Catalyst Activity Goes Under The Micrometer Microscope

Vienna University of Technology scientists have developed methods to watch catalytic reactions with micrometer resolution under the microscope – and the process is much more complex than previously thought.

Microscope setup graphic. For a more complete explanation scroll down to Figure 1 in the published study paper. There the components broken out more completely and the results are graphically indicated. Image Credit: © 2023 The study paper authors. Institut für Materialchemie TU Wien. The study paper is not behind a paywall at posting. There are more images and explanations available by clicking the study paper link.

Usually, catalytic reactions are analyzed by checking which chemicals go into a chemical reactor and which come out. But as it turns out, in order to properly understand and optimize catalysts, much more information is necessary.

Catalysts composed from tiny metal particles play an important role in many areas of technology – from fuel cells to production of synthetic fuels for energy storage. The exact behavior of catalysts depends, however, on many fine details and their interplay is often difficult to understand. Even when preparing exactly the same catalyst twice, it often occurs that these two will differ in minute aspects and therefore behave very different chemically.

At TU Wien, scientists try to identify reasons for such effects by imaging the catalytic reactions taking place on various locations on these catalysts, applying several different microscopy techniques. Such an approach yields a reliable, microscopically correct understanding of the catalytic processes. The results have been published in ACS Catalysis.

In doing so, it appeared that even relatively “simple” catalytic systems were more complex than expected. For example, it is not only the size of the employed metal particles or the chemical nature of the support material that define the catalytic properties. Even within a single metal particle, different scenarios can prevail on the micrometer scale. In combination with numeric simulations, the behavior of different catalysts could then be explained and correctly predicted.

Not all particles are the same

 Prof. Günther Rupprechter from the Institute of Materials Chemistry at TU Wien explained, “We investigate the combustion of the possible future energy carrier hydrogen with oxygen, forming pure water, by using rhodium particles as catalysts.” Various parameters play an important role in this process: How big are the individual rhodium particles? Which support material do they bind to? At which temperature and which reactant pressures does the reaction take place?

“The catalyst is made from supported rhodium particles, but it does not behave like a uniform object which can be described by a few simple parameters, as often tried in the past,” highlighted Prof. Rupprechter. “It soon became clear, that the catalytic behavior strongly varies at different catalyst locations. A given area on a given rhodium particle may be catalytically active, whereas another one, just micrometers away, maybe catalytically inactive. And a few minutes later, the situation may even have reversed.”

Nine catalysts at one sweep

 For the experiments, the first author of the study as published in the journal ACS Catalysis, Dr. Philipp Winkler, prepared a stunning catalyst sample, comprising nine different catalysts with differently sized metal particles and varying support materials. In a dedicated apparatus, all catalysts could therefore be observed and compared simultaneously in a single experiment.

“With our microscopes, we can determine if the catalyst is catalytically active, it’s chemical composition and electronic properties – and this for each and every individual spot on the sample,” said Dr. Winkler. “In contrast, traditional methods usually just measure an average value for the entire sample. However, as we have demonstrated, this is often by far not sufficient.”

Even more complex than anticipated

 Chemical analysis on the microscopic scale has shown that the catalyst composition can vary locally even more than expected: Even within the individual metal particles strong differences were observed. “Atoms of the support material can migrate onto or in the particles, or even form surface alloys,” stated Prof. Rupprechter. “At some point, there is even no clear boundary anymore, but rather a continuous transition between catalyst particle and support material. It is crucial to consider this fact – because it also affects the chemical activity.”

In a next step, the team at TU Wien will apply the gained insights and the successful methods to tackle even more complex catalytic processes, in their continuing mission to explain processes on a microscopic scale, to contribute to the development of improved catalysts, and to search for new catalysts.

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Every step made into catalysts seems to up rate the value and understanding while seemingly opening the door to many more questions.

It also looks like the field is still just getting opened up. Once in a while we’ll see something with a two stage catalyst and when that research gets going the field will get really interesting.

The best news of all is news like this – where researchers are getting a solid look at what is going on and can build a much better database of what and how to do the challenging work coming in the future.

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