Rhodium is a chemical element that is a rare, silvery-white, hard, and chemically inert
transition metal and a member of the platinum group. It has the chemical symbol Rh and atomic
number 45. It is composed of only one naturally-occurring isotope, 103Rh. Naturally occurring rhodium
is usually found as the free metal, alloyed 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 so-called noble metal, resistant
to corrosion, found in platinum- or nickel ores together 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, produced after it
reacted with the powerful acid mixture aqua regia.
The element's major use 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 for tarnish
resistance. Rhodium detectors are used in nuclear reactors
to measure the neutron flux level.
History
Rhodium meaning "rose") was discovered in 1803 by William Hyde Wollaston, soon after
his discovery of palladium. He used crude platinum ore presumably obtained from South
America. His procedure involved dissolving the ore in aqua regia and neutralizing the
acid with sodium hydroxide. He then precipitated the platinum as ammonium chloroplatinate by
adding ammonium chloride, NH 4Cl. Most other metals like copper, lead,
palladium and rhodium were precipitated with zinc. Diluted nitric acid dissolved all but
palladium and rhodium, which were dissolved in aqua regia, and the rhodium was precipitated
by the addition of sodium chloride as Na 3[RhCl
6]·nH 2O. 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. After the discovery, 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. The first major application was electroplating
for decorative uses and as corrosion resistant coating. The introduction of the three way
catalytic converter by Volvo in 1976 increased the demand for rhodium. The previous catalytic
converters used platinum or palladium while the three way catalytic converter used rhodium
to reduce the amount of NOx in the exhaust. Characteristics
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.
Chemical properties
Rhodium belongs to group 9 of the periodic table but has an atypical configuration in
its outermost electron shells compared to the rest of the members. This can also be
observed in the neighborhood of niobium, ruthenium, and palladium.
The common oxidation state of rhodium is +3, but oxidation states from +0 to +6 are also
observed. Unlike ruthenium and osmium, rhodium forms
no volatile oxygen compounds. The known stable oxides include Rh
2O 3, RhO
2, RhO 2·xH
2O, Na 2RhO
3, Sr 3LiRhO
6 and Sr 3NaRhO
6. Halogen compounds are known in nearly the full range of possible oxidation states. Rhodium(III)
chloride, rhodium(IV) fluoride, rhodium(V) fluoride and rhodium(VI) fluoride are some
examples. The lower oxidation states are only stable if ligands are present.
The best-known rhodium-halogen compound is the Wilkinson's catalyst chlorotris(triphenylphosphine)rhodium(I).
This catalyst is used, for example, in the hydroformylation or hydrogenation of alkenes.
Isotopes
Naturally occurring rhodium is composed of only one isotope, 103Rh. The most stable radioisotopes
are 101Rh with a half-life of 3.3 years, 102Rh with a half-life of 207 days, 102mRh with
a half-life of 2.9 years, and 99Rh with a half-life of 16.1 days. Twenty other radioisotopes
have been characterized with atomic weights ranging from 92.926 u to 116.925 u. Most of
these have half-lives shorter than an hour, except 100Rh and 105Rh. There are also numerous
meta states, the most stable being 102mRh with a half-life of about 2.9 years and 101mRh
with a half-life of 4.34 days. The primary decay mode before the only stable
isotope, 103Rh, is electron capture and the primary mode after is beta emission. The primary
decay product before 103Rh is ruthenium and the primary product after is palladium.
Occurrence Rhodium is one of the rarest elements in the
Earth's crust, of which it comprises an estimated 0.0002 parts per million. Its rarity affects
its price, and thus its usage in commercial applications.
