Editing Rhodium

{{Good article}}
{{Use dmy dates|date=January 2021}}
{{Infobox rhodium}}

'''Rhodium''' is a [[chemical element]]; it has [[Symbol (chemistry)|symbol]] '''Rh''' and [[atomic number]] 45. It is a very rare, silvery-white, hard, [[corrosion|corrosion-resistant]] [[transition metal]]. It is a [[noble metal]] and a member of the [[platinum group]]. It has only one naturally occurring [[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 [[bowieite]] and [[rhodplumsite]]. It is one of the rarest and most valuable [[precious metal]]s. Rhodium is a [[Group 9 element|group 9]] element (cobalt group).

Rhodium is found in platinum or nickel ores with the other members of the [[platinum group]] metals. It was [[discovery of the chemical elements|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 [[catalyst]]s in the [[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 [[alloy]]ed with [[platinum]] or [[palladium]] and applied in high-temperature and corrosion-resistive coatings. [[Colored gold#White gold|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 [[nuclear reactor]]s to measure the [[Neutron detection|neutron flux level]]. Other uses of rhodium include asymmetric hydrogenation used to form drug precursors and the processes for the production of [[acetic acid]].

==History==
[[File:Wollaston William Hyde Jackson color.jpg|thumb|upright=0.8|left|[[William Hyde Wollaston]]]]

Rhodium (from {{langx|el|ῥόδον}} {{tlit|el|rhodon}}, meaning 'rose') was [[discovery of the chemical elements|discovered]] in 1803 by [[William Hyde Wollaston]],<ref>{{cite journal |title= On a New Metal, Found in Crude Platina |first=W. H. |last=Wollaston|author-link=William Hyde 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 [[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| doi=10.1595/003214003X474175183 |doi-access = free}}</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 [[platinum]] ore presumably obtained from [[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 [[aqua regia]] and neutralized the acid with [[sodium hydroxide]] (NaOH). He then precipitated the platinum as [[ammonium chloroplatinate]] by adding [[ammonium chloride]] ({{chem|NH|4|Cl}}). Most other metals like [[copper]], [[lead]], [[palladium]], and rhodium were precipitated with [[zinc]]. Diluted [[nitric acid]] dissolved all but palladium and rhodium. Of these, palladium dissolved in [[aqua regia]] but rhodium did not,<ref>{{Greenwood&Earnshaw2nd|page=1113}}</ref> and the rhodium was precipitated by the addition of [[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 [[Displacement reaction|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|doi=10.1595/003214003X474175183 |doi-access=free }}</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| bibcode = 1904JAChS..26.1512H | 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 }}{{Dead link|date=June 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</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 [[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|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 = 395–403 | last2 = Ramnäs | first2 = Olle| bibcode = 2001JCPro...9..395A }}</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.| bibcode = 2001AppCA.221..443H }}</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.| bibcode = 2001ChEnJ..82..149H }}</ref>

== Characteristics ==
{| class="wikitable" style="float: right; margin: 0; margin-left: 1em;"
|-
![[Atomic number|Z]] !! [[Chemical element|Element]] !! [[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 [[reflectance]]. Rhodium metal does not normally form an [[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> [[Oxygen]] is absorbed from the [[atmosphere]] only at the [[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 = [[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 [[density]] than [[platinum]]. It is not attacked by most [[acid]]s: it is completely insoluble in [[nitric acid]] and dissolves slightly in [[aqua regia]].

Rhodium belongs to [[group 9 element|group 9]] of the periodic table, but exhibits an atypical [[ground state]] [[valence electron]] configuration for that group.  Like neighboring elements [[niobium]] (41), [[ruthenium]] (44), and [[palladium]] (46), it only has one electron in its outermost [[s orbital|''s'' orbital]].

===Chemical properties===
[[Image:Wilkinson's-catalyst-2D.png|thumb|left|upright|Structure of [[Wilkinson's catalyst]] (Ph = [[phenyl]] = C<sub>6</sub>H<sub>5</sub>).]]

