Simulated annealing study of the structure and reducibility in ceria clusters

The structure and reducibility of ceria clusters was investigated using simulated annealing and interionic potentials, given by Sayle and co-workers (Sayle, T. X. T.; Parker, S. C.; Catlow, C. R. A. Surf. Sci. 1994, 316, 329), which include terms for ionic and van der Waals interactions, overlap repulsions, and ion polarizability. For Ce_nO_2n, the lattice energies, pair-distribution functions, and angular-distribution functions were determined for the crystals at 0 K for n = 2-20 and n = 50. As expected, the lattice energy decreased with n. Only for the largest cluster was the fluorite structure clearly observed. This cluster also exhibited (111) surface facets, indicating that this is the most stable surface. The reducibility of the clusters was determined from the difference in lattice energies of Ce_nO_2n, and Ce_nO_2n-1, where charge neutrality was maintained by changing two Ce⁴⁺ ions to Ce³⁺. The energy required to reduce the cluster generally increased with increasing cluster size, but large fluctuations were also observed. These results generally agree with the experimental observations that reduction of ceria is structure sensitive and that larger crystals are more difficult to reduce.