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rutheniumrhodiumpalladium
Co

Rh

Ir
Appearance
silvery white metallic
General properties
Name, symbol, number rhodium, Rh, 45
Element category transition metal
Group, period, block 95, d
Standard atomic weight 102.90550g·mol−1
Electron configuration [Kr] 4d8 5s1
Electrons per shell 2, 8, 18, 16, 1 (Image)
Physical properties
Phase solid
Density (near r.t.) 12.41 g·cm−3
Liquid density at m.p. 10.7 g·cm−3
Melting point 2237 K, 1964 °C, 3567 °F
Boiling point 3968 K, 3695 °C, 6683 °F
Heat of fusion 26.59 kJ·mol−1
Heat of vaporization 494 kJ·mol−1
Specific heat capacity (25 °C) 24.98 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 2288 2496 2749 3063 3405 3997
Atomic properties
Oxidation states 6, 5, 4, 3, 2, 1[1], -1
(amphoteric oxide)
Electronegativity 2.28 (Pauling scale)
Ionization energies 1st: 719.7 kJ·mol−1
2nd: 1740 kJ·mol−1
3rd: 2997 kJ·mol−1
Atomic radius 134 pm
Covalent radius 142±7 pm
Miscellanea
Crystal structure face-centered cubic
Magnetic ordering paramagnetic[2]
Electrical resistivity (0 °C) 43.3 nΩ·m
Thermal conductivity (300 K) 150 W·m−1·K−1
Thermal expansion (25 °C) 8.2 µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 4700 m/s
Young's modulus 380 GPa
Shear modulus 150 GPa
Bulk modulus 275 GPa
Poisson ratio 0.26
Mohs hardness 6.0
Vickers hardness 1246 MPa
Brinell hardness 1100 MPa
CAS registry number 7440-16-6
Most stable isotopes
Main article: Isotopes of rhodium
iso NA half-life DM DE (MeV) DP
99Rh syn 16.1 d ε - 99Ru
γ 0.089, 0.353,
0.528
-
101mRh syn 4.34 d ε - 101Ru
IT 0.157 101Rh
γ 0.306, 0.545 -
101Rh syn 3.3 y ε - 101Ru
γ 0.127, 0.198,
0.325
-
102mRh syn 2.9 y ε - 102Ru
γ 0.475, 0.631,
0.697, 1.046
-
102Rh syn 207 d ε - 102Ru
β+ 0.826, 1.301 102Ru
β 1.151 102Pd
γ 0.475, 0.628 -
103Rh 100% 103Rh is stable with 58 neutrons
105Rh syn 35.36 h β 0.247, 0.260,
0.566
105Pd
γ 0.306, 0.318 -

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 one isotope, 103Rh. It is one of the rarest and, with the price of about $80,000/kg in 2010, is the most expensive precious metal.[3]

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.

Contents

Characteristics

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.[4] Oxygen is absorbed from the atmosphere at the melting point of rhodium, but on solidification, the oxygen is released.[5] 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.

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Chemical properties

Rhodium belongs to group 9 in the periodic table

Z Element No. of electrons/shell
27 cobalt 2, 8, 15, 2
45 rhodium 2, 8, 18, 16, 1
77 iridium 2, 8, 18, 32, 15, 2
109 meitnerium 2, 8, 18, 32, 32, 15, 2

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).)

Oxidation states
of rhodium
+0 Rh4(CO)12
+1 RhCl(PH3)2
+2 Rh2(O2CCH3)
+3 RhCl3, Rh2O3
+4 RhF4, RhO2
+5 RhF5, Sr3LiRhO6
+6 RhF6

Common oxidation states of rhodium is +3, but oxidation states from +0 to +6 are also observed.[6]

Unlike ruthenium and osmium, rhodium forms no volatile oxygen compounds. The known stable oxides include Rh2O3, RhO2, RhO2·xH2O, Na2RhO3, Sr3LiRhO6 and Sr3NaRhO6.[7] 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.[8]

Wilkinson's catalyst

The best known example is the Wilkinson's catalyst chlorotris(triphenylphosphine)rhodium(I). The catalyst is for example used for the hydrogenation of alkenes.[9]

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 (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.[10]

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.[11]

History

William Hyde Wollaston

Rhodium (Greek rhodon (ῥόδον) meaning "rose") was discovered in 1803 by William Hyde Wollaston,[12][13] soon after his discovery of palladium.[14][15] He made this discovery in England using crude platinum ore that he presumably obtained from South America.[16]

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[RhCl6nH2O. After washing with ethanol, the rose red precipitate was reacted with zinc forming rhodium metal.[17]

Applications

Rhodium foil and wire

The primary use of this element is in automobiles as a catalytic converter, which converts harmful emissions from the engine into less harmful gases.[18][19]

Catalyst

Cross section of a Metal-core Converter

In 2007 81%[18] of the world production of rhodium was consumed to produce three-way catalytic converters.[18] Rhodium shows some advantages over the other platinum metals in the reduction of nitrogen oxides to nitrogen and oxygen:[20]