{|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 [[oxidation state]]s of rhodium are +3 and +1. Oxidation states 0, +2, and +4 are also well known.<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> A few complexes at still higher oxidation states are known.<ref>{{cite journal |doi=10.1002/anie.202207688 |title=The Highest Oxidation State of Rhodium: Rhodium(VII) in &#91;RhO<sub>3</sub>&#93;<sup>+</sup> |date=2022 |last1=Da Silva Santos |first1=Mayara |last2=Stüker |first2=Tony |last3=Flach |first3=Max |last4=Ablyasova |first4=Olesya S. |last5=Timm |first5=Martin |last6=von Issendorff |first6=Bernd |last7=Hirsch |first7=Konstantin |last8=Zamudio-Bayer |first8=Vicente |last9=Riedel |first9=Sebastian |last10=Lau |first10=J. Tobias |journal=Angewandte Chemie International Edition |volume=61 |issue=38 |pages=e202207688 |pmid=35818987 |pmc=9544489 }}</ref>

The rhodium 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>  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, {{chem2|Na3RhCl6}}, and [[pentaamminechlororhodium dichloride]], {{chem2|[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 with [[sodium acetate]] give the blue-green [[rhodium(II) acetate]], {{chem2|Rh2(O2CCH3)4}}, which features a Rh-Rh bond.  This complex and related [[rhodium(II) trifluoroacetate]] have attracted attention as catalysts for [[cyclopropanation]] reactions. Hydrated rhodium trichloride is reduced by [[carbon monoxide]], [[ethylene]], and [[trifluorophosphine]] to give rhodium(I) complexes {{chem2|Rh2Cl2L4}} (L = CO, {{chem2|C2H4, PF3}}).  When treated with [[triphenylphosphine]], hydrated rhodium trichloride converts to the maroon-colored {{chem2|RhCl(P(C6H5)3)3}}, which is known as [[Wilkinson's catalyst]]. Reduction of [[rhodium carbonyl chloride]] gives [[hexarhodium hexadecacarbonyl]], {{chem2|Rh6(CO)16}}, and [[tetrarhodium dodecacarbonyl]], {{chem2|Rh4(CO)12}}, the two most common Rh(0) complexes.

As for other metals, rhodium forms high oxidation state [[binary phase|binary fluoride]]s.  These include [[rhodium pentafluoride]], a tetrameric complex with the true formula {{chem2|Rh4F20}}) and [[rhodium hexafluoride]].<ref>{{cite book|author=Griffith, W. P.|title=The Rarer Platinum Metals|publisher=John Wiley and Sons|location=New York|year=1976}}</ref>

=== Isotopes ===
{{Main|Isotopes of rhodium}}

Naturally occurring rhodium is composed of only one [[isotope]], <sup>103</sup>Rh. The most stable [[radioisotope]]s are <sup>101</sup>Rh with a [[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 [[atomic weight]]s ranging from 92.926 [[atomic mass unit|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 state]]s, 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 [[isotopes of rhodium]]).<ref name="nubase">{{NUBASE 2003}}</ref>

In isotopes weighing less than 103 (the stable isotope), the primary [[decay mode]] is [[electron capture]] and the primary [[decay product]] is [[ruthenium]]. In isotopes greater than 103, the primary decay mode is [[beta emission]] and the primary product is [[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 [[Abundance of elements in Earth's crust|rarest elements in the Earth's crust]], comprising an estimated 0.0002 [[Parts-per notation|parts per million]] (2 × 10<sup>−10</sup>).<ref>Barbalace, Kenneth, "[http://environmentalchemistry.com/yogi/periodic/Periodic Table of Elements]{{Dead link|date=June 2024 |bot=InternetArchiveBot |fix-attempted=yes }}". 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 [[meteorite]]s is typically 1 [[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 [[potato]]es with concentrations between 0.8 and 30 ppt.<ref>{{cite journal | doi =  10.3390/foods8020059 | volume=8 | title=Platinum and Rhodium in Potato Samples by Using Voltammetric Techniques | journal=Foods | page=59 | last1 = Orecchio | first1 = Santino | last2 = Amorello | first2 = Diana| date=2019 | issue=2 | doi-access=free | pmid=30764564 | pmc=6406736 | hdl=10447/349662 | hdl-access=free }}</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]]

