as catalysts for cyclopropanation reactions.
Heating treated with triphenylphosphine in ethanol, hydrated rhodium trichloride converts to . This square planar complex, which is known as Wilkinson's catalyst, is a common and early example of a well-defined homogeneous catalyst for hydrogenation of alkenes.
Cyclopentadienyl complexes of rhodium have been investigated as analogues of ferrocene. The parent is rhodocene, which participates in an unusual monomer-dimer equilibrium:
Related cyclopentadienyl compounds include the Rh(I) and Rh(III) half-sandwich complexes and [[Pentamethylcyclopentadienyl rhodium dichloride dimer|. The latter compound is prepared by the reaction of rhodium trichloride trihydrate and pentamethylcyclopentadiene in hot methanol.
A related but cationic family of hydrogenation catalysts arise from cyclooctadiene rhodium chloride dimer, Rh2Cl2(C8H12)2.Giordano, G.; Crabtree, R. H. "Di-μ-chloro-bis(η4-1,5-cyclooctadiene)dirhodium(I)" Inorganic Syntheses, 1990, volume 28, pages 88-90. The cyclooctadiene (C8H12) ligands are easily displaced, and this allows the easy introduction of chiral ligands, leading to asymmetric hydrogenations, including the Nobel Prize-winning route to the chiral drug L-DOPA.
When treated with sodium borohydride and carbon monoxide, converts to the pentacoordinate complex , which is used commercially for the hydroformylation of alkenes. Despite its much higher cost, tris(triphenylphosphine)rhodium carbonyl hydride has displaced cheaper cobalt-based catalysts for this application.J. F. Hartwig; Organotransition metal chemistry - from bonding to catalysis. University Science Books. 2009. 753, 757-578. .
The most stable are 101Rh with a half-life of 4.07 years, 102Rh with a half-life of 207 days, and 99Rh with a half-life of 16.1 days. Thirty-eight other radioisotopes have been characterized ranging from 90Rh to 128Rh; these have half-lives that are less than an hour except 100Rh (20.8 hours) and 105Rh (35.34 hours). Numerous are also known, of which the most stable are 102mRh (3.742 years) and 101mRh (4.343 days).
In isotopes lighter than 103Rh (the stable isotope), the primary decay mode is electron capture and the primary decay product is ruthenium. In isotopes heavier than 103Rh, the primary decay mode is beta emission and the primary product is palladium.
In 2008, net demand (with the recycling accounted for) of rhodium for automotive converters made up 84% of the world usage, with the number fluctuating around 80% in 2015−2021.
Rh-based hydroformylation underpins the industrial production of products as diverse as detergents, fragrances, and some drugs. Rhodium based catalysts have 1000 to 10000 times higher activity for hydroformylation than cheaper cobalt carbonyl-based catalysts, allowing reactions at lower temperatures and pressures.
Rhodium is also known to catalyze many reactions involving hydrogen gas and . These include hydrogenations and hydrosilylations of alkenes. Rhodium metal, but not rhodium complexes, catalyzes the hydrogenation of benzene to cyclohexane.
Rhodium plated by either electroplating or evaporation is extremely hard and useful for optical instruments. In automobile manufacturing, rhodium is also used in the construction of headlight reflectors.Stwertka, Albert. A Guide to the Elements, Oxford University Press, 1996, p. 125. 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.
People can be exposed to rhodium in the workplace by inhalation. The Occupational Safety and Health Administration (OSHA) has specified the legal limit (Permissible exposure limit) for rhodium exposure in the workplace at 0.1 mg/m3 over an 8-hour workday, and the National Institute for Occupational Safety and Health (NIOSH) has set the recommended exposure limit (REL), at the same level. At levels of 100 mg/m3, rhodium is immediately dangerous to life or health. For soluble compounds, the PEL and REL are both 0.001 mg/m3.
Organorhodium compounds
Rhodium is known for its many organometallic derivatives. Rhodium(I) complexes are commonly used as catalysis, with a few being BINAP-Rh(I), DIPAMP-Rh(I), and BDPP-Rh(I).
(2026). 9780470132609 ISBN 9780470132609
Isotopes
Naturally occurring rhodium is composed of only one isotope, 103Rh. With a nuclear spin of -1/2, 103Rh is well-suited for nuclear magnetic resonance spectroscopic studies. With a particularly low nuclear dipole moment, 103Rh exhibits very low receptivity.
Occurrence
Rhodium is one of the rarest elements in the Earth's crust, with less than one part per billion. Its rarity affects its price and its use in commercial applications. The concentration of rhodium in nickel is typically 1 part per billion.D.E.Ryan, J.Holzbecher and R.R.Brooks, Chemical Geology, Volume 85, Issues 3–4, 30 July 1990, Pages 295-303 Rhodium has been measured in some with concentrations between 0.8 and 30 ppt.
Mining and price
Rhodium ores are a mixture with other metals such as palladium, silver, platinum, and gold. Few rhodium are known. The separation of rhodium from the other metals poses significant challenges. Principal production is located in South Africa, at 75%, with much less production in Russia and Zimbabwe. The annual world production flucuates around 30 . The price of rhodium is highly variable, reaching a high of $14,197 in July 2022.
Used nuclear fuels
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.
Applications
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.
Carbonylation
Rhodium are used in some industrial processes, notably those involving carbon monoxide. In the Monsanto process, rhodium iodides catalyze the carbonylation of methanol to produce acetic acid. This technology has been significantly displaced by the iridium-based Cativa process, which effects the same conversion but more efficiently. Rhodium-based complexes are the dominant catalysts for hydroformylation, which converts alkenes to according to the following equation:
Ornamental uses
Rhodium finds use in jewelry and for decorations. It is electroplated on white 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 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. The high cost ensures that rhodium is typically applied as an electroplating. 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 and in 2008, Barack Obama gave his wife a rhodium ring.
Other uses
Rhodium is used as an alloying agent for hardening and improving the corrosion resistance of platinum and palladium. These alloys are used in furnace windings, bushings for glass fiber production, thermocouple elements, for aircraft , and laboratory crucibles. Other uses include electrical contacts, where it is valued for small electrical resistance, small and stable contact resistance, and great corrosion resistance, and filters and anodes in mammography systems that emit a lower Absorbed dose than comparable systems using molybdenum.
Precautions
Being a noble metal, pure rhodium is inert and harmless in elemental form. However, chemical complexes of rhodium can be reactive. For rhodium chloride, the median lethal dose (LD50) for rats is 198 mg () per kilogram of body weight.
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