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{{infobox rhodium}}

'''Rhodium''' is a [[Chemistry:Chemical element|chemical element]]; it has [[Chemistry:Symbol|symbol]] '''Rh''' and [[Physics:Atomic number|atomic number]] 45. It is a very rare, silvery-white, hard, [[Chemistry:Corrosion|corrosion-resistant]] [[Chemistry:Transition metal|transition metal]]. It is a [[Chemistry:Noble metal|noble metal]] and a member of the [[Earth:Platinum group|platinum group]]. It has only one naturally occurring [[Physics:Isotope|isotope]], which is <sup>103</sup>Rh. 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 [[Chemistry:Bowieite|bowieite]] and [[Chemistry:Rhodplumsite|rhodplumsite]]. It is one of the rarest and most valuable [[Chemistry:Precious metal|precious metal]]s.

Rhodium is found in platinum or nickel ores with the other members of the [[Earth:Platinum group|platinum group]] metals. It was discovered in 1803 by [[Biography:William Hyde Wollaston|William Hyde Wollaston]] in one such ore, and named for the rose color of one of its [[Chemistry:Chlorine|chlorine]] compounds.

The element's major use (consuming about 80% of world rhodium production) is as one of the [[Chemistry:Catalyst|catalyst]]s in the [[Chemistry:Catalytic converter#Three-way|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 [[Chemistry:Alloy|alloy]]ed with [[Chemistry:Platinum|platinum]] or [[Chemistry:Palladium|palladium]] and applied in high-temperature and corrosion-resistive coatings. [[Chemistry:Colored gold#White gold|White gold]] is often plated with a thin rhodium layer to improve its appearance, while [[Chemistry:Sterling silver|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.<ref>Armin Fehn and Juergen Weidinger, Wacker Chemie AG, US patent US7129309B2</ref>

Rhodium detectors are used in [[Physics:Nuclear reactor|nuclear reactor]]s to measure the [[Physics:Neutron detection|neutron flux level]]. Other uses of rhodium include asymmetric hydrogenation used to form drug precursors and the processes for the production of [[Chemistry:Acetic acid|acetic acid]].

==History==
[[File:Wollaston William Hyde Jackson color.jpg|thumb|upright|left|William Hyde Wollaston]]
Rhodium ([[Social:Greek language|Greek]] ''rhodon'' (ῥόδον) meaning "rose") was discovered in 1803 by [[Biography:William Hyde Wollaston|William Hyde Wollaston]],<ref>{{cite journal |title= On a New Metal, Found in Crude Platina |first=W. H. |last=Wollaston|journal=Philosophical Transactions of the Royal Society of London |volume=94 |year=1804 |pages=419–430 |doi=10.1098/rstl.1804.0019| url = https://books.google.com/books?id=7AZGAAAAMAAJ&pg=PA419|doi-access=free }}</ref> soon after he discovered [[Chemistry:Palladium|palladium]].<ref>{{cite journal|journal = Platinum Metals Review|url = http://www.platinummetalsreview.com/dynamic/article/view/47-4-175-183|title = Rhodium and Palladium – Events Surrounding Its Discovery|first = W. P. |last = Griffith|volume = 47|issue = 4|year = 2003|pages = 175–183}}</ref><ref>{{cite journal|title = On the Discovery of Palladium; With Observations on Other Substances Found with Platina|first = W. H.|last = Wollaston|author-link = William Hyde Wollaston|journal = Philosophical Transactions of the Royal Society of London|volume = 95|year = 1805|pages = 316–330|doi = 10.1098/rstl.1805.0024|doi-access = free}}</ref><ref name="contr">{{cite journal | doi = 10.1080/00033797800200431 | title = The Wollaston/Chenevix controversy over the elemental nature of palladium: A curious episode in the history of chemistry |year = 1978 | last1 = Usselman | first1 = Melvyn | journal = Annals of Science | volume = 35 | issue = 6 | pages = 551–579}}</ref> He used crude [[Chemistry:Platinum|platinum]] ore presumably obtained from [[Earth:South America|South America]].<ref>{{Cite book|author=Lide, David R.|title=CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data|year=2004|publisher=CRC Press|location=Boca Raton|isbn=978-0-8493-0485-9|pages=[https://archive.org/details/crchandbookofche81lide/page/4 4–26]|url-access=registration|url=https://archive.org/details/crchandbookofche81lide/page/4}}</ref> His procedure dissolved the ore in [[Chemistry:Aqua regia|aqua regia]] and neutralized the acid with [[Chemistry:Sodium hydroxide|sodium hydroxide]] (NaOH). He then precipitated the platinum as ammonium chloroplatinate by adding [[Chemistry:Ammonium chloride|ammonium chloride]] ({{chem|NH|4|Cl}}). Most other metals like [[Chemistry:Copper|copper]], [[Chemistry:Lead|lead]], [[Chemistry:Palladium|palladium]], and rhodium were precipitated with [[Chemistry:Zinc|zinc]]. Diluted [[Chemistry:Nitric acid|nitric acid]] dissolved all but palladium and rhodium. Of these, palladium dissolved in [[Chemistry:Aqua regia|aqua regia]] but rhodium did not,<ref>{{Greenwood&Earnshaw2nd|page=1113}}</ref> and the rhodium was precipitated by the addition of [[Chemistry:Sodium chloride|sodium chloride]] as {{chem| Na|3|[RhCl|6|]·''n''H|2|O}}. After being washed with ethanol, the rose-red precipitate was reacted with zinc, which displaced the rhodium in the ionic compound and thereby released the rhodium as free metal.<ref name="griffith">{{cite journal |title=Bicentenary of Four Platinum Group Metals: Osmium and iridium – events surrounding their discoveries |author=Griffith, W. P. |journal=Platinum Metals Review |volume=47 |issue=4 |year=2003 |pages=175–183}}</ref>

