Catalysts based on metal nanoparticles play an important role in energy conversion and environmental technologies. Their high catalytic efficiency is attributed to their large surface-to-volume ratio and their high number of low-coordination sites, such as edges, which can decrease kinetic barriers between reactants. A key factor influencing the reactivity is the interaction between the reactants and the catalyst, which is closely linked to structural changes of the catalyst itself. Indeed, several studies have reported changes of the overall shape or size of nanocatalysts during catalytic processes. Using measurements at the Advanced Photon Source (APS) of Argonne National Laboratory (ANL) in the USA and the PETRA III synchrotron source at the DESY research centre in Germany, an international team led by Sogang University in Korea has now observed a strong distortion of the crystal lattice at the edges of metal nanocrystals during catalysis. The results identified the edges as the active sites underlying the catalytic activity at the atomic scale.
The team studied the heterogeneous catalytic oxidation of methane on platinum nanocrystals as an example process. Their in situ Bragg coherent X-ray diffractive imaging (BCDI) measurements at PETRA III were performed using a LAMBDA detector, whose small pixel area of 55 µm by 55 µm – about half the one of conventional detectors – enabled the BCDI experiment to be carried out in the first place. This high spatial resolution allowed the team to observe a strong contraction at the edges of the nanoparticles during adsorption of the oxygen. The strain further increased when the methane was introduced and continued during the oxidation of the methane. After the catalytic process was completed, the nanoparticles returned to their original state. As the team demonstrated with their innovative BCDI study, reaction mechanisms obtained from in situ strain imaging provide important insights for improving catalysts and designing future nanostructured catalytic materials.
Active site localization of methane oxidation on Pt nanocrystals
Nature Communications 9, 3422 (2018)