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CATALYSIS

June 2023 ESRFnews

evolution of defects in the nanoparticles as they oxidised the carbon monoxide into carbon dioxide (Nat. Commun. 12 5385). Although the results did not give any direct clues about how to improve nanoparticles for exhaust catalysis, they did show the potential of the technique, with the clarity provided by the new X-ray source.

What the team really wanted to measure was the surface strain the local distortion of the crystal lattice, which determines where atomic reactants are adsorping during catalysis. So, in an experiment this year, they made an electrochemical cell with platinum nanoparticles 200 400 nm in size on a carbon substrate functioning as one of the electrodes. By changing the electric potential across the cell from one scan to the next, they could change the level of adsorption of (bi-)sulfate anions hydrogen (H) and oxygen (O) atoms as dilute sulphuric acid (H2SO4) flowed over. It s very demanding to work with an electrochemical cell, says Corentin Chatelier, a postdoc in Richard s team. If any of the nanoparticles move or rotate just a little bit, you don t see anything. Everything has to be perfect at every step.

Strain has been studied in catalysts since the 1990s, and scientists had always assumed it to be evenly distributed across nanoparticle surfaces. However, Richard s group found that it is, in fact, heterogeneously distributed, reaching its highest levels around a nanoparticle s edges and corners (Nat. Mater. doi:10.1038/s41563-023- 01528-x). Since strain promotes reactant adsorption, says Clément Atlan, the first author of the work, we might want to be growing nanoparticle crystals with a lot of corners.

The researchers next want to do a similar experiment in which electrocatalysis actually occurs, as that would truly mark the start of an era in which the shape of individual nanoparticles can be designed with specific catalytic reactions in mind. We re just at the beginning of what we can do, says Richard. 

Jon Cartwright

Around a decade ago, improvements in focusing optics and Bragg coherent diffraction imaging made it possible for Richard to image the structure of particles down to about 100 nm. Now, with its unprecedented X-ray brightness and coherence, the EBS has finally allowed Richard to study single catalyst particles down to just 20 nm, in working conditions  and she has a grant from the European Research Council to make the best of it. I needed the EBS to come to life, she says. She is not the only one: users have been exploiting the potential of the EBS for studies of catalysis at various ESRF beamlines (see The EBS: A catalyst for science , above).

Richard and her colleagues begin their experiments by drying out a platinum film on a substrate, such as sapphire or carbon, until they are left with individual platinum nanoparticles of various sizes. On a robotic nano-positioning platform, the substrate can then be automatically rastered through the ID01 beamline in increments as small as 2 nm. The ID01 X-ray beam can be focused down to 30 nm. The substrate gives no diffraction signal, but when the beam hits one of the nanoparticles, the detector lights up, and the researchers know they have struck gold  or in this case, platinum.

Bragg coherent diffraction imaging is especially sensitive to atomic displacement and strain. At ID01, the single scan of a nanoparticle takes two or three minutes; phase-retrieval software is then required to turn the diffraction peaks into real-space atomic coordinates, which reveal shape, strain and lattice displacement at picometre resolution. Multiple scans provide a way to see the evolution of a nanoparticle as it catalyses, even under changing conditions. ID01 is dedicated to this kind of experiment, says Richard. Everything is set up here to make sure it all runs smoothly.

In 2021, in the first year of EBS user operation, Richard and colleagues studied platinum nanoparticles exposed to carbon monoxide, argon and oxygen at temperatures between 450 and 500 °C, a similar environment to a car exhaust. The diffraction imaging revealed the 3D

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ID01 is dedicated to this kind of experiment. Everything is set up here to make sure it all runs smoothly

THE EBS: A CATALYST FOR SCIENCE

Besides ID01, various other ESRF beamlines are involved in the study of catalysis. In the past two years, for example:

the ESRF s Anastasia Molokova and others have employed X-ray absorption and emission spectroscopy at BM23 to find out why catalysts used to remove harmful nitrogen oxides from diesel exhaust are easily restricted by just small amounts of sulphur dioxide (JACS Au 2 787);

at ID26, the ESRF s Viktoriia Saveleva and colleagues have used X-ray absorption and emission spectroscopy to study the structural composition of iron nitrogen carbon a promising alternative catalyst to platinum for fuel cells before and after it has been crafted into an electrode (ACS Appl. Energy Mater. 6 611);

Asger Moss at the Technical University of Denmark and others have used X-ray diffraction at ID31 to understand why catalyst-containing devices used to convert carbon dioxide into useful chemicals can become unstable during operation (Energy Environ. Sci. 16 1631).

Nanoparticle catalysts offer good value, but until now the way their structures change during catalysis has been poorly understood.