In situ X-ray imaging of segregation and mixing in PtPd core-shell nanoparticles under methane oxidation conditions

The high global warming potential of methane requires the development of advanced catalysts for conversion with increased activity and a longer lifetime. Here we utilize in situ Bragg coherent diffraction imaging (BCDI) to follow the evolution of two PtPd model catalyst nanoparticles during methane oxidation with different initial arrangements, one core-shell and one partially alloyed nanoparticle. While heating the nanoparticles under reaction conditions we observe the mixing and segregation of Pt and Pd, indicating a partial alloying and de-alloying, and a preferred segregation of Pt towards {111} oriented surface facets with potential use as bifunctional catalyst. Our in situ BCDI data also indicate a complete core-shell inversion at elevated temperatures with Pd moving into the nanoparticle. This suggests that a maximum operation temperature should not be exceeded, since Pd oxide is the more active phase. In spite of such limitations, binary metal core-shell nanoparticles are promising candidates for novel catalysts, as the elemental arrangement at their surface may be better controlled - a pivotal step for improving future catalysts.