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= Palladium =
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Introduction
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Palladium is a chemical element; it has 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 (formally 2 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 (PGMs). 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 (hydrocarbons, carbon monoxide,
and nitrogen dioxide) into nontoxic substances (nitrogen, carbon
dioxide and water vapor). 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 PGMs 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
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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 5's' instead fill the 4'd' 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 (or 2, 8, 17, 1)
46 palladium 2, 8, 18, 18, 0
78 platinum 2, 8, 18, 32, 17, 1
110 darmstadtium 2, 8, 18, 32, 32, 16, 2 (predicted)
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 (and thus
does not tarnish in air). 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 having 'T'c = 3.2 K.
Isotopes
==========
Naturally occurring palladium is composed of seven isotopes, six of
which are stable. The most stable radioisotopes are 107Pd with a
half-life of 6.5 million years (found in nature), 103Pd with 17 days,
and 100Pd with 3.63 days. Eighteen other radioisotopes have been
characterized with atomic weights ranging from (91Pd) to (123Pd).
These have half-lives of less than thirty minutes, except 101Pd
(half-life: 8.47 hours), 109Pd (half-life: 13.7 hours), and 112Pd
(half-life: 21 hours).
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 Da 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 bodies, which have been
melted since accretion of the Solar System, 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 less concerning of the
long-lived fission products.
Compounds
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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.
File:Alpha-palladium(II)-chloride-xtal-3D-balls.png|Structure of
'α'-PdCl2
File:Pd6Cl12-from-xtal-1996-CM-3D-ellipsoids.png|
Palladium(II)
===============
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. Solutions of in nitric acid react with acetic
acid to give palladium(II) acetate, also a versatile reagent. reacts
with ligands (L) to give square planar complexes of the type . One
example of such complexes is the benzonitrile derivative
Bis(benzonitrile)palladium dichloride.
: (L = PhCN, PPh3, NH3, etc.)
The complex bis(triphenylphosphine)palladium(II) 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
tetrakis(triphenylphosphine)palladium(0):
:
Another major palladium(0) complex,
tris(dibenzylideneacetone)dipalladium(0) (), is prepared by reducing
sodium tetrachloropalladate in the presence of dibenzylideneacetone.
Palladium(0), as well as palladium(II), 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(II) acetate,
tetrakis(triphenylphosphine)palladium(0) (), and
tris(dibenzylideneacetone)dipalladium(0) () serve either as catalysts
or precatalysts.
Other oxidation states
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Although Pd(IV) compounds are comparatively rare, one example is
sodium hexachloropalladate(IV), . A few compounds of palladium(III)
are also known. Palladium(VI) was claimed in 2002, but subsequently
disproven.
Mixed valence palladium complexes exist, e.g. forms an infinite Pd
chain structure, with alternatively interconnected and Pd(acac)2
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
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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 (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. 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
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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 (concert or classical) 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:
* Heck reaction
* Suzuki coupling
* Tsuji-Trost reactions
* Wacker process
* Negishi reaction
* Stille coupling
* Sonogashira coupling
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.
Electronics
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The primary application of palladium in electronics is in multi-layer
ceramic capacitors in which palladium (and palladium-silver alloy) is
used for electrodes. Palladium (sometimes alloyed with nickel) is or
can be used for component and connector plating in consumer
electronics and in soldering materials. The electronic sector consumed
1.07 e6ozt of palladium in 2006, according to a Johnson Matthey
report. Palladium is used in the production of printed circuit boards.
Technology
============
Hydrogen easily diffuses through heated palladium, and membrane
reactors with Pd membranes are used in the production of high purity
hydrogen. Palladium is used in palladium-hydrogen electrodes in
electrochemical studies. Palladium(II) chloride readily catalyzes
carbon monoxide gas to carbon dioxide and is useful in carbon monoxide
detectors.
Palladium has been used to produce metallic glass by fast cooling
alloys, avoiding their crystallisation, thus reducing brittleness and
leading to stronger materials.
Hydrogen storage
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Palladium readily adsorbs hydrogen at room temperatures, forming
palladium hydride PdHx with x less than 1. 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. 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. The content of
hydrogen in palladium can be linked to magnetic susceptibility, which
decreases with the increase of hydrogen and becomes zero for PdH0.62.
At any higher ratio, the solid solution becomes diamagnetic.
Palladium is used for purification of hydrogen on a laboratory but not
industrial scale.
Medicine
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Palladium is used in small amounts (about 0.5%) in some alloys of
dental amalgam to decrease corrosion and increase the metallic lustre
of the final restoration. Palladium is also used in the production of
pacemakers.
Jewellery
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Palladium has been used as a precious 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 require rhodium
plating. Palladium, being much less dense than platinum, is similar to
gold in that it can be beaten into leaf as thin as 100 nm ( in).
Unlike platinum, palladium may discolor at temperatures above 400 C
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.
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).
Palladium-gold is more expensive than nickel-gold, but seldom causes
allergic reactions (though certain cross-allergies with nickel may
occur).
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 of casting. With the
casting problem resolved the use of palladium in jewellery increased,
originally because platinum increased in price whilst the price of
palladium decreased. In early 2004, when gold and platinum prices rose
steeply, China began fabricating volumes of palladium jewellery,
consuming 37 tonnes in 2005. Subsequent changes in the relative price
of platinum lowered demand for palladium to 17.4 tonnes in 2009.
