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Surface Segregation in Au–Ag Alloys Investigated by Atom Probe Tomography

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Abstract

Bimetallic alloys are increasingly used in heterogeneous catalysis because the synergistic effects between the two metals may create more efficient catalysts. As an example, a dilute amount of Ag within an Au catalyst allows to obtain a low temperature activity together with a large selectivity for selective oxidation reactions. In this context, we present a systematic study of segregations occurring on Au-8.8 at.% Ag samples by atom probe tomography (APT). The samples are prepared as sharp tips representing a single nanoparticle of catalyst. The segregation behaviours are studied by exposing the catalyst to unreactive (Ar), oxidative (O2 and N2O) and reductive (H2) atmospheres at different temperature (323, 373 and 473 K). With a dedicated reaction-cell mounted on the APT system, the exposure conditions are close to the operating conditions of applied formulations of the catalyst. The segregation lead to important surface composition changes, e.g. from Au91.2–Ag8.8 to Ag84–Au16. The obtained results highlight the presence of different Ag segregation behaviours due to surface diffusion after O2 exposure, and due to bulk diffusion after N2O exposure: the different dissociation properties of these two gases explain the different segregation behaviour. Gold segregation was also identified after sample treatments in H2 gas. These results are promising for the engineering of catalysts since it would be possible to design the surface composition by applying different treatments to the sample, therefore allowing a better control on the activity and selectivity of such designed catalysts.

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Acknowledgements

L.J. and C.B. thank the Fonds de la Recherche Scientifique (F.R.S.-FNRS) for financial support: PhD Grant from FRIA (L.J.) and postdoctoral fellowship from FNRS (C.B.).

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Authors and Affiliations

  1. Chemical Physics of Materials and Catalysis (CPMCT), Faculty of Sciences, Université Libre de Bruxelles, 2 Boulevard du Triomphe, CP243, 1050, Brussels, Belgium

    Natalia Gilis, Luc Jacobs, Cédric Barroo & Thierry Visart de Bocarmé

  2. Faculty of Sciences, Interdisciplinary Centre for Nonlinear Phenomena and Complex Systems (CENOLI), Université Libre de Bruxelles (ULB), Brussels, Belgium

    Cédric Barroo & Thierry Visart de Bocarmé

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  1. Natalia Gilis
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  2. Luc Jacobs
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  3. Cédric Barroo
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  4. Thierry Visart de Bocarmé
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Correspondence to Thierry Visart de Bocarmé.

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Gilis, N., Jacobs, L., Barroo, C. et al. Surface Segregation in Au–Ag Alloys Investigated by Atom Probe Tomography. Top Catal 61, 1437–1448 (2018). https://doi.org/10.1007/s11244-018-1040-0

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