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Laurent_06821801_2023.pdf
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- High entropy alloys (HEAs) are gaining attention due to their unique properties and potential as heterogeneous catalysts. However, their atomic-scale dynamics remains understudied. In this work, a macroporous Ru-Pt-Pd-Ir-Rh HEA and seven simpler alloys were investigated using time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and X-ray diffraction. Starting with pure Ru and its binary combinations, the research sequentially progressed through ternary and quaternary compositions, culminating with the HEA. The methodology was optimised through statistical analysis of ToF-SIMS conditions to improve the detection of large-mass clusters. Initial insights from binary samples revealed unequal atomic mixing among them, ranking as Ru-Rh > Ru-Pd > Ru-Pt > Ru-Ir. Further investigations of alloys with three, four, and five components revealed a direct positive correlation between the number of alloy components and their degree of atomic mixing. With each additional component integrated into the alloys, ToF-SIMS analyses also demonstrated a notable decrease in the oxides present and a shift toward a more equitable distribution of oxide constituents within the samples. In addition to these structural analyses, CO oxidation tests offered insight into the reactivity. The resulting hierarchy of catalytic activity, in decreasing order, was Ru-Pt-Pd > Ru-Pt-Pd-Ir-Rh > Ru-Pt ≃ Ru-Pd ≃ Ru-Rh > Ru > Ru-Ir > Ru-Pt-Pd-Ir. In an attempt to decode these results, this work presents two primary hypotheses. First, it is suggested that the CO oxidation reaction predominantly takes place at sites containing Pt and Pd atoms. The close proximity of these two elements within the FCC phase of the ternary alloy is believed to significantly enhance the CO oxidation reaction. Secondly, the presence of Ir, especially in a relatively low-mixed alloy, might lead to the formation of "oxygen traps." This phenomenon could monopolise oxygen within the alloy, thus constraining its interaction with CO during oxidation.