Catalysts more complex than expected

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

At the Vienna University of Technology (TU Wien) in Austria, scientists have been trying to identify the reasons for this by using several different microscopy techniques to image the reactions taking place at various locations on these catalysts. Such an approach can yield a reliable, microscopically correct understanding of the catalytic process.

In this study, the scientists have discovered that even relatively ‘simple’ catalytic systems are more complex than expected. They found that it is not just 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 occur on the micrometer scale.

This finding, in combination with numeric simulations, has allowed the scientists to explain and correctly predict the behavior of different catalysts. They report their work in a paper in ACS Catalysis.

“We investigate the combustion of the possible future energy carrier hydrogen with oxygen, forming pure water, by using rhodium particles as catalysts,” explains Günther Rupprechter from the Institute of Materials Chemistry at TU Wien. Various parameters play an important role in this process, including the size of the individual rhodium particles, the support materials they bind to, and the temperature and reactant pressure.

“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,” says 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, may be catalytically inactive. And a few minutes later, the situation may even have reversed.”

For the experiments, Philipp Winkler, first author of the paper, prepared a stunning sample comprising nine different catalysts with differently sized metal particles and varying support materials. Using a dedicated apparatus, this meant all the catalysts could 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,” says 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.”

Chemical analysis on the microscopic scale has shown that the catalyst composition can vary locally even more than expected: Even within individual metal particles, strong differences were observed.

“Atoms of the support material can migrate onto or in the particles, or even form surface alloys,” says 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 these insights and their successful methods to 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.

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.