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Temperature-Dependent Evolution of the Oxidation States of Cobalt and Platinum in Co1-xPtx Clusters under H2 and CO + H2 Atmospheres

Abstract : In this study, Co 1–xPt x clusters of 2.9-nm size with a range of atomically precise Pt/Co atomic ratios (x = 0, 0.25, 0.5, 0.75, 1) were synthesized using the mass-selected low-energy cluster beam deposition (LECBD) technique and soft-landed onto an amorphous alumina thin film prepared by atomic layer deposition (ALD). Utilizing ex situ X-ray photoemission spectroscopy (XPS), the oxidation state of the as-made clusters supported on Al 2O 3 was determined after both a 1-h-long exposure to air and aging for several weeks while exposed to air. Next, the aged cluster samples were characterized by grazing-incidence X-ray absorption spectroscopy (GIXAS) and then pretreated with diluted hydrogen and further exposed to the mixture of diluted CO and H 2 up to 225°C at atmospheric pressure, and the temperature-dependent evolutions of the particle size/shape and the oxidation states of the individual metal components within the clusters were monitored using in situ grazing-incidence small-angle X-ray scattering and X-ray absorption spectroscopy (GISAXS/GIXAS). The changes in the oxidation states of Co and Pt exhibited a nonlinear dependence on the Pt/Co atomic ratio of the clusters. For example, a low Pt/Co ratio (x ≤ 0.5) facilitates the formation of Co(OH) 2, whereas a high Pt/Co ratio (x = 0.75) stabilizes the Co 3O 4 composition instead through the formation of a Co–Pt core–shell structure where the platinum shell inhibits the reduction of cobalt in the core of the Co 1–xPt x alloy clusters. Finally, the obtained results indicate methods for optimizing the composition and structure of binary alloy clusters for catalysis.
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Submitted on : Monday, September 16, 2019 - 5:01:25 PM
Last modification on : Tuesday, December 8, 2020 - 10:35:16 AM

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Bing Yang, Ghassan Khadra, Juliette Tuaillon-Combes, Eric C. Tyo, Michael J. Pellin, et al.. Temperature-Dependent Evolution of the Oxidation States of Cobalt and Platinum in Co1-xPtx Clusters under H2 and CO + H2 Atmospheres. Journal of Physical Chemistry C, American Chemical Society, 2016, 120 (38), pp.21496-21504. ⟨10.1021/acs.jpcc.6b06483⟩. ⟨hal-02289454⟩

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