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Compositional segregation in shaped Pt alloy nanoparticles and their structural behaviour during electrocatalysis

Abstract

Shape-selective monometallic nanocatalysts offer activity benefits based on structural sensitivity and high surface area. In bimetallic nanoalloys with well-defined shape, site-dependent metal surface segregation additionally affects the catalytic activity and stability. However, segregation on shaped alloy nanocatalysts and their atomic-scale evolution is largely unexplored. Exemplified by three octahedral PtxNi1−x alloy nanoparticle electrocatalysts with unique activity for the oxygen reduction reaction at fuel cell cathodes, we reveal an unexpected compositional segregation structure across the {111} facets using aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy. In contrast to theoretical predictions, the pristine PtxNi1−x nano-octahedra feature a Pt-rich frame along their edges and corners, whereas their Ni atoms are preferentially segregated in their {111} facet region. We follow their morphological and compositional evolution in electrochemical environments and correlate this with their exceptional catalytic activity. The octahedra preferentially leach in their facet centres and evolve into ‘concave octahedra’. More generally, the segregation and leaching mechanisms revealed here highlight the complexity with which shape-selective nanoalloys form and evolve under reactive conditions.

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Figure 1: Atomic-scale Z-contrast STEM images and composition profile analysis of PtxNi1−x octahedral nanoparticles.
Figure 2: Electrochemical studies on carbon-supported PtxNi1−x octahedra.
Figure 3: STEM-EELS analysis of Pt1.5Ni and PtNi octahedra after 25 potential cycles.
Figure 4: HRTEM images of PtxNi1−x octahedra after stability tests.
Figure 5: Morphology and surface structural changes of PtxNi1−x octahedra.

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Acknowledgements

We thank the Zentraleinrichtung für Elektronenmikroskopie (Zelmi) of the Technical University Berlin for their support with TEM and energy-dispersive X-ray spectra techniques. This work was supported by US DOE EERE award DE-EE0000458 via subcontract through General Motors. P.S. acknowledges financial support through the cluster of excellence in catalysis (UniCat).

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P.S. and C.C. conceived and designed the experiments. C.C. carried out the chemical synthesis and the electrochemical experiments and analysed the results. L.G. and M.H. performed the HRTEM and STEM-EELS experiments, evaluated and analysed the results. P.S. and C.C. aggregated the figures and co-wrote the manuscript. All authors discussed the results, drew conclusions and commented on the manuscript.

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Correspondence to Peter Strasser.

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Cui, C., Gan, L., Heggen, M. et al. Compositional segregation in shaped Pt alloy nanoparticles and their structural behaviour during electrocatalysis. Nature Mater 12, 765–771 (2013). https://doi.org/10.1038/nmat3668

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