Frontiers in Energy Research: April 2014

Confined Catalysts Last Longer

For renewable fuels production, cage-like support improves stability without hindering performance

Methanol-producing catalysts consist of copper nanoparticles (light dots) on porous silica (grey structure). Comparing catalysts on commercially available SiO2 supports with open porosity (left) with those based on cage-like porosity (right) highlights the dependency of deactivation on pore structure and that narrow constrictions between cages lead to stable catalysts (middle).

Catalytic materials play an important role in many facets of the energy economy. One every-day example is the catalytic converter in your car, which uses a precious metal that is wash-coated to a ceramic support to convert toxic exhaust products from the engine to a less toxic form before they leave the tailpipe. Most solid catalysts that interact with gases or liquids, such as the one in your car's catalytic converter, are actually nanoparticles as they maximize the material's effectiveness. Nanoparticles are typically metal clusters that can be as small as a few atoms.

Unfortunately, the effectiveness or activity of catalysts tends to decrease over time. One way this occurs is that the nanoparticles tend to merge into larger particles when the temperature is increased, decreasing the total surface area available for reactions. Thus, scientists want to understand and limit nanoparticle growth during catalyst use. One of the factors controlling nanoparticle growth is the support structure upon which the catalyst is fixed. Recently, researchers at the Center for Atomic-Level Catalyst Design studied a direct relationship between the rate of catalyst deactivation and the support structure.

In their work, the researchers investigated the effect of two supports for copper-based catalysts on the production of methanol. Methanol is considered a future sustainable fuel and building block for sustainable chemicals if produced from biomass-derived materials or other renewable sources. An important challenge associated with the large-scale production of methanol is the limited lifetime of the low-melting copper-based catalysts. The first support was a silica structure with open porosity that did not restrict the motion of the nanoparticles. The second was a support with similar average pore sizes, but the catalyst nanoparticles were placed in cages connected by openings smaller than the catalytic nanoparticles.

The scientists measured the production rate of methanol while flowing synthesis gas, a mixture of hydrogen, carbon monoxide, and carbon dioxide, over the catalyst. They then investigated the effect of the support structure (silica or cage-like) on the stability of the catalyst nanoparticles by producing methanol continuously for 10 days under typical industrial reaction conditions.

The scientists showed that for catalysts with very similar overall characteristics in terms of particle and pore sizes, the specific introduction of narrow constrictions in the pores could significantly enhance the stability of the catalysts, while not affecting the activity or selectivity. Hence, controlling the pore structure of the support material provides an efficient strategy towards a fundamental understanding of the causes of catalyst deactivation and sustainable catalysts.

Acknowledgments: 

This material is based upon work supported as part of the Center for Atomic-Level Catalyst Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information: 

Prieto, G, M Shakeri, KP de Jong, and PE de Jongh. 2014. "Quantitative Relationship between Support Porosity and the Stability of Pore-Confined Metal Nanoparticles Studied on CuZnO/SiO2 Methanol Synthesis Catalysts." ACS Nano 8(3):2522-2531. DOI: 10.1021/nn406119j

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