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January 2013

With Nanoparticles, Distance Makes the Catalyst Last Longer

Advances in understanding at the nanoscale allow better material performance in the long run

Enoch Dames

Three-dimensional snapshots of catalysts' internal structure, shown on either side of the above picture: this imaging technology is enabling rational design of longer-living catalysts and other materials for numerous applications.

Chemical reactions can be sped up with the use of catalysts materials that lower the overall energy barrier for a reaction. The chemical industry uses catalysts consisting of metal nanoparticles dispersed in three-dimensional porous matrices for the synthesis of transportation fuels and various chemical feedstocks. One shortcoming of these catalysts, however, is their finite lifetime due to deactivation. This can occur when the metal nanoparticles fuse into larger crystals. Researchers at the Center for Atomic-Level Catalyst Design, CALCD, are demonstrating that control over the spatial distribution of metal nanoparticles is a powerful strategy for designing catalysts with longer lifetimes.

At the heart of every catalyst, be it the enzymes in your body or the converter in your car, lies an inorganic element, often a metal. Being engineers far less impressive than our own bodies, we have historically relied on trial and error to develop catalyst technologies, some applications of which include sensors, gas storage, batteries and fuel synthesis. The new tools developed by CALCD, further discussed in a recent study published in Nature Materials, pave the way towards the rational design of longer-living catalysts and other materials for these applications. As a result, scientists believe progress in these fields can be accelerated.

Smaller is better. Generally, a higher metal surface area in catalysts translates into improved overall performance. This increased surface area is realized through smaller metal particles, and in this case, down to the nanometer scale. However, freely suspended small nanoparticles don't last too long for most purposes because they tend to move around (even within solids) and can fuse into larger, less-effective clusters. This translates to a shorter catalyst lifetime, greater frequency of replacement and, ultimately, higher costs. It is therefore advantageous to set these nanoparticles within a solid three-dimensional structure. Scientists at CALCD were the first to prepare copper nanoparticles smaller than 6 nanometers embedded into a silica honeycomb-like matrix.

Out of order comes longevity. CALCD scientists are developing novel catalyst design tools and three-dimensional imaging technologies to explore and optimize the spatial distribution of copper nanoparticles within their silica matrices. Until now, it has been difficult to control both the particle size and the inter-particle distance in catalyst frameworks. In fact, this issue hasn't received much attention because no definitive correlation has been drawn between this characteristic and catalyst performance. Specifically, no methods previously existed to simultaneously elucidate individual properties of the nanoparticles, such as their size, in addition to collective features such as their positioning relative to their metal neighbors. The researchers at CALCD take a step toward overcoming these limitations, using a three-dimensional electron microscopy technique to visualize and measure inter-particle distances between copper nanoparticles. They found that, relative to many commercial catalysts, larger inter-particle distances enabled increased catalyst lifetime when applied to the conversion of syngas to methanol. Thus, larger inter-particle spacing prevents what may be migration of nanoparticles into larger, less efficient structures.

More Information

Prieto G, J Zecevic, H Friedrich, KP de Jong and PE de Jongh. 2013. "Towards Stable Catalysts by Controlling Collective Properties of Supported Metal Nanoparticles." Nature Materials 12:34-39. DOI: 10.1038/nmat3471

Acknowledgments

This material is largely based on work supported as part of the Center for Atomic-Level Catalyst Design, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences. The research of G Prieto was wholly funded by the Center for Atomic-Level Catalyst Design. J Zecevic and H Friedrich were funded by the NRSCC. PE de Jongh and KP de Jong were funded by Utrecht University.

About the author(s):

  • Enoch Dames is a postdoctoral fellow at the Combustion Energy Frontier Research Center. He is currently at Stanford University studying experimental design, uncertainty quantification and minimization, and gas-phase reaction rate theory.

Standing Alone to Get the Job Done Faster

Tiny catalytic copper particles produce methanol longer when evenly distributed

From their roles in petroleum refineries to solar and fuel cells, improving catalyst performance requires determining and controlling the arrangement of the active metal sites at the nanoscale. Scientists are utilizing imaging technology from biology to advance our understanding of what makes an efficient and long-lasting catalyst.

To create fuels, whether simple methanol or complex hydrocarbons, a catalyst can be used to increase the rate at which these fuels are produced by reducing the energy barriers to their formation. A well-designed catalyst can reduce costs for petroleum and chemical manufacturers. A stumbling block to designing catalysts has been the lack of methods to characterize the size of the catalytic particles and their collective features, such as their position relative to neighboring particles. Scientists have addressed this challenge using an instrument classically applied in biology to visualize cells. This tool, a three-dimensional electron microscope, allowed the team to obtain images and precise measurements of the distance between tiny catalytic copper particles. They found that, relative to many commercial catalysts, greater distances between the particles let the catalyst function longer, creating more methanol before the catalyst deactivated. Uniform particle spacing could be preventing migratory particle fusing, which leads to large, inefficient structures. As the need for energy grows, fundamental answers about how to design catalysts for petroleum refineries, solar cells or fuels cells will become increasingly vital. Scientists with the Center for Atomic-Level Catalyst Design, led by Louisiana State University, performed this research.

More Information

Prieto G, J Zecevic, H Friedrich, KP de Jong and PE de Jongh. 2013. "Towards Stable Catalysts by Controlling Collective Properties of Supported Metal Nanoparticles." Nature Materials 12:34-39. DOI: 10.1038/nmat3471

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.