Palladium

(chemistry) A chemical element, symbol Pd, atomic number 46, atomic weight 106.4.
(metallurgy) A white, ductile malleable metal that resembles platinum and follows it in abundance and importance of applications; does not tarnish at normal temperatures.

A chemical element, Pd, atomic number 46, and atomic weight 106.4. A transition metal, palladium occurs in combination with platinum (Pt) and is the second most abundant platinum-group metal, accounting for 38% of the reserves of these metals. See also Periodic table; Platinum.

Palladium is soft and ductile and can be fabricated into wire and sheet. The metal forms ductile alloys with a broad range of elements. Palladium is not tarnished by dry or moist air at ordinary temperatures. At temperatures from 350 to 790°C (660 to 1450°F) a thin protective oxide forms in air, but at temperatures from 790°C this film decomposes by oxygen loss, leaving the bright metal. In the presence of industrial sulfur-containing gases a slight brownish tarnish develops; however, alloying palladium with small amounts of iridium or rhodium prevents this action. Important physical properties of palladium are given in the table. See also Alloy; Metal.

At room temperature, palladium is resistant to nonoxidizing acids such as sulfuric acid, hydrochloric acid, hydrofluoric acid, and acetic acid. The metal is attacked by nitric acid, and a mixture of nitric acid and hydrochloric acid is a solvent for the metal. Palladium is also attacked by moist chlorine (Cl) and bromine (Br). See also Nonstoichiometric compounds.

The major applications of palladium are in the electronics industry, where it is used as an alloy with silver for electrical contacts or in pastes in miniature solid-state devices and in integrated circuits. Palladium is widely used in dentistry as a substitute for gold. Other consumer applications are in automobile exhaust catalysts and jewelry.

Palladium supported on carbon or alumina is used as a catalyst for hydrogenation and dehydrogenation in both liquid- and gas-phase reactions. Palladium finds widespread use in catalysis because it is frequently very active under ambient conditions, and it can yield very high selectivities. Palladium catalyzes the reaction of hydrogen with oxygen to give water. Palladium also catalyzes isomerization and fragmentation reactions. See also Catalysis.

Halides of divalent palladium can be used as homogeneous catalysts for the oxidation of olefins (Wacker process). This requires water for the oxygen transfer step, and a copper salt to reoxidize the palladium back to its divalent state to complete the catalytic cycle. See also Homogeneous catalysis; Transition elements.

Pallādium, an image of the Greek goddess Pallas Athena, identified by the Romans with Minerva, said to have been sent down from heaven by Zeus to Dardanus, the founder of Troy (or to his son Ilus). Since Troy could not be captured while it contained this image, legend related that the Greek heroes Diomedēs and Odysseus carried it off and thus made it possible for the Greeks to sack Troy. It is variously said to have found its way to Athens, or Argos, or Sparta, or Rome. The Romans believed that an image in the temple of Vesta, thought to have saved Rome from the attack of the Gauls in 390 BC, was the Palladium, either brought safely to Rome by the Trojan Aeneas after the sack (Diomedes having succeeded in stealing merely a copy) or else surrendered by Diomedes. When the temple of Vesta caught fire in 241 BC the image was rescued by the pontifex L. Metellus at the cost of his eyesight. Whatever its origin, it seems clear that such a talismanic object was kept in Rome. For the Palladium at Athens see ERECHTHEUM.