Palladium

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This article is about the chemical element. For other uses, see Palladium (disambiguation).

Palladium, 46Pd

General properties

Pronunciation /p le di m/

p -LAY-dee- m

Appearance silvery white

Standard atomic weight (Ar, standard) 106.42(1)[1]

Palladium in the periodic table

H H
yd eli
ro u
ge m N
n i
Li Be Bo Ca Ni Ox Fl N
thi ryl ro rb tro yg uo eo P
u liu n on ge en rin n d
m m n e
So M Al Sil Ph Sul Ch Ar P
di ag u ico os fur lor go t
u ne mi n ph in n
m siu ni or e
m us
 u
m
Po Ca Sc Tit V Ch M Iro Co Nic Co Zi Ga Ge Ar Sel Br Kr
ta lci an ani an ro an n bal kel pp nc lli rm se eni o yp
ssi u di um ad mi ga t er u ani nic um mi to
u m u iu um ne m u ne n
m m m se m
R Str Yt Zir Ni Mo Te Ru Rh Pal Sil Ca In Ti An Tel Io X
ub on tri con ob lyb ch th od lad ver dm di n ti luri di en
idi tiu u ium iu de net en iu iu iu u m um ne on
u m m m nu iu iu m m m on
m
m m m m y
C Ba La C Pra Ne Pr Sa Eu Ga Te Dy Ho Er Th Yt Lu Haf Ta Tu Rh Os Iri Pla Go Me Th Le Bi Pol As Ra
ae riu nt er seo od om m ro do rbi spr lm bi uli ter teti niu nt ngs eni mi di tin ld rcu all ad sm oni tat do
si m ha iu dy ym eth ari pi lin u osi iu u um bi um m al ten u u u um ry iu ut um in n
u nu m miu iu iu u u iu m um m m u u m m m (el m h e
m m m m m m m m m m em
ent
)
Fr Ra Ac T Pro Ur Ne Pl A Cu Be Ca Ei F Me N La Rut D Se Bo Ha M Dar Ro Co Ni Fl M Liv Te O
an di tin h tact ani ptu ut m riu rk lif nst er nde ob wr her ub ab hri ssi eit mst ent per ho er os er nn ga
ci u iu or iniu um niu on eri m eli or ein m lev eli en for ni org u u ne adti ge nic ni ov co mo es ne
u m m iu m m iu ci u niu iu iu iu u ciu diu u iu m m riu um niu iu u iu vi riu sin ss
m m m u m m m m m m m m m m m m m m m u m e on
m m
rhodium palladium silver

Atomic number (Z) 46

Group, period group 10, period 5

Block d-block

Element category transition metal

Electron configuration [Kr] 4d10

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 (near r.t.) 12.023 g/cm3

when liquid (at m.p.) 10.38 g/cm3

Heat of fusion 16.74 kJ/mol

Heat of vaporization 358 kJ/mol

Molar heat capacity 25.98 J/(molK)

Vapor pressure

P (Pa) 1 10 100 1k 10 k 100 k

at T (K) 1721 1897 2117 2395 2753 3234

Atomic properties

Oxidation states 0, +1, +2, +3, +4 (a mildly basic oxide)

Electronegativity Pauling scale: 2.20

1st: 804.4 kJ/mol

Ionization energies
2nd: 1870 kJ/mol

3rd: 3177 kJ/mol

Atomic radius empirical: 137 pm

Covalent radius 1396 pm

Van der Waals radius 163 pm

Spectral lines

Miscellanea
Crystal structure face-centered cubic (fcc)

Speed of soundthin rod 3070 m/s (at 20 C)

Thermal expansion 11.8 m/(mK) (at 25 C)

Thermal conductivity 71.8 W/(mK)

Electrical resistivity 105.4 nm (at 20 C)

Magnetic ordering paramagnetic[2]

Magnetic susceptibility +567.410 6 cm3/mol (288 K)[3]

Young's modulus 121 GPa

Shear modulus 44 GPa

Bulk modulus 180 GPa

Poisson ratio 0.39

Mohs hardness 4.75

Vickers hardness 400600 MPa

Brinell hardness 320610 MPa

CAS Number 7440-05-3

History

Naming after asteroid Pallas, itself named after Pallas Athena

Discovery and first isolation William Hyde Wollaston(1803)

Main isotopes of palladium

Iso- Abun- Half- Decay Pro-

tope dance life(t1/2) mode duct
Rh
100
100
Pd syn 3.63 d

102
Pd 1.02% stable
103
Pd syn 16.991 d Rh
103

104
Pd 11.14% stable
105
Pd 22.33% stable
 106
Pd 27.33% stable
107
Pd trace 6.5106 y Ag
107

108
Pd 26.46% stable
110
Pd 11.72% stable

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Palladium is a chemical element with symbol Pd and atomic number 46. It is a rare and lustrous
silvery-white metal 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.
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 less noxious substances (nitrogen, carbon dioxide and water
vapor). Palladium is also used in electronics, dentistry, medicine, hydrogen purification, chemical
applications, groundwater treatment, and jewelry. Palladium is a key component of fuel cells, which
react hydrogen with oxygen 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 investmentinterest.

Contents
[hide]

1Characteristics
o 1.1Isotopes
2Compounds
o 2.1Palladium(II)
o 2.2Palladium(0)
o 2.3Other oxidation states
3History
4Occurrence
5Applications
o 5.1Catalysis
o 5.2Electronics
o 5.3Technology
o 5.4Hydrogen storage
o 5.5Dentistry
 o 5.6Jewelry
o 5.7Photography
6Toxicity
7Precautions
8See also
9References
10External links

Characteristics[edit]
Palladium belongs to group 10 in the periodic table, but the configuration in the outermost electron
shells is atypical for group 10 (see also niobium (41), ruthenium (44), and rhodium (45)). Fewer
electron shells are filled than the elements directly preceding it (a phenomenon unique to palladium).
The valence shell has eighteen electrons ten more than the eight found in the valence shells of
the noble gases from neon onward.

