Gold supported on well-ordered ceria films: nucleation, growth and morphology in CO oxidation reaction

Structure of gold nanoparticles formed by physical vapor deposition onto thin ceria films was studied by scanning tunneling microscopy (STM). Gold preferentially nucleates on point defects present on the terraces of the well-ordered, fully oxidized films to a low density. The nucleation expands to the terrace step edges, providing a large variety of low-coordinated sites. Only at high coverage, the Au particles grow homogeneously on the oxygen-terminated CeO2(111) terraces. The morphology of Au particles was further examined by STM in situ and ex situ at elevated (up to 20 mbar) pressures of O-2, CO, and CO + O-2 at 300 K. The particles are found to be stable in O-2 ambient up to 10 mbar, meanwhile gold sintering emerges at CO pressures above similar to 1 mbar. Sintering of the Au particles, which mainly proceeds along the step edges of the CeO2(111) support, is observed in CO + O-2 (1: 1) mixture at much lower pressure (similar to 10(-3) mbar), thus indicating that the structural stability of the Au/ceria catalysts is intimately connected with its reactivity in the CO oxidation reaction.