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Palladium, ₄₆Pd

Palladium
Pronunciation	/pəˈleɪdiəm/ ​(pə-LAY-dee-əm)
Appearance	silvery white
Standard atomic weight Ar°(Pd)
	106.42±0.01 106.42±0.01 (abridged)
Palladium in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Ni ↑ Pd ↓ Pt rhodium ← palladium → silver
Atomic number (Z)	46
Group	group 10
Period	period 5
Block	d-block
Electron configuration	[Kr] 4d
Electrons per shell	2, 8, 18, 18
Physical properties
Phase at STP	solid
Melting point	1828.05 K ​(1554.9 °C, ​2830.82 °F)
Boiling point	3236 K ​(2963 °C, ​5365 °F)
Density (at 20° C)	12.007 g/cm
when liquid (at m.p.)	10.38 g/cm
Heat of fusion	16.74 kJ/mol
Heat of vaporization	358 kJ/mol
Molar heat capacity	25.98 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1721 1897 2117 2395 2753 3234
Atomic properties
Oxidation states	common: 0, +2, +4 +1, +3, +5
Electronegativity	Pauling scale: 2.20
Ionization energies	1st: 804.4 kJ/mol 2nd: 1870 kJ/mol 3rd: 3177 kJ/mol
Atomic radius	empirical: 137 pm
Covalent radius	139±6 pm
Van der Waals radius	163 pm
Spectral lines of palladium
Other properties
Natural occurrence	primordial
Crystal structure	​face-centered cubic (fcc) (cF4)
Lattice constant	a = 389.02 pm (at 20 °C)
Thermal expansion	11.77×10/K (at 20 °C)
Thermal conductivity	71.8 W/(m⋅K)
Electrical resistivity	105.4 nΩ⋅m (at 20 °C)
Magnetic ordering	paramagnetic
Molar magnetic susceptibility	+567.4×10 cm/mol (288 K)
Young's modulus	121 GPa
Shear modulus	44 GPa
Bulk modulus	180 GPa
Speed of sound thin rod	3070 m/s (at 20 °C)
Poisson ratio	0.39
Mohs hardness	4.75
Vickers hardness	400–600 MPa
Brinell hardness	320–610 MPa
CAS Number	7440-05-3
History
Naming	after asteroid Pallas, itself named after Pallas Athena
Discovery and first isolation	William Hyde Wollaston (1802)
Isotopes of palladium v e
Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct Pd synth 3.63 d ε Rh γ – Pd 1.02% stable Pd synth 16.991 d ε Rh Pd 11.1% stable Pd 22.3% stable Pd 27.3% stable Pd trace 6.5×10 y β Ag Pd 26.5% stable Pd 11.7% stable
Category: Palladium view talk edit | references

Palladium (/pəˈleɪdiəm/ pə-LAY-dee-əm) is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal and was discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired by her when she slew Pallas.

Palladium, platinum, rhodium, ruthenium, iridium and osmium form a group of elements referred to as the platinum group metals (PGMs). These have similar chemical properties, but palladium has the lowest melting point and is the least dense of them.

The unique properties of palladium and other platinum group metals account for their widespread use. A quarter of all goods manufactured today either contain PGMs or have a significant part in their manufacturing process played by PGMs. Over half of the supply of palladium and its congener platinum goes into catalytic converters, which convert up to 90% of harmful gases from auto exhaust (hydrocarbons, carbon monoxide and nitrogen oxide) into less harmful substances (nitrogen, carbon dioxide and water vapor). Palladium is found in many electronics including computers, mobile phones, multi-layer ceramic capacitors, component plating, low voltage electrical contacts and SED/OLED/LCD televisions. Palladium is also used in dentistry, medicine, hydrogen purification, chemical applications and groundwater treatment. Palladium plays a key role in the technology used for fuel cells, which combine hydrogen and oxygen to produce electricity, heat and water.

Ore deposits of palladium and other PGMs are rare, and the most extensive deposits have been found in the norite belt of the Bushveld Igneous Complex covering the Transvaal Basin in South Africa, the Stillwater Complex in Montana, United States, the Sudbury District of Ontario, Canada, and the Norilsk Complex in Russia. Recycling is also a source of palladium, mostly from scrapped catalytic converters. The numerous applications and limited supply sources of palladium result in palladium drawing considerable investment interest.

History

William Hyde Wollaston note the discovery of an new noble metal in July 1802 in his lab-book and named it palladium in August of the same year. Wollaston purified enough of the material and offered it, without naming the discoverer, in a small shop in Soho in April 1803. After a harsh criticism that palladium is an alloy of platinum an mercury by Richard Chenevix Wollaston anonymously offered a reward of 20 British pounds for 20 grains of synthetic palladium alloy. Chenevix received the Copley Medal in 1803 after he published his experiments on palladium. Wollaston published the discovery of rhodium in 1804 and mentions some of his work on palladium. He disclosed to be the discoverer of palladium in a publication in 1805.

It was named by Wollaston in 1802 after the asteroid Pallas, which had been discovered two years earlier. 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 0.065 g per day (approximately one milligram per kilogram of body weight). This treatment had many negative side-effects, and 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.