2 NOxx 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.[18]

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)[21] It is also used to catalyze addition of hydrosilanes to molecular double bonds, a process important in manufacture of certain silicone rubbers.[22] Rhodium catalysts are also used to reduce benzene to cyclohexane.[23]

The complex of a rhodium ion with BINAP gives a widely used chiral catalyst for chiral synthesis, as in the synthesis of menthol.[24]

Ornamental uses

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 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.[25]

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.[26]

Other uses

Rhodium is used as an alloying agent for hardening and improving the corrosion resistance[4] 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.[27] Other uses include:

  • It is also used as a filter in mammography systems because of the characteristic X-rays it produces.[30]
  • It is also used in high quality pen surfaces due to its high chemical and mechanical resistance. These pens include Graf von Faber-Castell[31] and Caran D'ache.[32]
  • 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, allowing fine tuning to most economically burn the nuclear fuel.[33]

Occurrence

Normal mining

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.[34] 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,[35] but in recent years, it has increased to about $4500/troy oz.[3] In 2008 the price briefly rose above $10,000 per ounce.[3] 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).[3]

Fission product

The radioactivity in MBq per gram of each of the platinum group metals which are formed by the fission of uranium, ruthenium is the most radioactive. Palladium has an almost constant activity due to the very long lived 107Pd, while rhodium is the least radioactive

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.[36]

Precautions

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.[37] 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.[38]

