Bimetallic Pd–Rh Nanoparticles Supported on Co3O4(111): Atomic Ordering and Stability

Abstract

We have investigated the atomic ordering and stability of monometallic Rh and Pd nanoparticles and bimetallic Pd@Rh and Rh@Pd core@shell nanoparticles supported on well-ordered Co₃O₄(111) films on Ir(100) by means of synchrotron radiation photoelectron spectroscopy and scanning tunneling microscopy. The thermal stabilities of these model systems are controlled by the electronic metal support interaction associated with charge transfer at the metal/oxide interface. This effect is most pronounced in the Rh/Co₃O₄(111) model system. It is associated with the formation of atomically dispersed Rh³⁺ species at the metal/oxide interface and the growth of highly dispersed Rh nanoparticles. The system is stable up to 450 K. Annealing of the Rh/Co₃O₄(111) model system triggers sintering of the Rh nanoparticles above 450 K and Rh dissolution into the Co₃O₄(111) substrate above 550 K. The morphologies of the Pd@Rh and Rh@Pd core@shell nanoparticles are similar to those observed for the Rh/Co₃O₄(111) model system. With respect to atomic ordering, the Rh@Pd core@shell nanoparticles are fairly stable, while segregation of Pd in the Pd@Rh core@shell nanoparticles occurs upon annealing to 550 K. Above 550 K, redistribution of the charge at the metal/oxide interface leads to sintering, dissolution of Rh into the Co₃O₄(111) substrate and collapse of the core@shell nanoparticles. In particular, phase separation in the Pd@Rh and Rh@Pd core@shell nanoparticles occurs upon annealing above 550 K, yielding Rh-rich and Pd-rich nanoparticles on Co₃O₄(111).