Different looks, different responses
Observing the shape of palladium nanoparticles during redox cycles
Catalysts are used to promote chemical reactions without being consumed themselves. In cars, for example, metals such as platinum, rhodium or palladium are used to strip the exhaust of harmful gases via reduction and oxidation (redox) reactions. Such catalysts often come in the shape of nanoparticles. Due to their small size, they have a large surface-to-volume-ratio and thus higher reactivity than materials which consist of larger units. However, not all nanoparticles of a specific material exhibit the same shape. They may have different sizes or orientations with respect to the substrate on which they were grown. This might alter their functionality, but little is known about the details of these relationships.
A team of researchers from Germany investigated the exact shapes of palladium nanoparticles and their alteration during oxidation and reduction with X-ray diffraction. For this purpose, they deposited palladium nanoparticles on a magnesium oxide substrate by means of molecular beam epitaxy. The scattering experiment was carried out at the P23 In situ X-ray diffraction and imaging beamline at PETRA III, DESY, and used a LAMBDA 750k detector. The sample was transferred into an ultra-high vacuum (UHV) chamber and exposed to changing levels of oxygen and recurring high temperatures, similar to the conditions in common catalyst applications. During the redox cycle, X-ray scattering images were recorded at a rate of 1 Hz. “The essentially noise-free photon counting capability of the LAMBDA detector enabled us to measure weak scattering signals from the different facets of the nanoparticle with low background levels”, said lead author Simon Chung from DESY. “This enabled us to capture large portions of the particle’s reciprocal space maps in a time efficient manner.”
Setup at the P23 beamline at PETRA III.
|PETRA III, DESY, P23 beamline
|1536 x 512 pixels
From the scattering signal, the researchers determined the nanoparticles’ dimensions and inferred that they exhibited the shape of a truncated octahedron. For most but not all particles, the main crystal axes were aligned with those of the substrate: They grew in a preferred orientation, cube on cube. However, around one per cent of the nanoparticles had a different orientation: they were rotated around the normal of the cube-on-cube surface by 3.7 degrees. Those particles also changed their shape differently during the redox cycles if compared to the cube-on-cube nanoparticles. The latter, in turn, developed considerable strain as concluded from their asymmetric scattering signals. All shape changes were reversible upon reduction. As of now, it is still uncertain what exactly this means for the catalytic functionality, but clearly, the heterogeneity of the nanoparticles’ shapes needs to be considered when investigating their catalytic response. With regards to the methods, the study demonstrated that X-ray scattering can be used as a tool not only to study the shape of nanoparticles in an ensemble, but also to observe strain of the nanoparticle facets in situ.
Two shapes of palladium nanoparticles.
Simon Chung, Jan-Christian Schober, Steffen Tober, Daniel Schmidt, Azat Khadiev, Dmitri V. Novikov, Vedran Vonk, & Andreas Stierle: Epitaxy and Shape Heterogeneity of a Nanoparticle Ensemble during Redox Cycles. ACS Nano 2021 15 (8), 13267-13278. DOI: 10.1021/acsnano.1c03002
This information sheet illustrates a real-world application of a LAMBDA 750k camera, developed and manufactured by X-Spectrum. We gratefully acknowledge the voluntary support by the scientists mentioned in this sheet.