We
carried out density functional theory calculations to investigate
the ripening of Pd clusters on CeO2(111). Starting from
stable Pdn 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 Pdn 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.
First-principles
calculations have been performed to explore the
charge transport process over defective CeO2(111). Charge
transport can proceed either by direct migration of the oxygen anion
(i.e., vacancy diffusion) or by a polaron-hopping-assisted mechanism.
On the basis of DFT+U calculations, we found that
the latter process is significantly more favorable than the former.
The overall barrier for charge transport involving polaron migration,
followed by oxygen diffusion, is determined by the barrier for polaron
hopping, which amounts to 0.18 eV. This computed value is in good
agreement with the experimental barrier for ceria with a low defect
density. We have shown by a careful analysis of the magnetization
density, the density of states, and the reaction pathway trajectory
that this process is phonon induced. Our results provide valuable
insights into carrier drift processes over defective metal oxide surfaces.
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