Gold is the noblest of all known metals: it does not react easily with substances such as oxygen, because the extremely compact arrangement of atoms on its surface prevents oxygen molecules from splitting and triggering oxidation.
There are several good reasons why gold is one of the year mais valioros of Earth.
Among them, and no less important, is its brilliant shine. Unlike many other metals, gold is extremely resistant to rusttarnishing and corrosion — thousands of years from now it will continue to shine with the same intense yellow hue that it has today.
This property is known as chemical nobilitywhich means the element has low reactivity.
Gold is the noblest of all known metals: does not react easily with substances such as oxygen, which binds to atoms in the surface layers of other metals to form rust or tarnish.
Now computational chemists Santu Biswas e Matthew M. Montemorefrom Tulane University, in the United States, discovered the reason.
According to yours, recently published in Physical Review Lettersa arrangement of atoms on the surface of gold forms a pattern so densely packed that the dioxygen moleculewho would otherwise interact with him, cannot divide easily enough to trigger oxidation.
Simply loosen this pattern slightly and the gold becomes dramatically more vulnerable to rust — which, in practice, could be something positive.
In chemistry, Oxygen activation is a crucial step thatwhich allows other reactions to occur. For example, to convert carbon monoxide to carbon dioxide, a free, reactive oxygen atom is required that can bind to CO to form CO₂.
To this end, scientists can “activate” dioxygenproceeding to a metallic surface that helps split the molecule into two highly reactive oxygen atoms.
Gold would be an especially desirable catalyst for this reaction precisely por be so inert — that is, by not reacting intensely with other atoms or molecules.
Some oxygen activation catalysts are much more reactive, which can generate undesirable by-products; in other cases, the catalyst itself binds too tightly to oxygen and corrodes over time.
One might think that gold would be a bad candidate for this type of work, but in the 1980s, scientists made a surprising discovery: although solid gold is unsuitable for oxygen catalysis, gold nanoparticles prove to be surprisingly effective at activating oxygen.
This discovery raised a big question. If gold resists oxygen so intensely, how can these tiny particles drive oxidation reactions?
The new study suggests the answer could reside in the form how the atoms are arranged on the surface of gold.
Biswas and Montemore turned to computer simulations to study what happens when oxygen molecules come into contact with gold surfaces. on a nanoscopic scale, with different arrangements of atoms.
In particular, they studied two distinct types of patterns: “surfaces.”rebuilt“, in which atoms are arranged in the densely packed hexagonal arrangement that gold naturally prefers; and surfaces “not rebuilt“, which form looser patterns, similar to squares.
The difference between the two types of surface turned out to be striking. On unreconstructed surfaces, the scenario couldn’t be more different. Oxygen molecules divide relatively easily.
Simulations suggest this is due to the fact that on the densely packed hexagonal surface, oxygen molecules don’t find space enough to divide easily.
Square patterns have a looser geometrywith this integrated space, the oxygen molecules can much more easily find the support they need to divide.
How much easier, after all?? Many orders of magnitude, the researchers concluded. Oxygen dissociation occurred between billions and billions times more readily on unreconstructed surfaces than on reconstructed ones.
This could help explain why tiny nanoparticles of gold behave so different from solid gold. Small particles may not fully develop the reconstructed, densely packed surfaces observed in larger gold fragments, leaving more reactive regions with square-like patterns exposed.
The compact arrangement of surface atoms in solid gold is not necessarily designed to resist oxidation; It’s simply the configuration more stable for metal. Corrosion resistance is just a curious side effect of this configuration.
The new findings could help scientists develop gold catalysts that balance the corrosion resistance with efficient activation of oxygen.
“This provides new insight into why gold is so inert toward dioxygen and suggests that creating surfaces with square or rectangular structures could significantly improve catalytic activity for oxidation reactions in gold,” the researchers conclude.
