Palladium (Pd)

Stable isotopes of palladium available from ISOFLEX

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Palladium was discovered in 1803 by William Hyde Wollaston. It is named for the asteroid Pallas, which was discovered at about the same time, as well as for the Greek name Pallas, goddess of wisdom.

A silver-white, ductile metal, palladium has a face-centered cubic crystalline structure and does not tarnish in air. It is the least noble (most reactive) of the platinum group and can absorb up to 800 times its own volume of hydrogen. Upon doing so, the metal swells, becoming brittle and cracked. Such absorption of hydrogen decreases the electrical conductivity of the metal. Also, such absorption activates molecular hydrogen, dissociating it to atomic hydrogen. It is attacked by hot, concentrated nitric acid and boiling sulfuric acid. It is soluble in aqua regia, fused alkalis, hot nitric acid and boiling sulfuric acid, and insoluble in organic acids and water. It is a good electrical conductor. It is nontoxic and noncombustible, except as dust. The metal forms mostly bivalent compounds, although a small number of tetravalent and fewer trivalent compounds are known. Palladium exhibits a strong tendency to form complexes, and it dissolves more oxygen in its molten state than in solid form. Palladium reacts with fluorine and chlorine at 500 ºC, forming its halides. Hydrochloric acid has no effect on the metal.

One of the most important applications of palladium is to catalyze hydrogenation, dehydrogenation and petroleum cracking. Such reactions are widely employed in organic syntheses and petroleum refining. Palladium and platinum are installed in catalytic converters in automobiles to cut down the emission of unsaturated hydrocarbon gases.

Properties of Palladium