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&nbsp;°C.<ref>{{cite journal |year = 1904 | last1 = Hulett | first1 = G. A. | title = Volatilization of Platinum | last2 = Berger | first2 = H. W. | journal = Journal of the American Chemical Society | volume = 26 | issue = 11 | pages = 1512–1515 | doi=10.1021/ja02001a012| url = https://zenodo.org/record/1428894 |via= Zenodo |url-status=live |archive-url=https://web.archive.org/web/20240124003101/https://zenodo.org/records/1428894/files/article.pdf?download=1 |archive-date= Jan 24, 2024 }}</ref><ref>{{cite book | chapter = Platinum Type | isbn = 978-0-8031-1466-1 | chapter-url = https://books.google.com/books?id=Pos-MXDWb6MC&pg=PA63 | title = Manual on the use of thermocouples in temperature measurement | series = ASTM Special Technical Publication | publisher = ASTM International | author1 =  ((ASTM Committee E.2.0. on Temperature Measurement)) |year = 1993| bibcode = 1981mutt.book.....B }}</ref> They have exceptionally good stability in the temperature range of 1300 to 1800&nbsp;°C.<ref>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&nbsp;°C and 1500&nbsp;°C, METROLOGIA, 2018, Volume: 55 Issue: 4 Pages: 558-567</ref>

The first major application was electroplating for decorative uses and as corrosion-resistant coating.<ref>{{cite journal | last = Kushner | first = Joseph B. | journal = Metals and Alloys | pages = 137–140 |year = 1940 | title = Modern rhodium plating | volume = 11 }}</ref> The introduction of the three-way [[Chemistry:Catalytic converter|catalytic converter]] by [[Company:Volvo|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 [[Earth:NOx|NO<sub>x</sub>]] in the exhaust.<ref>{{cite journal | doi = 10.1016/S0959-6526(00)00082-2 | title = Life cycle assessment of a catalytic converter for passenger cars |year = 2001 | last1 = Amatayakul | first1 = W. | journal = Journal of Cleaner Production | volume = 9 | issue = 5 | pages = 395 | last2 = Ramnäs | first2 = Olle}}</ref><ref>{{cite journal | doi = 10.1016/S0926-860X(01)00818-3 | title = Automobile exhaust catalysts |year = 2001 | last1 = Heck | first1 = R. | journal = Applied Catalysis A: General | volume = 221 | issue = 1–2 | pages = 443–457 | last2 = Farrauto | first2 = Robert J.}}</ref><ref>{{cite journal | doi =  10.1016/S1385-8947(00)00365-X | title = The application of monoliths for gas phase catalytic reactions |year = 2001 | last1 = Heck | first1 = R. | journal = Chemical Engineering Journal | volume = 82 | issue = 1–3 | pages = 149–156 | last2 = Gulati | first2 = Suresh | last3 = Farrauto | first3 = Robert J.}}</ref>

