Growth and characterization of Rh and Pd nanoparticles on oxidized and reduced CeO_x(111) thin films by scanning tunneling microscopy

The growth and structure of Rh and Pd nanoparticles on vapor-deposited ceria thin films grown epitaxially on Ru(001) were investigated by scanning tunneling microscopy as a function of coverage, postdeposition annealing temperatures, as well as ceria oxidation states. Both metals grow as three-dimensional nanoparticles on the fully oxidized CeO₂(111) thin films at room temperature. At low coverage, Rh particles preferentially decorate step edges on ceria, forming particles with smaller average size than those on the terraces. With increasing coverage, the number of Rh particles increases until near 2.3 monolayers, where the Rh particles cover most of the ceria surface and the particle size becomes relatively uniform. Larger Rh or Pd particles can be prepared by annealing the surface after deposition at 300 K; however, the particle size distribution becomes broader during this process. Similar growth behavior was observed on the reduced ceria surfaces. It is concluded that metal particle size or morphology is not responsible for previously reported differences in surface chemistry observed when Rh is deposited on reduced CeO_x compared to fully oxidized CeO₂. Instead, it is proposed that the enhanced dissociation occurs at the interface between the metal and the reduced support, which, coupled with rapid O diffusion, may lead to high dissociation fractions.