Nanoburgers show promising defects

Catalysts are indispensable in many industries: they speed up chemical reactions, making them economically viable. They often consist of tiny particles, just a few nanometres across, to which molecules attach themselves, making it easier for them to form a bond with another reagent.

Nanocatalysts made of platinum and rhodium are used for the purification of waste gases in hydrogen production and in fuel cells. Scientists led by DESY physicist Andreas Stierle had found that some of the nanoparticles are not tiny, homogeneous lumps but consist of an upper and a lower half – like the two halves of a burger bun. Although the two halves are stuck together, the nature of this bond and how it affects the catalytic properties of the nanoparticles was, until now, unclear.

In order to find answers, the team came to the ESRF’s ID01 beamline. Specifically, the researchers used a method known as Bragg Coherent Diffraction Imaging (BCDI), in which the X-ray beam creates a special diffraction pattern as it passes through the nanoparticle, and this is recorded by a detector. “Special algorithms can then be used to reconstruct how the atoms are arranged in the crystal lattice and where they deviate from the regular structure – distortions, defects and dislocations in the crystal lattice”, explains Ivan Vartanyants, the scientist at DESY who supervised the reconstructions.

The measurements were performed while the nanocatalysts were active. The group directed a stream of carbon monoxide and oxygen to pass over the nanoparticles, on which surface the gas was converted into CO2 – at temperatures of more than 400 degrees Celsius. “These experiments were extremely difficult; we had to keep the nanoparticles fixed to within ten nanometres so that the X-ray beam always illuminated the entire particle,” explains first author and PhD candidate Lydia Bachmann.

The outcome was unexpected: the experts discovered pronounced crystal defects where the upper and lower halves of the nanoburgers meet. The two boundary surfaces did not fit perfectly on top of each other; atoms were missing around the outer edges. These gaps cause all the atoms in the vicinity to shift, significantly distorting and displacing the crystal lattice.

What was truly remarkable was that these ‘flaws’ had an extremely positive effect on the catalytic properties of the nanoburgers. ‘The defects serve as unique absorption sites for molecules, ” explains co-author Thomas Keller. “Molecules such as oxygen adhere very well to them, which increases the effectiveness of the catalyst.” In the future, these findings could help industry to develop more efficient and effective catalysts – through deliberate ‘defect engineering’ to create as many binding sites as possible on the nanoparticles, where molecules can be converted.

The team plans to continue working towards this goal. Among other things, the scientists would like to know how the defects in the nanoburgers are created in the first place. “The particles are produced at temperatures of 1000 degrees Celsius, and we suspect that the defects form when the particles cool down rapidly,” says team leader Andreas Stierle. “It appears that, because the particles are so small, thermal stresses arise in them, which then disrupt the stacking order of the crystal layers.” If this process were understood in detail, it might be possible to optimise the manufacturing process to create specific crystal defects that are particularly effective at increasing the catalytic effect of the nanoparticles.

Reference:
Lydia Bachmann, et al, ACS Nano, 2025, DOI: 10.1021/acsnano.4c15457

Text adapted from DESY news