X-rays reveal compositional changes on active surface under reaction conditions – sciencedaily
A DESY-led research team used high-intensity X-rays to observe a single catalyst nanoparticle at work. The experiment revealed for the first time how the chemical composition of the surface of an individual nanoparticle changes under the reaction conditions, making it more active. The team led by Andreas Stierle from DESY presents its results in the journal Science Advances. This study marks an important step towards a better understanding of real industrial catalytic materials.
Catalysts are materials that promote chemical reactions without themselves being consumed. Today, catalysts are used in many industrial processes, from the production of fertilizers to the manufacture of plastics. For this reason, catalysts are of enormous economic importance. A well-known example is the catalytic converter installed in car exhaust systems. These contain precious metals such as platinum, rhodium and palladium, which convert highly toxic carbon monoxide (CO) into carbon dioxide (CO2) and reduce the amount of nitrogen oxides ( NOx) harmful.
“Despite their widespread use and great importance, we still ignore many important details about how different catalysts work,” explains Stierle, director of DESY NanoLab. “This is why we have long wanted to study real catalysts in operation.” This is not easy, because in order to make the active surface as large as possible, catalysts are usually used in the form of tiny nanoparticles, and changes that affect their activity occur on their surface.
Surface deformation is related to the chemical composition
As part of the European Nanoscience Foundries and Fine Analysis (NFFA) project, the DESY NanoLab team has developed a technique to label individual nanoparticles and thus identify them in a sample. “For the study, we cultured nanoparticles of a platinum-rhodium alloy on a substrate in the laboratory and labeled a specific particle,” explains co-author Thomas Keller of DESY NanoLab and project manager at DESY. “The diameter of the labeled particle is around 100 nanometers, and it is similar to the particles used in a car’s catalytic converter.” A nanometer is a millionth of a millimeter.
Using x-rays from the European ESRF synchrotron facility in Grenoble, France, the team was not only able to create a detailed image of the nanoparticle; it also measured the mechanical stress inside its surface. “Surface deformation is related to the composition of the surface, in particular the ratio of platinum atoms to rhodium atoms,” explains co-author Philipp Pleßow of the Karlsruhe Institute of Technology (KIT), whose group calculated the deformation depending on the composition of the surface. By comparing the observed and calculated facet-dependent strain, conclusions can be drawn regarding the chemical composition at the particle surface. The different surfaces of a nanoparticle are called facets, just like the facets of a cut gemstone.
When the nanoparticle grows, its surface is mainly made up of platinum atoms, because this configuration is energetically favored. However, scientists have studied the shape of the particle and its surface deformation under different conditions, including the operating conditions of an automotive catalytic converter. To do this, they heated the particle to around 430 degrees Celsius and allowed carbon monoxide and oxygen molecules to pass through it. “Under these reaction conditions, the rhodium inside the particle becomes mobile and migrates to the surface because it interacts more strongly with oxygen than platinum,” Pleßow explains. This is also predicted by theory.
“As a result, the surface deformation and shape of the particle changes,” reports co-author Ivan Vartaniants, of DESY, whose team converted x-ray diffraction data into three-dimensional spatial images. “A facet-dependent rhodium enrichment takes place, whereby additional corners and edges are formed.” The chemical composition of the surface, as well as the shape and size of the particles have a significant effect on their function and efficiency. However, scientists are only just beginning to understand exactly how these are connected and how to control the structure and composition of nanoparticles. X-rays allow researchers to detect changes of as little as 0.1 in a thousand in the strain, which in this experiment corresponds to an accuracy of around 0.0003 nanometers (0.3 picometers).
Crucial step towards the analysis of industrial catalytic materials
“We can now, for the first time, observe the details of the structural changes of these catalyst nanoparticles during their operation,” says Stierle, senior scientist at DESY and professor of nanosciences at the University of Hamburg. “This is a big step forward and it helps us understand a whole class of reactions that use alloy nanoparticles.” Scientists from KIT and DESY now want to explore this systematically in the new Collaborative Research Center 1441, funded by the German Research Foundation (DFG) and titled “Tracking the Active Sites in Heterogeneous Catalysis for Emission Control (TrackAct)”.
“Our investigation is an important step towards the analysis of industrial catalytic materials,” Stierle stresses. Until now, scientists had to develop model systems in the laboratory to conduct such investigations. “In this study, we have gone to the limit of what can be done. With the PETRA IV X-ray microscope provided by DESY, we will be able to observe individual particles ten times smaller in real catalysts and under reaction conditions. . “