== Characteristics ==

{| class="wikitable" style="float: right; margin: 0; margin-left: 1em;"
|-
![[Physics:Atomic number|Z]] !! [[Chemistry:Chemical element|Element]] !! [[Physics:Electron shell|No. of electrons/shell]]
|-
| 27 || cobalt || 2, 8, 15, 2
|-
| 45 || rhodium || 2, 8, 18, 16, 1
|-
| 77 || iridium || 2, 8, 18, 32, 15, 2
|-
| 109 || meitnerium || 2, 8, 18, 32, 32, 15, 2 (predicted)
|}
Rhodium is a hard, silvery, durable metal that has a high [[Physics:Reflectance|reflectance]]. Rhodium metal does not normally form an [[Chemistry:Oxide|oxide]], even when heated.<ref name="ASM13B">{{cite book|editor-last = Cramer|editor-first = Stephen D.|editor2-last = Covino |editor2-first=Bernard S. Jr.|title = ASM handbook|year = 1990|publisher = ASM International|location = Materials Park, OH|isbn=978-0-87170-707-9|pages = 393–396|url = https://books.google.com/books?id=QV0sWU2qF5oC&pg=PA396}}</ref> [[Chemistry:Oxygen|Oxygen]] is absorbed from the [[Astronomy:Atmosphere|atmosphere]] only at the [[Physics:Melting point|melting point]] of rhodium, but is released on solidification.<ref>{{cite book|last = Emsley|first = John|title = Nature's Building Blocks|edition = (Hardcover, First Edition)|publisher = [[Organization:Oxford University Press|Oxford University Press]]|year = 2001|page = [https://archive.org/details/naturesbuildingb0000emsl/page/363 363]|isbn = 978-0-19-850340-8|url = https://archive.org/details/naturesbuildingb0000emsl/page/363}}</ref> Rhodium has both a higher melting point and lower [[Physics:Density|density]] than [[Chemistry:Platinum|platinum]]. It is not attacked by most [[Chemistry:Acid|acid]]s: it is completely insoluble in [[Chemistry:Nitric acid|nitric acid]] and dissolves slightly in [[Chemistry:Aqua regia|aqua regia]].

===Chemical properties===
[[Image:Wilkinson's-catalyst-2D.png|thumb|left|upright|Wilkinson's catalyst]]
Rhodium belongs to [[Chemistry:Group 9 element|group 9]] of the periodic table, but exhibits an atypical [[Physics:Ground state|ground state]] [[Physics:Valence electron|valence electron]] configuration for that group.  Like neighboring elements [[Chemistry:Niobium|niobium]] (41), [[Chemistry:Ruthenium|ruthenium]] (44), and [[Chemistry:Palladium|palladium]] (46), it only has one electron in its outermost ''s'' orbital.

{|class="wikitable" style="float: right; margin: 0; margin-left: 1em;"
|-
! colspan=2|Oxidation states<br />of rhodium
|-
| +0 ||{{chem|Rh|4|(||CO)|12|}}
|-
| +1 ||{{chem|RhCl||(PH|3|)|2|}}
|-
| +2 ||{{chem|Rh|2|(O|2|CCH|3|)|4}}
|-
| '''+3''' ||{{chem|RhCl|3|, Rh|2|O|3|}}
|-
| +4 ||{{chem|RhO|2}}
|-
| +5 ||{{chem|RhF|5|, Sr|3|LiRhO|6}}
|-
| +6 ||{{chem|RhF|6}}
|}

The common [[Chemistry:Oxidation state|oxidation state]] of rhodium is +3, but oxidation states from 0 to +7 are also observed.<ref name="Holl">{{cite book|publisher = Walter de Gruyter|year = 1985|edition = 91–100|pages = 1056–1057|isbn = 978-3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|first = Arnold F.|last = Holleman|author2 = Wiberg, Egon|author3 = Wiberg, Nils}}</ref>
<ref>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)</ref>

Unlike [[Chemistry:Ruthenium|ruthenium]] and [[Chemistry:Osmium|osmium]], rhodium forms no volatile oxygen compounds. The known stable oxides include {{chem|link=Rhodium(III) oxide|Rh|2|O|3}}, {{chem|link=Rhodium(IV) oxide|RhO|2}}, {{chem|RhO|2|·''x''H|2|O}}, {{chem|Na|2|RhO|3}}, {{chem|Sr|3|LiRhO|6}} and {{chem|Sr|3|NaRhO|6}}.<ref>{{cite journal|first = B. A.|last = Reisner|author2=Stacy, A. M. |title={{chem|Sr|3|ARhO|6}} (A = Li, Na): Crystallization of a Rhodium(V) Oxide from Molten Hydroxide|doi = 10.1021/ja974231q|volume = 120|issue = 37|date = 1998|pages = 9682–9989|journal =Journal of the American Chemical Society }}</ref> Halogen compounds are known in nearly the full range of possible oxidation states. [[Chemistry:Rhodium(III) chloride|Rhodium(III) chloride]], [[Chemistry:Rhodium trifluoride|rhodium trifluoride]], [[Chemistry:Rhodium pentafluoride|rhodium pentafluoride]] and [[Chemistry:Rhodium hexafluoride|rhodium hexafluoride]] are examples. The lower oxidation states are stable only in the presence of ligands.<ref>Griffith, W. P. ''The Rarer Platinum Metals'', John Wiley and Sons: New
York, 1976, p. 313.</ref>

