For the first time in history, researchers have witnessed hydrogen and oxygen atoms coming together in real time and at the molecular level to form tiny nanoscale water bubbles. The event was part of a new Northwestern University study in which scientists sought to understand how palladium, a rare metallic element, catalyzes a gaseous reaction to form water. By seeing the reaction at the nanoscale, the Northwestern team understood how this process occurs and even discovered new strategies to speed it up.

Because the reaction does not require extreme conditions, the researchers say it could be used as a practical solution for rapid water formation in arid environments, including other planets.

The study was published on: Proceedings of the National Academy of Sciences.

“By directly imaging nanoscale water formation, we were able to identify optimal conditions for rapid water formation under ambient conditions,” said Northwestern’s Vinayak Dravid, senior author of the study. “These findings have important implications for practical applications, such as enabling rapid water production in deep space using gases and metal catalysts without requiring extreme reaction conditions.

“Think of Matt Damon’s character, Mark Watney, in The Martian. He burned rocket fuel to make hydrogen and then added oxygen from his oxygenator, but we skipped over the need for fire and other extreme conditions.

Dravid is the Abraham Harris Professor of Materials Science and Engineering at Northwestern University’s McCormick School of Engineering and founding director of the Northwestern University Experimental Center for Atomic and Nanoscale Characterization (NUANCE), where the research was conducted. He is also the Director of Global Initiatives at the International Nanotechnology Institute.

Allows discovery of new technologies

Since the early 1900s, researchers have known that palladium can act as a catalyst for rapid water formation. But exactly how this reaction occurs remains a mystery.

“This is a known phenomenon, but it has never been fully understood,” said Yukun Liu, a doctoral student and first author of the study. candidate in the Dravida laboratory. “Because you really need to be able to combine direct visualization of water formation with atomic-scale structural analysis to understand what’s happening in the reaction and how to optimize it.”

But until nine months ago it was absolutely impossible to see the process with atomic precision. In January 2024, Dravid’s team announced a new method to analyze gas molecules in real time. Dravid and his team developed an ultrathin glass membrane that traps gas molecules in honeycomb-shaped nanoreactors so they can be imaged in high-vacuum transmission electron microscopes.

Using a new technique previously published Science DevelopmentsResearchers can examine samples in atmospheric pressure gas with a resolution of only 0.102 nanometers, compared to 0.236 nanometers with other state-of-the-art instruments. The technique also enabled simultaneous analysis of spectral and mutual information for the first time.

“By using an ultrathin membrane, we get more information than the sample itself,” said Kunmo Ku, first author of the Science Advances paper and a research fellow at the NUANCE Center, where he is advised by Associate Professor Xiaobin Hu. “Otherwise, information coming from a thick container would hinder analysis.”

The smallest bubble ever seen

Dravid, Liu and Ku investigated the palladium reaction using the new technology. They first saw how hydrogen atoms entered palladium and expanded its square lattice. However, the researchers could not believe their eyes when they saw tiny water bubbles forming on the palladium surface.

“We think this may be the smallest bubble ever formed and directly visible,” Liu said. “This wasn’t what we expected. Luckily, we recorded it to prove to other people that we weren’t crazy.”

“We were skeptical,” Ku added. “We needed to continue our research to prove that water actually formed.”

The team applied a technique called electron energy loss spectroscopy to analyze the bubbles. By examining the energy loss of scattered electrons, the researchers discovered the oxygen-binding properties unique to water and confirmed that the bubbles were indeed water. The researchers then cross-checked this result by heating the balloon to estimate the boiling point.

“This is a nanoscale counterpart to the Chandrayaan-1 lunar rover experiment, which looked for evidence of water in lunar soil,” Ku said. “While studying the Moon, he used spectroscopy to analyze and identify molecules in the atmosphere and on the surface. We used a similar spectroscopic approach to determine whether the product created was actually water.”

Optimization recipe

After confirming that the reaction with palladium produced water, the researchers set out to optimize the process. They added hydrogen and oxygen separately or combined them at different times to determine which events produced water most quickly.

Dravid, Liu, and Ku found that adding hydrogen first and then oxygen resulted in the fastest reaction rate. Because the hydrogen atoms are so small, they can get stuck between the palladium atoms, causing the metal to expand. After filling the palladium with hydrogen, the researchers added oxygen.

“Oxygen atoms are energetically favorable for adsorption on the palladium surface, but they are too large to enter the lattice,” Liu said. “When we first used oxygen, its dissociated atoms covered the entire surface of the palladium, so the hydrogen could not be adsorbed to the surface to start the reaction. But when we first stored the hydrogen in the palladium and then added the oxygen, the reaction started. The hydrogen comes out to react with the oxygen, and the palladium contracts back to its original state.” It’s coming back.”

A stable system for deep space

The Northwestern team thinks others in the future could potentially prepare hydrogen-filled palladium for spaceflight. Then, travelers will only need to add oxygen to obtain water for drinking or watering plants. Although the research focused on studying bubble formation at the nanoscale, larger palladium sheets would produce much larger amounts of water.

“Palladium may seem expensive, but it is recyclable,” Liu said. “Our process does not consume it. All that is consumed is the gas, and hydrogen is the most abundant gas in the universe. After the reaction, we can use the palladium platform over and over again.”