Scientists Watch Palladium Create the Smallest Water Bubble Ever Seen

Everyone knows the recipe for water: combine two hydrogen atoms with one oxygen atom in precisely the right way and, voilà, you get a molecule of the compound most vital to life on Earth.

For the first time, researchers have now watched that reaction at close range. Working in real time at molecular scale, materials scientist Vinayak Dravid of Northwestern University and his team saw the smallest water bubble ever directly observed appear seemingly from nowhere.

They achieved this by exploiting the unusual properties of palladium, a metal known to catalyse the synthesis of water from the two elements.

"Think of Matt Damon's character, Mark Watney, in the movie The Martian," Dravid says. "He burned rocket fuel to extract hydrogen and then added oxygen from his oxygenator. Our process is analogous, except we bypass the need for fire and other extreme conditions. We simply mixed palladium and gasses together."

Palladium and the synthesis of water

Scientists have long known that palladium can produce substantial quantities of water in a comparatively short time. Precisely how it accomplishes this, however, has been difficult to establish, because the reaction occurs on scales that are challenging to observe.

"You really need to be able to combine the direct visualization of water generation and the structure analysis at the atomic scale in order to figure out what's happening," explains materials scientist Yukun Liu of Northwestern University.

To tackle that obstacle, the researchers created a membrane that holds molecules within minute hexagonal nanoreactor cells. The setup allows molecular activity to be imaged with transmission electron microscopy at nanometre-scale resolution.

Even then, the team did not know whether it would be possible to observe the palladium reaction directly. They placed hydrogen and oxygen atoms on the surface of a 20-nanometre-wide piece of palladium, then used the membrane to contain the interaction that followed.

A water molecule measures under one-third of a nanometre across, and its constituent atoms are much smaller still. To ‘see’ these minuscule compounds bond, the researchers used a type of electron microscopy that measured the energy lost as electrons scattered.

Watching the smallest water bubble form

To their surprise, they watched atoms join together in real time, forming a minute water bubble on the palladium surface.

"We think it might be the smallest bubble ever formed that has been viewed directly," Liu says. "It's not what we were expecting. Luckily, we were recording it, so we could prove to other people that we weren't crazy."

The team then investigated ways to make the process more efficient by supplying the hydrogen and oxygen gases separately. Their experiments showed that the reaction unfolds in two stages. First, hydrogen atoms force their way into the palladium surface, lodging in its atomic lattice and swelling the metal as it expands.

Once oxygen is introduced, the hydrogen leaves its hiding places in the palladium and combines with the oxygen to produce water droplets. At the same time, the palladium returns to its initial, unswollen state.

"Palladium might seem expensive, but it's recyclable," Liu says. "Our process doesn't consume it. The only thing consumed is gas, and hydrogen is the most abundant gas in the universe. After the reaction, we can reuse the palladium platform over and over."

Water production for space habitats

The method is remarkably straightforward and could be scaled up: it simply requires a larger piece of palladium. With a reliable supply of recycled oxygen and hydrogen, water molecules could be made from scratch in a lunar habitat, a Mars base, a space station, or perhaps during a long-distance flight to the stars.

A sufficiently large palladium platform could bolster water supplies away from Earth, where water is limited and recycled urine may be part of the menu. Palladium-produced water could make a welcome alternative.

The research was published in the Proceedings of the National Academy of Sciences.