First look inside nanoscale catalysts shows defects are useful

Research in Nature validates hypothesis correlating high density of atomic defects with high catalytic reactivity

Jerusalem, 11/1 – Scientists using one of the brightest light sources to peer inside some of the smallest particles have been able to confirm a longstanding hypothesis: that disorder or “defects” at the edges of nanoparticles makes them more effective as chemical change agents.

The process by which a change agent, or catalyst, accelerates a chemical reaction is key to the creation of many materials essential to daily life, such as plastics, fuels and fertilizers. Known as catalysis, this process is a basic pillar of the chemical industry, making chemical reactions more efficient and less energy-demanding, and reducing or even eliminating the use and generation of hazardous substances.

Although catalysts have been used in industry for more than a century, scientists have yet to observe how their structure impacts their effectiveness as change agents. That’s because catalysts are typically tiny metallic nanoparticles made of precious metals such as Platinum, Palladium or Rhenium. The extreme smallness that makes nanoparticles such effective catalysts also makes it hard to see how they work.

If scientists could peer inside individual nanoparticles’ chemical reactions at a nanoscopic level, they would gather a treasure of useful knowledge for the design of improved catalysts to address the pressing energy needs of the 21st century.

That type of knowledge may now be close at hand, thanks to new research to be published in the prestigious journal Nature. In the new study, led by Dr. Elad Gross from the Institute of Chemistry and the Center for Nanoscience and Nanofabrication at the Hebrew University of Jerusalem, and Prof. F. Dean Toste from the College of Chemistry at University of California, Berkeley, and Chemical Science Division at Lawrence Berkeley National Laboratory, researchers directly observed for the first time how metallic nanoparticles, used as catalysts in numerous industrial processes, activate catalytic processes.

Using a light source one million times brighter than the sun, the researchers were able to observe chemical reactivity on single Platinum particles similar to those used as industrial catalysts. What they found is that chemical reactivity primarily occurs on the particles’ periphery or edges, while lower reactivity occurs at the particles’ center.

The different reactivity observed at the center and edges of Platinum particles corresponds to the different properties of the Platinum atoms in the two locations. The atoms are mostly flat at the center, while they’re corrugated and less-ordered at the edges. This disorderly or “defective” structure means that Platinum atoms at the edges are not totally surrounded by other Platinum atoms, and will therefore form stronger interactions with reactant molecules. Stronger interactions activate the reactant molecules and initiate a chemical reaction that will transform the reactant molecule into a desired product.

The research findings validate a well-known hypothesis in the world of catalysis, which correlates high catalytic reactivity with high density of atomic defects. It also shows, for the first time, that the enhanced reactivity of defected sites can be identified at the single-particle level.

To peer into individual nanoparticles, researchers focused a bright infrared beam generated in a synchrotron source (Advanced Light Source, Lawrence Berkeley National Laboratory) into a thin probe with an apex diameter of 20 nanometers. The probe acts as an antenna, localizes the infra-red light in a specific range, and by that provides the capabilities to identify molecules which reside on the surface of the catalytic nanoparticles. By scanning the particles with the nanometric probe while it is being radiated by the infrared light, the researchers identified the locations and conditions in which chemical reaction occurs on the surface of single particle.

Chemical reactivity was identified on the surface of single Platinum nanoparticles, which are similar to the particles which are used as catalysts in various industrial processes. The chemical reactivity was measured by focusing a bright infrared beam into the apex of a thin tip with a diameter of 20 nm that monitored the chemical reactivity on the particle’s surface. (Credit: Hebrew University of Jerusalem)

CITATION: High-spatial-resolution mapping of catalytic reactions on single particles. Chung-Yeh Wu, William J. Wolf, Yehonatan Levartovsky, Hans A. Bechtel, Michael C. Martin, F. Dean Toste, & Elad Gross…

The Hebrew University of Jerusalem is Israel’s leading academic and research institution, producing one-third of all civilian research in Israel. For more information, visit https://new.huji.ac.il/en.