There are several compelling reasons why gold stands as one of the most prized metals on Earth. Its dazzling sheen is certainly a significant factor. Unlike many other metals, gold demonstrates remarkable resistance to rust, tarnish, and corrosion – it will maintain its brilliant yellow hue for millennia, just as it does today.
This characteristic is known as noble status, signifying that the element exhibits low reactivity.
Gold is the noblest of all known metals; it does not react with substances like oxygen, which bonds with atoms on the surface layers of other metals, leading to the formation of rust or tarnish.
Now, computational chemists Santu Biswas and Matthew M. Montefald from Tulane University in the US have uncovered the underlying reasons for this behavior. Their findings have been published in the journal Physical Review Letters.
According to their research, the atomic arrangement on a gold surface forms such a dense structure that oxygen molecules, which would otherwise interact with it, cannot easily break apart to initiate oxidation.
Slightly altering this arrangement can render gold significantly more susceptible to rust, but this could actually be advantageous.
In chemistry, oxygen activation is a crucial step that enables other reactions to proceed. For instance, converting carbon monoxide into carbon dioxide requires a free, reactive oxygen atom that can attach to CO and form CO₂.
To achieve this, scientists can “activate” diatomic oxygen by using a metal surface that facilitates the splitting of the molecule into two highly reactive oxygen atoms.
Gold would be an especially suitable catalyst for this reaction due to its high inertness, meaning it doesn’t strongly react with other atoms or molecules.
Some oxygen-activating catalysts possess much higher reactivity, which can lead to the formation of undesirable byproducts, or the catalyst itself binds too strongly to oxygen and corrodes over time.
One might assume gold is unsuitable for such tasks, but in the 1980s, scientists made a surprising discovery.
While bulk gold is not ideal for oxygen catalysis on its own, gold nanoparticles are surprisingly effective at activating oxygen.
This revelation presented a crucial question.
If gold strongly resists oxygen’s effects, how can these minuscule particles possibly initiate oxidation reactions?
The new research suggests that the answer may lie in the specific way atoms are arranged on the gold surface.
Biswas and Montefald employed computer simulations to investigate the interactions between oxygen molecules and nanoscale gold surfaces with varying atomic arrangements.
Specifically, they examined two distinct surface structures: “reconstructed” surfaces, where atoms adopt the closely packed hexagonal arrangement that gold naturally favors; and “unreconstructed” surfaces, which exhibit looser, square-like arrangements.
The difference between these two surface types was striking.
On reconstructed surfaces, the interaction occurred as anticipated. The oxygen molecule struggled to split into two oxygen atoms, mirroring observations with bulk gold.
On unreconstructed surfaces, the situation was entirely different. Oxygen molecules dissociated quite readily.
The simulation results indicate this is because, on the densely packed hexagonal surface, oxygen molecules lack sufficient space to split easily.
The square patterns, with their more open geometry and available space, allow oxygen molecules to find the necessary room for dissociation much more readily.
How much more readily? The researchers found the difference to be on the order of many magnitudes. Oxygen dissociation occurred billions and trillions of times more easily on unreconstructed surfaces compared to reconstructed ones.
This could help explain why tiny gold nanoparticles behave so differently from gold in general. Small particles might not fully form the densely packed reconstructed surfaces seen in larger gold pieces, leaving more reactive square regions exposed.
The dense atomic arrangement on bulk gold surfaces is not inherently designed to resist oxidation; it’s simply the metal’s most stable configuration. Its resistance to corrosion is merely a fortunate byproduct of this phenomenon.
The novel findings could aid scientists in developing gold catalysts that strike a balance between corrosion resistance and efficient oxygen activation.
“This provides a new perspective on why gold is so inert towards oxygen and suggests that creating surfaces with square or rectangular structures could significantly enhance catalytic activity in oxidation reactions on gold,” the researchers write. “Our results offer a novel strategy for designing gold-based catalysts that minimize reconstruction or stabilize square motifs for enhanced diatomic oxygen activation.”
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