# Rhodium

Rhodium is a chemical element; it has symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, 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 $^{103}$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 metals. Rhodium is a group 9 element (cobalt group).
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 platinum or palladium 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 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.

## History

Rhodium (from Greek: ῥόδον rhodon, meaning 'rose') was discovered in 1803 by William Hyde Wollaston, soon after he discovered palladium. He used crude platinum ore presumably obtained from South America. 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 (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, and the rhodium was precipitated by the addition of sodium chloride as Na $_3$[RhCl $_6$]·nH $_2$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.
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. They have exceptionally good stability in the temperature range of 1300 to 1800 °C.
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 NO$_x$ in the exhaust.

## Characteristics

| Z | Element | 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. Oxygen is absorbed from the atmosphere only at the melting point of rhodium, but is released on solidification. 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.
Rhodium belongs to 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.

### Chemical properties

Oxidation states of rhodium :
| +0 | Rh $_4$(CO) $_{12}$ |
| +1 | RhCl(PH $_3$) $_2$ |
| +2 | Rh $_2$(O $_2$CCH $_3$) $_4$ |
| +3 | RhCl $_3$, Rh $_2$O $_3$ |
| +4 | RhO $_2$ |
| +5 | RhF $_5$, Sr $_3$LiRhO $_6$ |
| +6 | RhF $_6$ |
The common oxidation states of rhodium are +3 and +1. Oxidation states 0, +2, and +4 are also well known. A few complexes at still higher oxidation states are known.
The rhodium oxides include Rh $_2$O $_3$, RhO $_2$, RhO $_2$·xH $_2$O, Na $_2$RhO $_3$, Sr $_3$LiRhO $_6$ and Sr $_3$NaRhO $_6$. 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, Na$_3$RhCl$_6$, and pentaamminechlororhodium dichloride, [Rh(NH$_3$)$_5$Cl]Cl$_2$. 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, Rh$_2$(O$_2$CCH$_3$)$_4$, 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 Rh$_2$Cl$_2$L$_4$ (L = CO, C$_2$H$_4$, PF$_3$). When treated with triphenylphosphine, hydrated rhodium trichloride converts to the maroon-colored RhCl(P(C$_6$H$_5$)$_3$)$_3$, which is known as Wilkinson's catalyst. Reduction of rhodium carbonyl chloride gives hexarhodium hexadecacarbonyl, Rh$_6$(CO)$_{16}$, and tetrarhodium dodecacarbonyl, Rh$_4$(CO)$_{12}$, the two most common Rh(0) complexes.
As for other metals, rhodium forms high oxidation state binary fluorides. These include rhodium pentafluoride, a tetrameric complex with the true formula Rh$_4$F$_{20}$) and rhodium hexafluoride.

### Isotopes

Naturally occurring rhodium is composed of only one isotope, $^{103}$Rh. The most stable radioisotopes are $^{101}$Rh with a half-life of 3.3 years, $^{102}$Rh with a half-life of 207 days, $^{102m}$Rh with a half-life of 2.9 years, and $^{99}$Rh with a half-life of 16.1 days. Twenty other radioisotopes have been characterized with atomic weights ranging from 92.926 u ($^{93}$Rh) to 116.925 u ($^{117}$Rh). Most of these have half-lives shorter than an hour, except $^{100}$Rh (20.8 hours) and $^{105}$Rh (35.36 hours). Rhodium has numerous meta states, the most stable being $^{102m}$Rh (0.141 MeV) with a half-life of about 2.9 years and $^{101m}$Rh (0.157 MeV) with a half-life of 4.34 days (see isotopes of rhodium).
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.

## Occurrence

Rhodium is one of the rarest elements in the Earth's crust, comprising an estimated 0.0002 parts per million (2 × 10$^{−10}$). Its rarity affects its price and its use in commercial applications. The concentration of rhodium in nickel meteorites is typically 1 part per billion. Rhodium has been measured in some potatoes 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 minerals 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 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. The annual world production is 30 tonnes. The price of rhodium is highly variable.

### 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 ($^{101}$Rh with a half-life of 3.3 years, and $^{102m}$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.

## 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.
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.

### Carbonylation

Rhodium catalysts 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 aldehydes according to the following equation:
    RCH=CH$_2$ + H$_2$ + CO → RCH$_2$−CH$_2$CHO
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 hydrosilanes. These include hydrogenations and hydrosilylations of alkenes. Rhodium metal, but not rhodium complexes, catalyzes the hydrogenation of benzene to cyclohexane.

### 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$_2$S, produced from atmospheric hydrogen sulfide, H$_2$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. 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.

### 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, electrodes for aircraft spark plugs, 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.
* Rhodium plated by either electroplating or evaporation is extremely hard and useful for optical instruments.
* Filters in mammography systems for the characteristic X-rays it produces.
* 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.
In automobile manufacturing, rhodium is also used in the construction of headlight reflectors.
* A 78 g sample of rhodium
* Cut-away of a metal-core catalytic converter
* Rhodium-plated white gold wedding ring
* Rhodium foil and wire

## 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 (LD$_{50}$) for rats is 198 mg (RhCl $_3$) per kilogram of body weight. 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 mg/m$^3$ 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/m$^3$, rhodium is immediately dangerous to life or health. For soluble compounds, the PEL and REL are both 0.001 mg/m$^3$.