Physical properties
Palladium is the middle element in group 10 of the transition elements (periods 4, 5, and6). Many of its properties are similar to nickel located above it and platinum just below it inthis group.
Palladium is a soft, silvery-white metal whose chemical and physical properties closelyresemble platinum. It is mostly found with deposits of other metals. It is malleable and ductile,which means it can be worked into thin sheets and drawn through a die to form very thinwires. It does not corrode. Its melting point is 1,554°C, its boiling point is 3,140°C, and itsdensity is 12.02 g/cm3.
Isotopes
There are 42 isotopes of palladium, ranging from Pd-91 to Pd-124. All but sixare radioactive and artificially produced in nuclear reactors with half-lives ranging from159 nanoseconds to 6.5×10+6 years. The six stable isotopes of palladium and theirproportional contribution to their existence in the Earth’s crust are as follows: Pd-102 =1.02%, Pd-104 = 11.14%, Pd-105 = 22.23%, Pd-106 = 27.33%, Pd-108 = 26.46%,and Pd-110 = 11.72%.
Origin of Name
Palladium is named after the asteroid Pallas, which was discovered at
about the same time as the element. Pallas is the name of two mythological Greek figures,
one male and the other female.
Occurrence
Palladium is considered a rare metal, making up only about 1 part per 100 million partsof the Earth’s crust. It is considered the 77th most abundant element on Earth, although it is seldom found in pure states. Rather, it is mixed with other metals or in compounds of palladium.
It was originally found in gold ores from Brazil, where the miners thought the gold wascontaminated by what they referred to as “white gold.” Later, it was considered an alloy combinationof palladium and gold.
Deposits of ores containing palladium, as well as other metals, are found in Siberia and theUral Mountains of Russia, Canada, and South Africa, as well as in South America.
Characteristics
One of palladium’s unique characteristics is its ability to absorb 900 times its own volumeof hydrogen gas. When the surface of the pure metal is exposed to hydrogen gas (H2), the gasmolecules break into atomic hydrogen. These hydrogen atoms then seep into the holes in thecrystal structure of the metal. The result is a metallic hydride (PdH0.5) that changes palladiumfrom an electrical conductor to a semiconductor. The compound palladium dichloride (PdCl2)also has the ability to absorb large quantities of carbon monoxide (CO). These characteristicsare useful for many commercial applications. Palladium is the most reactive of all the platinumfamily of elements (Ru, Rh, Pd, Os, Is, and Pt.)
Application
Palladium is used in the preparation of Palladium catalysts. Palladium based nanomaterials can also be applied in cancer diagnosis and therapy.
Definition
ChEBI: Chemical element (nickel group element atom) with atomic number 46.
Definition
palladium: Symbol Pd. A soft whiteductile transition element (see alsoplatinum metals); a.n. 46; r.a.m.106.4; r.d. 12.02; m.p. 1552°C; b.p.3140±1°C. It occurs in some copperand nickel ores and is used in jewelleryand as a catalyst for hydrogenationreactions. Chemically, itdoes not react with oxygen at normaltemperatures. It dissolves slowly inhydrochloric acid. Palladium is capableof occluding 900 times its ownvolume of hydrogen. It forms fewsimple salts, most compounds beingcomplexes of palladium(II) with somepalladium(IV). It was discovered byWilliam Woolaston (1766–1828) in1803.
Production Methods
Palladium and platinum have been used since 1974 as
catalyst to oxidize carbon monoxide to carbon dioxide in
automobile engines and in hydrocarbon conversion. This use
of palladium results in palladium being emitted as solid
particles with automotive exhaust gases.
General Description
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Hazard
Palladium is not combustible except as fine powder or dust. Several of palladium’s compoundsare oxidizing agents, and some react violently with organic substances.
Flammability and Explosibility
Palladium on carbon catalysts containing adsorbed hydrogen are pyrophoric,
particularly when dry and at elevated temperatures. Palladium on carbon catalysts
prepared by formaldehyde reduction are less pyrophoric than those reduced with
hydrogen. Finely divided carbon, like most materials in powder form, is capable of
creating a dust explosion.
Industrial uses
A rare metal, palladium (symbol Pd) is foundin the ores of platinum. It resembles platinum,but is slightly harder and lighter in weight andhas a more beautiful silvery luster. It is onlyhalf as plentiful but is less costly. The specificgravity is 12.10 and the melting point is1552°C. Annealed, the metal has a hardness ofBrinell 40 and a tensile strength of 186 MPa.
