Rh and Pt nanoparticles supported by CeO2: Metal-support interaction upon high-temperature reduction observed by electron microscopy

Pt and Rh nanoparticles obtained by epitaxial growth on NaCl (001) were coated with a crystalline ceria support and subjected to systematic hydrogen reduction in the temperature range 573 Kless than or equal toTless than or equal to973 K. The structural and morphological changes were examined by high resolution electron microscopy and selected area electron diffraction. Metal-support interaction between Pt particles and ceria becomes apparent after reduction in hydrogen at 723 K and results in alloy formation and reconstruction of the metal into regular cube-like Pt3Ce particles which are stable up to 973 K. In contrast, the reduction of the comparable rhodium-ceria system leads to larger particles of undefined shape which start to develop below 673 K and increase in size up to reduction at 973 K. Electron diffraction reveals again alloy formation, but unlike on Pt, several alloy phases (Rh3Ce, Rh2Ce and RhCe) are developed simultaneously and a defined topotactic growth does not occur. The thermal stability of the resulting phases and their resistance toward oxidation are discussed.