Demand for palladium as a catalyst has increased the price of
palladium to about 50% higher than that of platinum in January 2019.
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.
Fountain pen nibs made from gold are sometimes plated with palladium
when a silver (rather than gold) appearance is desired. Sheaffer has
used palladium plating for decades, either as an accent on otherwise
gold nibs or covering the gold completely.
Palladium is also used by the luxury brand Hermès as one of the metals
plating the hardware on their handbags, the most famous of which is
Birkin.
Photography
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In 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. But
palladium is more inert than the silver used in silver bromide prints,
so such photographs are better archived than conventional prints and
convey details more clearly.
Art
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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 owing 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 silver leaf with similar working properties, but it
is not as commercially available on demand in leaf form.
-->
Cold fusion
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Palladium plays an important role in the ongoing research into
cold-fusion energy.
Super tough metallic glass
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Research is being done to develop metallic glass as a microalloy
featuring palladium, a metal with a high "bulk-to-shear" stiffness
ratio that counteracts the intrinsic brittleness of glassy materials.
The initial samples of the new metallic glass were microalloys of
palladium with phosphorous, silicon and germanium that yielded glass
rods approximately one millimeter in diameter. Adding silver to the
mix enabled the Cal Tech researchers to expand the thickness of the
glass rods to six millimeters.
Alternatives
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Pseudo palladium (RhAg) is a binary alloy consisting of equal parts of
rhodium (atomic number 45) and silver (atomic number 47). This alloy
exhibits properties of palladium (atomic number 46).
-->
Toxicity
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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. 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 the human body when ingested. Plants such as the water
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
on rodents suggest it may be carcinogenic, though until the recent
research cited above, no clear evidence indicated that the element
harms humans.
Precautions
=============
Like other platinum-group metals, bulk Pd is quite inert. Although
contact 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.
Some palladium is emitted with the exhaust gases of cars with
catalytic 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 as palladium
chloride, 99% is eliminated from the body within three 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.
History
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William Hyde Wollaston noted the discovery 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 asteroid 2 Pallas, which had been
discovered two months earlier (and which was previously considered a
planet). Wollaston purified a quantity of the material and offered it,
without naming the discoverer, in a small shop in Soho in April 1803.
After harsh criticism from Richard Chenevix, who claimed that
palladium was an alloy of platinum and mercury, Wollaston anonymously
offered a reward of £20 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 that
he was the discoverer of palladium in a publication in 1805.
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(II) 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.
Most palladium is used for catalytic converters in the automobile
industry. 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. The ensuing market panic drove the price to an
all-time high of 1340 $/ozt in January 2001. Around that time, the
Ford Motor Company, fearing that automobile production would be
disrupted by a palladium shortage, stockpiled the metal. 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.
Many were concerned about a steady supply of palladium in the wake of
Russia's annexation 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. Those concerns pushed palladium prices to
their highest level since 2001. 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. In January
2019 palladium futures climbed past $1,344 per ounce for the first
time on record, mainly due to the strong demand from the automotive
industry. Palladium reached 2024.64 $/ozt on 6 January 2020, passing
$2,000 per troy ounce the first time. The price rose above $3,000 per
troy ounce in May 2021 and March 2022.
Palladium as investment
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Global palladium sales were 8.84 e6ozt in 2017, of which 86% was used
in the manufacturing of automotive catalytic converters, followed by
industrial, jewellery, and investment usages. More than 75% of global
platinum and 40% of palladium are mined in South 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 per troy
ounce on May 3, 2021, driven mainly on speculation of the catalytic
converter demand from the automobile industry. Palladium is traded in
the spot market with the code "XPD". When settled in USD, the code is
"XPDUSD". A later surplus of the metal was caused by the Russian
government selling stockpiles from the Soviet era, at a rate of about
1.6 to a year. The amount and status of this stockpile are a state
secret.
During the Russo-Ukrainian War in March 2022, prices for palladium
increased 13%, since the first of March. Russia is the primary
supplier to Europe and the country supplies 37% of the global
production.
Palladium producers
=====================
* Norilsk Nickel
* Sibanye-Stillwater
* Anglo American Platinum
* Impala Platinum
* Northam Platinum
Exchange-traded products
==========================
WisdomTree Physical Palladium () is backed by allocated palladium
bullion and was the world's first palladium ETF. It is listed on the
London Stock Exchange as PHPD, Xetra Trading System, Euronext and
Milan. ETFS Physical Palladium Shares () is an ETF traded on the New
York Stock Exchange.
Bullion coins and bars
========================
A traditional way of investing in palladium is buying bullion coins
and bars made of palladium. Available palladium coins include the
Canadian Palladium Maple Leaf, the Chinese Panda, and the American
Palladium Eagle. The liquidity of direct palladium bullion investment
is poorer than that of gold, platinum, and silver because there is a
lower circulation of palladium coins than the big three precious
metals.
See also
======================================================================
* 2000s commodities boom
* 2020s commodities boom
* Bullion
* Bullion coin
* Inflation hedge
* Pseudo palladium
* Rare materials as an investment:
** Silver as an investment
** Gold as an investment
** Platinum as an investment
** Diamonds as an investment
External links
======================================================================
* [
http://www.periodicvideos.com/videos/046.htm Palladium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
https://www.kitco.com/price/precious-metals/palladium Current and
Historical Palladium Price]
*
License
=========
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License URL:
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Original Article:
http://en.wikipedia.org/wiki/Palladium
