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46 rhodiumpalladiumsilver


Name, Symbol, Number palladium, Pd, 46
Chemical series transition metals
Group, Period, Block 10, 5, d
Appearance silvery white metallic
Standard atomic weight 106.42(1)  g·mol−1
Electron configuration [Kr] 4d10
Electrons per shell 2, 8, 18, 18, 0
Physical properties
Phase solid
Density (near r.t.) 12.023  g·cm−3
Liquid density at m.p. 10.38  g·cm−3
Melting point 1828.05 K
(1554.9 °C, 2830.82 °F)
Boiling point 3236 K
(2963 °C, 5365 °F)
Heat of fusion 16.74  kJ·mol−1
Heat of vaporization 362  kJ·mol−1
Heat capacity (25 °C) 25.98  J·mol−1·K−1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 1721 1897 2117 2395 2753 3234
Atomic properties
Crystal structure cubic face centered
Oxidation states 2, 4
(mildly basic oxide)
Electronegativity 2.20 (scale Pauling)
Ionization energies 1st: 804.4 kJ/mol
2nd: 1870 kJ/mol
3rd: 3177 kJ/mol
Atomic radius 140pm
Atomic radius (calc.) 169  pm
Covalent radius 131  pm
Van der Waals radius 163 pm
Magnetic ordering no data
Electrical resistivity (20 °C) 105.4 n Ω·m
Thermal conductivity (300 K) 71.8  W·m−1·K−1
Thermal expansion (25 °C) 11.8  µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 3070 m/s
Young's modulus 121  GPa
Shear modulus 44  GPa
Bulk modulus 180  GPa
Poisson ratio 0.39
Mohs hardness 4.75
Vickers hardness 461  MPa
Brinell hardness 37.3  MPa
CAS registry number 7440-05-3
Selected isotopes
Main article: Isotopes of palladium
iso NA half-life DM DE (MeV) DP
100Pd syn 3.63 d ε - 107Rh
γ 0.084, 0.074,
102Pd 1.02% Pd is stable with 56 neutrons
103Pd syn 16.991 d ε - 103Rh
104Pd 11.14% Pd is stable with 58 neutrons
105Pd 22.33% Pd is stable with 59 neutrons
106Pd 27.33% Pd is stable with 60 neutrons
107Pd syn 6.5×106 y β- 0.033 107Ag
108Pd 26.46% Pd is stable with 62 neutrons
110Pd 11.72% Pd is stable with 64 neutrons

Palladium (IPA: /pəˈleɪdiəm/) is a chemical element with symbol Pd and atomic number 46. It is a rare silver white transition metal of the platinum group, resembling platinum chemically. It was discovered in platinum ores in 1803, and named after the asteroid Pallas by William Hyde Wollaston.

Palladium is usually found as a free metal, alloyed with others in the platinum group. It is commercially extracted from copper-nickel ores. Palladium has a great affinity for hydrogen, being able to absorb 900 times its own volume of the gas. Palladium metal and its complexes are often used in catalysis such as in catalytic converters on cars, palladium on carbon used in organic chemistry, and other coupling reactions. As a precious metal, it is sometimes used in jewelry, and has the ISO currency code of XPD. Palladium as an investment has attracted recent investment interests. In early 2007, several ETFs backed by physical palladium were launched, including London ETF Security and ZKB Palladium ETF.


[edit] History

Palladium was discovered by William Hyde Wollaston in 1803.[1][2] This element was named by Wollaston in 1804 after the asteroid Pallas, which was discovered two years earlier.[3]

Wollaston found palladium in crude platinum ore from South America by dissolving the ore in aqua regia, neutralizing the solution with sodium hydroxide, and precipitating platinum as ammonium chloroplatinate with ammonium chloride. He added mercuric cyanide to form the compound palladium cyanide, which was heated to extract palladium metal.

Palladium chloride was at one time prescribed as a tuberculosis treatment at the rate of 0.065g per day (approximately one milligram per kilogram of body weight). This treatment did not have many negative side effects, but was later replaced by more effective drugs.

Palladium's affinity for hydrogen led it to play an essential role in the Fleischmann-Pons experiment in 1989, also known as cold fusion. The cold fusion was quickly discredited but recently gained attention again due to some experimental support.

In the run up to 2000, Russian supply of palladium to global market was repeatedly delayed and disrupted[4] because the export quota was not granted on time, due to political reasons. The ensuring market panic buying drove the palladium price to an all time high of almost $1100, reached in January, 2001.[5] During the time period, Ford Motor Company, in fear of auto vehicle production disruption due to possible palladium shortage, stockpiled large amounts of the metal, purchased near the price high. As prices subsequently fell in early 2001, Ford lost nearly $1 billion U.S. dollars.

