What really happens during catalysis

Using a combination of spectromicroscopy at Bessy II and microscopic analyzes at the DESY NANOLAB, a team has acquired new information on the chemical behavior of nanocatalysts during catalysis.

Research is published in the journal Nano ACS.

Nanoparticles consisted of a platinum nucleus with a rhodium shell. This configuration allows a better understanding of structural changes, for example, for example, Rhodium catalysts – platinum for emission control. The results show that under typical catalytic conditions, part of the rhodium in the shell can diffuse inside nanoparticles. However, most of it remains on the surface and oxidizes. This process strongly depends on the surface orientation of nanoparticles facets.

Nanoparticles measure less than ten thousandth in a millimeter in diameter and have enormous surfaces compared to their mass. This makes them attractive as catalysts: metallic nanoparticles can facilitate chemical conversions, whether for environmental protection, industrial synthesis or production of (sustainable) fuel₂ and hydrogen.

Platinum kernel with rhodium shell

Platinum (PT) is one of the best known metallic catalysts and is used in catalysis in heterogeneous gas phase for emissions control, for example to convert toxic carbon monoxide into car exhaust gases from non -toxic combustion motors₂.

“The mixture of platinum particles with the rhodium element (HR) can further increase efficiency,” explains Jagarti Dwivedi, the first author of the publication. The location of the two elements plays an important role in this process. The so -called nucleus nanoparticles with a platinum nucleus and an extremely thin rhodium shell can help in search of the optimal distribution of elements that can prolong the lifespan of nanoparticles.

Experiences at Bessy II and Desy Nanolab

Until now, however, there was little known about how the chemical composition of the surface of a catalyst changes during operation. A team led by Dr. Thomas F. Keller, leader of the microscopy group in Desy Nanolab, has now studied these crystalline PT-RH nanoparticles at Bessy II and has acquired new information on changes to the facets of polyhedric nanoparticles.

The nanoparticles were first characterized and marked in their neighborhood using electron scanning microscopy and atomic force microscopy at DESY NANOLAB. These markers were then used to analyze the same spectroscopic nanoparticles and microscopically simultaneous images using X -ray light on a special instrument in Bessy II.

The intelligent instrument of the Fritz Haber Institute of the Max Planck Society allows the electron microscopy with Xpeemo photo (Xpeem) in microscope mode. This makes it possible to distinguish individual elements with a high spatial resolution, allowing the observation of chemical processes to atomic layers close to the surface.

“The instrument allows the chemical analysis of individual elements with a resolution of 5 to 10 nanometers, which is unique,” explains Keller.

The survey has shown that rhodium can be partially spreading in platinum nuclei during catalysis: the two elements are miscible at the typical operating temperatures of the catalyst. The mixture is improved in a reducing environment (h₂) and slows down in an oxidizing environment (o₂) without reverse the net flow of rhodium in the platinum.

“At higher temperatures, this process even increases considerably,” said Keller.

Different reaction rates

Reaction rates also depend on the orientation of the facets of nanoparticles.

“They are particularly high on certain facets,” said Jagrati Dwivedi. “Our study resolved by facets shows that the oxidation of the rhodium is the highest on the facets with many atomic stages, where atoms are more easily linked.”

This detailed analysis of the oxidation behavior will contribute to the additional optimization of these nanocatalysis, which can undergo irreversible changes during use.