Synthesis and stability of Pd–Rh nanoalloys with fully tunable particle size and composition

• Carbon supported Pd–Rh nanoalloys are synthesized by direct and indirect methods.
• Nanoparticles are well distributed and average particle size can be easily tuned.
• Pd–Rh phase diagram at nanoscale is determined by Nano-Calphad approach.
• These calculation confirms that Pd–Rh nanoalloys are non-equilibrium phases.

Bulk immiscible Pd–Rh nanoalloys supported on carbon are easily formed with tunable compositions and average particle sizes by two different methods: an indirect method (incipient wetness impregnation followed by reduction under H2 at 300 and 500 °C) along with a direct one (one-pot route heat treated at 600 °C under Ar). The main advantage of our synthetic methods is the easy implementation as well as the formation of nanoalloys with well controlled composition and particle size distribution. For the indirect route at 300 °C, the nanoalloys have an average particle size of around 2 nm, irrespective of the composition. However, the average particle sizes show a strong composition dependence for higher synthetic temperatures: it decreases from around 6 to 2 nm with increasing Rh content for the indirect method at 500 °C, whereas it increases from around 6 to 10 nm for the direct method at 600 °C with increasing Rh content. Moreover, our calculation of the phase diagram taking into account the size effect within the Nano-Calphad approach confirms that these nanoalloys are non-equilibrium phases. The change of the binary phase diagram due to the size effect (decrease of the melting points of pure elements, reduction of the critical temperature along with modifications in the solubility limits of the miscibility gap) cannot account for the formation of Pd–Rh solid solutions at nanoscale.

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