The best-known rhodium-halogen compound is the [[Chemistry:Wilkinson's catalyst|Wilkinson's catalyst]] chlorotris(triphenylphosphine)rhodium(I). This catalyst is used in the [[Chemistry:Hydroformylation|hydroformylation]] or [[Chemistry:Hydrogenation|hydrogenation]] of [[Chemistry:Alkene|alkene]]s.<ref>{{cite journal| title=The Preparation and Properties of Tris(triphenylphosphine)halogenorhodium(I) and Some Reactions Thereof Including Catalytic Homogeneous Hydrogenation of Olefins and Acetylenes and Their Derivatives| journal= Journal of the Chemical Society A|year = 1966|pages = 1711–1732|doi = 10.1039/J19660001711| author1=Osborn, J. A.| author2=Jardine, F. H.| author3=Young, J. F.| author4=Wilkinson, G.}}</ref>

=== Isotopes ===

{{main|Physics:Isotopes of rhodium}}
Naturally occurring rhodium is composed of only one [[Physics:Isotope|isotope]], <sup>103</sup>Rh. The most stable radioisotopes are <sup>101</sup>Rh with a [[Physics:Half-life|half-life]] of 3.3 years, <sup>102</sup>Rh with a half-life of 207 days, <sup>102m</sup>Rh with a half-life of 2.9 years, and <sup>99</sup>Rh with a half-life of 16.1 days. Twenty other radioisotopes have been characterized with [[Physics:Atomic weight|atomic weight]]s ranging from 92.926 u (<sup>93</sup>Rh) to 116.925 u (<sup>117</sup>Rh). Most of these have half-lives shorter than an hour, except <sup>100</sup>Rh (20.8 hours) and <sup>105</sup>Rh (35.36 hours). Rhodium has numerous meta states, the most stable being <sup>102m</sup>Rh (0.141&nbsp;MeV) with a half-life of about 2.9 years and <sup>101m</sup>Rh (0.157&nbsp;MeV) with a half-life of 4.34 days (see [[Physics:Isotopes of rhodium|isotopes of rhodium]]).<ref name="nubase">{{NUBASE 2003}}</ref>

In isotopes weighing less than 103 (the stable isotope), the primary decay mode is [[Physics:Electron capture|electron capture]] and the primary [[Physics:Decay product|decay product]] is [[Chemistry:Ruthenium|ruthenium]]. In isotopes greater than 103, the primary decay mode is [[Physics:Beta emission|beta emission]] and the primary product is [[Chemistry:Palladium|palladium]].<ref>David R. Lide (ed.), Norman E. Holden in ''CRC Handbook of Chemistry and Physics, 85th Edition'' CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes.</ref>

==Occurrence==
Rhodium is one of the [[Earth:Abundance of elements in Earth's crust|rarest elements in the Earth's crust]], comprising an estimated 0.0002 [[Physics:Parts-per notation|parts per million]] (2 × 10<sup>−10</sup>).<ref>Barbalace, Kenneth, "[http://environmentalchemistry.com/yogi/periodic/Periodic Table of Elements]". Environmental Chemistry.com; retrieved 2007-04-14.</ref> Its rarity affects its price and its use in commercial applications. The concentration of rhodium in nickel [[Astronomy:Meteorite|meteorite]]s is typically 1 [[Physics:Parts-per notation|part per billion]].<ref>D.E.Ryan, J.Holzbecher and R.R.Brooks, Chemical Geology, Volume 85, Issues 3–4, 30 July 1990, Pages 295-303</ref> Rhodium has been measured in some [[Biology:Potato|potato]]es with concentrations between 0.8 and 30 ppt.<ref>Orecchio and Amorello, Foods, 2019, volume 8, issue 2, {{doi| 10.3390/foods8020059}}</ref>

