A research team has taken a close look at catalytic surfaces at the DESY NanoLab. In doing so, they have not only learnt more about the exact processes that take place during catalysis, but have also tried out ways of positively influencing these processes. The team presents its results in the scientific journal ACS Nano.

Catalysts are important promotors in many industrial processes. They increase efficiency in the production of basic chemicals and continue to play a central role in their ‘classic’ application as exhaust gas purifiers in combustion engines because they reduce emissions of climate-damaging gases such as carbon dioxide or methane during combustion processes. They can therefore make important contributions to environmental protection and more cost-efficient production. 

However, what exactly happens at the atomic level when a catalyst accelerates a chemical reaction or enables it to take place at lower temperatures without changing itself is in many cases not yet well understood. The Collaborative Research Centre SFB1441 (‘Tracking the Active Site in Heterogeneous Catalysis for Emission Control’), funded by the German Research Foundation (DFG), in which DESY works closely with the Karlsruhe Institute of Technology (KIT) and the Technical University of Munich (TUM), has set out to get to the bottom of it. With the results that have just been published, they have come a good deal closer to their goal - but the researchers still have a lot of work ahead of them.

There is one material of choice for catalytic converters used for exhaust gas purification in combustion processes: platinum-palladium alloys. They sit on the surface of a carrier material, for example aluminium oxide, are non-reactive and therefore very stable. The number of surface atoms of the metal alloy therefore remains constant during the catalytic processes. And since the processes take place on the surface, creating as large a surface as possible is desirable on the one hand in order to have as much reaction surface as possible; on the other hand, one wants to know exactly how its surface composition changes in order to be able to influence the activity of the catalyst. ‘However, this is very difficult to investigate; the catalytic reactions take place under rather harsh environmental conditions such as high temperatures and high gas pressures,’ says Andreas Stierle, head of the DESY NanoLab group, senior scientist at DESY and professor of nanosciences at Universität Hamburg.

But this is exactly what the team of researchers has achieved: they have sent gas molecules into nanospace like tiny exploratory probes to observe where they accumulate on the surface. The gas, in this case carbon monoxide, binds to the metal atoms in the alloy in different places. Some settle directly on top of a platinum (Pt) atom, some between two palladium (Pd) atoms, while others feel most comfortable in the triangle between three Pd atoms. ‘With the help of infrared spectroscopy, we were able to recognise exactly where the carbon monoxide is located in the nanoworld and how its properties change depending on the location,’ says DESY researcher Heshmat Noei.

However, it is not only the observations from the DESY NanoLab that are important for understanding the processes on the catalyst surface - the team relies on extensive theoretical calculations carried out at the Karlsruhe Institute of Technology (KIT) to understand what is happening. ‘The direct comparison of theory with experiment is enormously important for such complex systems,’ says Philipp Plessow, head of the theory group at KIT.

The team found that the nature of the alloy also plays a key role. If there is more platinum on the surface, the carbon monoxide molecules only attach themselves directly on top of the atoms. If palladium dominates, they prefer to insert themselves between two or three atoms. ‘The surface composition of the material is key to its function,’ says first author Silvan Dolling, who carried out the study as part of his Master's thesis at Universität Hamburg. Platinum has a tendency to accumulate on the surface; however, the catalytic reactions run better with a good mixture of platinum and palladium. If the catalytic converter is pretreated accordingly, its surface properties can be modified to make it even more efficient.

Catalysts will also remain important for fuels of the future. ‘Green’ methane could be such a fuel for ships, and there are also models and test operations for “green” ammonia. However, neither should be released into the air - the first is unfortunately a highly effective greenhouse gas, while the other is toxic and also harmful to the climate. Well-understood and controllable catalytic converters will bring us a good deal closer to low-emission drive systems.  

Original publication: DOI 10.1021/acsnano.4c08291