Edges and corners increase efficiency of catalytic converters

X-rays reveal oxide islands on noble metal nanoparticles.

With increasing oxygen (red) concentration, an oxide sandwich forms on the surface of the metallic nanoparticles, inhibiting the desired reaction of carbon monoxide to carbon dioxide. At the edges, however, the oxide sandwich brakes up, leaving free active sites for catalysis. The more edges the nanoparticles posses, the more efficient will the catalytic converter work. Credit: DESY, Lucid Berlin
With increasing oxygen (red) concentration, an oxide sandwich forms on the surface of the metallic nanoparticles, inhibiting the desired reaction of carbon monoxide to carbon dioxide. At the edges, however, the oxide sandwich brakes up, leaving free active sites for catalysis. The more edges the nanoparticles posses, the more efficient will the catalytic converter work. Credit: DESY, Lucid Berlin

Catalytic converters for cleaning exhaust emission are more proficient when they utilize nanoparticles with numerous edges. This is one of the discoveries of an investigation completed at DESY’s X-beam source PETRA III. A group of researchers from the DESY NanoLab observed live as harmful carbon monoxide (CO) was changed over into regular carbon dioxide (CO2) on the surface of respectable metal nanoparticles like those utilized as a part of exhaust systems of autos.

The researchers are introducing their discoveries in the diary Physical Review Letters. Their outcomes propose that having countless expands the effectiveness of synergist responses, as the diverse features of the nanoparticles are frequently secured by developing islands of a nano-oxide, at last rendering these aspects latent. At the edges, the oxide islands can’t interface, leaving dynamic destinations for the synergist response and a productive oxygen supply.

Catalytic converters, for the most part, utilize nanoparticles in light of the fact that these have a far more noteworthy surface territory for a given measure of the material, on which the synergist response can happen. For the investigation introduced here, the researchers at DESY’s NanoLab developed platinum-rhodium nanoparticles on a substrate such that for all intents and purposes every one of the particles was adjusted a similar way and had a similar state of truncated octahedrons.

The researchers at that point considered the synergist properties of this example under the run of the mill working states of a car exhaust system, with various vaporous syntheses in a response chamber that was presented to exceptional X-beams from PETRA III on the P09 beamline.

Uta Hejral, the first author of the paper said, “We carry out a kind of emission test on the nanoparticles. Because of the parallel alignment of the nanoparticles, the scientists were also able to determine those surfaces of the nanoparticles on which the reaction went particularly well. “Here we can really follow the reaction on an atomic scale.”

Andreas Stierle, who is a lead scientist at DESY and a professor of nanoscience at the University of Hamburg said, “Normally, the noble metal nanoparticles in a car’s catalytic converter are attached to tiny crumbs of a substrate, which stick together forming complex structures. These are difficult to examine using X-rays because the noble metals only account for a few weight percent and in particular because the nanoparticles are aligned in all sorts of different directions.”

“Under X-ray illumination, every particle produces a separate diffraction pattern and these overlap to create a blurred image. By having them aligned in parallel to each other, on the other hand, the diffraction patterns of all the nanoparticles are superimposed and amplify each another. This allows the different facets of the nanoparticles, in other words, their individual surfaces, to be identified and specifically observed.”

“The investigation showed that the reactivity of the nanoparticles increases sharply at a certain oxygen concentration. This happens when just enough oxygen is available to oxidize each carbon monoxide molecule and turn it into carbon dioxide. Beyond that concentration, the reactivity gradually drops again because a thick oxide layer grows on the surface of the particles, impeding the reaction.”

“The X-ray analysis reveals the atomic structure of the surface of the nanoparticles at the best resolution yet under the conditions at which the reaction occurs. This shows that once a certain oxygen concentration is exceeded, the different crystal faces of the nanoparticles become coated with an oxygen-rhodium-oxygen sandwich until eventually the surface of the metal is completely covered by this nano-oxide layer.”

“The surface oxide eventually forms a closed layer over the nanoparticles. This is unfavorable for the desired reaction at first because it makes it difficult for carbon monoxide molecules to attach themselves to the surface. However, the oxygen is unable to form a closed film along the edges between the faces of the nanoparticles, which means that the reactivity along the edges is higher.”

This finding suggests a direct pathway to making catalytic converters more efficient: “We would expect catalytic converters to be increasingly efficient the more edges the nanoparticles have for a given surface area.”