===Mining and price===
[[File:Rh price.png|thumb|left|upright=1.6|Rh price evolution]]
[[File:Rhodium daily Price 1992-2022.webp|thumb|350px|left|Rhodium daily price 1992–2022]]
The industrial extraction of rhodium is complex because the ores are mixed with other metals such as [[Chemistry:Palladium|palladium]], [[Chemistry:Silver|silver]], [[Chemistry:Platinum|platinum]], and [[Chemistry:Gold|gold]] and there are very few rhodium-bearing [[Earth:Mineral|mineral]]s. 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 [[Chemistry:Copper|copper]]-[[Chemistry:Nickel sulfide|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.<ref name="USGSYB08" /> The annual world production is 30 [[Tonne|tonne]]s. The price of rhodium is highly variable. In 2007, rhodium cost approximately eight times more than gold, 450 times more than silver, and 27,250 times more than copper by weight. In 2008, the price briefly rose above $10,000 per ounce ($350,000 per kilogram). The economic slowdown of the 3rd quarter of 2008 pushed rhodium prices sharply back below $1,000 per ounce ($35,000 per kilogram); the price rebounded to $2,750 by early 2010 ($97,000 per kilogram) (more than twice the gold price), but in late 2013, the prices were less than $1,000. Political and financial problems{{clarify|reason=which problems, where, when?|date=February 2016}} led to very low oil prices and over supply, causing most metals to drop in price. The economies of China, India and other emerging countries slowed in 2014 and 2015. In 2014 alone, 23,722,890 motor vehicles were produced in China, excluding motorbikes.{{clarify|reason=not clear whether this random figure represents normal production, increase or decrease|date=February 2016}} This resulted in a rhodium price of 740.00 US-$ per Troy ounce (31.1 grams) in late November 2015.<ref>{{Cite web|url=https://www.finanzen.net/rohstoffe/rhodiumpreis|title=Rhodiumpreis aktuell in Euro und Dollar &#124; Rhodium &#124; Rhodiumkurs|website=finanzen.net}}</ref>

Owners of rhodium—a metal with a highly volatile market price—are periodically put in an extremely advantageous market position: extracting more rhodium-containing ore from the ground will necessarily also extract other much more abundant precious metals—notably platinum and palladium—which would oversupply the market with those other metals, lowering their prices. Since it is economically infeasible to simply extract these other metals just to obtain rhodium, the market is often left hopelessly squeezed for rhodium supply, causing prices to spike. Recovery from this supply-deficit position may be quite problematic in the future for many reasons, notably because it is not known how much rhodium (and other precious metals) actually was placed in catalytic converters during the many years when manufacturers' emissions-cheating software was in use. Much of the world supply of rhodium is obtained from recycled catalytic converters obtained from scrapped vehicles. As of early November 2020, the spot price of rhodium was US$14,700 per troy ounce.{{cn|date=November 2022}} In early March 2021, rhodium reached a price of US$29,400 per troy ounce{{cn|date=November 2022}} on Metals Daily{{rs?|date=November 2022}} (a precious metals commodity listing).

===Used nuclear fuels===
{{Main|Physics:Synthesis of precious metals}}
Rhodium is a fission product of [[Physics:Uranium-235|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 (<sup>101</sup>Rh with a [[Physics:Half-life|half-life]] of 3.3 years, and <sup>102m</sup>Rh with a [[Physics:Half-life|half-life]] of 2.9 years), or about 10 years. These factors make the source unattractive and no large-scale extraction has been attempted.<ref>{{cite journal|doi = 10.1595/147106705X35263|url = http://www.platinummetalsreview.com/pdf/79-90-pmr-apr05.pdf|title = Potential Applications of Fission Platinoids in Industry|year = 2005| last1 = Kolarik|first1 = Zdenek|last2 = Renard|first2 = Edouard V.|journal = Platinum Metals Review|volume = 49|issue = 2|pages = 79|doi-access = free}}</ref><ref>{{cite journal|title = Recovery of Value Fission Platinoids from Spent Nuclear Fuel. Part I PART I: General Considerations and Basic Chemistry|url =http://www.platinummetalsreview.com/pdf/pmr-v47-i2-074-087.pdf| first1 =Zdenek|last1 =Kolarik|first2 =Edouard V.|last2 =Renard| journal = Platinum Metals Review|volume = 47|issue = 2|year = 2003|pages = 74–87}}</ref><ref>{{cite journal|title = Recovery of Value Fission Platinoids from Spent Nuclear Fuel. Part II: Separation Process| url =http://www.platinummetalsreview.com/pdf/pmr-v47-i2-074-087.pdf|first1 =Zdenek|last1 =Kolarik|first2 =Edouard V.|last2 =Renard| journal = Platinum Metals Review|volume = 47|issue = 2|year = 2003|pages = 123–131}}</ref>

