Unprecedented view of a single catalyst nanoparticle at work

An X-ray analysis of a catalyst nanoparticle provided a 3D image that showed changes in its surface tension, surface chemical composition and operating modes. Credit: Science Communication Lab, DESY
A DESY-led research group has used high-intensity Xrays to see a single catalyst nanoparticle in action. This experiment revealed how the chemical composition of a single nanoparticle's surface changes under reaction conditions. It made it more active. Andreas Stierle, DESY's lead scientist, is presenting the team's findings in Science Advances. This research is an important step in the direction of better understanding industrial catalytic materials.

Catalysts can be materials that stimulate chemical reactions but are not consumed. Catalysts are used in many industrial processes today, including fertilizer production and plastic manufacturing. Catalysts are therefore of great economic value. One example of this is the catalytic convertor that is found in cars' exhaust systems. These precious metals, such as palladium, rhodium, and platinum, allow carbon monoxide (CO), which is highly toxic, to be converted into carbon dioxide (2CO 2 ), and reduce harmful nitrogen oxides (1NO x).

Stierle, the head of the DESY NanoLab, says that despite their widespread use and importance, many details about how different catalysts work are not known. "This is why we long wanted to see real catalysts in action. It is difficult because catalysts are often used as tiny nanoparticles to increase the active surface. The changes that affect their activity take place on their surface.

Close-up view (artist’s impression): The surface of the nanoparticle reacts with carbon monoxide to produce carbon dioxide. Credit: Science Communication Lab for DESY

Chemical composition is related to surface strain

The DESY NanoLab team has created a method for labeling nanoparticles and thereby identifying them within a sample. This was done in the framework of the EU project Nanoscience Foundries and Fine Analysis. Thomas Keller, co-author and project manager at DESY NanoLab, said that the study involved the growth of nanoparticles made of a platinum-rhodium-alloy alloy on a lab substrate. The labeled particle has a diameter of around 100 nanometres and is very similar to particles in a car's catalytic convertor. One millionth of a mile is one nanometre.

The team used X-rays taken at the European Synchrotron Radiation Facility ESRF in Grenoble (France) to take a detailed picture of the nanoparticle and also measure the mechanical strain. Philipp Pleow, co-author from the Karlsruhe Institute of Technology (KIT), explained that the surface strain was related to surface composition. His group calculated strain as a function of surface content. Comparing the computed and observed facet-dependent strain can help to determine the chemical composition of the particle's surface. A nanoparticle's different surfaces are known as facets. They are similar to the facets on a cut gemstone.

The nanoparticle's surface is mainly composed of platinum atoms. This configuration is highly energetically preferred. Scientists studied the particle's shape and surface strain under various conditions, including those of an automobile catalytic converter. They heated the particle to 430°C and let carbon monoxide, oxygen and molecules of oxygen pass through it. Pleow explains that the reaction conditions created a situation where the rhodium in the particle is mobile and migrates to its surface. This happens because it interacts with oxygen more strongly than platinum. This prediction is also supported by theory.

"As a consequence, the surface strain as well as the shape of particle change," says co-author Ivan Vartaniants from DESY. Vartaniants' team converted the Xray diffraction data to three-dimensional spatial images. "Facet-dependent enrichment occurs, which results in the formation of additional edges and corners." The surface's chemical composition and the size and shape of the particles can have significant effects on their efficiency and function. Scientists are still learning how these interact and how to control their structure and composition. Researchers can detect small changes in the strain using X-rays. This corresponds to a precision about 0.0003 nanometres (0.3 picometres)

Animation: In operation (diatomic carbon monoxide molecules oxidise into (triatomic carbon dioxide molecules on a particle). The characteristic diffraction pattern produced by X-ray radiation can be used to read changes in surface tension and chemical composition during operation. Credit: Science Communication Lab, DESY

Critical step towards the analysis of industrial catalyst materials

Stierle, Lead Scientist of DESY, and Professor for Nanoscience at the University of Hamburg, said that "we can now observe the details of structural changes in such catalyst-nanoparticles while they are in operation." This is a significant step forward, and it is helping us understand an entire class reaction that makes use of alloy nanoparticles. KIT and DESY scientists now plan to investigate this systematically at the Collaborative Research Centre (1441), funded by the German Research Foundation, which is entitled "Tracking Active Sites in HeterogeneousCatalysis for Emission Control" (TrackAct).

Stierle says, "Our investigation represents an important step towards the analysis of industrial catalytic material." To conduct these investigations, scientists had to create model systems in the lab. "In this study we have gone beyond the limits of what is possible. DESY's PETRA IV X-ray microscope will allow us to see individual particles ten times smaller in reaction conditions and real catalysts. DESY is one the most prominent particle accelerator centers in the world. It studies the structure and function matter, from the interaction of small elementary particles to the behavior of novel nanomaterials to the mysteries of the universe. The unique research tools that DESY creates and builds at its Hamburg and Zeuthen locations are the particle accelerators, detectors, and other devices. They produce the strongest X-ray radiation anywhere in the world, accelerate particles to record energies, and provide new insights into the universe. DESY is a member of Germany's largest scientific association, the Helmholtz Association. It receives funding from the German Federal Ministry of Education and Research, (BMBF), (90%) and the German Federal States of Hamburg and Brandenburg (10%).

Continue reading Edges and corners increase the efficiency of catalytic convertors

More information: Single Alloy Nanoparticle X-Ray Imaging during a Catalytic Reaction; Young Yong Kim, Thomas F. Keller, Tiago J. Goncalves, Manuel Abuin, Henning Runge, Luca Gelisio, Jerome Carnis, Vedran Vonk, Philipp N. Plessow, Ivan A. Vartanyants, Andreas Stierle; Science Advances, 2021; 10.1126/sciadv.abh0757 Journal information: Science Advances Single Alloy Nanoparticle X-Ray Imaging during a Catalytic Reaction; Young Yong Kim, Thomas F. Keller, Tiago J. Goncalves, Manuel Abuin, Henning Runge, Luca Gelisio, Jerome Carnis, Vedran Vonk, Philipp N. Plessow, Ivan A. Vartanyants, Andreas Stierle;, 2021; 10.1126/sciadv.abh0757