Electron Microscopy Advances in Catalysis
Written by Prachi Patel
Stig Helveg set the stage for his talk by emphasizing the importance of catalysis for a sustainable future. Catalysis is the backbone of chemical reactions to produce fertilizers, chemicals, and fuels. It will be critical for increasing food supply, reducing air pollutant emissions, and for harnessing renewable energy.
“It’s a field we cannot live without today and also, of course, tomorrow,” he said, before delving into the goal and the promise of his pioneering work on atomic-scale transmission electron microscopy under working reaction conditions to deepen the understanding of catalysis and advance catalyst research.
Helveg first highlighted how recent advances in electron microscopy have been able to address challenges in catalyst research. With tremendous advances in electron microscopy over the past 10 years, scientists can now observe materials at the atomic scale. Helveg and his colleagues, for example, published an article in 2014 showing how they were able to resolve the sites of single cobalt atoms in a Co‐Mo‐S catalyst that is used in desulfurization processes at oil refineries by analyzing the material at the single‐atom level. “There has been a debate for decades where Co atoms lie,” he said. “This really shows the power of electron microscopy.”
His work has been devoted to harnessing this power for insights into nanostructured catalysts. Researchers have in recent years focused on developing nanocatalysts given their abundant sites for catalysis, and to increase the activity of those sites by sculpturing and composing them. The challenge is that nanostructures are very sensitive to changes in their surroundings, he said. Chemical reactions can alter their surface, morphology, phase, and make them clump together or break apart. Understanding what happens to catalysts at the nanoscale under industrial chemical reaction conditions is critical to improve catalysis, and has driven his research at Haldor Topsoe A/S, Helveg said.
“Introducing reaction conditions into a microscope is really like mixing fat and water,” he said. Microscopes are high-vacuum machines that work at room temperature, while chemical plants work at high pressures and temperatures. Nonetheless, he and others have developed technologies such as aperture gas cells that have allowed performing reaction conditions from an industrial plant in a microscope. These tools have yielded key insight into several phenomena in catalysts, such as generation of active sites, restructuring, and compositional changes. Helveg offered a more detailed look on two case studies that illustrate the practical application of these techniques.
In one, they used high-resolution TEM to study vanadium oxide supported on titanium dioxide, which is a catalyst for removing nitrogen oxide emissions. They found that the vanadium oxide (001) facets oscillate between a clean edge and a smeared edge as the researchers switched between oxidizing and reducing conditions. Using this information on the catalyst system, Helveg is now working with researchers at Aarhus University and Innovation Fund of Denmark to create a better NOx reduction catalyst.
Another case study involves the use of a specialized window cell that the Haldor Topsoe A/S team developed with researchers at TU Delft and ThermoFisher Scientific. The cell is a tiny gas flow channel made with two 1-µm-thick silicon nitride membranes separated by 5-µm-tall pillars. Windows that are just 15-nm wide act like a nanoreactor, allowing atomic-resolution imaging of sub-nanoliter volumes of catalyst at ambient pressures of up to 14 bar and elevated temperatures of 660°C.
This device helped the research group elucidate previously unseen phenomenon during the oxidation of carbon monoxide by platinum. Many catalytic reactions oscillate, and it has been known for decades that this is because of dynamic changes to the catalyst surface. But no one had observed these changes in nanoparticle catalysts. Using their nanoreactor, Helveg and his colleagues saw that the nanocrystals undergo reversible refacetting, going back and forth between a rounded shape to a more pointed shape as the reaction oscillates.
To sum up, Helveg stressed how advances in electron microscopy have allowed the study of nanoparticle dynamics under meaningful catalytic conditions, and how he and his colleagues have linked their observations to functional analysis. Going forward, he appeals to those in the electron microscopy field to look into two critical issues: improving the understanding of the right electron dose for a chemically relevant signal, and understanding the mass-temperature distribution inside commercial reactor devices. “The future certainly looks very bright from where we are,” he concluded.
The Innovation in Materials Characterization Award honors an outstanding advance in materials characterization that notably increases knowledge of the structure, composition, in situ behavior under outside stimulus, electronic behavior, or other characterization feature, of materials. It is not limited to the method of characterization or the class of materials observed.
Helveg’s award citation is “for pioneering atomic-scale transmission electron microscopy under reactive gas environments, leading to groundbreaking insights in catalysis, crystal growth and corrosion.”
The Innovation in Materials Characterization Award has been endowed by Dr. Gwo-Ching Wang and Dr. Toh-Ming Lu.