| AVS 62nd International Symposium & Exhibition | |
| In-Situ Spectroscopy and Microscopy Focus Topic | Monday Sessions |
| Session IS+AS+SS-MoM |
| Session: | Fundamental Studies of Surface Chemistry of Single Crystal and Nanomaterials under Reaction Conditions |
| Presenter: | Bing Yang, Material Science Division, Argonne National Laboratory |
| Authors: | B. Yang, Material Science Division, Argonne National Laboratory G. Khadra, Institut Lumière Matière, University Lyon & CNRS, France J. Tuaillon-Combes, Institut Lumière Matière, University Lyon & CNRS, France E. Tyo, Material Science Division, Argonne National Laboratory S. Seifert, X-ray Science Division, Argonne National Laboratory X. Chen, Department of Mechanical Engineering, Northwestern University V. Dupuis, Institut Lumière Matière, University Lyon & CNRS, France S. Vajda, Material Science Division, Argonne National Laboratory |
| Correspondent: | Click to Email |
CoPt alloy particles have recently attracted great interests for their excellent catalytic and magneticproperties. The alloy phase of cobalt and platinum may create dual-functional sites at the mixed interface which enables novel catalytic properties and synergic effect at nanometer scale. In-situ characterization is thus essential to probe the structure and composition of bimetallic clusters under reaction conditions in a catalytic process of interest.
Co1-xPtx bimetallic clusters with atomic-precise Pt/Co atomic ratio (x=0, 0.25, 0.5, 0.75, 1) were synthesized using mass-selected low energy clusters beam deposition (LECBD) technique and soft-landed onto the amorphous alumina thin film prepared by atomic layer deposition (ALD). The median diameter of size-selected Co1-xPtx alloy clusters is 3nm with size dispersion lower than 10 % according to transmission electron microscopy (TEM). Utilizing X-ray photoemission spectroscopy (XPS), the oxidation state of as-made clusters as well as the aged particles after extended exposure to air was characterized. After exposure to air, both cobalt and platinum species in the bimetallic clusters are found to be oxidized, while the shift of their covalent state exhibits a non-linear correlation with their atomic composition (Pt/Co).
Utilizing in-situ grazing incidence small-angle X-ray scattering and X-ray absorption spectroscopy (GISAXS/GIXAS), the evolution of particle size/shape and the oxidation state of the individual metals are monitored under atmosphere reaction conditions. The as-made Co1-xPtx clusters were first pretreated with hydrogen and further exposed to CO and H2 mixture up to 225oC. The change in the oxidation state of Co and Pt of the supported bimetallic clusters exhibited a non-linear dependency on the Pt/Co atomic ratio. For example, low Pt/Co ratio (x≤0.5) facilitates the formation of Co(OH)2, whereas, high Pt/Co ratio (x=0.75) stabilizes Co3O4 composition instead, due to the formation of Co@Pt core-shell structure where the platinum shell inhibits the reduction of cobalt in the core of the Co1-xPtx alloy clusters.
In this work, we have demonstrated in-situ measurement of particle size/shape and the oxidation state of supported Co1-xPtx bimetallic clusters under operating conditions, and elucidated the different surface structure and chemical state with respect to their atomic ratio. The obtained results indicate ways for optimizing the composition of binary alloy clusters for catalysis.