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= Rhenium =
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Introduction
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Rhenium is a chemical element; it has symbol Re and atomic number 75.
It is a silvery-gray, heavy, third-row transition metal in group 7 of
the periodic table. With an estimated average concentration of 1 part
per billion (ppb), rhenium is one of the rarest elements in the
Earth's crust. It has one of the highest melting and boiling points of
any element. It resembles manganese and technetium chemically and is
mainly obtained as a by-product of the extraction and refinement of
molybdenum and copper ores. It shows in its compounds a wide variety
of oxidation states ranging from −1 to +7.
Rhenium was originally discovered in 1908 by Masataka Ogawa, but he
mistakenly assigned it as element 43 (now known as technetium) rather
than element 75 and named it 'nipponium'. It was rediscovered in 1925
by Walter Noddack, Ida Tacke and Otto Berg, who gave it its present
name. It was named after the river Rhine in Europe, from which the
earliest samples had been obtained and worked commercially.
Nickel-based superalloys of rhenium are used in combustion chambers,
turbine blades, and exhaust nozzles of jet engines. These alloys
contain up to 6% rhenium, making jet engine construction the largest
single use for the element. The second-most important use is as a
catalyst: it is an excellent catalyst for hydrogenation and
isomerization, and is used for example in catalytic reforming of
naphtha for use in gasoline (rheniforming process). Because of the low
availability relative to demand, rhenium is expensive, with price
reaching an all-time high in 2008-09 of US$10,600 per kilogram
(US$4,800 per pound). As of 2018, its price had dropped to US$2,844
per kilogram (US$1,290 per pound) due to increased recycling and a
drop in demand for rhenium catalysts.
History
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In 1908, Japanese chemist Masataka Ogawa announced that he had
discovered the 43rd element and named it 'nipponium' (Np) after Japan
('Nippon' in Japanese). In fact, he had found element 75 (rhenium)
instead of element 43: both elements are in the same group of the
periodic table. Ogawa's work was often incorrectly cited, because some
of his key results were published only in Japanese; it is likely that
his insistence on searching for element 43 prevented him from
considering that he might have found element 75 instead. Just before
Ogawa's death in 1930, Kenjiro Kimura analysed Ogawa's sample by X-ray
spectroscopy at the Imperial University of Tokyo, and said to a friend
that "it was beautiful rhenium indeed". He did not reveal this
publicly, because under the Japanese university culture before World
War II it was frowned upon to point out the mistakes of one's seniors,
but the evidence became known to some Japanese news media regardless.
As time passed with no repetitions of the experiments or new work on
nipponium, Ogawa's claim faded away. The symbol Np was later used for
the element neptunium, and the name "nihonium", also named after
Japan, along with symbol Nh, was later used for element 113. Element
113 was also discovered by a team of Japanese scientists and was named
in respectful homage to Ogawa's work. Today, Ogawa's claim is widely
accepted as having been the discovery of element 75 in hindsight.
Rhenium ( meaning: "Rhine") received its current name when it was
rediscovered by Walter Noddack, Ida Noddack, and Otto Berg in Germany.
In 1925 they reported that they had detected the element in platinum
ore and in the mineral columbite. They also found rhenium in
gadolinite and molybdenite. In 1928 they were able to extract 1 g of
the element by processing 660 kg of molybdenite. It was estimated in
1968 that 75% of the rhenium metal in the United States was used for
research and the development of refractory metal alloys. It took
several years from that point before the superalloys became widely
used.
The original mischaracterization by Ogawa in 1908 and final work in
1925 makes rhenium perhaps the last stable element to be understood.
Hafnium was discovered in 1923 and all other new elements discovered
since then are radioactive.
Characteristics
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Rhenium is a silvery-white metal with one of the highest melting
points of all elements, exceeded by only tungsten. (At standard
pressure carbon sublimes rather than melts, though its sublimation
point is comparable to the melting points of tungsten and rhenium.) It
also has one of the highest boiling points of all elements, and the
highest among stable elements. It is also one of the densest, exceeded
only by platinum, iridium and osmium. Rhenium has a hexagonal
close-packed crystal structure.
