By Ron B. Davis Jr., Georgetown University
Below the Iron Triad, on the periodic table, are six very special elements, often referred to as the ‘platinum group’ that are practically unreactive, even without forming a thin protective layer. These elements—ruthenium, rhodium, palladium, osmium, iridium and platinum—make up the lion’s share of another cluster of elements sometimes referred to as ‘noble metals’. Rhenium and gold sometimes also join this group.
Resistance to Combine
The term noble metals is akin to noble gases, in that both groups are resistant to combining chemically with other elements. These metals are hopelessly distant from obtaining an octet and strongly retain to their electrons with large positive nuclear charges. As a result, they are remarkably unreactive, even in the presence of oxygen—no passivation is necessary to protect the noble metals.
The platinum group elements are characterized by some desirable properties including high melting points and corrosion resistance.
Its position, near the bottom of the activity series, explains why platinum, like gold, occurs as a native metal in nature. It was discovered by pre-Columbian Indians and, for centuries, was highly valued and used in South America as an ornamental material. With a similar density and inertness to that of gold, platinum can, in the right place, be found simply by panning—just the same way one can search for gold.
Rarest Non-radioactive Elements
Their larger nuclei and ‘iron-loving’ character of the six ‘platinum group’ elements make them—along with hanger-on neighbor Rhenium—the rarest non-radioactive elements in the Earth’s crust.
These precious metals cluster together, not only in the periodic table, but also in nature. All five of the other metals in this cluster were only discovered in the first half of the 19th century, each of them hiding in platinum samples. Rhodium, palladium, osmium, and iridium were all identified in quick succession by chemists working to develop ways to dissolve platinum metal. When they succeeded, they found residues left behind that contained each of these elements.
Elements from this region of the table also have an interesting ability to accelerate certain desirable chemical reactions, simply by being present. Chemists call the process ‘catalysis’.
Part of the reason that these metals make such desirable catalysts is their own inertness. To be a catalyst, the material itself must not be consumed or altered in the overall reaction—it must simply promote chemical reactions without actually being consumed or produced.
The second part of the equation is again those d-orbital electrons. With a large reservoir of d-electrons and orbitals, platinum group metals can push and pull electrons from reagents that adsorb onto their exposed surfaces. This push-and-pull can make the chemical bonds in those reagents break and re-form more easily.
In short—a metal not itself easy to modify chemically, yet able to act as a temporary sink or source of electrons. Combine those two properties, and we have a recipe for a potent catalyst.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Hydrogenated oils are produced by the reaction of unsaturated fats with hydrogen to produce a more shelf-stable product. Paul Sabatier accomplished this in 1899, inspired by the already-known ability of highly valuable metals like platinum and palladium to catalyze such hydrogenations. The twist was that he decided to investigate their lighter (and much more available and inexpensive) group-10 member, nickel.
Sabatier was able to develop a reaction in which freshly produced nickel metal promoted the addition of hydrogen to oils, providing a means of mass-producing edible fats that would last much longer in storage than the original oils themselves.
Platinum’s Catalytic Powers
But probably nowhere has the catalytic power of the actual platinum group metals been more influential in the past century than in the automotive industry.
Fuel chemistry can be complicated, and the exact chemical composition of a motor fuel such as gasoline or diesel fuel can profoundly impact fuel performance and cleanliness when used. The American chemist, Vladimir Haensel, knew this, and he also knew, as many others in the field did, that platinum’s catalytic powers seemed to know no bounds.
At that time, it was already well-known that platinum had the ability to alter the chemical composition of hydrocarbons in petroleum products as part of a process called, reforming. However, most balked at the remarkably high cost of platinum. How could such an expensive metal ever be part of a cost-effective means of refining petroleum?
Clearly, Haensel understood two important things:
First, that platinum’s effect was catalytic—that the platinum itself would not be consumed in any process it promoted. Secondly, the catalysis took place only at the very surface of the metal. The key was using as little of this expensive stuff as possible.
Thus, Haensel went about developing highly dispersed platinum coatings on tough, inexpensive support materials like alumina. In doing so, he created a solid material that had the same catalytic power as a pure platinum ingot with similar surface area, but at a vast fraction of the cost.
Hence, platinum group metals contribute to the quality of the fuel that goes into our car. But they also contribute to the quality of what comes out the other end.
Protecting the Environment
Since the mid-1970s, practically every vehicle produced in America, and many around the world, has been putting a small sample of some of the most expensive metals on earth in its tailpipe. Finely dispersed platinum group metal coatings, on heat-resistant supports, are placed in a small box, just before the exhaust exits the system.
This made it possible to harness the catalytic power of platinum—and later, palladium and rhodium—ensuring that the gaseous by-products of combustion are completely oxidized to less harmful compounds before they leave the automobile.
Precious metals helping protect the environment, while not being used up in the process. What’s not to like?
Common Questions about the ‘Platinum Group’ Elements
The term noble metals is akin to noble gases, in that both groups are resistant to combining chemically with other elements.
Like the elements of the Iron Triad, the metals of the platinum group cluster together, not only in the periodic table, but also in nature.
Platinum has the ability to alter the chemical composition of hydrocarbons in petroleum products as part of a process called reforming.