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Rhodium ( /ˈroʊdiəm/ ROH-dee-əm) 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. Naturally-occurring rhodium is composed of only one isotope, ¹⁰³Rh. It is one of the rarest precious metals and, with a price of about US$80,000/kg in 2010, is the most expensive member of that class.

Rhodium was discovered in 1803 by William Hyde Wollaston. It is found in platinum- or nickel ores together with the other members of the platinum group metals. Rhodium is mostly used as a catalyst in the three-way catalytic converter. Rhodium is inert against corrosion and most aggressive chemicals, and because of that and 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 optical impression while sterling silver is often rhodium plated for tarnish resistance. Rhodium detectors are used in nuclear reactors to measure the neutron flux level.

Rhodium (Greek rhodon (ῥόδον) 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 (NaOH). He then precipitated the platinum by adding ammonium chloride, NH₄Cl, as ammonium chloroplatinate. 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 Na₃[RhCl₆]·nH₂O. After washing with ethanol, the rose red precipitate was reacted with zinc forming rhodium 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.

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 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 (41), ruthenium (44), rhodium (45), and palladium (46).

Common oxidation states 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 Rh₂O₃, RhO₂, RhO₂·xH₂O, Na₂RhO₃, Sr₃LiRhO₆ and Sr₃NaRhO₆.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.

Naturally-occurring rhodium is composed of only one isotope, ¹⁰³Rh. The most stable radioisotopes are Rh 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 (⁹³Rh) to 116.925 u (¹¹⁷Rh). Most of these have half-lives shorter than an hour, except ¹⁰⁰Rh (half-life: 20.8 hours) and ¹⁰⁵Rh (half-life: 35.36 hours). There are also numerous meta states, the most stable being ^102mRh (0.141 MeV) with a half-life of about 2.9 years and ^101mRh (0.157 MeV) with a half-life of 4.34 days. (See isotopes of rhodium).

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.

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 (>80%) followed by Russia.The annual world production of this element is only about 25 tonnes and there are very few rhodium-bearing minerals. As of October 2007^[update], rhodium cost approximately eight times more than gold, 450 times more than silver, and 27,250 times more than copper by weight. Rhodium's typical historical price is about $1,000/troy oz, but in recent years, it has increased to about $4500/troy oz.

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, but they rebounded to $2,750 by early 2010 (over twice the gold price).

Main article: Synthesis of precious metals

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 decaying isotopes in order to reduce the radioactivity. This makes this source of rhodium unattractive and no large-scale extraction has been attempted.

The primary use of this element is in automobiles as a catalytic converter, which changes harmful emissions from the engine into less polluting gases.

Cross section of a metal-core converter

In 2007, 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 NO_x → x O₂ + N₂

The recycling of catalytic converters also became a valuable source for rhodium. In 2007, 5.7 t were extracted from this source. Compared to the 22 t which had been mined, this is a relatively high recycling rate.

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.

Rhodium-plated white gold wedding ring

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 in order to strengthen the metal against tarnish, which is silver sulfide (Ag₂S) produced from the atmospheric hydrogen sulfide (H₂S). Solid (pure) rhodium jewelry is very rare, because the metal has both high melting point and poor malleability (making such jewelry very hard to fabricate) 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 (commonly called rhodium sponge) dissolved into electroplating solutions.

Rhodium has also been used for honors, or to symbolize wealth, 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.

Many opponents of fiat money advocate the use of rhodium in high-value coins. The Free Competition in Currency Act sponsored by United States Representative Ron Paul would legalize private minting of rhodium, among other precious metals.

Rhodium foil and wire

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 is also used to coat some parts of high-quality fountain pens due to its high reflectance, as well as it's chemical and mechanical resistance. These pens include Graf von Faber-Castell^[42] and Caran D'ache.

• 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 5 vertical levels, providing an accurate 3-D "picture" of reactivity and allowing fine tuning to most economically burn the nuclear fuel.

Being a noble metal, pure rhodium is inert. However, chemical complexes of rhodium can be reactive. Median lethal dose (LD₅₀) for rats is 12.6 mg of rhodium chloride (RhCl₃) per kilogram of body weight.^[45] Rhodium compounds can strongly stain human skin. The element plays no biological role in humans. If used in elemental form rather than as compounds, the metal is harmless.

引用出處：http://en.wikipedia.org/wiki/Rhodium

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