Palladium

Palladium is a rare platinum group metal with a remarkable ability to absorb enormous quantities of hydrogen gas, up to 900 times its own volume. It is a key component of catalytic converters, a catalyst in pharmaceutical manufacturing, and was briefly used in the controversial claim of "cold fusion" in 1989.

• Atomic Number4646 protons, 46 electrons
• Atomic Mass106.42 u46× heavier than hydrogen
• State at Room TempSolidSolid
• Density12.0 g/cm³
• Melting / Boiling1554.9°C / 2962.8°C
• Discovered1803

What is Palladium?

Palladium is a transition metal in Group 10 of the periodic table, sitting below nickel. With 46 protons, it has a bright silver appearance and is the least dense and lowest-melting of the platinum group metals. It is remarkably good at forming coordination complexes with organic molecules, which is why palladium catalysts are so useful in making pharmaceuticals and fine chemicals.

Palladium gets its name from the asteroid Pallas, discovered just before the element in 1802. The asteroid itself was named after Pallas Athena, the Greek goddess of wisdom. William Hyde Wollaston discovered palladium in 1803 while processing platinum ore and chose to name it after the recently discovered asteroid, an unusual choice that was briefly mocked by sceptics who doubted the discovery before it was confirmed.

Fact Palladium can absorb up to 900 times its own volume of hydrogen at room temperature, with hydrogen atoms fitting into spaces between palladium atoms in the metal lattice. This remarkable property sparked the controversial "cold fusion" claim in 1989 and is now studied for hydrogen storage in future fuel cell vehicles.

Where you find Palladium

In space

Palladium is produced in stellar nucleosynthesis and is found in the Sun.

On Earth

Palladium is one of the rarest elements in the Earth's crust, just approx. 0.015 parts per million.

• Platinum ores. Palladium occurs with platinum, rhodium and other PGMs in sulfide nickel-copper ores. Russia's Norilsk deposits and South Africa's Bushveld complex supply most of the world's palladium.
• By-product of nickel refining. Palladium is recovered from the anode slime produced during electrolytic nickel refining.

How we use Palladium

• Catalytic converters.. Palladium, with platinum, oxidises CO and hydrocarbons in catalytic converters. Palladium demand has surged with the shift from diesel (platinum-heavy) to petrol (palladium-heavy) cars.
• Pharmaceutical synthesis.. Palladium catalysts enable "cross-coupling" reactions that are essential for making complex drug molecules. The 2010 Nobel Prize in Chemistry was awarded for palladium-catalysed cross-coupling.
• Hydrogen purification.. Palladium membranes allow only hydrogen to pass through (other gases are blocked), enabling ultra-pure hydrogen production for fuel cells and semiconductor manufacture.
• Dentistry.. Palladium alloys are used in dental crowns and bridges as alternatives to gold alloys.

Did you know? In 2010, Richard Heck, Ei-ichi Negishi and Akira Suzuki shared the Nobel Prize in Chemistry for palladium-catalysed cross-coupling reactions. These reactions, Heck coupling, Negishi coupling and Suzuki coupling, allow chemists to join carbon atoms together with great precision. They are used to make about a third of all pharmaceutical drugs and are among the most widely used reactions in modern synthetic chemistry.

How it was discovered

Palladium was discovered in 1803 by William Hyde Wollaston while analysing platinum ore. He initially tried to sell it anonymously under the name "new silver" before formally announcing it. The announcement was greeted with scepticism by some chemists who doubted the claim, but Wollaston provided the pure element as proof.

Deeper dive: palladium chemistry and applications

Palladium-catalysed cross-coupling reactions work by forming temporary palladium-carbon bonds that allow two organic molecules to be joined together. The palladium catalyst cycles through oxidation states (+0 to +2) as it facilitates the bond formation, then returns to its original state ready for another cycle. This catalytic cycle is extraordinarily efficient, tiny amounts of palladium can facilitate millions of reactions. The Suzuki coupling is particularly widely used, joining aryl halides with boronic acids to make biaryl compounds found in many drugs, liquid crystal displays and conducting polymers.

Moving to 47 protons on the periodic table brings us to Silver.