World demand for palladium increased from 100 ton in 1990 to nearly 300 ton in 2000. The global production from mines was 222 metric tons in 2006 according to USGSdata. Most palladium is used for catalytic converters in the automobile industry.[6]

[edit] Occurrence

Palladium output in 2005
Palladium output in 2005

In 2005, Russia was the top producer of palladium with atleast 50% world share followed by South Africa, USA and Canada, reports the British Geological Survey.

Palladium is found as a free metal and alloyed with platinum and gold with platinum group metals in placer deposits of the Ural Mountains, Australia, Ethiopia, South and North America. It is commercially produced from nickel-copper deposits found in South Africa, Ontario and Siberia; the huge volume of ore processed makes this extraction profitable despite the low proportion of palladium in these ores. The world's largest single producer of palladium is MMC Norilsk Nickel produced from the Norilsk–Talnakh nickel deposits. The Merensky Reef of the Bushveld Igneous Complex of South Africa contains significant palladium in addition to other platinum group elements.

Palladium is also produced in nuclear fission reactors and can be extracted from spent nuclear fuel, see Synthesis of noble metals.

Palladium is found in the rare minerals cooperite and polarite.

[edit] Characteristics


Palladium is a soft silver-white metal that resembles platinum. It is the least dense and has the lowest melting point of the platinum group metals. It is soft and ductile when annealed and greatly increases its strength and hardness when it is cold-worked. Palladium is chemically attacked by sulfuric, nitric and hydrochloric acid in which it dissolves slowly.[3] This metal also does not react with oxygen at normal temperatures (and thus does not tarnish in air). Palladium heated to 800°C will produce a layer of palladium(II) oxide (PdO). It lightly tarnishes in moist atmosphere containing sulfur.

This metal has the uncommon ability to absorb up to 900 times its own volume of hydrogen at room temperatures. It is thought that this possibly forms palladium hydride (PdH2) but it is not yet clear if this is a true chemical compound.[3]

When palladium has absorbed large amounts of hydrogen, it can swell up, like a sponge full of water, visible to the naked eye.[citation needed]

Common oxidation states of palladium are 0,+1, +2 and +4. Although originally +3 was thought of as one of the fundamental oxidation states of palladium, there is no evidence for palladium occurring in the +3 oxidation state; this has been investigated via X-ray diffraction for a number of compounds, indicating a dimer of palladium(II) and palladium(IV) instead. Recently, compounds with an oxidation state of +6 were synthesised.

[edit] Isotopes

Main article: isotopes of palladium

Naturally-occurring palladium is composed of six isotopes. The most stable radioisotopes are 107Pd with a half-life of 6.5 million years, 103Pd with a half-life of 17 days, and 100Pd with a half-life of 3.63 days. Eighteen other radioisotopes have been characterized with atomic weights ranging from 92.936 u (93Pd) to 119.924 u (120Pd). Most of these have half-lives that are less than a half an hour except 101Pd (half-life: 8.47 hours), 109Pd (half-life: 13.7 hours), and 112Pd (half-life: 21 hours).

The primary decay mode before the most abundant stable isotope, 106Pd, is electron capture and the primary mode after is beta decay. The primary decay product before 106Pd is rhodium and the primary product after is silver.

Radiogenic 107Ag is a decay product of 107Pd and was first discovered in the Santa Clara, California meteorite of 1978.[7] The discoverers suggest that the coalescence and differentiation of iron-cored small planets may have occurred 10 million years after a nucleosynthetic event. 107Pd versus Ag correlations observed in bodies, which have clearly been melted since accretion of the solar system, must reflect the presence of short-lived nuclides in the early solar system.[8]

[edit] Applications

Palladium is also used in dentistry,[9][10] watch making, in aircraft spark plugs and in the production of surgical instruments and electrical contacts.[citation needed] Palladium is also used to make professional transverse flutes.[citation needed] Palladium is often used by traditional sign makers [11] as it does not tarnish in normal conditions.

[edit] Electronics

The biggest application of palladium in electronics is making the Multi-Layer Ceramic Capacitor (MLCC). It is also used in plating of electronic components and in soldering materials. The electronic sector consumed 1.07 million troy ounces of palladium in 2006, according to a Johnson Matthey report.

