Why gold never tarnishes has finally been explained

The study, published in Physical Review Letters, found that atoms on certain gold surfaces naturally shift into protective patterns that make it extremely difficult for oxygen to react with the metal.

This newly identified behavior helps explain why gold jewelry, coins, and other objects can retain their luster for centuries. It could also help scientists develop more effective gold based catalysts for industrial manufacturing and clean energy technologies.

Gold's Hidden Atomic Defense

"People have generally thought gold doesn't tarnish simply because it doesn't interact strongly with oxygen," said Matthew Montemore, associate professor in Chemical Engineering in Tulane's School of Science and Engineering. "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."

To investigate the process, Montemore and co-author Santu Biswas, a postdoctoral fellow in Tulane's Department of Chemical & Biomolecular Engineering, used computer simulations to model how atoms and electrons behave. They examined how oxygen molecules interact with two common types of gold surfaces.

The simulations revealed that if the surface atoms did not rearrange themselves, oxygen molecules could split apart much more easily and react with the gold. Instead, the atomic restructuring dramatically limits those reactions.

According to the researchers, these reorganized surfaces reduce oxygen reactions by a factor of a billion to a trillion. In effect, they create an atomic scale protective barrier that allows gold to remain shiny almost indefinitely.

What the Discovery Means for Gold Catalysts

Beyond explaining one of gold's most recognizable characteristics, the research could have important implications for catalysis.

Gold based catalysts, which speed up chemical reactions, are already used in several industrial oxidation processes. However, the same property that makes gold highly resistant to oxygen, making it ideal for jewelry and electronics, also reduces its effectiveness for some chemical manufacturing and energy related reactions.

For example, gold palladium catalysts are used to produce vinyl acetate, an essential ingredient in many plastics and other products. Scientists are also exploring gold catalysts for applications such as removing carbon monoxide from vehicle exhaust and producing propylene oxide, another widely used industrial chemical.

"If you can trick gold into dissociating oxygen, it can actually become a very effective catalyst for certain reactions," Montemore said. "Our work suggests a new strategy for potentially doing that by preventing or reversing these surface rearrangements."

A New Strategy for Better Catalysts

Until now, efforts to improve gold catalysts have largely focused on combining gold with other metals or using tiny gold nanoparticles on oxide surfaces.

The new findings suggest there may be another path forward. By controlling the geometry of gold's surface and the way its atoms arrange themselves, researchers may be able to enhance the metal's catalytic performance while building on a deeper understanding of why gold has remained untarnished throughout history.