Palladium is achemical element; it has thesymbolPd andatomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemistWilliam Hyde Wollaston. He named it after theasteroid Pallas (formally 2 Pallas), which was itself named after theepithet of the Greek goddessAthena, acquired by her when she slewPallas. Palladium,platinum,rhodium,ruthenium,iridium andosmium form together a group of elements referred to as theplatinum group metals. They have similar chemical properties, but palladium has the lowest melting point and is the least dense of them.
Palladium belongs togroup 10 in the periodic table, but the configuration in the outermost electrons is in accordance withHund's rule. Electrons that by theMadelung 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.[clarification needed]
This 5s0 configuration, unique inperiod 5, makes palladium the heaviest element having onlyone incompleteelectron 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 lowestmelting point of the platinum group metals. It is soft andductile whenannealed and is greatly increased in strength and hardness when cold-worked. Palladium dissolves slowly in concentratednitric acid, in hot, concentratedsulfuric acid, and when finely ground, inhydrochloric acid.[18] It dissolves readily at room temperature inaqua regia.
Palladium does not react withoxygen at standard temperature (and thus does not tarnish inair). Palladium heated to 800 °C will produce a layer of palladium(II) oxide (PdO). 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 havingTc = 3.2 K.[19]
Naturally occurring palladium is composed of six stableisotopes. The most stableradioisotopes are107Pd with ahalf-life of 6.5 million years (traces found in nature),103Pd with a half-life of 16.99 days, and100Pd with a half-life of 3.63 days. There are 25 other radioisotopes characterized ranging from91Pd to129Pd. These have half-lives of less than thirty minutes, except101Pd (8.47 hours),109Pd (13.6 hours), and112Pd (21.0 hours).[20]
For isotopes with atomic masses less than that of the most abundant stable isotope,106Pd, the primarydecay mode iselectron capture with the primarydecay product being rhodium. The primary mode of decay for those isotopes of Pd with atomic mass greater than 106 isbeta decay with the primary product of this decay beingsilver.[20]
Radiogenic107Ag is a decay product of107Pd and was first discovered in 1978[21] in theSanta Clara[22] meteorite of 1976. The discoverers suggest that the coalescence and differentiation of iron-cored small planets may have occurred 10 million years after anucleosynthetic event.107Pd versus Ag correlations observed inSolar System bodies must reflect the presence of short-lived nuclides in the early Solar System.[23]
107 Pd is also produced as afission product in spontaneous or induced fission of235 U. As it is not very mobile in the environment and has a relatively lowdecay energy,107 Pd is usually considered to be among the more benign of thelong-lived fission products.
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) chloride 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.[24] Solutions ofPdCl2 in nitric acid react withacetic acid to givepalladium(II) acetate, also a versatile reagent.PdCl2 reacts withligands (L) to give square planar complexes of the typePdCl2L2. One example of such complexes is thebenzonitrile derivativePdCl2(PhCN)2.[25][26]
Palladium(II) acetatePlatinum-palladium ore from the Stillwater mine in the Beartooth Mountains, Montana, USSulfidic serpentintite (platinum-palladium ore) from the Stillwater mine in Montana
Palladium forms a range of zerovalent complexes with the formulaPdL4,PdL3 andPdL2. For example, reduction of a mixture ofPdCl2(PPh3)2 andPPh3 givestetrakis(triphenylphosphine)palladium(0):[28]
Although Pd(IV) compounds are comparatively rare, one example issodium hexachloropalladate(IV),Na2[PdCl6]. Pd(II) and Pd(IV) can transform into each other under certain electrochemical conditions.[31][32] A fewcompounds of palladium(III) are also known.[33] Palladium(VI) was claimed to have been synthesized in 2002,[34][35] but subsequently disproven.[36][37]
Mixed valence palladium complexes exist, e.g.Pd4(CO)4(OAc)4Pd(acac)2 forms an infinite Pd chain structure, with alternatively interconnectedPd4(CO)4(OAc)4 andPd(acac)2 units.[38]
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.[41] Russia's companyNorilsk Nickel ranks first among the largest palladium producers globally, accounting for 39% of the world's production.[42]
When dispersed on conductive materials, palladium is an excellent electrocatalyst for oxidation of primary alcohols in alkaline media.[56] Palladium is also a versatile metal forhomogeneous catalysis, used in combination with a broad variety ofligands for highly selective chemical transformations.