[edit] Technology

Hydrogen easily diffuses through heated palladium; thus, it provides a means of purifying the gas.[3] Palladium (and palladium-silver alloys) are used as electrodes in multi-layer ceramic capacitors.[9] Palladium (sometimes alloyed with nickel) is used in connector platings in consumer electronics.

It is also used as Palladium-Hydrogen electrode in electrochemical studies. Palladium dichloride can absorb large amounts of carbon monoxide gas, and is used in carbon monoxide detectors.

[edit] Catalysis

When it is finely divided, such as in palladium on carbon, palladium forms a good catalyst and is used to speed up hydrogenation and dehydrogenation reactions, as well as in petroleum cracking. A large number of carbon-carbon bond forming reactions in organic chemistry (such as the Heck and Suzuki coupling) are facilitated by catalysis with palladium compounds. The largest use of palladium today is in catalytic converters.[9] Much research is in progress to discover ways to replace the much more expensive platinum with palladium in this application.[citation needed]

[edit] Hydrogen storage

See Palladium hydride

[edit] Jewelry

Since 1939[citation needed] palladium itself has occasionally been used as a precious metal in jewelry, as replacement for platinum or white gold.

A Palladium plated belt buckle.
A Palladium plated belt buckle.

This is due to its naturally white properties giving it no need for a rhodium plating. It is slightly whiter, much lighter and about 12% harder. Similar to gold, palladium can be beaten into a thin leaf form as thin as 100 nm (1/250,000 in).[3] Like platinum, it will develop a hazy patina over time. Unlike platinum, however, palladium will discolor at soldering temperatures, become brittle with repeated heating and cooling, and react with strong acids.

It can also be used as a substitute for nickel when making white gold.[12] Palladium is one of three most used metals which can be alloyed with gold to produce white gold.[9] (Nickel and silver can also be used.) Palladium-gold is a much more expensive alloy than nickel-gold but is hypoallergenic and holds its white color better.

When platinum was declared a strategic government resource during World War II, many jewellery bands were made out of palladium. As recently as September 2001,[13] palladium was more expensive than platinum and rarely used in jewellery also due to the technical obstacle of casting. However the casting problem has been resolved and its use in jewelry has increased because of a large spike in the price of platinum and a drop in the price of palladium.[14]

[edit] Photography

With the platinotype printing process photographers make fine-art black-and-white prints using platinum or palladium salts. Often used with platinum, palladium provides an alternative to silver.[15]

[edit] See also

[edit] References

  1. ^ Platinum Metals Review, Rhodium and Palladium - Events Surrounding Its Discovery, accessed 5 Feb 2007.
  2. ^ W. H. Wollaston (1804). "On a New Metal, Found in Crude Platina". Philosophical Transactions of the Royal Society of London 94: 419-430.. 
  3. ^ a b c d e Los Alamos National Laboratory, Palladium, accessed 5 Feb 2007.
  4. ^ Williamson, A.. Russian PGM Stocks.
  5. ^ Historical Palladium Charts and Data. Kitco. Retrieved on 2007-08-09.
  6. ^ J. Kielhorn, C. Melber, D. Keller, I. Mangelsdorf (2002). "Palladium – A review of exposure and effects to human health". International Journal of Hygiene and Environmental Health 205 (6). DOI:10.1078/1438-4639-00180. 
  7. ^ W. R. Kelly, G. J. Wasserburg, (1978). "Evidence for the existence of 107Pd in the early solar system". Geophysical Research Letters 5: 1079–1082. 
  8. ^ J. H. Chen, G. J. Wasserburg (1990). "The isotopic composition of Ag in meteorites and the presence of 107Pd in protoplanets". Geochimica et Cosmochimica Acta 54 (6): 1729-1743. DOI:10.1016/0016-7037(90)90404-9. 
  9. ^ a b c d United Nations Conference on Trade and Development, Palladium accessed 5 Feb 2007.
  10. ^ Platinum Metals Review, Palladium in Restorative Dentistry
  11. ^ Clover Signs Signs, accessed 31 March 2007.
  12. ^
  13. ^ Johnson Matthey NY: Daily Metal Prices, September 2001
  14. ^ Holmes, E., "Palladium, Platinum's Cheaper Sister, Makes a Bid for Love", Wall Street Journal (Eastern edition). Feb 13, 2007. p. B.1
  15. ^ Mike Ware (2005). "Book Review of : Photography in Platinum and Palladium". Platinum Metals Review 49 (4): 190-195. DOI:10.1595/147106705X70291. 

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