Researchers witness nanoscale water formation in real time
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For the first time ever, researchers have witnessed—in real time and at the molecular-scale—hydrogen and oxygen atoms merge to form tiny, nano-sized bubbles of water.
The event occurred as part of a new Northwestern University study, during which scientists sought to understand how palladium, a rare metallic element, catalyzes the gaseous reaction to generate water. By witnessing the reaction at the nanoscale, the Northwestern team unraveled how the process occurs and even uncovered new strategies to accelerate it.
Because the reaction does not require extreme conditions, the researchers say it could be harnessed as a practical solution for rapidly generating water in arid environments, including on other planets.
The research is published in the Proceedings of the National Academy of Sciences.
"By directly visualizing nanoscale water generation, we were able to identify the optimal conditions for rapid water generation under ambient conditions," said Northwestern's Vinayak Dravid, senior author of the study. "These findings have significant implications for practical applications, such as enabling rapid water generation in deep space environments using gases and metal catalysts, without requiring extreme reaction conditions.
"Think of Matt Damon's character, Mark Watney, in the movie 'The Martian.' 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 gases together."
Dravid is the Abraham Harris Professor of Materials Science and Engineering at Northwestern's McCormick School of Engineering and founding director of the Northwestern University Atomic and Nanoscale Characterization Experimental (NUANCE) Center, where the study was conducted. He is also director of global initiatives at the International Institute for Nanotechnology.
New technology enables discovery
Since the early 1900s, researchers have known that palladium can act as a catalyst to rapidly generate water. But exactly how this reaction occurs has remained a mystery.
"It's a known phenomenon, but it was never fully understood," said Yukun Liu, the study's first author and a Ph.D. candidate in Dravid's laboratory. "Because 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 with the reaction and how to optimize it."
But viewing the process with atomic precision was simply impossible—until nine months ago. In January 2024, Dravid's team unveiled a novel method to analyze gas molecules in real time. Dravid and his team developed an ultra-thin glassy membrane that holds gas molecules within honeycomb-shaped nanoreactors, so they can be viewed within high-vacuum transmission electron microscopes.
With the new technique, previously published in Science Advances, researchers can examine samples in atmospheric pressure gas at a resolution of just 0.102 nanometers, compared to a 0.236-nanometer resolution using other state-of-the-art tools. The technique also enabled, for the first time, concurrent spectral and reciprocal information analysis.
"Using the ultrathin membrane, we are getting more information from the sample itself," said Kunmo Koo, first author of the Science Advances paper and a research associate at the NUANCE Center, where he is mentored by research associate professor Xiaobing Hu. "Otherwise, information from the thick container interferes with the analysis."
Smallest bubble ever seen
Using the new technology, Dravid, Liu and Koo examined the palladium reaction. First, they saw the hydrogen atoms enter the palladium, expanding its square lattice. But when they saw tiny water bubbles form on the palladium surface, the researchers couldn't believe their eyes.
"We think it might be the smallest bubble ever formed that has been viewed directly," Liu said. "It's not what we were expecting. Luckily, we were recording it, so we could prove to other people that we weren't crazy."
"We were skeptical," Koo added. "We needed to investigate it further to prove that it was actually water that formed."
The team implemented a technique, called electron energy loss spectroscopy, to analyze the bubbles. By examining the energy loss of scattered electrons, researchers identified oxygen-bonding characteristics unique to water, confirming the bubbles were indeed water. The researchers then cross-checked this result by heating the bubble to evaluate the boiling point.
"It's a nanoscale analog of the Chandrayaan-1 moon rover experiment, which searched for evidence of water in lunar soil," Koo said. "While surveying the moon, it used spectroscopy to analyze and identify molecules within the atmosphere and on the surface. We took a similar spectroscopic approach to determine if the generated product was, indeed, water."
Recipe for optimization
After confirming that the palladium reaction generated water, the researchers next sought to optimize the process. They added hydrogen and oxygen separately at different times or mixed together to determine which sequence of events generated water at the fastest rate.
Dravid, Liu and Koo discovered that adding hydrogen first, followed by oxygen, led to the fastest reaction rate. Because hydrogen atoms are so small, they can squeeze between palladium's atoms—causing the metal to expand. After filling the palladium with hydrogen, the researchers added oxygen gas.
"Oxygen atoms are energetically favorable to adsorb onto palladium surfaces, but they are too large to enter the lattice," Liu said. "When we flowed in oxygen first, its dissociated atoms covered the entire surface of the palladium, so hydrogen could not adsorb onto the surface to trigger the reaction. But when we stored hydrogen in the palladium first, and then added oxygen, the reaction started. Hydrogen comes out of the palladium to react with the oxygen, and the palladium shrinks and returns to its initial state."
Sustainable system for deep space
The Northwestern team imagines that others, in the future, potentially could prepare hydrogen-filled palladium before traveling into space. Then, to generate water for drinking or for watering plants, travelers will only need to add oxygen. Although the study focused on studying bubble generation at nanoscale, larger sheets of palladium would generate much larger quantities of water.
"Palladium might seem expensive, but it's recyclable," Liu said. "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."
More information: Yukun Liu et al, Unraveling the adsorption-limited hydrogen oxidation reaction at palladium surface via in situ electron microscopy, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2408277121
Journal information: Proceedings of the National Academy of Sciences , Science Advances
Provided by Northwestern University