Mining and price
The industrial extraction of rhodium is complex, as the metal occurs in ores mixed with other
metals such as palladium, silver, platinum, and gold. It is found in platinum ores and
extracted as a white inert metal which is very difficult to fuse. Principal sources
are located in South Africa; in river sands of the Ural Mountains; and in North America,
including the copper-nickel sulfide mining area of the Sudbury, Ontario, region. Although
the quantity 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
followed by Russia. The annual world production of this element is 30 tonnes and there are
very few rhodium-bearing minerals. The price of rhodium is historically 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. The economic slowdown of the 3rd quarter of 2008 pushed rhodium
prices sharply back below $1,000 per ounce; they rebounded to $2,750 by early 2010, but
in late 2013, the prices were a bit lower than $1000.
Used nuclear fuels
Rhodium is a fission product of uranium-235; therefore, each kilogram of fission products
contains significant amounts of the lighter platinum group metals including rhodium. Used
nuclear fuel might be a possible source for rhodium. However, the extraction is complex
and expensive, and the also present radioactive isotopes of rhodium would require a storage
for several half-lives of the longest-lived decaying isotope to reduce the radioactivity.
This makes this source of rhodium unattractive and no large-scale extraction has been attempted.
Applications The primary use of this element is in automobiles
as a catalytic converter, which changes harmful unburned hydrocarbons, carbon monoxide, and
nitrogen oxide emissions from the engine into less noxious gases. Of 30,000 kg of rhodium
consumed worldwide in 2012, some 24,300 kg went into and 8,060 kg recovered from this
application. About 964 kg of rhodium was used in the glass industry, mostly for production
of fiberglass and flat-panel glass, and 2,520 kg in the chemical industry.
Catalyst In 2012, 81% of the world production of rhodium
was consumed to produce three-way catalytic converters. Rhodium shows some advantages
over the other platinum metals in the reduction of nitrogen oxides to nitrogen and oxygen:
2 NOx → x O 2 + N
2 Rhodium-based catalysts are used in a number
of industrial processes; notably, in the automobile catalytic converters and for catalytic carbonylation
of methanol to produce acetic acid by the Monsanto process. It is also used to catalyze
addition of hydrosilanes to molecular double bonds, a process important in manufacture
of certain silicone rubbers. Rhodium catalysts are also used to reduce benzene to cyclohexane.
The complex of a rhodium ion with BINAP gives a widely used chiral catalyst for chiral synthesis,
as in the synthesis of menthol. 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. This is known as rhodium flashing in the jewelry
business. It may also be used in coating sterling silver to protect against tarnish, which is
silver sulfide produced from the atmospheric hydrogen sulfide. Solid rhodium jewelry is
very rare, because the metal has both high melting point and poor malleability rather
than due to its high price. Additionally, its high cost assures that most of its jewelry
usage is in the form of tiny amounts of powder dissolved into electroplating solutions.
Rhodium has also been used for honors or to signify elite status, when more commonly used
metals such as silver, gold, or platinum are 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:
An electrical contact material due to its low electrical resistance, low and stable
contact resistance, and high corrosion resistance. Plated rhodium, made by electroplating or
evaporation, is extremely hard and is used for optical instruments.
It is also used as a filter in mammography systems because of the characteristic X-rays
it produces. Rhodium neutron detectors are used in combustion
engineering nuclear reactors to measure neutron flux levels – a method that requires a digital
filter to determine the current neutron flux level, as there are three signals generated:
immediate, a few seconds later, and a minute later, each with its own signal level, and
all three are combined in the rhodium detector signals. 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 most economically
burn the nuclear fuel.
Precautions Being a noble metal, pure rhodium is inert.
However, chemical complexes of rhodium can be reactive. Median lethal dose for rats is
198 mg of rhodium chloride (RhCl 3) per kilogram of body weight. Rhodium compounds
can strongly stain human skin. Like the other noble metals, all of which are too inert to
occur as chemical compounds in nature, rhodium has not been found to play, or suspected to
play, any biological role. If used in elemental form rather than as compounds, the metal is
harmless. See also
Rhodium compounds 2000s commodities boom
References
External links Rhodium at The Periodic Table of Videos
Current Rhodium price Rhodium Technical and Safety Data
CDC - NIOSH Pocket Guide to Chemical Hazards