Catalytic cycle for Kumada cross coupling reaction, which is widely used in the synthesis of fine chemicals
Palladium catalysis is primarily employed in organic chemistry and industrial applications, although its use is growing as a tool forsynthetic biology; in 2017, effectivein vivo catalytic activity of palladiumnanoparticles was demonstrated in mammals to treat disease.[58]
Palladium is also used as a catalyst in the production ofbiofuels.[59]
The primary application of palladium in electronics is inmulti-layer ceramic capacitors[60] in which palladium (and palladium-silver alloy) is used for electrodes.[49] Palladium (sometimes alloyed with nickel) is or can be used for component and connector plating in consumer electronics[61][62] and in soldering materials. The electronic sector consumed 33 tonnes (1.07 million troy ounces) of palladium in 2006, according to aJohnson Matthey report.[63] Palladium is used in the production ofprinted circuit boards.[64]
Palladium has been used to producemetallic glass by fast cooling alloys, avoiding their crystallisation, thus reducing brittleness and leading to stronger materials.[67]
Palladium readilyadsorbs hydrogen at room temperatures, formingpalladium hydride PdHx with x less than 1.[68] While this property is common to many transition metals, palladium has a uniquely high absorption capacity and does not lose its ductility until x approaches 1.[69] This property has been investigated in designing an efficient and safe hydrogen fuel storage medium, though palladium itself is currently prohibitively expensive for this purpose.[70] The content of hydrogen in palladium can be linked tomagnetic susceptibility, which decreases with the increase of hydrogen and becomes zero for PdH0.62. At any higher ratio, thesolid solution becomesdiamagnetic.[71]
Palladium is used for purification of hydrogen on a laboratory[72] but not industrial scale.[73]
Palladium is used in small amounts (about 0.5%) in some alloys ofdental amalgam to decrease corrosion and increase themetallic lustre of the final restoration.[74][75] Palladium is also used in the production ofpacemakers.[76]
Palladium has been used as aprecious metal in jewellery since 1939 as an alternative to platinum in the alloys called "white gold", where the naturally white color of palladium does not requirerhodium plating. Palladium, being much less dense than platinum, is similar to gold in that it can be beaten intoleaf as thin as 100 nm (1⁄250,000 in).[18] Unlike platinum, palladium may discolor at temperatures above 400 °C (752 °F)[77] due to oxidation, making it more brittle and thus less suitable for use in jewellery; to prevent this, palladium intended for jewellery is heated under controlled conditions.[78]
Prior to 2004, the principal use of palladium in jewellery was the manufacture of white gold. Palladium is one of the three most popular alloying metals in white gold (nickel and silver can also be used).[49] Palladium-gold is more expensive than nickel-gold, but seldom causes allergic reactions (though certain cross-allergies with nickel may occur).[79]
When platinum became a strategic resource during World War II, many jewellery bands were made out of palladium. Palladium was little used in jewellery because of the technical difficulty ofcasting. With the casting problem resolved[80] the use of palladium in jewellery increased, originally because platinum increased in price whilst the price of palladium decreased.[81] In early 2004, when gold and platinum prices rose steeply, China began fabricating volumes of palladium jewellery, consuming 37tonnes in 2005. Subsequent changes in the relative price of platinum lowered demand for palladium to 17.4 tonnes in 2009.[82][83] Demand for palladium as a catalyst has increased the price of palladium to about 50% higher than that of platinum in January 2019.[84]
In January 2010,hallmarks for palladium were introduced by assay offices in the United Kingdom, and hallmarking became mandatory for all jewellery advertising pure or alloyed palladium. Articles can be marked as 500, 950, or 999 parts of palladium per thousand of the alloy.