==Applications==
The primary use of this element is in automobiles as a [[Chemistry:Catalytic converter|catalytic converter]], changing harmful unburned hydrocarbons, carbon monoxide, and nitrogen oxide exhaust emissions into less noxious gases. Of 30,000&nbsp;kg of rhodium consumed worldwide in 2012, 81% (24,300&nbsp;kg) went into this application, and 8,060&nbsp;kg was recovered from old converters. About 964&nbsp;kg of rhodium was used in the glass industry, mostly for production of fiberglass and flat-panel glass, and 2,520&nbsp;kg was used in the chemical industry.<ref name="USGSYB08">{{cite web|url = http://minerals.usgs.gov/minerals/pubs/commodity/platinum/myb1-2012-plati.pdf|publisher = United States Geological Survey|access-date = 2012-07-16|title = Commodity Report: Platinum-Group Metals|first = Patricia J.|last = Loferski|year=2013}}</ref>

===Catalyst===
Rhodium is preferable to the other platinum metals in the [[Chemistry:Redox|reduction]] of nitrogen oxides to [[Chemistry:Nitrogen|nitrogen]] and [[Chemistry:Oxygen|oxygen]]:<ref name="Whyrhodium">{{cite journal|last = Shelef|first =M.|author2=Graham, G. W. |year = 1994|title = Why Rhodium in Automotive Three-Way Catalysts?|journal = Catalysis Reviews|volume = 36|issue = 3|pages=433–457|doi = 10.1080/01614949408009468}}</ref>

{{block indent|2 {{chem|NO|''x''}} → ''x'' {{chem|O|2}} + {{chem|N|2}}}}

In 2008, net demand (with the recycling accounted for) of rhodium for automotive converters made up 84% of the world usage,<ref>{{Cite thesis |last=Murray |first=Angela Janet |title=Recovery of Platinum Group Metals from Spent Furnace Linings and Used Automotive Catalysts |date=2012 |degree=PhD |publisher=University of Birmingham |url=https://etheses.bham.ac.uk/id/eprint/7210/1/Murray12PhD.pdf}}</ref> with the number fluctuating around 80% in 2015−2021.<ref>{{cite web | url=https://www.sfa-oxford.com/platinum-group-metals/rhodium-market-and-rhodium-price-drivers | title=The Rhodium Market and Rhodium Price }}</ref>

Rhodium [[Chemistry:Catalyst|catalyst]]s are used in a number of industrial processes, notably in catalytic carbonylation of [[Chemistry:Methanol|methanol]] to produce [[Chemistry:Acetic acid|acetic acid]] by the [[Chemistry:Monsanto process|Monsanto process]].<ref>{{cite journal|title = Rhodium Catalysed Carbonylation of Methanol|first = James F.|last = Roth|journal = Platinum Metals Review|volume = 19|issue = 1 January|year = 1975|pages = 12–14|url = http://www.platinummetalsreview.com/pdf/pmr-v19-i1-012-014.pdf}}</ref> It is also used to catalyze addition of hydrosilanes to molecular [[Chemistry:Double bond|double bond]]s, a process important in manufacture of certain silicone rubbers.<ref>{{cite journal|author=Heidingsfeldova, M.|author2=Capka, M.|name-list-style=amp |title=Rhodium complexes as catalysts for hydrosilylation crosslinking of silicone rubber|doi=10.1002/app.1985.070300505 |journal=Journal of Applied Polymer Science |volume=30|issue=5 |year=2003|page=1837}}</ref> Rhodium catalysts are also used to reduce [[Chemistry:Benzene|benzene]] to [[Chemistry:Cyclohexane|cyclohexane]].<ref>{{cite journal|author=Halligudi, S. B.|display-authors=0 |title=Hydrogenation of benzene to cyclohexane catalyzed by rhodium(I) complex supported on montmorillonite clay|doi=10.1007/BF02162706 |journal=Reaction Kinetics and Catalysis Letters|volume=48|issue=2|year=1992|page= 547|bibcode=1992RKCL...48..505T |s2cid=97802315 }}</ref>

The complex of a rhodium ion with BINAP is a widely used chiral catalyst for chiral synthesis, as in the synthesis of [[Chemistry:Menthol|menthol]].<ref>{{cite journal|author=Akutagawa, S. |title=Asymmetric synthesis by metal BINAP catalysts|doi=10.1016/0926-860X(95)00097-6 |journal=Applied Catalysis A: General|volume=128|issue=2 |page=171|year=1995}}</ref>

