Palladium

Palladium is a chemical element; it has the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist 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 together a group of elements referred to as the platinum group metals. They have similar chemical properties, but palladium has the lowest melting point and is the least dense of them.
More than half the supply of palladium and its congener platinum is used in catalytic converters, which convert as much as 90% of the harmful gases in automobile exhaust into nontoxic substances. Palladium is also used in electronics, dentistry, medicine, hydrogen purification, chemical applications, electrochemical sensors, electrosynthesis, groundwater treatment, and jewellery. Palladium is a key component of fuel cells, in which hydrogen and oxygen react to produce electricity, heat, and water.
Ore deposits of palladium and other platinum group metals are rare. 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 Basin and Thunder Bay District of Ontario, Canada; and the Norilsk Complex in Russia. Recycling is also a source, mostly from scrapped catalytic converters. The numerous applications and limited supply sources result in considerable investment interest.

Characteristics

Palladium belongs to group 10 in the periodic table, but the configuration in the outermost electrons is in accordance with Hund's rule. Electrons that by the Madelung rule would be expected to occupy the 5s instead fill the 4d orbitals, as it is more energetically favorable to have a completely filled 4d¹⁰ shell instead of the 5s² 4d⁸ configuration.

Z	Element	No. of electrons/shell
28	nickel	2, 8, 16, 2
46	palladium	2, 8, 18, 18, 0
78	platinum	2, 8, 18, 32, 17, 1
110	darmstadtium	2, 8, 18, 32, 32, 16, 2

This 5s⁰ configuration, unique in period 5, makes palladium the heaviest element having only one incomplete electron shell, with all shells above it empty.
Palladium has the appearance of 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 is greatly increased in strength and hardness when cold-worked. Palladium dissolves slowly in concentrated nitric acid, in hot, concentrated sulfuric acid, and when finely ground, in hydrochloric acid. It dissolves readily at room temperature in aqua regia.
Palladium does not react with oxygen at standard temperature. Palladium heated to 800 °C will produce a layer of palladium oxide. It may slowly develop a slight brownish coloration over time, likely due to the formation of a surface layer of its monoxide.
Palladium films with defects produced by alpha particle bombardment at low temperature exhibit superconductivity having T_c = 3.2 K.

Isotopes

Naturally occurring palladium is composed of six stable isotopes. The most stable radioisotopes are ¹⁰⁷Pd with a half-life of 6.5 million years, ¹⁰³Pd with a half-life of 16.99 days, and ¹⁰⁰Pd with a half-life of 3.63 days. There are 25 other radioisotopes characterized ranging from ⁹¹Pd to ¹²⁹Pd. These have half-lives of less than thirty minutes, except ¹⁰¹Pd, ¹⁰⁹Pd, and ¹¹²Pd.
For isotopes with atomic masses less than that of the most abundant stable isotope, ¹⁰⁶Pd, the primary decay mode is electron capture with the primary decay product being rhodium. The primary mode of decay for those isotopes of Pd with atomic mass greater than 106 is beta decay with the primary product of this decay being 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 Solar System bodies must reflect the presence of short-lived nuclides in the early Solar System.
is also produced as a fission product in spontaneous or induced fission of. As it is not very mobile in the environment and has a relatively low decay energy, is usually considered to be among the more benign of the long-lived fission products.

Compounds

Palladium compounds exist primarily in the 0 and +2 oxidation state. Other less common states are also recognized. Generally the compounds of palladium are more similar to those of platinum than those of any other element.

Palladium(II)

is the principal starting material for other palladium compounds. It arises by the reaction of palladium with chlorine. It is used to prepare heterogeneous palladium catalysts such as palladium on barium sulfate, palladium on carbon, and palladium chloride on carbon. Solutions of in nitric acid react with acetic acid to give palladium acetate, also a versatile reagent. reacts with ligands to give square planar complexes of the type. One example of such complexes is the benzonitrile derivative Bispalladium dichloride|.
The complex bispalladium dichloride is a useful catalyst.

Palladium(0)

Palladium forms a range of zerovalent complexes with the formula, and. For example, reduction of a mixture of and gives tetrakispalladium:
Another major palladium complex, trisdipalladium, is prepared by reducing sodium tetrachloropalladate in the presence of dibenzylideneacetone.
Palladium, as well as palladium, are catalysts in coupling reactions, as has been recognized by the 2010 Nobel Prize in Chemistry to Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki. Such reactions are widely practiced for the synthesis of fine chemicals. Prominent coupling reactions include the Heck, Suzuki, Sonogashira coupling, Stille reactions, and the Kumada coupling. Palladium acetate, tetrakispalladium, and trisdipalladium serve either as catalysts or precatalysts.

Other oxidation states

Although Pd compounds are comparatively rare, one example is sodium hexachloropalladate,. Pd and Pd can transform into each other under certain electrochemical conditions. A few compounds of palladium are also known. Palladium was claimed to have been synthesized in 2002, but subsequently disproven.
Mixed valence palladium complexes exist, e.g. forms an infinite Pd chain structure, with alternatively interconnected and Pd₂ units.
When alloyed with a more electropositive element, palladium can acquire a negative charge. Such compounds are known as palladides, such as gallium palladide. Palladides with the stoichiometry exist where R is scandium, yttrium, or any of the lanthanides.

Occurrence

As overall mine production of palladium reached 210,000 kilograms in 2022, Russia was the top producer with 88,000 kilograms, followed by South Africa, Canada, the U.S., and Zimbabwe. Russia's company Norilsk Nickel ranks first among the largest palladium producers globally, accounting for 39% of the world's production.
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 most important commercial 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. Many more Pd minerals are known, but all of them are very rare.
Palladium is also produced in nuclear fission reactors and can be extracted from spent nuclear fuel, 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. A complication for the recovery of palladium in spent fuel is the presence of, a slightly radioactive long-lived fission product. Depending on end use, the radioactivity contributed by the might make the recovered palladium unusable without a costly step of isotope separation.

Applications

The largest use of palladium today is in catalytic converters. Palladium is also used in jewellery, dentistry, watch making, blood sugar test strips, aircraft spark plugs, surgical instruments, and electrical contacts. Palladium is also used to make some 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. Because it adsorbs hydrogen, palladium was a key component of the controversial cold fusion experiments of the late 1980s.

Catalysis

When it is finely divided, as with palladium on carbon, palladium forms a versatile catalyst; it speeds heterogeneous catalytic processes like hydrogenation, dehydrogenation, and petroleum cracking. Palladium is also essential to the Lindlar catalyst, also called Lindlar's Palladium. A large number of carbon–carbon bonding reactions in organic chemistry are facilitated by palladium compound catalysts. For example:

When dispersed on conductive materials, palladium is an excellent electrocatalyst for oxidation of primary alcohols in alkaline media. Palladium is also a versatile metal for homogeneous catalysis, used in combination with a broad variety of ligands for highly selective chemical transformations.
In 2010 the Nobel Prize in Chemistry was awarded "for palladium-catalyzed cross couplings in organic synthesis" to Richard F. Heck, Ei-ichi Negishi and Akira Suzuki. A 2008 study showed that palladium is an effective catalyst for carbon–fluorine bonds.
Palladium catalysis is primarily employed in organic chemistry and industrial applications, although its use is growing as a tool for synthetic biology; in 2017, effective in vivo catalytic activity of palladium nanoparticles was demonstrated in mammals to treat disease.
Palladium is also used as a catalyst in the production of biofuels.