Its usual commercial form is a powder, but this element can be
consolidated by pressing and sintering in a vacuum or hydrogen
atmosphere. This procedure yields a compact solid having a density
above 90% of the density of the metal. When annealed this metal is
very ductile and can be bent, coiled, or rolled. Rhenium-molybdenum
alloys are superconductive at 10 K; tungsten-rhenium alloys are also
superconductive around 4-8 K, depending on the alloy. Rhenium metal
superconducts at .
In bulk form and at room temperature and atmospheric pressure, the
element resists alkalis, sulfuric acid, hydrochloric acid, nitric
acid, and aqua regia. It will however, react with nitric acid upon
heating.
Isotopes
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Rhenium has one stable isotope, rhenium-185, which nevertheless occurs
in minority abundance, a situation found only in two other elements
(indium and tellurium). Naturally occurring rhenium is only 37.4%
185Re, and 62.6% 187Re, which is unstable but has a very long
half-life (~1010 years). A kilogram of natural rhenium emits 1.07 MBq
of radiation due to the presence of this isotope. This lifetime can be
greatly affected by the charge state of the rhenium atom. The beta
decay of 187Re is used for rhenium-osmium dating of ores. The
available energy for this beta decay (2.6 keV) is the second lowest
known among all radionuclides, only behind the decay from 115In to
excited 115Sn* (0.147 keV). The isotope rhenium-186m is notable as
being one of the longest lived metastable isotopes with a half-life of
around 200,000 years. There are 33 other unstable isotopes that have
been recognized, ranging from 160Re to 194Re, the longest-lived of
which is 183Re with a half-life of 70 days.
Compounds
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Rhenium compounds are known for all the oxidation states between −3
and +7 except −2. The oxidation states +7, +4, and +3 are the most
common. Rhenium is most available commercially as salts of perrhenate,
including sodium and ammonium perrhenates. These are white,
water-soluble compounds. Tetrathioperrhenate anion [ReS4]− is
possible.
Halides and oxyhalides
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The most common rhenium chlorides are ReCl6, ReCl5, ReCl4, and ReCl3.
The structures of these compounds often feature extensive Re-Re
bonding, which is characteristic of this metal in oxidation states
lower than VII. Salts of [Re2Cl8]2− feature a quadruple metal-metal
bond. Although the highest rhenium chloride features Re(VI), fluorine
gives the d0 Re(VII) derivative rhenium heptafluoride. Bromides and
iodides of rhenium are also well known, including rhenium pentabromide
and rhenium tetraiodide.
Like tungsten and molybdenum, with which it shares chemical
similarities, rhenium forms a variety of oxyhalides. The oxychlorides
are most common, and include ReOCl4, ReOCl3.
Oxides and sulfides
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The most common oxide is the volatile yellow Re2O7. The red rhenium
trioxide ReO3 adopts a perovskite-like structure. Other oxides include
Re2O5, ReO2, and Re2O3. The sulfides are ReS2 and Re2S7. Perrhenate
salts can be converted to tetrathioperrhenate by the action of
ammonium hydrosulfide.
Other compounds
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Rhenium diboride (ReB2) is a hard compound having a hardness similar
to that of tungsten carbide, silicon carbide, titanium diboride or
zirconium diboride.
Organorhenium compounds
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Dirhenium decacarbonyl is the most common entry to organorhenium
chemistry. Its reduction with sodium amalgam gives Na[Re(CO)5] with
rhenium in the formal oxidation state −1. Dirhenium decacarbonyl can
be oxidised with bromine to bromopentacarbonylrhenium(I):
:Re2(CO)10 + Br2 → 2 Re(CO)5Br
Reduction of this pentacarbonyl with zinc and acetic acid gives
pentacarbonylhydridorhenium:
:Re(CO)5Br + Zn + HOAc → Re(CO)5H + ZnBr(OAc)
Methylrhenium trioxide ("MTO"), CH3ReO3 is a volatile, colourless
solid that has been used as a catalyst in some laboratory experiments.
It can be prepared by many routes, a typical method is the reaction of
Re2O7 and tetramethyltin:
:Re2O7 + (CH3)4Sn → CH3ReO3 + (CH3)3SnOReO3
Analogous alkyl and aryl derivatives are known. MTO catalyses for the
oxidations with hydrogen peroxide. Terminal alkynes yield the
corresponding acid or ester, internal alkynes yield diketones, and
alkenes give epoxides. MTO also catalyses the conversion of aldehydes
and diazoalkanes into an alkene.
