Rhodium is a chemical element; it has symbol Rh and atomic number 45. It is a very rare, 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).
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-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 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 , 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 (). 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 . 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.[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.
Characteristics
|
|
| 2, 8, 15, 2 |
| 2, 8, 18, 16, 1 |
| 2, 8, 18, 32, 15, 2 |
| 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.
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 anomaly is also observed in the neighboring elements niobium (41), ruthenium (44), and palladium (46).
Chemical properties
The common of rhodium are +3 and +1. Oxidation states 0, +2, and +4 are also well known.
The rhodium oxides include , , , , and .
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, , 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 give 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. Hydrated rhodium trichloride is reduced by carbon monoxide, ethylene, and trifluorophosphine to give rhodium(I) complexes (L = CO, ). When treated with triphenylphosphine, hydrated rhodium trichloride converts to the maroon-colored , which is known as Wilkinson's catalyst. Reduction of rhodium carbonyl chloride gives hexarhodium hexadecacarbonyl, , and tetrarhodium dodecacarbonyl, , the two most common Rh(0) complexes.
As for other metals, rhodium forms high oxidation state binary phase. These include rhodium pentafluoride, a tetrameric complex with the true formula ) and rhodium hexafluoride.
Isotopes
Naturally occurring rhodium is composed of only one
isotope,
103Rh. 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 other radioisotopes have been characterized ranging from
89Rh to
122Rh - these have half-lives that are less than an hour except
100Rh (20.8 hours) and
105Rh (35.34 hours). Numerous
are also found; those of notable stability being
102mRh (3.742 years) and
101mRh (4.343 days).
In isotopes 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).
[Barbalace, Kenneth, " Table of Elements". Environmental Chemistry.com; retrieved 2007-04-14.] 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 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, 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.
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.
In 2008, net demand (with the recycling accounted for) of rhodium for automotive converters made up 84% of the world usage,
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:
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 . These include hydrogenations and hydrosilylations of alkenes.
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
2S, produced from atmospheric
hydrogen sulfide, H
2S). 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
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.
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.
-
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.[Stwertka, Albert. A Guide to the Elements, Oxford University Press, 1996, p. 125. ]
File:Rhodium 78g sample.jpg|A 78 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
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 () 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/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.
See also
External links
