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Multifaceted Catalysts

Platinum polyhedra outshine their spherical cousins in catalysis

by Bethany Halford
May 7, 2007 | A version of this story appeared in Volume 85, Issue 19

Credit: Courtesy of S.-G. Sun & Z. L. Wang
Tetrahexahedral platinum particles about 200 nm across.
Credit: Courtesy of S.-G. Sun & Z. L. Wang
Tetrahexahedral platinum particles about 200 nm across.

TO FIND THE ACTION in catalysis reactions, you've got to go to the edge—the edge of the catalyst, that is. That's because the catalyst's most reactive regions can be found at the edges' open sites, where an atom is missing one or more neighbors to which it could bind. Now, researchers at Xiamen University, in China, and Georgia Institute of Technology have created new efficient platinum nanocrystal catalysts with 24 facets (Science 2007, 316, 732).

The rough facets on these edgy "tetrahexahedral" structures provide unsaturated steps, ledges, and kinks, which help make the catalysts 200 to 400% more efficient than spherical platinum nanoparticles at oxidizing organic fuels such as formic acid and ethanol. That increased efficiency could bring a boost to hydrogen fuel cells, according to Xiamen University's Shi-Gang Sun and Georgia Tech's Zhong Lin Wang, who led the study.

"If we are going to have a hydrogen economy, we are going to need better catalysts," Wang explains. "This new shape for platinum catalyst nanoparticles greatly improves their activity." The nanoparticles also exhibit remarkable robustness, Sun and Wang point out. They remain stable at temperatures up to 800 oC, so they would be recyclable in many applications.

The new multifaceted nanocrystals are made electrochemically. The researchers first deposit platinum nanospheres onto an electrode under a constant potential. Next, they subject the deposited platinum to a potential pulse sequence that alternates between reducing and oxidizing potentials. The alternating potentials transform the spheres into smaller polyhedra.

Daniel L. Feldheim, a nanomaterials expert at the University of Colorado, Boulder, calls the work "a breakthrough in the synthesis of nanoscale catalysts." One major challenge facing catalysis researchers, he explains in a commentary that accompanies the paper, is to understand how to control nanoparticle shape to maximize the number of edges and corner sites available for catalytic reactions.

Feldheim notes that it will be "exciting to explore the electrochemical synthesis of mixed metal and metal oxide nanoparticles, because the presence of other atoms at edge sites in the lattice could enhance catalytic reactivity further while lessening the amount of expensive precious metal used."

Although they're still fine-tuning the process, Sun and Wang note that they can control the particle size so that fewer than 5% of the nanocrystals are larger or smaller than desired.

Next, they hope to make the tetrahexahedral crystals even smaller. "If we can reduce the size through better control of processing conditions, we will have a catalytic system that would allow production of hydrogen with greater efficiency," Wang says.



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