Nonahydridorhenate
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A distinctive derivative of rhenium is nonahydridorhenate, originally
thought to be the 'rhenide' anion, Re−, but actually containing the
anion in which the oxidation state of rhenium is +7.
Occurrence
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Molybdenite
Rhenium is one of the rarest elements in Earth's crust with an average
concentration of 1 ppb; other sources quote the number of 0.5 ppb
making it the 77th most abundant element in Earth's crust. Rhenium is
probably not found free in nature (its possible natural occurrence is
uncertain), but occurs in amounts up to 0.2% in the mineral
molybdenite (which is primarily molybdenum disulfide), the major
commercial source, although single molybdenite samples with up to
1.88% have been found. Chile has the world's largest rhenium reserves,
part of the copper ore deposits, and was the leading producer as of
2005. It was only recently (in 1994) that the first rhenium mineral
was found and described, a rhenium sulfide mineral (ReS2) condensing
from a fumarole on Kudriavy volcano, Iturup island, in the Kuril
Islands. Kudriavy discharges up to 20-60 kg rhenium per year mostly in
the form of rhenium disulfide. Named rheniite, this rare mineral
commands high prices among collectors.
Production
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Ammonium perrhenate
Approximately 80% of rhenium is extracted from porphyry molybdenum
deposits. Some ores contain 0.001% to 0.2% rhenium. Roasting the ore
volatilizes rhenium oxides. Rhenium(VII) oxide and perrhenic acid
readily dissolve in water; they are leached from flue dusts and gasses
and extracted by precipitating with potassium or ammonium chloride as
the perrhenate salts, and purified by recrystallization. Total world
production is between 40 and 50 tons/year; the main producers are in
Chile, the United States, Peru, and Poland. Recycling of used Pt-Re
catalyst and special alloys allow the recovery of another 10 tons per
year. Prices for the metal rose rapidly in early 2008, from
$1000-$2000 per kg in 2003-2006 to over $10,000 in February 2008. The
metal form is prepared by reducing ammonium perrhenate with hydrogen
at high temperatures:
:2 NH4ReO4 + 7 H2 → 2 Re + 8 H2O + 2 NH3
There are technologies for the associated extraction of rhenium from
productive solutions of underground leaching of uranium ores.
Applications
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Rhenium is added to high-temperature superalloys that are used to make
jet engine parts, using 70% of the worldwide rhenium production.
Another major application is in platinum-rhenium catalysts, which are
primarily used in making lead-free, high-octane gasoline.
Alloys
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The nickel-based superalloys have improved creep strength with the
addition of rhenium. The alloys normally contain 3% or 6% of rhenium.
Second-generation alloys contain 3%; these alloys were used in the
engines for the F-15 and F-16, whereas the newer single-crystal
third-generation alloys contain 6% of rhenium; they are used in the
F-22 and F-35 engines. Rhenium is also used in the superalloys, such
as CMSX-4 (2nd gen) and CMSX-10 (3rd gen) that are used in industrial
gas turbine engines like the GE 7FA. Rhenium can cause superalloys to
become microstructurally unstable, forming undesirable topologically
close packed (TCP) phases. In 4th- and 5th-generation superalloys,
ruthenium is used to avoid this effect. Among others the new
superalloys are EPM-102 (with 3% Ru) and TMS-162 (with 6% Ru), as well
as TMS-138 and TMS-174.
For 2006, the consumption is given as 28% for General Electric, 28%
Rolls-Royce plc and 12% Pratt & Whitney, all for superalloys,
whereas the use for catalysts only accounts for 14% and the remaining
applications use 18%. In 2006, 77% of rhenium consumption in the
United States was in alloys. The rising demand for military jet
engines and the constant supply made it necessary to develop
superalloys with a lower rhenium content. For example, the newer CFM
International CFM56 high-pressure turbine (HPT) blades will use Rene
N515 with a rhenium content of 1.5% instead of Rene N5 with 3%.
