Promoters control pacemakers on catalytic nanoparticles

Catalysts are essential for numerous chemical technologies, ranging from cleaning vehicle exhausts to producing valuable chemicals and energy carriers. Often, tiny traces of additional substances, known as ‘promoters’, are used alongside catalysts to make them even more effective.

Although they play a crucial role in catalyst technology, promoters have proven notoriously difficult to study. In most cases, determining the effect that a certain quantity of a promoter has on a catalyst has been a trial-and-error process.

Now, however, researchers at the Vienna University of Technology (TU Wien) in Austria have managed to directly observe the role of lanthanum (La) promoters in hydrogen oxidation. Using high-tech microscopy methods, they were able to visualize the role of individual La atoms.

This revealed that two surface areas of the catalyst act as pacemakers, similar to the conductors in an orchestra, and that the promoter plays a vital role in this process, by controlling the pacemakers. The researchers report their findings in a paper in Nature Communications.

"Many chemical processes use catalysts in the form of tiny nanoparticles," says Günther Rupprechter from the Institute of Materials Chemistry at TU Wien. While the performance of a catalyst can be easily determined from analyzing the products generated by the catalytic reaction, microscopic insights cannot be gained by this approach.

This has now changed. Over several years, Rupprechter and his team have developed sophisticated methods for directly observing individual nanoparticles during a chemical reaction. These methods allow them to see how the activity changes at different locations on the nanoparticle surface over the course of the reaction.

"We use rhodium nanotips that behave like nanoparticles," says Rupprechter. "They can serve as catalysts, for example when hydrogen and oxygen are combined to form water molecules – the reaction we are examining in detail."

In recent years, the TU Wien team has demonstrated that specific regions of nanoparticle surfaces oscillate between an active and an inactive state. This means that sometimes the desired chemical reaction occurs at certain locations, while at other times it does not.

Using dedicated microscopes, the researchers have shown that these oscillations occur on each nanoparticle in parallel, and they all influence each other. Certain regions of the nanoparticle surface, often only a few atom diameters wide, play a more significant role than others: they act as highly efficient ‘pacemakers’, even controlling the chemical oscillations of other regions.

Promoters can interfere in this pacemaker behavior, and that is precisely what the methods developed at TU Wien allowed the researchers to investigate.

When rhodium is used as a catalyst, lanthanum can serve as a promoter for catalytic reactions. The researchers placed individual lanthanum atoms on the tiny surface of a rhodium nanoparticle and then investigated the nanoparticle’s catalytic activity in both the presence and absence of the promoter. This approach revealed in detail the specific effect of individual lanthanum atoms on the progress of the chemical reaction.

Maximilian Raab, Johannes Zeininger and Carla Weigl at TU Wien performed the experiments. "The difference is enormous," says Raab. "A lanthanum atom can bind oxygen, and that changes the dynamics of the catalytic reaction." The tiny amount of lanthanum alters the coupling between different areas of the nanoparticle.

"Lanthanum can selectively deactivate certain pacemakers," explains Zeininger. "Imagine an orchestra with two conductors – we would hear quite complex music. The promoter ensures that there is only one pacemaker left, making the situation simpler and more ordered."

In addition to the measurements, the team, supported by Alexander Genest and Yuri Suchorski, developed a mathematical model to simulate the coupling between the nanoparticle's individual areas. This approach offers a more powerful way to describe chemical catalysis, producing a complex model that considers how different areas of the catalyst switch between activity and inactivity, and how, controlled by promoters, they mutually influence each other.

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.