"At first, that result was confusing," said Andrew Peterson, professor 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 CO2 to CO 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 CO2 on a commercial scale.
"Because we're using nanoparticles, we're using a lot less gold than in a bulk metal catalyst," Sun said. "That lowers the cost for making such a catalyst and gives the potential to scale up."
|Contact: Kevin Stacey|