In theplatinotype 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.[85] But palladium is more inert than the silver used insilver bromide prints, so such photographs are better archived than conventional prints and convey details more clearly.[86][87]
Palladium is a metal with low toxicity as conventionally measured (e.g.LD50). Recent research on the mechanism of palladium toxicity suggests high toxicity if measured on a longer timeframe and at the cellular level in the liver and kidney.[89] Mitochondria appear to have a key role in palladium toxicity via mitochondrial membrane potential collapse and depletion of the cellular glutathione (GSH) level. Until that recent work, it had been thought that palladium was poorly absorbed by thehuman body wheningested. Plants such as thewater hyacinth are killed by low levels of palladium salts, but most other plants tolerate it, although tests show that, at levels above 0.0003%, growth is affected. High doses of palladium could be poisonous; tests onrodents suggest it may becarcinogenic, though until the recent research cited above, no clear evidence indicated that the element harms humans.[90]
Like otherplatinum-group metals, bulk Pd is quite inert. Althoughcontact dermatitis has been reported, data on the effects are limited. It has been shown that people with an allergic reaction to palladium also react to nickel, making it advisable to avoid the use of dental alloys containing palladium on those so allergic.[91][92][93][94][95]
Some palladium is emitted with the exhaust gases of cars withcatalytic converters. Between 4 and 108 ng/km of palladium particulate is released by such cars, while the total uptake from food is estimated to be less than 2 μg per person a day. The second possible source of palladium is dental restoration, from which the uptake of palladium is estimated to be less than 15 μg per person per day. People working with palladium or its compounds might have a considerably greater uptake. For soluble compounds such aspalladium chloride, 99% is eliminated from the body within three days.[91]
William Hyde Wollaston noted thediscovery of a new noble metal in July 1802 in his lab book and named it palladium in August of the same year. He named the element after the asteroid2 Pallas, which had been discovered two months earlier (and which waspreviously considered a planet).[18] Wollaston purified a quantity of the material and offered it, without naming the discoverer, in a small shop inSoho in April 1803. After harsh criticism fromRichard Chenevix, who claimed that palladium was an alloy of platinum and mercury, Wollaston anonymously offered a reward of £20 for 20 grains of synthetic palladiumalloy.[96] Chenevix received theCopley Medal in 1803 after he published his experiments on palladium. Wollaston published the discovery ofrhodium in 1804 and mentions some of his work on palladium.[97][98] He disclosed that he was the discoverer of palladium in a publication in 1805.[96][99]
Palladium chloride was at one time prescribed as atuberculosis treatment at the rate of 0.065 g per day (approximately one milligram per kilogram of body weight). This treatment had many negativeside-effects, and was later replaced by more effective drugs.[100]
Most palladium is used forcatalytic converters in the automobile industry.[91] Catalytic converters are targets for thieves because they contain palladium and other rare metals. In the run up to year 2000, the Russian supply of palladium to the global market was repeatedly delayed and disrupted; for political reasons, the export quota was not granted on time.[101] The ensuing market panic drove the price to an all-time high of $1,340 pertroy ounce ($43/g) in January 2001.[102] Around that time, theFord Motor Company, fearing that automobile production would be disrupted by a palladium shortage, stockpiled the metal. When prices fell in early 2001, Ford lost nearlyUS$1 billion.[103]
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 theUnited States Geological Survey.[43] Many were concerned about a steady supply of palladium in the wake of Russia'sannexation of Crimea, partly as sanctions could hamper Russian palladium exports; any restrictions on Russian palladium exports could have exacerbated what was already expected to be a large palladium deficit in 2014.[104] Those concerns pushed palladium prices to their highest level since 2001.[105] In September 2014 they soared above the $900 per ounce mark. In 2016 however palladium cost around $614 per ounce as Russia managed to maintain stable supplies.[106] In January 2019 palladiumfutures climbed past $1,344 per ounce for the first time on record, mainly due to the strong demand from the automotive industry.[107] Palladium reached $2,024.64 per troy ounce ($65.094/g) on 6 January 2020, passing $2,000 per troy ounce the first time.[108] The price rose above $3,000 per troy ounce in May 2021 and March 2022.[109]
Global palladium sales were 8.84 million troy ounces (275 t) in 2017,[110] of which 86% was used in the manufacturing of automotive catalytic converters, followed by industrial, jewellery, and investment usages.[111] More than 75% of global platinum and 40% of palladium are mined inSouth Africa. Russia's mining company,Norilsk Nickel, produces another 44% of palladium, with US and Canada-based mines producing most of the rest.
The price for palladium reached an all-time high of $2,981.40 pertroy ounce on May 3, 2021,[112][113] driven mainly on speculation of thecatalytic converter demand from theautomobile industry. Over the following few years the price fell by over two-thirds. Palladium is traded in thespot market with the code "XPD". When settled in USD, the code is "XPDUSD". A later surplus of the metal was caused by theRussian government selling stockpiles from theSoviet era, at a rate of about 1.6 to 2 million troy ounces (50 to 62 t) a year. The amount and status of this stockpile are astate secret.
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