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Palladium (
/pəˈleɪdiəm/ pə-LAY-dee-əm) is a chemical element with the chemical symbol Pd and an atomic number of 46. Palladium is a rare and lustrous silvery-white metal that was discovered in 1803 by William Hyde Wollaston, who named it after the asteroid Pallas, which was named after the epithet of the Greek goddess Athena, acquired by her when she slew Pallas.
Palladium, along with platinum, rhodium, ruthenium, iridium and osmium form a group of elements referred to as the platinum group metals (PGMs). Platinum group metals share similar chemical properties, but palladium has the lowest melting point and is the least dense of these precious metals.
The unique properties of palladium and other platinum group metals account for their widespread use. One in four goods manufactured today either contain platinum group metals or the platinum group metals play a key role during their manufacturing process. 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 combines hydrogen and oxygen to produce electricity, heat and water.
Ore deposits of palladium and other platinum group metals are rare, and the most extensive deposits have been found in the norite belt of the Bushveld Igneous Complex in the Transvaal in South Africa, the Stillwater Complex in Montana, United States, the Sudbury District of Ontario, Canada, and the Norilsk Complex in Russia. In addition to mining, 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.
Palladium was discovered by William Hyde Wollaston in 1803. This element was named by Wollaston in 1804 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 did have 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, 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 palladium 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 metric tons in 2006 according to USGS data. Most palladium is used for catalytic converters in the automobile industry.
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 may 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 deposits from which palladium is produced 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 also produced in nuclear fission reactors and can be extracted from spent nuclear fuel (see synthesis of precious metals) though the quantity produced is insignificant.
Palladium is found in the rare minerals cooperite and polarite.
Palladium belongs to group 10 in the periodic table:
| Z | Element | No. of electrons/shell |
|---|---|---|
| 28 | nickel | 2, 8, 16, 2 |
| 46 | palladium | 2, 8, 18, 18 |
| 78 | platinum | 2, 8, 18, 32, 17, 1 |
| 110 | darmstadtium | 2, 8, 18, 32, 32, 17, 1 |
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.[5] 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.[5] 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.
Naturally occurring palladium is composed of seven isotopes, which includes six stable isotopes. The most stable radioisotopes are 107Pd with a half-life of 6.5 million years (found in nature), 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 90.94948(64) u (91Pd) to 122.93426(64) u (123Pd). Most of these have half-lives that are less than a half-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 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. 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.
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), [PdCl6]2−.
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, e.g.:
- 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 cataly Where desired, the catalyst may be formed in situ.
- PdCl2 + 2PPh3 → 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:
- PdCl2(PPh3)2 + 2 PPh3 + 2.5 N2H4 → Pd(PPh3)4 + 0.5 N2 + 2 N2H5+Cl−
The other major palladium(0) complex, tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3), is prepared by reducing sodium hexachloropalladate(IV) 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 reaction, 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. One troublesome problem with palladium catalysis is that the catalysts may decompose at high temperature to give elemental palladium, either as a black precipitate ("palladium black") or deposited as a mirror on the reaction flask.
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.
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 In 2010, palladium-catalysed organic reactions were recognised by the Nobel Prize in Chemistry
Pd 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.
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 metric tons) of palladium in 2006, according to a Johnson Matthey report.
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.
It 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.
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.
Palladium itself has been used as a precious metal in jewelry since 1939, as an alternative to platinum or white gold. This is due to its naturally white properties, giving it no need for rhodium plating. It is 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). Like platinum, it will develop a hazy patina over time. Unlike platinum, however, palladium may discolor at high soldering temperatures, become brittle with repeated heating and cooling, and react with strong acids.
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 as an alloy in the manufacture of white gold jewelry, but, beginning early in 2004 when gold and platinum prices began to rise steeply, Chinese jewelers began fabricating significant volumes of palladium jewelry. Johnson Matthey estimated that in 2004, with the introduction of palladium jewelry in China, demand for palladium for jewelry fabrication was 920,000 ounces, or approximately 14% of the total palladium demand for 2004—an increase of almost 700,000 ounces from the previous year. This growth continued during 2005, with estimated worldwide jewelry demand for palladium of about 1.4 million ounces, or almost 21% of net palladium supply, again with most of the demand centered in China. The popularity of palladium jewelry is expected to grow in 2008 as the world's biggest producers embark on a joint marketing effort to promote palladium jewelry worldwide.
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.
Palladium leaf is one of several alternatives to silver leaf used in manuscript illumination. The use of silver leaf is problematic because it tarnishes quickly, dulling the appearance and requiring constant cleaning. Palladium is a suitable substitute due to its resistance to tarnishing. Aluminium leaf is another inexpensive alternative, but aluminium is much more difficult to work than gold or silver and results in less than optimal results when employing traditional metal leafing techniques, so palladium leaf is considered the best substitute despite its considerable cost. Platinum leaf may be used to the same effect as palladium leaf with similar working properties, but it is not as commercially available on demand in leaf form.
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
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