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Most metals found in nature are actually in their oxide forms. To extract those metals for use in critical applications—ranging from infrastructure such as bridges and buildings to advanced technologies like airplanes, semiconductors or even quantum materials—those oxides must be reduced with gases.

A new study illuminating how different gases can affect oxide reduction, however, has the potential to revamp scientific understandings and current industrial practices.

Hydrogen or carbon monoxide are typically used as reductants, presumed to get the job done similarly enough. This research highlights, for the first time, distinct variations between the two that affect the critical chemical reactions fueling metal production.

Published in Nature, the was a collaboration between Binghamton University and Brookhaven National Laboratory, as well as Stony Brook University and Columbia University.

"For metal production, the key challenge is efficiently removing atomic oxygen from metal oxides to yield pure metals," said Guangwen Zhou, a SUNY distinguished professor at the Thomas J. Watson College of Engineering and Applied Science and deputy director of Binghamton University's Materials Science and Engineering program.

"The goal is to drive this reduction process using less energy, at lower temperatures, and with minimal carbon dioxide emissions. Our study offers insights that can help guide the choice of gases or reductants to accelerate reaction kinetics, making metal extraction faster, cleaner and more energy efficient."

Carbon monoxides have raised concerns for their role in releasing harmful greenhouse gases during manufacturing. The findings of this study point to hydrogen as a greener alternative for metal production, capable of speeding up the process in a more sustainable manner. All this happens while generating benign water vapor as a much more benign chemical consequence.

Members of Zhou's research group have been working on oxides for a long time, according to Binghamton doctoral student and first author Xiaobo Chen, but they gradually began noticing discrepancies in reduction reactions when using one gas versus another.

After prying into the mechanisms of each reducing agent, they found that carbon monoxide and hydrogen reductants aren't actually so similar. When carbon monoxide was used to reduce nickel oxide, the oxide's surface gradually grew coated with a thin layer of metal—essentially stopping any more catalytic reactions from occurring as oxygen depleted from the top.

Trapped and unable to migrate into the bulk, those pockets lacking oxygen accumulated at the surface and drove the local conversion of nickel oxide into metallic nickel.

This newly formed metallic "crust" further blocked oxygen from being removed deeper within the oxide, slowing the overall reduction process. In addition to carbon dioxide emissions, continuing to wring any reactions out of a now inactive oxide would be even more costly and time-consuming.

"If we look at CO—because it's mostly used as a method for metal production—if metal forms on the surface, it can block active sites and slow down the reaction kinetics," Zhou said. "That makes the extraction process more difficult, which means you need to use more energy and higher temperatures."

In contrast, when hydrogen was used, oxygen vacancies formed at the surface could migrate into the bulk of the oxide, enabling metal formation throughout the interior. Importantly, the surface remained largely intact with hydrogen, still capable of the catalytic reactions that are crucial for jumpstarting chemical reactions.

"All this difference is related to the difference in the fundamental mechanisms," Zhou said. "I think that's the reason the community has a strong interest in this work, because we've provided this fundamental insight to understand these two basic reductant gases in controlling reactions—in both kinetics and reaction products."

And because hydrogen protons help oxygen vacancies more easily migrate away from the surface, that also raises the possibility of replenishing them through counterdiffusion of atomic oxygen from the oxide's interior to its surface—a self-healing behavioral quirk that oxides exhibit.

Zhou has also studied this mechanism in a paper published, in the Proceedings of the National Academy of Sciences.

"If we use hydrogen, we can facilitate this process. For industrial applications, we can have that catalyst regeneration, without interrupting the catalytic process," Zhou said. "The reaction itself may actually build or provide some self-healing capabilities to make the catalyst last longer."

A longtime collaboration

More than its potential to improve industrial practices, this study also recontextualizes how scientists can understand the very basic principles of oxide reduction, according to Judith Yang, a scientist at the Brookhaven National Laboratory's Center for Functional Nanomaterials (CFN).

The previous belief held that reductions are more influenced by the partial pressure of oxygen, rather than the reductants themselves. You might wonder, for example, what's better for baking a good dessert: the temperature of the oven or the foundational ingredients.

"With these new tools and scientific insights, like from Professor Zhou, we're really seeing a great richness in these systems, which have a classical and standard description that is still taught in the classroom," Yang said. "We are now developing a new paradigm."

Zhou and his students conducted their research using instruments, coupled with staff scientific support, at the shared user facilities housed in Brookhaven National Laboratory, which is sponsored by the U.S. Department of Energy's Basic Energy Sciences program. First, they used CFN's environmental transmission electron microscope (TEM) to observe in situ reactions in real time, atom by atom.

"There are only a few [of these tools] with such a capability in the entire country," Zhou said. "That's why we are lucky to have this opportunity to access this tool."

They complemented this with synchrotron X-ray diffraction (XRD) to study reactions on a larger scale.

"The combination of these techniques provides a comprehensive, multi-scale understanding of the reaction," explained Lu Ma, lead beamline scientist at the Quick X-ray Absorption and Scattering beamline at Brookhaven National Lab's National Synchrotron Light Source II (NSLS-II).

"While in-situ TEM reveals whether nucleation initiates on the surface or within the interior at the nanoscale, it cannot probe larger-scale samples. Conversely, ensemble XRD offers bulk-scale insights. Together, these methods deliver consistent and complementary evidence of the reaction dynamics across different length scales."

A project like this required many hands and heads, Zhou said, but the partnership between Binghamton and Brookhaven has extended across multiple studies. Moreover, the CFN and NSLS-II are both shared-user facilities with cutting-edge instrumentation and scientific expertise that are free for use by the wider research community.

"I've been collaborating with people from Brookhaven National Lab since I started my faculty position here in Binghamton, so it's probably closer to 20 years," he said. "CFN at Brookhaven National Lab has been really instrumental to my career and research."

Many of Zhou's students also work on-site at Brookhaven, gaining crucial hands-on experience navigating complicated instruments and experiments while establishing rapport with seasoned scientists.

"We cannot guarantee, every time, to successfully perform the experiments. Sometimes, we need a lot of chances to try," Chen said. "We cannot guarantee we can get a result every time, but CFN and NSLS-II are a fundamentally friendly environment. We can have a lot of chances to try those kinds of things."

Studies like these don't just benefit industries, Yang said, but also scientists like herself who get to work with ever-advancing technologies for a living.

"It's the science that Xiaobo and Professor Zhou are doing that motivates the next generation of infrastructure development," she said. "This interest in getting the chemistry and structure in real time, at the atomic scale, in a controlled environment, motivated our next instrument."

In this case, it's a first-in-the-world specialized environmental scanning transmission electron microscope capable of handling angstrom-level resolution, exceptional energy and temporal resolution, and gases ranging from ultra-high vacuum pressures to only a few torr.

Zhou and his team now plan to expand their experimental materials, from copper to iron oxides—reminiscent of the same Bronze and Iron Ages that once characterized much of ancient history, Yang added.

"It's just really fascinating that Guangwen's work ties into the history of humankind," she said. "We're finding new fascination in what's defined the material ages of human history."