Atom-sized craters make catalyst much more active

Atom-sized craters make a catalyst much more active

8:52 AM, 27th November 2015
Atom-sized craters make a catalyst much more active
The MoS2 catalyst being bombarded with argon atoms to create holes where chemical reactions can take place. The bombardment removed about one-tenth of the sulphur atoms (yellow) on its surface. Then they are draped over microscopic bumps to change the spacing of the atoms.

MENLO PARK, US: Bombarding and stretching an important industrial catalyst opens up tiny holes on its surface where atoms can attach and react, greatly increasing its activity as a promoter of chemical reactions, according to a study by scientists at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory.

The method could offer a much cheaper way to rev up the production of clean hydrogen fuel from water, the researchers said, and should also apply to other catalysts that promote useful chemical reactions.

The study is published in the journal Nature Materials.

“This is just the first indication of a new effect, very much in the research stage,” said Xiaolin Zheng, an associate professor of mechanical engineering at Stanford who led the study. “But it opens up totally new possibilities yet to be explored.”

The catalyst studied here, molybdenum disulfide (MoS2); helps remove sulphur from petroleum in refineries. But scientists think it might also be a good alternative to platinum as a catalyst for a reaction that joins hydrogen atoms together to make hydrogen gas for fuel.

“We know platinum is good at catalyzing this reaction,” said Jens Norskov, study co-author and director of the SUNCAT center for Interface Science and Catalysis, a joint Stanford/SLAC institute. “But it’s a non-starter because of its rarity. There isn’t enough of it on Earth for large-scale hydrogen fuel production.”

MoS2 is much cheaper and made of abundant ingredients, and it comes in flexible sheets just one molecule thick, which is stacked together to make catalyst particles, Zheng said. All the catalytic action takes place on the edges of those sheets, where dangling chemical bonds can grab passing atoms and hold them together until they react.

In the new approach, Stanford postdoctoral researcher Hong Li used an instrument in the Stanford Nanocharacterization Laboratory to bombard a sheet of MoS2 with argon atoms. This knocked about 1 out of 10 sulphur atoms out of the surface of the sheet, leaving holes surrounded by dangling bonds.

Then he stretched the holey sheet over microscopic bumps made of silicon dioxide coated with gold. He wet the sheet with a solvent, and when it dried the sheet was permanently deformed: The spacing of the atoms had changed in a way that made the holes much more chemically reactive.

“Before, the top surface of the sheet was not reactive. It was inert – zero, almost,” Zheng said. “Now the surface is more catalytically active than the edges. And we can tune this activity so the bonds that form on the catalyst are just right – strong enough to hold the reacting atoms in place, but weak enough so they’ll let go of the finished product once the atoms have joined together.”

The research was supported by the Samsung Advanced Institute of Technology (SAIT) and Samsung R&D Center America, Silicon Valley, and by SUNCAT and the Center on Nanostructuring for Efficient Energy Conversion at Stanford, both funded by the DOE Office of Science.

© Stanford University News 

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