Palladium (Pd)

Palladium is a chemical element with symbol Pd and atomic number 46. A rare silver-white transition metal of the platinum group, palladium resembles platinum chemically and is extracted from some copper and nickel ores. It is primarily used as an industrial catalyst and in jewelry.      

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Notable Characteristics

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 greatly increases its strength and hardness when it is cold-worked. Palladium is chemically attacked by sulfuric and nitric acid but dissolves slowly in hydrochloric acid. This metal also does not react with oxygen at normal temperatures (and thus does not tarnish in air).


This metal has the uncommon ability to absorb up to 900 times its own volume of hydrogen at room temperatures. It is thought that this possibly forms palladium hydride (PdH2) but it is not yet clear if this is a true chemical compound.


Common oxidation states of palladium are 0,+1, +2 and +4. Although originally +3 was thought of as one of the fundamental oxidation states of palladium, there is no evidence for palladium occurring in the +3 oxidation state; this has been investigated via X-ray diffraction for a number of compounds, indicating a dimer of palladium(II) and palladium(IV) instead. Recently, compounds with an oxidation state of +6 were synthesised.

 

Applications

When it is finely divided, palladium forms a good catalyst and is used to speed up hydrogenation and dehydrogenation reactions, as well as in petroleum cracking. A large number of reactions in organic chemistry are facilitated by palladium catalysis, many of which involve carbon-carbon bond formation. It is also alloyed and used in jewelry. Other uses;

  • The largest use of palladium today is in catalytic converters. Much research is in progress to discover ways to replace the much more expensive platinum with palladium in this application.
  • Palladium is one of the two metals which can be alloyed with gold to produce White gold. (Nickel can also be used.)
  • Similar to gold, palladium can be beaten into a thin leaf form as thin as 100 nm (1/250,000 in).
  • Since 1939 palladium itself has occasionally been used as a precious metal in jewelry, often as a replacement for platinum.
  • Hydrogen easily diffuses through heated palladium; thus, it provides a means of purifying the gas.
  • Telecommunications switching-system equipment uses palladium.
  • Palladium is also used in dentistry, watch making, in aircraft spark plugs and in the production of surgical instruments and electrical contacts.
  • It is also used as Palladium-Hydrogen electrode in electrochemical studies.
  • Palladium dichloride can absorb large amounts of carbon monoxide gas and is used in carbon monoxide detectors

 

History

Palladium was discovered by William Hyde Wollaston in 1803. This element was named by Wollaston in 1804 after the asteroid Pallas, which was discovered two years earlier.


Wollaston found element 46 in crude platinum ore from South America. He did this by dissolving the ore in aqua regia, neutralizing the solution with sodium hydroxide, NaOH, precipitating platinum as ammonium chloroplatinate through treatment with ammonium chloride, NH4Cl, and then adding mercuric cyanide to form the compound palladium cyanide. Finally, he heated the resulting compound in order to extract palladium metal.


The compound palladium chloride was at one time prescribed as a tuberculosis treatment at the rate of 0.065g per day (approximately one milligram per kilogram of body weight). This treatment did not have many negative side effects, but was later replaced by more effective drugs.


The element played an essential role in the Fleischmann-Pons experiment, also known as cold fusion.


In 2000, Ford Motor Company created a price bubble in palladium by stockpiling large amounts of the metal, fearing interrupted supplies from Russia. As prices fell in early 2001, Ford lost nearly $1 billion U.S. dollars.

 

Occurrence

Palladium is found as a free metal and alloyed with platinum and gold with platinum group metals in placer deposits of the Ural Mountains, Australia, Ethiopia, South and North America. However it is commercially produced from nickel-copper deposits found in South Africa and Ontario; the huge volume of ore processed makes this extraction profitable in spite of the low proportion of palladium in these ores.


About the possibility of producing palladium in reactors or extracting it from spent nuclear fuel, see Synthesis of noble metals.

 

Isotopes

Naturally-occurring palladium is composed of six isotopes. The most stable radioisotopes are 107Pd with a half-life of 6.5 million years, 103Pd with a half-life of 17 days, and 100Pd with a half-life of 3.63 days. Eighteen other radioisotopes have been characterized with atomic weights ranging from 92.936 u (93Pd) to 119.924 u (120Pd). Most of these have half-lifes that are less than a half an hour except 101Pd (half-life: 8.47 hours), 109Pd (half-life: 13.7 hours), and 112Pd (half-life: 21 hours).


The primary decay mode before the most abundant stable isotope, 106Pd, is electron capture and the primary mode after is beta decay. The primary decay product before 106Pd is rhodium and the primary product after is silver.


Radiogenic 107Ag is a decay product of 107Pd and was first discovered in the Santa Clara, California meteorite of 1978. 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 clearly been melted since accretion of the solar system, must reflect the presence of short-lived nuclides in the early solar system.

 

References

Los Alamos National Laboratory – Palladium

Original article at http://en.wikipedia.org/wiki/Palladium