Theoretical Study of Ripening Mechanisms of Pd Clusters on Ceria.

We carried out density functional theory calculations to investigate the ripening of Pd clusters on CeO2 (111). Starting from stable Pd n clusters ( n = 1-21), we compared how these clusters can grow through Ostwald ripening and coalescence. As Pd atoms have mobility higher than that of Pd n clusters on the CeO2 (111) surface, Ostwald ripening is predicted to be the dominant sintering mechanism. Particle coalescence is possible only for clusters with less than 5 Pd atoms. These ripening mechanisms are facilitated by adsorbed CO through lowering barriers for the cluster diffusion, detachment of a Pd atom from clusters, and transformation of initial planar clusters.