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A study nearly 10 years in the making has shed new insight into how oxides can regularly sustain themselves, using the oxygen inherent in their own structures.

This paper reflects a mammoth effort by nearly 20 authors and multiple institutions, including Binghamton University, the Brookhaven National Laboratory, Lawrence Berkeley National Laboratory, the University of Pittsburgh, National Institute of Standards and Technology, and the University of the Chinese Academy of Sciences.

"The work involves a lot of experimental and modeling efforts, and the first several authors are all Ph.D. students from my group," said SUNY Distinguished Professor Guangwen Zhou, a faculty member at the Thomas J. Watson College of Engineering and Applied Science's Department of Mechanical Engineering who oversaw the project. "From the beginning of the experiment to the paper now, it took about 10 years."

The resulting paper is in the Proceedings of the National Academy of Sciences.

Oxides are widely used as catalysts to jumpstart chemical reactions, forming compounds such as methane or even simply water. But when those catalysts run out, manufacturing processes often need to be shut down to update them.

Zhou's latest research, however, could lead to new energy- and cost-saving measures by eliminating the need to pause operations to update catalysts. That change would be helpful for fields from the automotive industry, which uses catalytic converters to reduce energy emissions, to the energy industry that uses methane in gas turbines.

"Practically, this may design more sustainable catalysts that can heal themselves and recover their catalytic behavior. This saves a lot of money for the industry level, because you don't have to shut down the reactor," Zhou said.

How this works is that certain properties allow oxides to fuel their own reactions themselves, using the atomic oxygen embedded within their lattice structures.

If, for example, scientists expose a metal oxide to hydrogen in order to form water, the oxygen required to join that hydrogen atom to become a fully-fledged water molecule can be pulled from the oxide itself, rather than from an external source.

"In other words, the oxide itself is actively participating in the reaction process," Zhou said.

This kind of behavior is called the Mars-van Krevelen (MvK) mechanism. But while it has been named and widely theorized, Zhou said experimental evidence proving it directly has been much more challenging to come by. There are so many catalyst particles involved in such a reaction that under a microscope it can become one washed-out deluge of data—with too much information to form a clear picture.

Using a combination of Zhou's novel in-situ transmission electron microscopy as well as computational modeling, the researchers could isolate a single surface and observe its behavior in real time.

"We can directly visualize how the catalyst itself evolves or changes at an atomic scale," Zhou said. "So we can particularly see the topmost atomic layer of a catalyst, and the structure changes over time with a basic kind of oscillatory behavior."

What they found was not only the confirmation that the MvK mechanism is at play, but also a unique behavior in which this self-sustaining cycle intrinsically regulates itself.

When most of the oxygen in an oxide's topmost layer is pulled away to form water, it becomes oxygen-deficient, slowing down its reactivity. During this lull period, the oxygen embedded within the oxide's internal structure begins to diffuse upward, repopulating the surface area until it becomes oxygen-rich again—thus restarting the cycle of reactivity. It's similar to periodically taking a break while running to replenish energy, before starting up a sprint again.

"This [MvK] mechanism itself does not tell us about any kind of self-oscillation behavior, because this method just tells us the oxide itself can supply oxygen to oxygenate products," Zhou said. "This oscillatory behavior or mechanism is new. We actually figured this out based on our experimental evidence."

For this experiment, Zhou's metal of choice was copper oxide. His team placed a piece of copper inside a transmission electron microscope—which beams a concentrated stream of electrons through a sample less than 100 nanometers thick—and cleaned it using hydrogen. Then, the researchers formed the oxide on site by combining the sample with high-purity oxygen inside the tool.

The entire reaction process is contained within the microscope, where scientists can carefully observe and control experimental conditions. But while the microscope operates in high resolutions and can take videos at 30 frames per second, the direct view it provides of atoms as they move and change does not necessarily explain what precise factors led to those movements. Because of that, this study required additional computational modeling and analysis on top of microscopic imaging.

"From the experiment, we can see the phenomena and reactions now," Zhou said. "From the modeling side, we can better understand how much energy we need to supply to make it happen first."

While oxides cannot sustain themselves perpetually—they only last as long as there's still oxygen in the structure—Zhou says this can be circumvented strategically, such as by using oxygen to continually replenish the oxide's reservoir itself. In a similar vein, Zhou will be tinkering with reaction conditions to see if there are other ways to change or even control oscillatory behavior next.

"I think this will help the community to better understand this MvK mechanism, and this part of this work is also the first to provide experimental evidence at an atomic level," Zhou said. "I believe this will provide a deep and fundamental understanding of this phenomenon."