===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 [[Chemistry:Sterling silver|sterling silver]] to protect against tarnish ([[Chemistry:Silver sulfide|silver sulfide]], Ag<sub>2</sub>S, produced from atmospheric hydrogen sulfide, H<sub>2</sub>S). 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.<ref>{{cite journal|doi = 10.1111/j.1600-0536.1984.tb00056.x|title = Contact sensitivity to nickel in white gold|year = 1984|last1 = Fischer|first1 = Torkel|journal = Contact Dermatitis|volume = 10|pages = 23–24|pmid = 6705515|last2 = Fregert|first2 = S.|last3 = Gruvberger|first3 = B.|last4 = Rystedt|first4 = I.|issue = 1|s2cid = 46626556}}</ref> The high cost ensures that rhodium is applied only as an [[Engineering:Electroplating|electroplate]].<!--, Text seems to add nothing, left for second opinion: "...where tiny amounts of powder, commonly called rhodium sponge,&nbsp;[clarification needed] are in solution."-->
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.<ref>{{cite news| url = https://www.independent.co.uk/news/people/hit-and-run/hit--run-ring-the-changes-1044166.html| work= The Independent |access-date=2009-06-06| title= Hit & Run: Ring the changes |location=London|date=2008-12-02}}</ref>

===Other uses===
Rhodium is used as an alloying agent for hardening and improving the corrosion resistance<ref name="ASM13B" /> of [[Chemistry:Platinum|platinum]] and [[Chemistry:Palladium|palladium]]. These alloys are used in furnace windings, bushings for glass fiber production, [[Physics:Thermocouple|thermocouple]] elements, [[Physics:Electrode|electrode]]s for aircraft [[Physics:Spark plug|spark plug]]s, and laboratory crucibles.<ref>{{Cite book|author= Lide, David R|title= CRC handbook of chemistry and physics 2004–2005: a ready-reference book of chemical and physical data|year=2004|publisher=CRC Press|location=Boca Raton|isbn=978-0-8493-0485-9 |edition=85th |pages=4–26|url=https://books.google.com/books?id=WDll8hA006AC}}</ref> Other uses include:
* [[Engineering:Switch#Contacts|Electrical contacts]], where it is valued for small [[Physics:Electrical resistance|electrical resistance]], small and stable [[Physics:Contact resistance|contact resistance]], and great [[Chemistry:Corrosion|corrosion]] resistance.<ref>{{cite journal|journal = Metal Finishing|volume = 97|issue = 1|year = 1999|pages =296–299| title = Rhodium plating|doi = 10.1016/S0026-0576(00)83088-3|first = Alfred M.|last = Weisberg}}</ref>
* Rhodium plated by either [[Engineering:Electroplating|electroplating]] or evaporation is extremely hard and useful for optical instruments.<ref>{{cite book|title = Modern optical engineering: the design of optical systems|first = Warren J.|last = Smith|publisher = McGraw-Hill|year = 2007|isbn = 978-0-07-147687-4|chapter-url = https://books.google.com/books?id=DrtM_bAnf_YC|pages = 247–248|chapter = Reflectors}}</ref>
* Filters in mammography systems for the characteristic X-rays it produces.<!--produces or filters/transmits?--><ref>{{cite journal|author=McDonagh, C P|display-authors=0|title=Optimum x-ray spectra for mammography: choice of K-edge filters for tungsten anode tubes|doi=10.1088/0031-9155/29/3/004 |pmid=6709704|journal=Phys. Med. Biol. |volume=29|issue=3 |pages=249–52|year=1984|bibcode = 1984PMB....29..249M |s2cid=250873106 }}</ref>
* 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.<ref>{{cite journal
|first1 = A. P. |last1 = Sokolov
|last2 = Pochivalin|first2 = G. P.
|last3 = Shipovskikh|first3 = Yu. M.
|last4 = Garusov|first4 = Yu. V.
|last5 = Chernikov|first5 = O. G.
|last6 = Shevchenko|first6 = V. G.
|title = Rhodium self-powered detector for monitoring neutron fluence, energy production, and isotopic composition of fuel|doi = 10.1007/BF00844622|journal = Atomic Energy|volume = 74|year = 1993|issue = 5 |pages = 365–367 |s2cid = 96175609
}}</ref>

In automobile manufacturing, rhodium is also used in the construction of headlight reflectors.<ref name="Stwertka">Stwertka, Albert. ''A Guide to the Elements'', Oxford University Press, 1996, p. 125. {{ISBN|0-19-508083-1}}</ref>

<gallery widths="200px" heights="180px">
File:Rhodium 78g sample.jpg|A 78&nbsp;g sample of rhodium
Image:Aufgeschnittener Metall Katalysator für ein Auto.jpg|Cut-away of a metal-core catalytic converter
Image:White-gold--rhodium-plated.jpg|Rhodium-plated white gold wedding ring
Image:Rhodium foil and wire.jpg|Rhodium foil and wire
</gallery>