See also

References

  1. ^ "Rhodium: rhodium(I) fluoride compound data". OpenMOPAC.net. http://openmopac.net/data_normal/rhfr_jmol.html. Retrieved 2007-12-10. 
  2. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
  3. ^ a b c d KITCO Rhodium Price Charts
  4. ^ a b Cramer, Stephen; S., Jr Covino, Bernard (1990). ASM handbook. Materials Park, OH: ASM International. pp. 393–396. ISBN 0-87170-707-1. http://books.google.com/books?id=QV0sWU2qF5oC&pg=PA396. 
  5. ^ Emsley, John (2001). Nature's Building Blocks ((Hardcover, First Edition) ed.). Oxford University Press. pp. 363. ISBN 0198503407. 
  6. ^ Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils; (1985). Lehrbuch der Anorganischen Chemie (91–100 ed.). Walter de Gruyter. pp. 1056–1057. ISBN 3-11-007511-3. 
  7. ^ Reisner, B. A.; Stacy, A. M. (1998). "Sr3ARhO6(A = Li, Na):  Crystallization of a Rhodium(V) Oxide from Molten Hydroxide". Of the American Chemical Society 120: 9682–9989. doi:10.1021/ja974231q. 
  8. ^ Griffith, W. P. The Rarer Platinum Metals; John Wiley and Sons: New York, 1976; p 313.
  9. ^ Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. (1966). "The Preparation and Properties of Tris(triphenylphosphine)halogenorhodium(I) and Some Reactions Thereof Including Catalytic Homogeneous Hydrogenation of Olefins and Acetylenes and Their Derivatives". Journal of the Chemical Society A: 1711–1732. doi:10.1039/J19660001711. 
  10. ^ Audi, G. (2003). "The NUBASE Evaluation of Nuclear and Decay Properties". Nuclear Physics A (Atomic Mass Data Center) 729: 3–128. doi:10.1016/j.nuclphysa.2003.11.001. 
  11. ^ David R. Lide (ed.), Norman E. Holden in CRC Handbook of Chemistry and Physics, 85th Edition CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes.
  12. ^ "WebElements – The History of Rhodium". http://www.webelements.com/webelements/elements/text/Rh/hist.html. Retrieved 2009-02-06. 
  13. ^ Wollaston, W. H. (1805). "On the Discovery of Palladium; With Observations on Other Substances Found with Platina". Philosophical Transactions of the Royal Society of London 95: 316–330. doi:10.1098/rstl.1805.0024. 
  14. ^ W. P. Griffith (2003). "Rhodium and Palladium – Events Surrounding Its Discovery". Platinum Metals Review 47 (4): 175–183. http://www.platinummetalsreview.com/dynamic/article/view/47-4-175-183. 
  15. ^ Wollaston, W. H. (1804). "On a New Metal, Found in Crude Platina". Philosophical Transactions of the Royal Society of London 94: 419–430. doi:10.1098/rstl.1804.0019. 
  16. ^ Lide, David R (2004). CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. Boca Raton: CRC Press. pp. 4–26. ISBN 0-8493-0485-7. 
  17. ^ Griffith, W. P. (2003). "Bicentenary of Four Platinum Group Metals: Osmium and iridium – events surrounding their discoveries". Platinum Metals Review 47 (4): 175–183. 
  18. ^ a b c d George, Micheal W.. "Commodity Report: Platinum-Group Metals". United States Geological Survey USGS. http://minerals.usgs.gov/minerals/pubs/commodity/platinum/mcs-2008-plati.pdf. Retrieved 2008-09-16. 
  19. ^ George, Micheal W.. "2006 Minerals Yearbook: Platinum-Group Metals". United States Geological Survey USGS. http://minerals.usgs.gov/minerals/pubs/commodity/platinum/myb1-2006-plati.pdf. Retrieved 2008-09-16. 
  20. ^ Shelef, M.; Graham, G. W. (1994). "Why Rhodium in Automotive Three-Way Catalysts?". Catalysis Reviews 36 (3): 433–457. doi:10.1080/01614949408009468. 
  21. ^ Roth, James F. (1975). "Rhodium Catalysed Carbonylation of Methanol" (PDF). Platinum Metals Review 19 (1 January): 12–14. http://www.platinummetalsreview.com/pdf/pmr-v19-i1-012-014.pdf. 
  22. ^ Heidingsfeldova, M. and Capka, M. (2003). "Rhodium complexes as catalysts for hydrosilylation crosslinking of silicone rubber". Journal of Applied Polymer Science 30: 1837. doi:10.1002/app.1985.070300505. 
  23. ^ Halligudi, S. B. et al. (1992). "Hydrogenation of benzene to cyclohexane catalyzed by rhodium(I) complex supported on montmorillonite clay". Reaction Kinetics and Catalysis Letters 48: 547. doi:10.1007/BF02162706. 
  24. ^ Akutagawa, S.. "Asymmetric synthesis by metal BINAP catalysts". Applied Catalysis A 128: 171. doi:10.1016/0926-860X(95)00097-6. 
  25. ^ Fischer, Torkel; Fregert, S; Gruvberger, B; Rystedt, I (1984). "Contact sensitivity to nickel in white gold". Contact Dermatitis 10 (1): 23–24. doi:10.1111/j.1600-0536.1984.tb00056.x. PMID 6705515. 
  26. ^ "Hit & Run: Ring the changes". The Independent. http://www.independent.co.uk/news/people/hit-and-run/hit--run-ring-the-changes-1044166.html. Retrieved 2009-06-06. 
  27. ^ Lide, David R (2004). CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. Boca Raton: CRC Press. pp. 4–26. ISBN 0-8493-0485-7. http://books.google.com/books?id=WDll8hA006AC. 
  28. ^ Weisberg, Alfred M. (1999). "Rhodium plating". Metal Finishing 97 (1): 296–299. doi:10.1016/S0026-0576(00)83088-3. 
  29. ^ Smith, Warren J. (2007). "Reflectors". Modern optical engineering: the design of optical systems. McGraw-Hill. pp. 247–248. ISBN 9780071476874. http://books.google.de/books?id=DrtM_bAnf_YC. 
  30. ^ McDonagh, C P et al. (1984). "Optimum x-ray spectra for mammography: choice of K-edge filters for tungsten anode tubes". Phys. Med. Biol. 29: 249. doi:10.1088/0031-9155/29/3/004. 
  31. ^ Guilloche luxury pen range by Graf von Faber-Castell
  32. ^ Caran D'Ache Ecridor Type 55 Rhodium Fountain Pen
  33. ^ Sokolov Pochivalin, G. P.; Shipovskikh, Yu. M.; Garusov, Yu. V.; Chernikov O. G.; Shevchenko V. G., A. P. (1993). "Rhodium self-powered detector for monitoring neutron fluence, energy production, and isotopic composition of fuel". Atomic Energy 74: 365–367. doi:10.1007/BF00844622. 
  34. ^ Chevalier, Patrick (?). "Mineral Yearbook: Platinum Group Metals". Natural Resources Canada. http://www.nrcan-rncan.gc.ca/mms-smm/busi-indu/cmy-amc/content/2004/71.pdf. Retrieved 2008-10-17. 
  35. ^ Lide, D. R., ed. (2005), CRC Handbook of Chemistry and Physics (86th ed.), Boca Raton (FL): CRC Press, ISBN 0-8493-0486-5 
  36. ^ Bush, R. P. (1991). "Recovery of Platinum Group Metals from High Level Radioactive Waste". Platinum Metals Review 35 (4): 202–208. 
  37. ^ Landolt, Robert R.; Berk Harold W.; Russell, Henry T. (1972). "Studies on the toxicity of rhodium trichloride in rats and rabbits". Toxicology and Applied Pharmacology 21 (4): 589–590. doi:10.1016/0041-008X(72)90016-6. PMID 5047055. 
  38. ^ Leikin, Jerrold B.; Paloucek Frank P. (2008). Poisoning and Toxicology Handbook. Informa Health Care. pp. 846. ISBN 9781420044799. http://books.google.com/books?id=0Bw2UJTC_uMC. 

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Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also rhodium

German

Chemical Element: Rh (atomic number 45)

Noun

Rhodium n

  1. rhodium

Simple English

File:Rhodium foil and
Rhodium foil and wire.

Rhodium is a chemical element. It has the chemical symbol Rh. It has the atomic number 45. It is a rare metal. It is silver white and hard. In chemistry it is placed in a group of metal elements named the transition metals. It is also part of the platinum group. Rhodium is found in platinum ores.

Rhodium is used as a catalyst in some platinum alloys. It is the most expensive precious metal.[1]

Reference

  1. Precious Metal Prices Index taxfreegold.co.uk (HTML). Accessed 19 November 2006.


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