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Stanford Team Applies Nanotechnology to Enhance Performance of Key Industrial Catalyst

According to a new research led by Scientists at Stanford University and SLAC National Accelerator Laboratory a small amount of stretching or squeezing can produce a big increase in catalytic performance.  

Chirranjeevi Gopal, former Stanford postdoctoral researcher, is lead author of the ceria study being published in Nature Communications. (Image credit: Courtesy Chirranjeevi Gopal)

The discovery concentrates on an industrial catalyst known as cerium oxide, or ceria, a spongy material frequently used in self-cleaning ovens, catalytic converters and various green-energy applications, such as solar water splitters and fuel cells. Details of the research have been published in the May 18th issue of Nature Communications.

Ceria stores and releases oxygen as needed, like a sponge. We discovered that stretching and compressing ceria by a few percent dramatically increases its oxygen storage capacity. This finding overturns conventional wisdom about oxide materials and could lead to better catalysts.

Will Chueh, Study Co-Author

Catalytic converters

For a long time ceria has been used in catalytic converters to help eliminate air pollutants from vehicle exhaust systems.

“In your car, ceria grabs oxygen from poisonous nitrogen oxide, creating harmless nitrogen gas,” said study lead author Chirranjeevi Balaji Gopal, a former Postdoctoral Researcher at Stanford. Gopal also stated,“Ceria then releases the stored oxygen and uses it to convert lethal carbon monoxide into benign carbon dioxide.”

Studies have revealed that squeezing and stretching ceria causes nanoscale alterations that impact its ability to store oxygen.

The oxygen storage capacity of ceria is critical to its effectiveness as a catalyst. The theoretical expectation based on previous studies is that stretching ceria would increase its capacity to store oxygen, while compressing would lower its storage capacity.

Aleksandra Vojvodic, Study Co-Author

To evaluate this estimation, the research team grew ultrathin films of ceria, each only a few nanometers thick, on top of substrates composed of diverse materials. This process exposed the ceria to stress equal to 10,000 times the Earth’s atmosphere. This massive stress caused the molecules of ceria to separate and squeeze together a distance of less than one nanometer.

Surprise results

Usually, materials like ceria release stress by forming defects in the film. But atomic-scale examination revealed a surprise.

Using high-resolution transmission electron microscopy to resolve the position of individual atoms, we showed that the films remain stretched or compressed without forming such defects, allowing the stress to remain in full force,” said Robert Sinclair, a Professor of Materials Science and Engineering at Stanford.

To measure the influence of stress under real-world operating conditions, the Researchers tested the ceria samples using the intense beams of X-ray light produced at Lawrence Berkeley National Laboratory’s Advanced Light Source.

The results were even more astonishing.

We discovered that the strained films exhibited a fourfold increase in the oxygen storage capacity of ceria,” Gopal said. He also said, “It doesn’t matter if you stretch it or compress it. You get a remarkably similar increase.”

The high-stress method used by the research team is easily realizable through nanoengineering, Chueh added.

This discovery has significant implications on how to nanoengineer oxide materials to improve catalytic efficiency for energy conversion and storage. It’s important for developing solid oxide fuel cells and other green-energy technologies, including new ways to make clean fuels from carbon dioxide or water.

Will Chueh, Study Co-Author

Other Stanford Co-Authors of the research are Max Garcia-Melchor, now at Trinity College Dublin (Ireland), and Graduate Students Sang Chul Lee, Zixuan Guan, Yezhou Shi and Matteo Monti. Additional Co-Authors are Andrey Shavorskiy of Lund University (Sweden) and Hendrik Bluhm of Lawrence Berkeley National Laboratory.

This research was supported by the U.S. Department of Energy, the National Science Foundation and the Stanford Precourt Institute for Energy.

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