Rhodium ores are a mixture with other metals such as [[palladium]], [[silver]], [[platinum]], and [[gold]]. Few rhodium [[mineral]]s are known. The separation of rhodium from the other metals poses significant challenges. Principal sources are located in South Africa, river sands of the [[Ural Mountains]] in Russia, and in North America, especially the [[copper]]-[[nickel sulfide]] mining area of the [[Greater Sudbury|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]]s. The price of rhodium is highly variable.

===Used nuclear fuels===
{{Main|Synthesis of precious metals}}

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 (<sup>101</sup>Rh with a [[half-life]] of 3.3 years, and <sup>102m</sup>Rh 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.<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 = 79–90|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|doi =10.1595/003214003X4727487}}</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| doi =10.1595/003214003X473123131}}</ref>

==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&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|archive-date = 10 January 2019|archive-url = https://web.archive.org/web/20190110111705/https://minerals.usgs.gov/minerals/pubs/commodity/platinum/myb1-2012-plati.pdf|url-status = dead}}</ref><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>

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>

===Carbonylation===
[[image:HRh(CO)P3again.png|thumb|140px|[[Tris(triphenylphosphine)rhodium carbonyl hydride]], a widely used catalyst for hydroformylation (Ph = C<sub>6</sub>H<sub>5</sub>)]]

Rhodium [[catalyst]]s 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]].<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|doi = 10.1595/003214075X1911214|url = http://www.platinummetalsreview.com/pdf/pmr-v19-i1-012-014.pdf|access-date = 5 February 2009|archive-date = 24 September 2015|archive-url = https://web.archive.org/web/20150924074253/http://www.platinummetalsreview.com/pdf/pmr-v19-i1-012-014.pdf|url-status = dead}}</ref> 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 [[aldehyde]]s according to the following equation:<ref>{{cite book | title=Organotransition Metal Chemistry: From Bonding to Catalysis | publisher=University Science Books | author=Hartwig, John | year=2010 | location=New York | pages=1160 | isbn=978-1-938787-15-7}}</ref><ref>{{cite book|title=Organometallics|author=C. Elschenbroich|publisher=VCH|year=2006|isbn=  978-3-527-29390-2}}</ref>

: {{chem2|RCH\dCH2  +  H2  +  CO  ->  RCH2\sCH2CHO}}

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 and [[hydrosilane]]s. These include hydrogenations and hydrosilylations of alkenes.<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|pages=1837–1846}}</ref> Rhodium metal, but not rhodium complexes, catalyzes the hydrogenation of [[benzene]] to [[cyclohexane]].<ref>{{cite journal|author=Halligudi, S. B.|display-authors=etal |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|pages= 547–552|bibcode=1992RKCL...48..505T |s2cid=97802315 }}</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 [[sterling silver]] to protect against tarnish ([[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 [[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 [[platinum]] and [[palladium]]. These alloys are used in furnace windings, bushings for glass fiber production, [[thermocouple]] elements, [[electrode]]s for aircraft [[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:

* [[Switch#Contacts|Electrical contacts]], where it is valued for small [[electrical resistance]], small and stable [[contact resistance]], and great [[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 [[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=etal|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 Generating Station|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 [[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 [[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| bibcode=1972ToxAP..21..589L }}</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 [[Occupational Safety and Health Administration]] (OSHA) has specified the legal 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 [[National Institute for Occupational Safety and Health]] (NIOSH) has set the [[recommended exposure limit]] (REL), at the same level. At levels of 100&nbsp;mg/m<sup>3</sup>, rhodium is [[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 [[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]]
* [[Bullion]]
* [[Bullion coin]]
* [[:Category:Rhodium compounds|Rhodium compounds]]

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

==External links==
{{Wiktionary|rhodium}}
{{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}}
{{Authority control}}

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