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Facts About Palladium

Palladium
Palladium
Credit: Andrei Marincas | Shutterstock

Shiny, silvery palladium is the element that gives white gold its color and a crucial component of fuel cells and catalytic converters.

Discovered in 1803 and named after an asteroid, palladium is more rare than gold or platinum. The only palladium mine in the United States is the Stillwater Mine in Montana; other mines are in Canada, South Africa and Russia, the world's largest palladium producer.

Palladium is one of six elements in the platinum group, along with platinum, rhodium, ruthenium, osmium and iridium. These metals are known for being excellent catalysts, or substances that speed up chemical reactions.

On Aug. 18, 2014, palladium's price hit $900 per troy once, the highest seen since 2001. That year, political instability in Russia sent palladium prices to $1,125 per troy ounce, according to the Wall Street Journal.

Just the facts

  • Atomic Number (number of protons in the nucleus): 46
  • Atomic Symbol (on the Periodic Table of Elements): Pd
  • Atomic Weight (average mass of the atom): 106.4
  • Density: 12.02 grams per cubic centimeter
  • Phase at Room Temperature: Solid
  • Melting Point: 2,831 degrees Fahrenheit (1,555 degrees Celsius)
  • Boiling Point: 5,360 degrees F (2,960 degrees C)
  • Number of isotopes (atoms of the same element with a different number of neutrons): 29, and six are stable
  • Most common isotopes: Pd-102, Pd-104, Pd-105, Pd-106, Pd-108, Pd-110

Rare metal

Palladium was first isolated in 1803 by English chemist William Hyde Wollaston, who would go on to discover another element, rhodium, shortly thereafter. Wollaston discovered palladium by dissolving platinum in a mixture of nitric acid and hydrochloric acid and noticing that a second element was left behind, according to the Royal Society of Chemistry (RSC).

Instead of simply reporting his find, Wollaston made a game of it: He put some up for sale with a London mineral dealer and advertised the new metal's properties on handbills posted around the city, according to Chemicool. Another chemist, Richard Chenevix, declared the new metal to be a hoax, but Wollaston got the last laugh. In 1805, he described palladium to the Royal Society of London, revealing himself as the element's discoverer only at the end of the speech.

Palladium gets its name from Pallas, the second-largest asteroid in the asteroid belt, which had just been discovered in 1802. The asteroid itself was named after the ancient Greek goddess Pallas Athena. (Pallas was visible from Earth in February 2014.)

Palladium is malleable and doesn't tarnish in air, making it a popular metal for jewelry. Since 1989, however, the main use for palladium has been in catalytic converters for automobiles. When a car's internal combustion engine burns fuel, the reaction isn't complete; before catalytic converters were developed, all sorts of nasty compounds escaped through the exhaust pipe, including unburned hydrocarbons and nitrogen oxides. 

A catalytic converter is a tubelike structure, usually ceramic, coated with palladium (or platinum or rhodium, which have similar properties). The exhaust from the engine travels through the structure, reacting with the nanoparticles of palladium along the way.

"The unwanted molecule sticks to the nanoparticle, and it literally becomes something else," said Mike Wong, a professor of chemical and biomechanical engineering at Rice University in Houston, Texas. "The bonds of the molecule break, basically."

Most of what is left is water and carbon dioxide, instead of the smoggy stuff that choked the air before catalytic converters became standard. Stringent emissions laws and expanding markets for automobiles have put palladium for catalytic converters in greater demand than ever, according to the Stillwater Mining Company.

Finding this rare element can be a challenge. All six platinum-group elements put together make up only 0.0005 parts per million of Earth's crust, according to U.S. Geological Survey. Most platinum-group elements are found in ore deposits formed by the cooling of magma, according to the USGS.

Who knew?

  • The U.S. imports about 90 percent of the platinum-group elements it uses, including palladium, according to the USGS.
  • Palladium's most incredible ability is that it can absorb up to 900 times its volume of hydrogen, according to the Thomas Jefferson National Accelerator Facility. Because of this ability, palladium is used to store and filter hydrogen.
  • Want a long-lasting photograph? Look into platinotyping. This method of photo printing using platinum and palladium salts, which become embedded in the photo paper, according to Tillman Crane photography. Platinotypes are prized for their matte appearance and archival stability — as long as the paper remains intact, so will the photograph.
  • Palladium is relatively biologically inactive, but can cause allergic reactions in some people. If you have a nickel allergy, you're at higher risk of irritation from palladium, too, according to a 2002 paper in the International Journal of Hygiene and Environmental Health.
  • Iron Man knows a thing or two about the metal. His amazing powers supposedly come from the "arc reactor" embedded in his chest. Turns out, the superhero is being slowly poisoned from the reactor's palladium. In reality, palladium can be toxic to the body. For instance, a study in rat hearts published in the journal Medical Chemistry found that an inorganic palladium compound decreased heart rate and blood pressure.

Current research

More and more, palladium is popping up in places it never did before. Take fuel cells: Platinum has long been used as a catalyst in these devices, but in 2009, researchers at Brown University in Rhode Island reported that they'd found a way to use cheaper palladium nanoparticles instead. The researchers created 4.5 nanometer-diameter palladium particles that remain intact four times longer than the particles previously available, they reported in the Journal of the American Chemical Society.  

Meanwhile, Wong and his colleagues have found a way to use palladium-gold nanoparticles as a sort of catalytic converter for water instead of exhaust. Partnering with DuPont, their lab has created a prototype water-filtration system that uses 6-foot-tall (1.8 meters) columns of gold-and-palladium nanoparticles to remove nasty chemicals like carcinogenic perchloroethylene (PCE), a dry-cleaning fluid, from water.

The technology, dubbed PGClear, works very much like the catalytic converter in a car, Wong told Live Science.

"We take water that contains this PCE, in this case, and it would just flow through this column of palladium-gold, and then those molecules bind to those nanoparticles of palladium-gold and become something else," he said. "It's as easy as that."

The clean water can then be recycled, reused or deposited in groundwater, Wong said.

The prototype unit is currently in testing, Wong said, and the goal is to find out if the system can scale up and become an economically feasible way to filter water. Palladium-gold can catalyze other types of water clean-up reactions, Wong added.

"It's not a one-trick pony," he said.

Researchers are also working to understand how palladium can absorb so much hydrogen. In 2012, MIT researchers reported in the journal Materials Science Engineering that hydrogen-drenched palladium warps under shear forces, turning almost liquidlike. The effect is far less pronounced when the metal is subject to tension forces (imagine stretching out the metal like taffy versus bending it like a ruler). The differing responses to shear and tension forces remain mysterious.

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Stephanie Pappas

Stephanie interned as a science writer at Stanford University Medical School, and also interned at ScienceNow magazine and the Santa Cruz Sentinel. She has a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science writing from the University of California, Santa Cruz.
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