Palladium, a unique metallic element, can swiftly convert hydrogen and oxygen into water. This remarkable process was observed at the nanoscale for the first time using an electron microscope. Through high-precision observation, researchers have identified ways to enhance the process for quicker water production. This method could potentially provide water on-demand in dry regions, including locations on other planets.
In a groundbreaking study, researchers have observed — in real time and at a molecular scale — the fusion of hydrogen and oxygen atoms resulting in the formation of minuscule, nano-sized water bubbles.
This exciting event was part of a recent Northwestern University investigation, where scientists aimed to better understand how palladium, a rare metallic element, catalyzes the reaction between gases to create water. By observing this process at the nanoscale, the Northwestern research team uncovered the mechanisms behind it and developed new techniques to speed it up.
Since the reaction can occur under mild conditions, the researchers believe it could serve as a viable solution for quickly producing water in arid regions, including extraterrestrial environments.
The findings will be featured in the upcoming issue of the Proceedings of the National Academy of Sciences on September 27.
“By visually monitoring water generation at the nanoscale, we were able to determine the best conditions for rapid water production under normal circumstances,” explained Vinayak Dravid from Northwestern, who is the senior author of the study. “These insights have far-reaching consequences for practical applications, such as facilitating swift water generation in outer space using gases and metal catalysts, without needing extreme conditions.”
“Consider Matt Damon’s character, Mark Watney, from ‘The Martian.’ He used rocket fuel to produce hydrogen and then supplemented it with oxygen from his oxygenator. Our method is similar, but we eliminate the requirement for fire and other extreme conditions; we simply mix palladium and gases,” Dravid added.
Dravid holds the position of Abraham Harris Professor of Materials Science and Engineering at Northwestern’s McCormick School of Engineering and is the founding director of the Northwestern University Atomic and Nanoscale Characterization Experimental (NUANCE) Center, where this research took place. He is also the director of global initiatives at the International Institute for Nanotechnology.
New technology enabled discovery
Since the early 1900s, it’s been known that palladium can act as a catalyst to quickly produce water. However, the specifics of how this reaction occurs have remained largely unclear.
“It’s a well-known phenomenon, but never fully comprehended,” stated Yukun Liu, the lead author of the study and a Ph.D. candidate in Dravid’s lab. “True understanding requires both direct visualization of water creation and detailed structural analysis at the atomic level to reveal the reaction’s intricacies and how to improve it.”
However, observing this process with atomic-level precision was previously unachievable — that is, until nine months ago. In January 2024, Dravid’s team introduced a groundbreaking approach to analyze gas molecules live. They crafted an ultra-thin glassy membrane that captures gas molecules within hexagonally shaped nanoreactors, allowing for examination in high-vacuum transmission electron microscopes.
This new technique, detailed in *Science Advances*, provides researchers with the ability to study samples in atmospheric gas pressure with a resolution of merely 0.102 nanometers, a significant improvement from the previous 0.236 nanometer resolution offered by other contemporary technologies. This advancement also allows for simultaneous analysis of spectral and reciprocal information.
“With the help of the ultrathin membrane, we are extracting more detailed information from the sample,” explained Kunmo Koo, the first author of the *Science Advances* paper and a research associate at the NUANCE Center, who is under the mentorship of research associate professor Xiaobing Hu. “Otherwise, the thick container would obstruct the analysis.”
Smallest bubble ever seen
Using the innovative technology, Dravid, Liu, and Koo investigated the reaction involving palladium. Initially, they observed hydrogen atoms infiltrating the palladium and expanding its square lattice structure. However, when they witnessed the formation of tiny bubbles of water on the surface of the palladium, they were astonished.
“We believe it may be the smallest bubble ever directly observed,” Liu remarked. “It was unexpected. Fortunately, we were capturing it on video, which allowed us to prove we weren’t imagining things.”
“We were initially doubtful,” Koo added. “We needed to conduct further investigation to confirm that what formed was indeed water.”
The team employed electron energy loss spectroscopy to characterize the bubbles. By analyzing the energy lost by scattered electrons, they identified oxygen bonding properties specific to water, confirming that the bubbles were indeed water. They further validated this finding by heating the bubble to examine its boiling point.
“This process parallels the Chandrayaan-1 moon rover experiment that sought water evidence in lunar soil,” Koo explained. “While navigating the moon, it utilized spectroscopy to identify molecules in the atmosphere and on the surface. We adopted a similar spectroscopic method to ascertain if the produced product was water.”
Recipe for optimization
After confirming that the palladium reaction produced water, the research team set out to optimize the method. They tested adding hydrogen and oxygen at different times or mixing them together to see which sequence yielded the fastest water production rate.
Dravid, Liu, and Koo found that introducing hydrogen first, followed by oxygen, resulted in the quickest reaction. Given the minuscule size of hydrogen atoms, they can penetrate between the atoms of palladium, causing the metal to expand. Once the palladium was filled with hydrogen, they introduced oxygen gas.
“Oxygen atoms readily adhere to the palladium surfaces, but their size prevents them from entering the lattice,” Liu noted. “If we infused oxygen first, its dissociated atoms would cover the palladium’s surface, limiting hydrogen’s ability to attach and trigger the reaction. However, by storing hydrogen in the palladium beforehand and then adding oxygen, the reaction commenced. Hydrogen escapes the palladium to react with oxygen, resulting in the palladium contracting back to its original form.”
Sustainable system for deep space
The Northwestern research team envisions a future where travelers could prepare palladium filled with hydrogen prior to embarking on space missions. To create drinking water or for plant hydration, they would only need to introduce oxygen. Although this study concentrated on tiny bubble generation, larger sheets of palladium could produce significantly greater volumes of water.
“While palladium may appear costly, it is recyclable,” Liu explained. “Our method does not deplete it. The only resource consumed is gas, and hydrogen is the most prevalent gas in the universe. After the reaction, the palladium can be reused multiple times.”
The study, titled “Unraveling the adsorption-limited hydrogen oxidation reaction at palladium surface via in situ electron microscopy,” received support from the Air Force Office of Scientific Research (grant number AFOSR FA9550-22-1-0300) and hydrogen-related initiatives by the Center for Hydrogen in Energy and Information Sciences, an Energy Frontier Research Center backed by the U.S. Department of Energy, Office of Science (grant number DE-SC0023450).