It is highly resistant to corrosion and toattack by acids, but, like gold, it is dissolved inaqua regia. It alloys readily with gold and isused in some white golds. It alloys in all proportionswith platinum and the alloys are harderthan either constituent.
Palladium is soft and ductile and can be fabricatedinto wire and sheet. The metal forms ductilealloys with a broad range of elements. Palladiumis not tarnished by dry or moist air atordinary temperatures. At temperatures from350 to 790°C a thin protective oxide forms inair, but at temperatures from 790°C this filmdecomposes by oxygen loss, leaving the brightmetal. In the presence of industrial sulfur-containinggases a slight brownish tarnish develops;however, alloying palladium with small amountsof iridium or rhodium prevents this action.
The major applications of palladium are in theelectronics industry, where it is used as an alloywith silver for electrical contacts or in pastes inminiature solid-state devices and in integratedcircuits. Palladium is widely used in dentistryas a substitute for gold. Other consumer applicationsare in automobile exhaust catalysts andjewelry.
Environmental Fate
Routes and Pathways, and Relevant Physicochemicals
Properties
Melting point＝ 1554.9 °C
Boiling point＝ 2963 °C
Solubilities: soluble in aqua regia and fused alkalis; insoluble
in organic acids
Partition Behavior in Water, Sediment, and Soil
Palladium compounds can enter the environment during the
mining and production of platinum group metals and is usually
found with platinum. The mobility of palladium compounds in
soil, therefore, is similar to that of platinum compounds. In soil, mobility depends on low pH and high chloride concentration.
The reactions of palladium compounds in water are depending
on pH level, redox potential, and ligand availability. It usually
has low affinity for hard ligands, such as fluoride and oxygen.
Palladium compounds are ionic; therefore, volatilization from
water and soil surfaces is not expected to be an important
environmental fate process.
Environmental Persistency
Palladium occurs in the earth’s crust at the concentration of
w0.2 ppm. This group of metals can be transferred from the
earth’s interior, where they are mostly concentrated in high
volume, to the lithosphere by volcanic activity. Environmental
levels of palladium in water, soil, and ambient air are not high.
Palladium, as a metal, tends to persist in the upper soil layer.
Long-Range Transport
Transfer from water column to sediment is assumed through
exchange, complexation, or precipitation reactions.
Bioaccumulation and Biomagnifications
Palladium has low to moderate bioaccumulation in terrestrial
and aquatic organisms, respectively.
storage
In
particular, palladium on carbon should always be handled under an inert atmosphere
(preferably argon), and reaction vessels should be flushed with inert gas before the
catalyst is added. Dry catalyst should never be added to an organic solvent in the
presence of air. Palladium on carbon recovered from catalytic hydrogenation
reactions by filtration requires careful handling because it is usually saturated with
hydrogen and will ignite spontaneously on exposure to air. The filter cake should
never be allowed to dry, and the moist material should be added to a large quantity
of water and disposed of properly.
Toxicity evaluation
Palladium ions, because of their ability to form strong
complexes with both organic and inorganic ligands, have the
potential to disturb cellular equilibria and interact with
macromolecules functional groups, such as proteins or DNA/
RNA, which interrupt various cellular processes. Palladium and
its compounds also have the ability to inhibit many enzymatic
activities as shown in in vitro and in vivo studies. These enzymes
include prolyl hydroxylase, creatine kinase, aldolase, succinic
dehydrogenase, carbonic anhydrase, and alkaline phosphatase.
Palladium, furthermore, has the capability of occupying
different oxidation states, which in turn may have different
effects. The ability of Pd4+ ions to change their oxidation states
to Pd2+ may also contribute to harmful effects. Studies have
shown that exposure to Pd2+ may disturb the organisms’
energy metabolism, acid–base, and electrolytic equilibria. Also,
exposure to high concentration of palladium during developmental
period may retard fetal growth and development.
Incompatibilities
Catalysts prepared on high surface area supports are highly active and readily cause
ignition of hydrogen/air and solvent/air mixtures. Methanol is notable for easy
ignition because of its high volatility. Addition of catalyst to a tetrahydroborate
solution may cause ignition of liberated hydrogen.
Waste Disposal
Excess palladium on carbon and waste material containing this substance should be covered in water,
placed in an appropriate container, clearly labeled, and handled according to your institution's waste
disposal guidelines.