In the run up to 2000, the Russian supply of palladium to the global market was repeatedly delayed and disrupted because the export quota was not granted on time, for political reasons. The ensuing market panic drove the price to an all-time high of $1100 per troy ounce in January 2001. Around this time, the Ford Motor Company, fearing auto vehicle production disruption due to a possible palladium shortage, stockpiled large amounts of the metal purchased near the price high. When prices fell in early 2001, Ford lost nearly US$1 billion. World demand for palladium increased from 100 tons in 1990 to nearly 300 tons in 2000. The global production of palladium from mines was 222 tonnes in 2006 according to the United States Geological Survey. Most palladium is used for catalytic converters in the automobile industry.

Occurrence

In 2007, Russia was the top producer of palladium, with a 44% world share, followed by South Africa with 40%. Canada with 6% and the U.S. with 5% are the only other substantial producers of palladium.

Palladium can be found as a free metal alloyed with gold and other platinum group metals in placer deposits of the Ural Mountains, Australia, Ethiopia, North and South America. For the production of palladium these deposits play only a minor role. The commercially most important sources are nickel-copper deposits found in the Sudbury Basin, Ontario, and the Norilsk–Talnakh deposits in Siberia. The other large deposit is the Merensky Reef platinum group metals deposit within the Bushveld Igneous Complex South Africa. The Stillwater igneous complex of Montana and the Roby zone ore body of the Lac des Îles igneous complex of Ontario are the two other sources of palladium in Canada and the United States. Palladium is found in the rare minerals cooperite and polarite.

Palladium is also produced in nuclear fission reactors and can be extracted from spent nuclear fuel (see synthesis of precious metals) though this source for palladium is not used. None of the existing nuclear reprocessing facilities are equipped to extract palladium from the high-level radioactive waste.

Characteristics

Palladium belongs to group 10 in the periodic table:

but has a very atypical configuration in its outermost electron shells compared to the rest of the members of group 10, if not to all elements (see also niobium (41), ruthenium (44) and rhodium (45)).

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 dissolves slowly in sulfuric, nitric and hydrochloric acid. 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.

The 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. When palladium has absorbed large amounts of hydrogen, it will expand slightly in size.

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. In 2002, palladium(VI) was first reported.

Isotopes

Naturally occurring palladium is composed of seven isotopes, which includes six stable isotopes. The most stable radioisotopes are Pd with a half-life of 6.5 million years (found in nature), Pd with a half-life of 17 days, and Pd with a half-life of 3.63 days. Eighteen other radioisotopes have been characterized with atomic weights ranging from 90.94948(64) u (Pd) to 122.93426(64) u (Pd). Most of these have half-lives that are less than a half-hour, except Pd (half-life: 8.47 hours), Pd (half-life: 13.7 hours), and Pd (half-life: 21 hours).

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

Radiogenic Ag is a decay product of Pd and was first discovered in 1978 in the Santa Clara meteorite of 1976. The discoverers suggest that the coalescence and differentiation of iron-cored small planets may have occurred 10 million years after a nucleosynthetic event. Pd 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.

Compounds

Palladium primarily exists in the 0, +2, +4 oxidation states; the +4 oxidation state is comparatively rare. One major example of palladium(IV) is hexachloropalladate(IV), .

Elemental palladium reacts with chlorine to give palladium(II) chloride; it dissolves in nitric acid and precipitates palladium(II) acetate on addition of acetic acid. These two compounds and the bromide are reactive and relatively inexpensive, making them convenient entry points to palladium chemistry. All three are not monomeric; the chloride and bromide often need to be refluxed in acetonitrile to obtain the more reactive acetonitrile complex monomers, for example:

PdX₂ + 2 MeCN → PdX₂(MeCN)₂ (X = Cl, Br)

Palladium(II) chloride is the principal starting material for many other palladium catalysts. It is used to prepare heterogeneous palladium catalysts: palladium on barium sulfate, palladium on carbon, and palladium chloride on carbon. It reacts with triphenylphosphine in coordinating solvents to give bis(triphenylphosphine)palladium(II) dichloride, a useful catalyst. Where desired, the catalyst may be formed in situ.

PdCl₂ + 2 PPh₃ → PdCl₂(PPh₃)₂

Reduction of this phosphine complex with hydrazine with more phosphine gives tetrakis(triphenylphosphine)palladium(0), one of the two major palladium(0) complexes:

2 PdCl₂(PPh₃)₂ + 4 PPh₃ + 5 N₂H₄ → 2 Pd(PPh₃)₄ + N₂ + 4 N₂H₅Cl

The other major palladium(0) complex, tris(dibenzylideneacetone)dipalladium(0) (Pd₂(dba)₃), is prepared by reducing sodium tetrachloropalladate in the presence of dibenzylideneacetone.

The great many reactions in which palladium compounds serve as catalysts are collectively known as palladium-catalyzed coupling reactions. Prominent examples include the Heck, Suzuki and Stille reactions. Palladium(II) acetate, tetrakis(triphenylphosphine)palladium(0) (Pd(PPh₃)₄, and tris(dibenzylideneacetone)dipalladium(0) (Pd₂(dba)₃) are useful in this regard, either as catalysts or as starting points to catalysts.

