1. Introduction

Au-Rh nanoalloys were prepared from colloidal solution [1]. The growth mechanism was studied in situ by TEM in a graphene liquid cell [2-3]. Then, in an environmental sample holder [4],the influence of oxygen or hydrogen adsorption on the structure of AuRh/TiO₂ catalysts was observed at a pressure of a few mbar and at room temperature.

2. Growth mechanism

The colloidal solution was encapsulated in a graphene oxide liquid cell and directly observed in a standard electron microscope. The particle growth was initiated under the electron beam.

The same area was observed during about 80 s and images were recorded every 5 s. The average particle size increases until 4 nm. The density number plotted in fig. 1, indicates a maximal density after 25 s, then a drop after 50 s, corresponding to a growth mechanism by direct adsorption of the monomers. This process is followed by coalescence of the  particles, as directly seen in fig. 2.

3. Segregation in hydrogen

Au-Rh nanoparticles with an homogeneous structure, and supported on rutile TiO₂ nanorods powders, were observed during oxidation- reduction cycles in an environmental sample holder. In the largest NPs, core- shell formation was clearly observed during hydrogen adsorption, as seen in figure 3. On the other hand, the observation of Au-Rh NPs annealed ex situ in H₂ at 400 °C, also shows the core- shell contrast which was not visible before H₂ treatment.

The strong interaction between hydrogen and Rh is certainly responsible for the surface segregation of Rh.

Acknowledgments:

References

Figures:

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