Rhodium (pronounced /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 ores and is mostly used as a catalyst. Because of its rarity, rhodium is usually alloyed with platinum or palladium and applied in high-temperature and corrosion-resistive coatings. Rhodium detectors are used in nuclear reactors to measure the neutron flux level.
Rhodium is a hard silvery white and durable metal that has a high reflectance. Rhodium metal does not normally form an oxide, even when heated. Oxygen is absorbed from the atmosphere at the melting point of rhodium, but on solidification, the oxygen is released. Rhodium has both a higher melting point and lower density than platinum. It is not attacked by acids: it is completely insoluble in nitric acid and dissolves slightly in aqua regia.
Rhodium belongs to group 9 in 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 example is the Wilkinson's catalyst chlorotris(triphenylphosphine)rhodium(I). The catalyst is for example used for the hydroformylation or hydrogenation of alkenes.
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 (93Rh) to 116.925 u (117Rh). Most of these have half-lives that are less than an hour except 100Rh (half-life: 20.8 hours) and 105Rh (half-life: 35.36 hours). There are also numerous meta states with 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.
Rhodium (Greek rhodon (ῥόδον) meaning "rose") was discovered in 1803 by William Hyde Wollaston, soon after his discovery of palladium. He made this discovery in England using crude platinum ore that he 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. All 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.
The primary use of this element is in automobiles as a catalytic converter, which converts harmful emissions from the engine into less harmful gases.
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 NOx → 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 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 also may be used in coating sterling silver in order to strengthen the metal from tarnish (silver sulfide, Ag2S—caused by hydrogen sulfide, H2S in the atmosphere). 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.
Rhodium has also been used for honours, 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.
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.
* It is also used as a filter in mammography systems because of the characteristic X-rays it produces.
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 obtained free as a white inert metal which is very difficult to fuse. Principal sources of this element 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 tons and there are very few rhodium-bearing minerals. As of October 2007, 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 3rd quarter 2008 economic slowdown pushed rhodium prices sharply back below $1,000 per ounce, however, bouncing up to $2,750 by early 2010 (over twice the gold price).
It is also possible to extract rhodium from used nuclear fuel, which contains rhodium (1 kg of the fission products of 235U contains 13.3 grams of 103Rh). As a typical used fuel has 3% fission products by weight, it will contain about 400 grams of rhodium per ton of used fuel. The longest lived radioisotope of rhodium is 102mRh which has a half life of 2.9 years, whereas the ground state (102Rh) has a half life of 207 days.
One kilogram of fission rhodium will contain 6.62 ng of 102Rh and 3.68 ng of 102mRh. As 102Rh decays by beta decay to either 102Ru (80%) (some positron emission will occur) or 102Pd (20%) (gamma ray photons with about 500 keV are generated) and the excited state decays by beta decay (electron capture) to 102Ru (gamma ray photons with about 1 MeV are generated). If the fission occurs in an instant then 13.3 grams of rhodium will contain 67.1 MBq (1.81 mCi) of 102Rh and 10.8 MBq (291 μCi) of 102mRh. As it is normal to allow used nuclear fuel to rest for about five years before reprocessing, much of this activity will decay leaving 4.7 MBq of 102Rh and 5.0 MBq of 102mRh. If the rhodium metal was then left for 20 years after fission, then the 13.3 grams of rhodium metal would contain 1.3 kBq of 102Rh and 500 kBq of 102mRh. At first glance, the rhodium might be adding to the resource value of reprocessed fission waste, but the cost of the separation of rhodium from other metals needs to be considered.
Rhodium metal is, as a noble metal, 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. 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.
* Rhodium compounds
1. ^ "Rhodium: rhodium(I) fluoride compound data". OpenMOPAC.net. http://openmopac.net/data_normal/rhfr_jmol.html. Retrieved 2007-12-10.