Rhenium improves the properties of tungsten. Tungsten-rhenium alloys
are more ductile at low temperature, allowing them to be more easily
machined. The high-temperature stability is also improved. The effect
increases with the rhenium concentration, and therefore tungsten
alloys are produced with up to 27% of Re, which is the solubility
limit. Tungsten-rhenium wire was originally created in efforts to
develop a wire that was more ductile after recrystallization. This
allows the wire to meet specific performance objectives, including
superior vibration resistance, improved ductility, and higher
resistivity. One application for the tungsten-rhenium alloys is X-ray
sources. The high melting point of both elements, together with their
high atomic mass, makes them stable against the prolonged electron
impact. Rhenium tungsten alloys are also applied as thermocouples to
measure temperatures up to 2200 °C.
The high temperature stability, low vapor pressure, good wear
resistance and ability to withstand arc corrosion of rhenium are
useful in self-cleaning electrical contacts. In particular, the
discharge that occurs during electrical switching oxidizes the
contacts. However, rhenium oxide Re2O7 is volatile (sublimes at ~360
°C) and therefore is removed during the discharge.
Rhenium has a high melting point and a low vapor pressure similar to
tantalum and tungsten. Therefore, rhenium filaments exhibit a higher
stability if the filament is operated not in vacuum, but in
oxygen-containing atmosphere. Those filaments are widely used in mass
spectrometers, ion gauges and photoflash lamps in photography.
Catalysts
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Rhenium in the form of rhenium-platinum alloy is used as catalyst for
catalytic reforming, which is a chemical process to convert petroleum
refinery naphthas with low octane ratings into high-octane liquid
products. Worldwide, 30% of catalysts used for this process contain
rhenium. The olefin metathesis is the other reaction for which rhenium
is used as catalyst. Normally Re2O7 on alumina is used for this
process. Rhenium catalysts are very resistant to chemical poisoning
from nitrogen, sulfur and phosphorus, and so are used in certain kinds
of hydrogenation reactions.
Other uses
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The isotopes 186Re and 188Re are radioactive and are used for
treatment of liver cancer. They both have similar penetration depth in
tissue (5 mm for 186Re and 11 mm for 188Re), but 186Re has the
advantage of a longer half-life (90 hours vs. 17 hours).
188Re is also being used experimentally in a novel treatment of
pancreatic cancer where it is delivered by means of the bacterium
'Listeria monocytogenes'. The 188Re isotope is also used for the
rhenium-SCT (skin cancer therapy). The treatment uses the isotope's
properties as a beta emitter for brachytherapy in the treatment of
basal cell carcinoma and squamous cell carcinoma of the skin.
Related by periodic trends, rhenium has a similar chemistry to that of
technetium; work done to label rhenium onto target compounds can often
be translated to technetium. This is useful for radiopharmacy, where
it is difficult to work with technetium - especially the
technetium-99m isotope used in medicine - due to its expense and short
half-life.
Rhenium is used in manufacturing high precision equipment like
gyroscopes. Its high density, mechanical stability and corrosion
resistance characteristics ensure the equipment's durability and
precise performance in demanding conditions. Rhenium cathodes are also
used for their stability and precision in spectral analysis.
Rhenium is used in aerospace, nuclear, and electronic industries, and
it shows potential for application in medical instrumentation. In the
rocket industry, it is used in engine components for booster rockets.
Additionally, rhenium was employed in the SP-100 program due to its
low-temperature ductility.
Rhenium's stiffness and high melting point makes it a common gasket
material for high pressure experiments in diamond anvil cells.
Precautions
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Very little is known about the toxicity of rhenium and its compounds
because they are used in very small amounts. Soluble salts, such as
the rhenium halides or perrhenates, could be hazardous due to elements
other than rhenium or due to rhenium itself. Only a few compounds of
rhenium have been tested for their acute toxicity; two examples are
potassium perrhenate and rhenium trichloride, which were injected as a
solution into rats. The perrhenate had an LD50 value of 2800 mg/kg
after seven days (this is very low toxicity, similar to that of table
salt) and the rhenium trichloride showed LD50 of 280 mg/kg.
External links
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* [
http://www.periodicvideos.com/videos/075.htm Rhenium] at 'The
Periodic Table of Videos' (University of Nottingham)
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Rhenium
