Space and Physics
PUBLISHED
Gold has been a treasured material for various cultures for thousands of years. This enduring appeal comes from a combination of two factors – its physical scarcity and its resistance to tarnishing, rusting or corrosion. In short, a gold object created hundreds of years ago will have the same luster today as it did when it was made.
For a long time, gold’s ability to remain perfect – making it the most “noble” of all known metals – was put down to its unique chemical composition, but a new study suggests that its very atoms rearrange themselves to protect it from oxidation.
Beyond being a valuable, aesthetically pleasing resource, gold is playing an increasingly important role as a catalyst for engineering and nanoparticles, where it helps speed up chemical reactions. But gold’s ability to resist oxidation – the process by which metal becomes tarnished or rusts – also limits its usefulness in chemical manufacturing and energy applications.
This is because many industrial and energy-relevant reactions depend on metals that can temporarily oxidize and reduce again. Gold’s exceptional resistance to oxidation means that it is reluctant to activate molecules such as oxygen that are essential for large-scale chemical transformations.
In order to improve gold’s performance as a catalyst, researchers have tried to combine it with other metals or to use small gold nanoparticles on oxide surfaces. However, this has had limited success, resulting in systems that rely on specific particle sizes, surface defects, or other materials that make it difficult to control or scale up meaningfully.
But the discovery that nanoparticles of gold can be used for limited oxygen activation has raised important questions as to why tiny amounts of it behave like this. Perhaps the answer lies in the way atoms are arranged on the material’s surface.
In a new study, researchers at Tulane University used computer simulations that predict how atoms and electrons will behave to see how oxygen molecules interact with common gold surface structures. They found that certain gold surfaces can naturally rearrange themselves to form protective patterns that suppress oxygen activation.
"People have generally thought gold doesn't tarnish simply because it doesn't interact strongly with oxygen," Matthew Montemore at Tulane University in Louisiana explained in a statement.
"What we show is that for two of the most common gold surface types, the surface atoms actually rearrange themselves in a way that makes the gold much more resistant to oxidation."
When Montemore and colleagues ran the simulations, they discovered that oxygen molecules could be much more reactive with gold if it did not have this ability to rearrange. It turns out that this ability actually suppresses oxygen reactions by a factor of a billion to a trillion, basically making an atomic-scale barrier that keeps gold pristine.
This explanation may open the door for new ways to exploit gold for advanced catalysis.
Gold-palladium catalysts are already being used for vinyl acetate, which serves as a building block of plastics and other materials. Scientists are also exploring ways to use gold catalysts to clean up carbon monoxide in car exhausts, and to make propylene oxide – a valuable industrial chemical.
“If you can trick gold into dissociating oxygen, it can actually become a very effective catalyst for certain reactions,” Montemore explained.
“Our work suggests a new strategy for potentially doing that by preventing or reversing these surface rearrangements.”
The paper is published in Physical Review Letters.
