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

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).