Applications

The largest use of palladium today is in catalytic converters. Palladium is also used in jewelry, in dentistry, watch making, in blood sugar test strips, in aircraft spark plugs and in the production of surgical instruments and electrical contacts. Palladium is also used to make professional transverse flutes. As a commodity, palladium bullion has ISO currency codes of XPD and 964. Palladium is one of only four metals to have such codes, the others being gold, silver and platinum.

Catalysis

When it is finely divided, such as in palladium on carbon, palladium forms a versatile catalyst and speeds 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. (see #Compounds and palladium-catalyzed coupling reactions) In addition palladium, when dispersed on conductive materials, proves to be an excellent electrocatalyst for oxidation of primary alcohols in alkaline media. In 2010, palladium-catalysed organic reactions were recognised by the Nobel Prize in Chemistry. Palladium is also a versatile metal for homogeneous catalysis. It is used in combination with a broad variety of ligands for highly selective chemical transformations. A 2008 study showed that palladium is an effective catalyst for making carbon-fluoride bonds. Palladium is found in the Lindlar catalyst, also called Lindlar's Palladium.

Electronics

The second biggest application of palladium in electronics is making the multilayer ceramic capacitor. Palladium (and palladium-silver alloys) are used as electrodes in multi-layer ceramic capacitors. Palladium (sometimes alloyed with nickel) is used in connector platings in consumer electronics.

It is also used in plating of electronic components and in soldering materials. The electronic sector consumed 1.07 million troy ounces (33.2 tonnes) of palladium in 2006, according to a Johnson Matthey report.

Technology

Hydrogen easily diffuses through heated palladium; thus, it provides a means of purifying the gas. Membrane reactors with Pd membranes are therefore used for the production of high purity hydrogen. Palladium is a part of the palladium-hydrogen electrode in electrochemical studies. Palladium(II) chloride can oxidize large amounts of carbon monoxide gas, and is used in carbon monoxide detectors.

Hydrogen storage

Palladium hydride is metallic palladium that contains a substantial quantity of hydrogen within its crystal lattice. At room temperature and atmospheric pressure, palladium can adsorb up to 900 times its own volume of hydrogen in a reversible process. This property has been investigated because hydrogen storage is of such interest and a better understanding of what happens at the molecular level could give clues to designing improved metal hydrides. A palladium based store, however, would be prohibitively expensive due to the cost of the metal.

Jewelry

Palladium itself has been used as a precious metal in jewelry since 1939, as an alternative to platinum or white gold. This use resulted in the naturally white color of palladium which required no rhodium plating. Palladium is proportionally much lighter than platinum. Similar to gold, palladium can be beaten into a thin leaf form as thin as 100 nm (1/250,000 in). Unlike platinum, palladium may discolor upon heating to above 400 °C; it is relatively brittle and reacts with strong acids such as nitric acid or aqua regia.

Palladium is one of the three most popular metals used to make white gold alloys (nickel and silver can also be used). Palladium-gold is a more expensive alloy than nickel-gold, but seldom causes allergic reactions (though certain cross-allergies with nickel may occur).

When platinum was declared a strategic government resource during World War II, many jewelry bands were made out of palladium. As recently as September 2001, palladium was more expensive than platinum and rarely used in jewelry 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.

Prior to 2004, the principal use of palladium in jewelry was the manufacture of white gold. In early 2004, when gold and platinum prices rose steeply, China began fabricating significant volumes of palladium jewelry and used 37 tonnes of palladium for this purpose in 2005. Changes of the relative price between palladium and platinum after 2008 lowered demand for palladium to 17.4 tonnes in 2009.

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.

Precautions

Finely divided palladium metal can be pyrophoric. As a platinum-group metal, the bulk material is quite inert. Although contact dermatitis has been reported, the amount of data on the effects of exposure to palladium is limited. It has been shown that people reaction allergic on palladium also react on nickel, making it possible to avoid the use of dental alloys containing palladium on those so allergic.

A considerable amount of palladium is distributed by exhaust of cars with catalytic converters. Between 4 and 108 ng/km of palladium particulate is released by those cars. The total uptake of from food is estimated to be lower than 2 µg per person and day. The second possible source for palladium are alloys for dental restoration, there the possible uptake of palladium is estimated to be lower than 15 µg per person and day. People working with palladium or its compounds might have a considerable higher uptake. For soluble compounds like palladium chloride 99% is eliminated from the body within 3 days.

The median lethal dose (LD₅₀) of soluble palladium compounds in mice is 200 mg/kg for oral and 5 mg/kg for intravenous administration.

See also

• 2000s commodities boom
• Palladium as an investment
• Palladium coin
• Periodic table
• Platinum
• Precious metal

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External links

• Current and Historical Palladium Price
• a picture of pure Palladium
• Special Market Report on Palladium and Precious Metals

Template:Chemical elements named after places

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• Ill-formatted IPAc-en transclusions
• Palladium
• Chemical elements
• Noble metals
• Transition metals
• Precious metals