Interaction of Gold with Cerium Oxide Supports: CeO2(111) Thin Films vs CeOx Nanoparticles

Morphology, electronic structure, and CO adsorption of gold supported on well-ordered CeO2(111) thin films and CeOx nanoparticles were studied by scanning tunneling microscopy (STM), photoelectron spectroscopy (XPS), and infrared reflection absorption spectroscopy (IRAS). Ceria nanoparticles grown on crystalline thin silica films possess Cc in both the 3+ and 4+ oxidation states, with the former dominating in smaller particles. Upon deposition on CeO2(111) films, most of the Au particles are formed at the step edges. The particles on terraces grow presumably through the decoration of point defects, which are more numerous on reduced surfaces. Combined XPS and IRAS data show that partially charged Au delta+ species are formed by deposition at low temperatures (similar to 100 K) and low coverages on both ceria supports. Formation of Au delta+ on CeO2(111) films is kinetically limited and is attributed to the interaction of the gold ad-atoms with defects. In variance to extended ceria surfaces, where only metallic Au nanoparticles are observed at 300 K, the "cationic" gold species are formed in abundance on nano-CeOx and exhibit enhanced thermal stability. It is shown that nanoceria stabilizes small Au clusters, which may even be incorporated into the ceria nanoparticles at elevated temperatures.