Modelling free and oxide-supported nanoalloy catalysts: comparison of bulk-immiscible Pd–Ir and Au–Rh systems and influence of a TiO2 support

Abstract

The relative stabilities of different chemical arrangements of Pd–Ir and Au–Rh nanoalloys (and their pure metal equivalents) are studied, for a range of compositions, for fcc truncated octahedral 38- and 79-atom nanoparticles (NPs). For the 38-atom NPs, comparisons are made of pure and alloy NPs supported on a TiO₂(110) slab. The relative energies of different chemical arrangements are found to be similar for Pd–Ir and Au–Rh nanoalloys, and depend on the cohesive and surface energies of the component metals. For supported nanoalloys on TiO₂, the interaction with the surface is greater for Ir (Rh) than Pd (Au): most of the pure NPs and nanoalloys preferentially bind to the TiO₂ surface in an edge-on configuration. When Au–Rh nanoalloys are bound to the surface through Au, the surface binding strength is lower than for the pure Au NP, while the Pd-surface interaction is found to be greater for Pd–Ir nanoalloys than for the pure Pd NP. However, alloying leads to very little difference in Ir-surface and Rh-surface binding strength. Comparing the relative stabilities of the TiO₂-supported NPs, the results for Pd–Ir and Au–Rh nanoalloys are the same: supported Janus NPs, whose Ir (Rh) atoms bind to the TiO₂ surface, bind most strongly to the surface, becoming closer in energy to the core–shell configurations (Ir@Pd and Rh@Au) which are favoured for the free particles.