Transient Activity of Surface Reaction

Transient Activity of Surface Reactions Metal surfaces act as catalysts for many important applications, from fuel cells to purification of vehicle exhaust gases. However, their behavior is precisely influenced by the oxygen atoms involved in the surface. This phenomenon has long been known, but until now it has not been possible to fully investigate point by point the role of oxygen on complex surfaces in order to understand the precise chemical background at the atomic level. This has now been achieved at TU Wien in collaboration with a team from Elettra Synchrotron in Trieste. It was possible to explain why partially contradictory results had been obtained in previous work; oxygen atoms are not distributed evenly, but readily settle in very specific places.

Precise Measurements Instead of Average Values

Prof. Günther Rupprechter from the Institute of Materials Chemistry at TU Wien said, "Directly examining a metal surface during catalysis is a major challenge. Of course, you can put the entire catalyst in a reactor and measure exactly what chemical products are produced - but you only get an average value. You cannot know which regions of the catalyst contribute to the chemical reaction and in what way." Another possibility is not to use a real catalyst, but to use a simple, extremely clean, idealized piece of it - for example, a small single crystal with well-known properties that you can then examine under a microscope. In this case, you get precise, reproducible results, but they have little relevance to practical applications. The research group led by Günther Rupprechter and Yuri Suchorski therefore combined the advantages of both approaches. They used thin foils made of rhodium composed of small grains. On each grain, surface atoms can be arranged differently. On a single grain, they form a smooth, regular surface with outer atoms lying in exactly the same plane; next to it, atoms can arrange themselves to form a more complex structure consisting of many atomic steps. These steps are what really matter. Philipp Winkler, first author of the paper, said, "For catalytic activity, the oxidation state of the catalyst plays a central role - that is, whether oxygen binds to the metal atoms or not," and added "In previous experiments, we usually found ourselves dealing with a certain state between 'oxidized' and 'not oxidized' - a state that is difficult to interpret." But this became understandable when it was realized that not every grain of the rhodium foil was oxidized to the same degree. Oxidation preferentially starts at corners, edges and steps - places where oxygen atoms particularly easily bond to the surface. Therefore, different grains with different surface structures oxidize to different degrees.

Electron Microscopy and Synchrotron in Trieste

This can be examined using a combination of extremely advanced technologies: "In a special electron microscope, the sample is irradiated with UV light during the catalytic reaction and the resulting electron emission is recorded with micrometer spatial resolution," explained Yuri Suchorski. "This allows us to precisely determine which grains of the rhodium foil are particularly catalytically active. The same sample is then examined again with a completely different microscope; at the synchrotron with X-rays, grain by grain, very precise information about the sample's surface oxidation is obtained." If you combine both results, you can determine exactly what chemical behavior is characteristic for specific structures. The key advantage is that it is possible to examine the entire rhodium foil, which contains hundreds of different grains, in a single experiment. Instead of examining small single crystals separately, a sample containing many different structures used for catalysis is examined under real conditions and information about the properties of these structures is obtained at once. Rupprechter said, "Now we no longer have to content ourselves with measuring an average value that inadequately describes the entire sample, but can really understand in detail which atomic structures show which effects. This will also make it possible to specifically develop important catalysts required for many applications in energy and environmental technology." Source: More information: P. Winkler et al. How the anisotropy of surface oxide formation influences the transient activity of a surface reaction, Nature Communications (2021). DOI: 10.1038/s41467-020-20377-9 Journal information: Nature Communications / Provided by Vienna University of Technology / https://phys.org/news/2021-01-anisotropysurface- oxide-formation-transient.html