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Gold nanoparticles give an edge in recycling carbon dioxide

Fri, 10/25/2013 - 8:00am

Gold nanoparticles make better catalysts for carbon dioxide recycling than bulk gold metal. Size is crucial though, since edges produce more desired results than corners (red points, above). Nanoparticles of 8 nm appear to have a better edge-to-corner ratio than 4-nm, 6-nm or 10-nm nanoparticles. Image: Sun laboratory/Brown Univ.By tuning gold nanoparticles to just the right size, researchers from Brown Univ. have developed a catalyst that selectively converts carbon dioxide to carbon monoxide, an active carbon molecule that can be used to make alternative fuels and commodity chemicals.

“Our study shows potential of carefully designed gold nanoparticles to recycle carbon dioxide into useful forms of carbon,” said Shouheng Sun, prof. of chemistry and one of the study’s senior authors. “The work we’ve done here is preliminary, but we think there’s great potential for this technology to be scaled up for commercial applications.”

The findings are published in the Journal of the American Chemical Society.

The idea of recycling carbon dioxide is enticing, but there are obstacles. Carbon dioxide is an extremely stable molecule that must be reduced to an active form like carbon monoxide to make it useful. Carbon monoxide is used to make synthetic natural gas, methanol and other alternative fuels.

Converting carbon dioxide to carbon monoxide isn’t easy. Prior research has shown that catalysts made of gold foil are active for this conversion, but they don’t do the job efficiently. The gold tends to react both with the carbon dioxide and with the water in which the carbon dioxide is dissolved, creating hydrogen byproduct rather than the desired carbon monoxide.

The Brown experimental group, led by Sun and Wenlei Zhu, a graduate student in Sun’s group, wanted to see if shrinking the gold down to nanoparticles might make it more selective for carbon dioxide. They found that the nanoparticles were indeed more selective, but that the exact size of those particles was important. Eight nanometer particles had the best selectivity, achieving a 90% rate of conversion from carbon dioxide to carbon monoxide. Other sizes the team tested—4, 6 and 10 nm—didn’t perform nearly as well.

“At first, that result was confusing,” said Andrew Peterson, prof. of engineering and also a senior author on the paper. “As we made the particles smaller we got more activity, but when we went smaller than eight nanometers, we got less activity.”

To understand what was happening, Peterson and postdoctoral researcher Ronald Michalsky used a modeling method called density functional theory. They were able to show that the shapes of the particles at different sizes influenced their catalytic properties.

“When you take a sphere and you reduce it to smaller and smaller sizes, you tend to get many more irregular features—flat surfaces, edges and corners,” Peterson said. “What we were able to figure out is that the most active sites for converting carbon dioxide to carbon monoxide are the edge sites, while the corner sites predominantly give the by-product, which is hydrogen. So as you shrink these particles down, you’ll hit a point where you start to optimize the activity because you have a high number of these edge sites but still a low number of these corner sites. But if you go too small, the edges start to shrink and you’re left with just corners.”

Now that they understand exactly what part of the catalyst is active, the researchers are working to further optimize the particles. “There’s still a lot of room for improvement,” Peterson said. “We’re working on new particles that maximize these active sites.”

The researchers believe these findings could be an important new avenue for recycling carbon dioxide on a commercial scale.

Source: Brown Univ.

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