Wherever you look in the chemical, material, and energy industries, behind the scenes you will likely find a catalyst at work. In basic terms, catalysts are materials used to improve the rate of chemical reactions. Although this sounds like a simple definition, the result is that catalysts are versatile and valuable materials which underpin a range of important processes we depend on every day. One example is the automotive catalytic convertor, responsible for treating harmful emissions from internal combustion engines. Billions of cars are on the road each day around the world, so even small efficiency increases in removing harmful chemicals such as carbon monoxide, nitrogen oxides, and particulate matter from their exhausts, can have a huge impact on air quality and human health. However, to improve on existing catalytic materials and processes, we first need to know exactly how they work, and this is no simple task. "Whether in a large chemical reactor, in a battery, or in the back of your car, technical and industrial catalysts often have a highly complex structure," says Thomas Sheppard from the Institute for Chemical Technology and Polymer Chemistry (ITCP) at KIT, who led the study. "But often they are considered as a uniform ‘black box’. To really understand how these materials function, we need to take a look inside the reactor when the catalyst is working, so to shine a light inside this black box. But the dream scenario would be an analytical tool to detect the complex 3D structure of the active catalyst at the same time, which is pretty challenging."
3D images of a sample
The team led by Sheppard (KIT) turned to X-ray computed spectrotomography approaches developed by Dario Ferreira Sanchez from PSI to solve this puzzle, performing experiments in close cooperation with synchrotron radiation facilities. The experiments were performed at the microXAS beamline in Swiss Light Source SLS in Switzerland, and pilot experiments were also done at the ID24 beamline in the ESRF, in France. "Computed tomography produces 3D images of a sample, including the exterior and interior, without needing to cut it open", says Dario Ferreira Sanchez. By using a special reactor, they were able to perform tomography and X-ray spectroscopy of a catalytic process at work, and therefore track the 3D structure of an emission control catalyst under conditions just like those in a real automotive exhaust. This approach, known as operando X-ray spectrotomography, provides not only the 3D structure of the sample but also important chemical information. "Since catalysts often show such a complex and non-uniform structure, it is important to know whether the entire catalyst volume or only parts of it are performing their chemical function as intended," explains Johannes Becher from ITCP, one of the main authors of the study. "Operando spectrotomography lets us see exactly that. We can take 3D images of the active catalyst, looking at the specific structure and function of every single piece. This tells us if the catalyst is performing at maximum efficiency or not, and more importantly it helps us to understand the reasoning behind this." The team observed structural gradients of the active copper species within the catalyst during reaction, which have not previously been detected using conventional analytical tools. This is important diagnostic information in the performance of emissions control catalysts, but the method itself can be applied to many different catalysts and chemical processes. "Until now it was not possible to freely select any piece of a working catalyst and understand which reactions take place in there without disturbing it. Here we can follow which reactions are occurring, and why," explains Jan-Dierk Grunwaldt, also from ITCP. "This opens the door to evaluation of technical catalysts in a manner which was not possible before. This is really the key to improving our understanding of chemical processes and designing better and more efficient catalysts in future."
Based on a media release by Karlsruhe Institute of Technology (KIT).