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Posted: Jul 23, 2015

Nanocages offer new direction in sustainable catalyst design

(Nanowerk News) University of Wisconsin-Madison engineers have developed a new approach to structuring the catalysts used in essential reactions in the chemical and energy fields. The advance offers a pathway for industries to wean themselves off of platinum, one of the scarcest metals in the earth's crust.
In an effort to reduce the catalysis world's dependence on this highly reactive and versatile -- but also quite expensive -- metal, UW-Madison chemical engineering Professor Manos Mavrikakis and his collaborators have turned to the nanoscale structure of particles, arranging atoms to achieve more potent chemical reactions while using less material.
In a paper to be published July 24 in the journal Science ("Platinum-based nanocages with subnanometer-thick walls and well-defined, controllable facets"), the researchers describe how they teased a small number of platinum atoms into hollow "cage" structures that prove to be 5.5 times as potent as conventional platinum non-hollowed particles in an oxygen-reduction reaction crucial to low temperature fuel cells.
Materials researchers at the Georgia Institute of Technology initially came across the nano cage as a potentially powerful approach, and Mavrikakis' group used its quantum mechanical modeling expertise to synthesize cages with a tiny amount of platinum.
The real significance of this research, Mavrikakis says, is less about basic chemistry and more about offering a way forward as chemical engineers work to predict and synthesize new catalytic materials, with the ultimate goal of replacing platinum and palladium with more affordable metals.
"This demonstrates a completely new concept about how you can make materials that would utilize a minimal amount of precious metals," Mavrikakis says. "Platinum is likely the most widely used catalyst in the chemical industry, which means that using less of it helps make that industry more sustainable."
To create the nano cages, researchers start with a nanoscale cube or octahedron of less expensive palladium, then deposit a few layers of platinum atoms on top of it.
Calculations by Mavrikakis' group show that platinum atoms have a tendency to burrow into the palladium during the deposition. This allows the palladium to be removed by etching agents, leaving behind a cagelike structure in the initial shape of the palladium template with faces formed by layers of platinum just three to five atoms thick.
Reactants can flow into the hollow structure through holes in the faces, interacting with more platinum atoms in the chemical reaction than would be the case on a flat sheet of platinum or traditional, nonhollowed nanoparticles.
"Because of this new structure they're taking on, they're naturally shortening the distances between platinum atoms, which makes the platinum more active for the oxygen reduction reaction," says Luke Roling, a graduate student in Mavrikakis' lab. "We're also able to use more of the platinum atoms than we were before -- at best, you could get up to twice as much of your platinum exposed."
Mavrikakis points out that, in a scaled-up version of this process, it would be possible to reuse palladium atoms after etching agents remove them from the nanoparticle. Jeff Herron, a postdoctoral researcher in Mavrikakis' group, adds that this process gives engineers a great deal of control over the shape and structure of the particle -- details that make a tremendous difference in how reactive the particle ultimately is.
"Instead of having maybe not-so-well-defined nanoparticles, you can have these well-defined facets," Herron says.
One challenge in developing the nano cage was to determine just how many atomic layers of platinum the structure needs to efficiently catalyze reactions and to be stable in the reactive environment. If it's too thin -- for example, two atomic layers -- the cage collapses. If it's too thick -- six or more layers -- it's harder to remove the palladium atoms and obtain the desired hollowed cages.
Next, researchers hope to determine the optimal nano cage facet thickness for other metallic pairings, beyond platinum on palladium.
While the UW-Madison and Georgia Tech groups have recently made other significant strides in synthesizing material structures that offer greater reactivity, Mavrikakis sees the nano cage structure has opened up a whole new avenue of investigation in synthesizing new catalysts.
"The fundamental understanding of how these materials can be formed and how their reactivity can be modulated allows us now to go well beyond this pair of elements and explore other possibilities to go after even more potent catalytic materials," says Mavrikakis, whose work was supported by the U.S. Department of Energy and UW-Madison's College of Engineering. "If your goal is to construct platinum nano cages, the question is, can you start with something even cheaper than palladium? The challenges will be there, but we've established a framework for how to address these challenges."
The discovery represents a collaborative effort between the Mavrikakis group and researchers at Georgia Tech -- led by professor Younan Xia -- Oak Ridge National Laboratory, Arizona State University and Xiamen University in China.
Source: University of Wisconsin-Madison
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