== Precautions ==
{{Chembox
| container_only = yes
|Section7={{Chembox Hazards
| ExternalSDS =
| GHSPictograms =
| GHSSignalWord = 
| HPhrases = {{H-phrases|413|}}
| PPhrases = {{P-phrases|273|501}}<ref>{{Cite web|url=https://www.sigmaaldrich.com/MSDS/MSDS/DisplayMSDSPage.do?country=US&language=en&productNumber=357340&brand=ALDRICH&PageToGoToURL=https://www.sigmaaldrich.com/catalog/product/aldrich/357340?lang=en|title=MSDS - 357340|website=www.sigmaaldrich.com}}</ref>
| NFPA-H = 0
| NFPA-F = 0
| NFPA-R = 0
| NFPA-S = 
| NFPA_ref = 
 }}
}}
Being a [[Chemistry:Noble metal|noble metal]], pure rhodium is inert and harmless in elemental form.<ref>{{cite book|page = 846|title = Poisoning and Toxicology Handbook|first = Jerrold B.|last = Leikin|author2=Paloucek Frank P. |publisher = Informa Health Care|year = 2008|isbn = 978-1-4200-4479-9|url = https://books.google.com/books?id=0Bw2UJTC_uMC}}</ref> However, chemical complexes of rhodium can be reactive. For rhodium chloride, the [[Chemistry:Median lethal dose|median lethal dose]] (LD<sub>50</sub>) for rats is 198&nbsp;mg ({{chem|RhCl|3}}) per kilogram of body weight.<ref>{{cite journal|doi = 10.1016/0041-008X(72)90016-6|journal = Toxicology and Applied Pharmacology|volume = 21| issue = 4|year = 1972|pages = 589–590|title = Studies on the toxicity of rhodium trichloride in rats and rabbits|first = Robert R.|last = Landolt|author2 = Berk Harold W.|author3 = Russell, Henry T.|pmid = 5047055}}</ref> Like the other noble metals, rhodium has not been found to serve any biological function.

People can be exposed to rhodium in the workplace by inhalation. The [[Organization:Occupational Safety and Health Administration|Occupational Safety and Health Administration]] (OSHA) has specified the legal limit ([[Earth:Permissible exposure limit|Permissible exposure limit]]) for rhodium exposure in the workplace at 0.1&nbsp;mg/m<sup>3</sup> over an 8-hour workday, and the [[Organization:National Institute for Occupational Safety and Health|National Institute for Occupational Safety and Health]] (NIOSH) has set the [[Earth:Recommended exposure limit|recommended exposure limit]] (REL), at the same level. At levels of 100&nbsp;mg/m<sup>3</sup>, rhodium is [[Chemistry:Immediately dangerous to life or health|immediately dangerous to life or health]].<ref>{{Cite web|title = NIOSH Pocket Guide to Chemical Hazards - Rhodium (metal fume and insoluble compounds, as Rh)|url = https://www.cdc.gov/niosh/npg/npgd0544.html|website = CDC |access-date = 2015-11-21}}</ref> For soluble compounds, the [[Earth:Permissible exposure limit|PEL]] and REL are both 0.001&nbsp;mg/m<sup>3</sup>.<ref>{{Cite web|title = NIOSH Pocket Guide to Chemical Hazards - Rhodium (soluble compounds, as Rh)|url = https://www.cdc.gov/niosh/npg/npgd0545.html|website = CDC |access-date = 2015-11-21}}</ref>

==See also==
* 2000s commodities boom
* 2020s commodities boom
* [[Chemistry:Bullion|Bullion]]
* Bullion coin
* [[:Category:Rhodium compounds|Rhodium compounds]]

==References==
{{Reflist|30em}}

==External links==
{{Commons|Rhodium}}
* [http://www.periodicvideos.com/videos/045.htm Rhodium] at ''The Periodic Table of Videos'' (University of Nottingham)
* [https://www.americanelements.com/rh.html Rhodium Technical and Safety Data]
* [https://www.cdc.gov/niosh/npg/npgd0544.html CDC – NIOSH Pocket Guide to Chemical Hazards]

{{Periodic table (navbox)}}
{{Rhodium compounds}}
{{Jewellery}}



[[Category:Chemical elements]]
[[Category:Noble metals]]
[[Category:Transition metals]]
[[Category:Native element minerals]]
[[Category:Chemical elements with face-centered cubic structure]]

{{Sourceattribution|Rhodium}}