Islands in the exhaust stream: X-rays get nano

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  • Published: Apr 15, 2018
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Islands in the exhaust stream: X-rays get nano

Edging efficiency

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

X-ray diffraction data sensitive to metal nanoparticle surface structure has been combined with in situ mass spectrometry to observe the ambient pressure carbon monoxide oxidation over a platinum-rhodium catalyst.

Catalytic converters are increasingly important in cleaning exhaust emissions from vehicles. Improving their efficiency at removing noxious gases, such as NOx and CO can be improved by the use of catalytic nanoparticles having many edges rather than their being smooth or spherical. This detail has emerged from an X-ray study carried out at the Deutsches Elektronen-Synchrotron's PETRA III source. The team from the DESY NanoLab observed carbon monoxide molecules being converted into carbon dioxide on the surface of noble metal nanoparticles similar to those used in standard vehicle catalytic converters. The results of the study detailed in the journal Physical Review Letters suggest that edge effects are key to improving efficiency. Given that the facets of nanoparticle catalysts ultimately become poisoned by the accumulation of an oxide layer which accrues in tiny islands on the facets, the edges preclude the merging of such islands and sustain active sites on the nanoparticle surface where catalytic reaction and thus an effective oxygen supply can keep the oxidation going.

Growing cats

The scientists at DESY’s NanoLab grew platinum-rhodium (PtRh) nanoparticles on a substrate so that almost all of the particles were in alignment. The particles were truncated octahedral. They then put the nanocatalyst to work under typical working conditions for a vehicle catalytic converter, with different exhaust gas compositions in a reaction chamber. They used the P09 beamline at PETRA III to probe the reactions, while mass spectrometry revealed the proportions of certain types of molecules in the exhaust emissions, specifically, the relative concentrations of carbon monoxide, oxygen, and carbon dioxide.

“We carry out a kind of emission test on the nanoparticles,” explains first author Uta Hejral, who has since moved to Lund University in Sweden. Because the nanoparticles are aligned in parallel, the scientists could also work out which surfaces were the best for the oxidation reaction. “Here, we can really follow the reaction on an atomic scale,” Hejral explains.

In a vehicle catalytic converter, the noble metal nanoparticles are attached to tiny crumbs of substrate, which stick together forming complex structures that are rather difficult to study, but less costly and easier to make than pristine laboratory setups. “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,” adds Andreas Stierle, who is a lead scientist at DESY and a professor of nanoscience at the University of Hamburg, Germany. “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.”

Activity sharpener

The team saw a sharp increase in activity at a specific oxygen concentration. “This happens when just enough oxygen is available to oxidize each carbon monoxide molecule and turn it into carbon dioxide,” adds Stierle. If the oxygen concentration is higher than that optimum, then reactivity falls because a rhodium oxide sandwich layer grows on the surface of the particles, poisoning the catalyst and blocking further activity.

“The surface oxide eventually forms a closed layer over the nanoparticles,” explains Hejral. “This is unfavourable 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.” The team suggests that this finding could lead to a more effective catalytic converter design. “We would expect catalytic converters to be increasingly efficient the more edges the nanoparticles have for a given surface area,” says Stierle. The discovery could also have implications for industrial catalyst design.

Related Links

Phys Rev Lett 2018, 120, online: "Identification of a Catalytically Highly Active Surface Phase for CO Oxidation over PtRh Nanoparticles under Operando Reaction Conditions"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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