Rhenium () is a chemical element with the symbol Re and atomic number 75. A rare silvery-white, heavy, polyvalent transition metal, rhenium resembles manganese chemically and is used in some alloys. Rhenium is obtained as a by-product of molybdenum refinement and rhenium-molybdenum alloys are superconducting. ( (#1) ) It was the last naturally occurring stable element to be discovered ( (#2) ) and is among the ten most expensive metals on Earth (over US$ 7500.-/kg). ( (#3) )
Notable characteristics
Rhenium is a silvery white metal, lustrous, and has one of the highest melting points of all elements, exceeded by only tungsten and carbon. It is also one of the most dense, exceeded only by platinum, iridium and osmium. Rhenium has the widest range of oxidation states of any known element: -3, -1, 0, +1, +2, +3, +4, +5, +6 and +7. The oxidation states +7, +6, +4, +2 and -1 are the most common.Its usual commercial form is a powder, but this element can be consolidated by pressing and resistance-sintering in a vacuum or hydrogen atmosphere. This procedure yields a compact shape that is in excess of 90 percent 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, ( (#4) ) around 4-8 K depending on the alloy. Rhenium metal superconducts at 2.4 K. ( (#5) )
Applications
This element is used in platinum-rhenium catalysts which in turn are primarily used in making lead-free, high-octane gasoline and in high-temperature superalloys that are used to make jet engine parts. ( (#6) ) Other uses:- Widely used as filaments in mass spectrographs and in ion gauges.
- An additive to tungsten and molybdenum-based alloys to increase ductility in these alloys.
- An additive to tungsten in some x-ray sources.
- Rhenium catalysts are very resistant to chemical poisoning, and so are used in certain kinds of hydrogenation reactions.
- Electrical contact material due to its good wear resistance and ability to withstand arc corrosion.
- Thermocouples containing alloys of rhenium and tungsten are used to measure temperatures up to 2200 °C.
- Rhenium wire is used in photoflash lamps in photography.
- Rhenium forms rhenium diboride with boron. It is a compound noted for its extreme hardness. (
- Isotopes of rhenium are radioactive. The 188 isotope, with a half-life of 69 days, has been tested for treatment of liver cancer. The 188 isotope may be obtained in the form of a generator. (
- Related by periodic trends, rhenium has a similar chemistry with 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 on technetium, especially the 99m isotope used in medicine, due to its expense and short half-life.
History
Rhenium (Latin Rhenus meaning "Rhine") was the next-to-last naturally occurring element to be discovered and the last element to be discovered having a stable isotope. The existence of a yet undiscovered element at this position in the periodic table had been predicted by Henry Moseley in 1914. It is generally considered to have been discovered by Walter Noddack, Ida Tacke, and Otto Berg in Germany. In 1925 they reported that they 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.The process was so complicated and the cost so high that production was discontinued until early 1950 when tungsten-rhenium and molybdenum-rhenium alloys were prepared. These alloys found important applications in industry that resulted in a great demand for the rhenium produced from the molybdenite fraction of porphyry copper ores.
In 1908, Japanese chemist Masataka Ogawa announced that he discovered the 43rd element, and named it nipponium (Np) after Japan (which is Nippon in Japanese). However, later analysis indicated the presence of rhenium (element 75), not element 43. The symbol Np was later used for the element neptunium.
Occurrence
Rhenium is not found free in nature, and it was only recently that the first rhenium mineral was found. In 1994, Nature published a letter describing a rhenium sulfide mineral found condensing from a fumarole on Russia's Kudriavy volcano. ( (#10) ) This is not an economically viable source of the element. Rhenium is widely spread through the Earth's crust at approximately 1 ppb.Chile has the world's largest reserves and was the leading producer as of 2005. ( (#11) )
Production
Commercial rhenium is extracted from molybdenum roaster-flue gas obtained from copper-sulfide ores. Some molybdenum ores contain 0.002% to 0.2% rhenium. Rhenium(VII) oxide and perrhenic acid readily dissolve in water; it is extracted by precipitating with potassium or ammonium chloride as the perrhenate salts, and purified by recrystallization. ( (#12) ) Total world production is between 40 and 50 tons/year; the main producers are in Chile, USA and Kazakhstan. ( (#6) ) Recycling of used Pt-Re catalyst and special alloys allow the recovery of another 10 tons/year. Prices for the metal rose rapidly in early 2008, from a price of $1000-$2000 per kg in 2003-2006 to over $10,000 in February 2008. ( (#14) )The metal form is prepared by reducing ammonium perrhenate with hydrogen at high temperatures:
:2 NH4ReO4 + 7 H2O → 2 Re + 8 H2O + 2 NH3
Isotopes
Naturally occurring rhenium is 37.4% 185Re, which is stable, and 62.6% 187Re, which is unstable but has a very long half-life which can be affected by its electron density (http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/decay_rates.html How to Change Nuclear Decay Rates) . The beta decay of 187Re is used for rhenium-osmium dating of ores. It is interesting that the available energy for this beta decay (2.6 keV) is the lowest known among all radionuclides. There are twenty-six other radioactive isotopes of rhenium recognized.Compounds
Rhenium is most available commercially as the sodium and ammonium perrhenates. It is also readily available as dirhenium decacarbonyl; these three compounds are common entry points to rhenium chemistry.Various perrhenate salts may be easily converted to tetrathioperrhenate by the action of ammonium hydrosulfide. ( (#15) )
The hardest Boron compound is created synthetically. Rhenium diboride (ReB2) can actually scratch diamond, giving it a higher than 10 rank in the Mohs scale of mineral hardness and making it one of the three hardest substances known to man - the other two being ultrahard fullerite and aggregated diamond nanorods.
Other compounds:
- Bromopentacarbonylrhenium(I)
- Pentacarbonylhydridorhenium
References
External links
Category:Chemical elements Category:Transition metals
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