Z Element No. of electrons/shell

28 nickel 2, 8, 16, 2 (or 2, 8, 17, 1)

46 palladium 2, 8, 18, 18

78 platinum 2, 8, 18, 32, 17, 1

110 darmstadtium 2, 8, 18, 32, 32, 16, 2 (predicted)

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 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.[4] It dissolves readily at room temperature in aqua regia.
Palladium does not react with oxygen at standard temperatures (and thus does not tarnish in air).
Palladium heated to 800 C will produce a layer of palladium(II) oxide (PdO). It tarnishes lightly in a
moist atmosphere containing sulfur.[clarification needed][5]
Palladium films with defects produced by alpha particle bombardment at low temperature exhibit
superconductivity having Tc=3.2 K.[6]
Isotopes[edit]
Main article: Isotopes of palladium
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 90.94948(64) u (91Pd) to 122.93426(64) u (123Pd).[7] 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).[8]
For isotopes with atomic mass unit values 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.[8]
Radiogenic 107Ag is a decay product of 107Pd and was first discovered in 1978[9] in the Santa
Clara[10] 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.[11]

Compounds[edit]
See also: Category:Palladium compounds.
Palladium compounds primarily exist 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.

Structure of -PdCl2 Structure of -PdCl2

Palladium(II)[edit]
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.[12] Solutions of PdCl2 in nitric acid react with acetic acid to give palladium(II) acetate, also
a versatile reagent. PdCl2 reacts with ligands (L) to give square planar complexes of the type
PdCl2L2. One example of such complexes is the benzonitrile derivative PdX2(PhCN)2.[13][14]
PdCl2 + 2 L PdCl2L2 (L = PhCN, PPh3, NH3, etc)
The complex bis(triphenylphosphine)palladium(II) dichloride is a useful catalyst.[15]

Palladium(II) acetate

Palladium(0)[edit]
Palladium forms a range of zerovalent complexes with the formula PdL4, PdL3, and PdL2. For
example, reduction of a mixture of PdCl2(PPh3)2 and
PPh3 gives tetrakis(triphenylphosphine)palladium(0):[16]
2 PdCl2(PPh3)2 + 4 PPh3 + 5 N2H4 2 Pd(PPh3)4 + N2 + 4 N2H5+Cl
Another major palladium(0)
complex, tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3), is prepared by
reducing sodium tetrachloropalladate in the presence of dibenzylideneacetone.[17]
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, Stille reactions, and
the Kumada coupling. Palladium(II)
acetate, tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4,
and tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) serve either as catalysts or
precatalysts.[18]
Other oxidation states[edit]
Although Pd(IV) compounds are comparatively rare, one example is sodium
hexachloropalladate(IV), Na2[PdCl6]. A few compounds of palladium(III) are also
known.[19]Palladium(VI) was first observed in 2002.[20][21]
Mixed valence palladium complexes exist, e.g. Pd4(CO)4(OAc)4Pd(acac)2 forms an
infinite Pd chain structure, with alternatively interconnected Pd4(CO)4(OAc)4 and
Pd(acac)2units.[22]

History[edit]

William Hyde Wollaston

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. Wollaston purified enough
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 that palladium is an alloy of
platinum and mercury, Wollaston anonymously offered a reward of 20 British pounds for
20 grains of synthetic palladium alloy.[23] 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.[24][25] He disclosed that
he was the discoverer of palladium in a publication in 1805.[23][26]
It was named by Wollaston in 1802 after the asteroid 2 Pallas, which had been
discovered two months earlier.[4] 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.[24]
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.[27]
Most palladium is used for catalytic converters in the automobile industry.[28] In the run
up to year 2000, the Russian supply of palladium to the global market was repeatedly
delayed and disrupted[29] because for political reasons, the export quota was not granted
on time. The ensuing market panic drove the price to an all-time high of $1100 per troy
ounce in January 2001.[30] 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.[31]
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.[32] Many are concerned about a steady supply of
palladium in the wake of Russia's military maneuvers in Ukraine, partly as sanctions
could hamper Russian palladium exports; any restrictions on Russian palladium exports
would exacerbate what is already expected to be a large palladium deficit in 2014.[33][needs
update]

Occurrence[edit]

Palladium output in 2005

With a 44% world share of palladium in 2007, Russia was the top producer, followed by
South Africa with 40%. Canada with 6% and the U.S. with 5% are the other substantial
producers of palladium.[32][34]
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 NorilskTalnakh 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 les igneous complex of Ontario are the two other sources of
palladium in Canada and the United States.[32][34] Palladium is found in the rare
minerals cooperite[35] and polarite.[36]
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.[37]

Applications[edit]
 Cross section of a metal-core catalytic converter

The Soviet 25-rouble commemorative palladium coin is a rare example of the monetary
usage of palladium.

The largest use of palladium today is in catalytic converters.[38] Palladium is also used in
jewelry, dentistry,[38][39] watch making, blood sugar test strips, aircraft spark
plugs, surgical instruments, and electrical contacts.[40] Palladium is also used to make
professional transverse (concert or classical) flutes.[41] 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.[42] Because it
absorbs hydrogen, palladium is a key component of the controversial cold
fusion experiments that began in 1989.