Simulated Annealing Study of the Structure and Reducibility in Ceria Clusters

Abstract

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 n O 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 n O 2n and Ce n O 2n-1 , where charge neutrality was maintained by changing two Ce 4+ ions to Ce 3+ . 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.

Catalytic oxidation of volatile organic compounds (n-hexane, benzene, toluene, o-xylene) promoted by cobalt catalysts supported on γ-Al2O3-CeO2

“…It is important to note that the greater reducibility of Co 10 /γ-Al 2 O 3 -CeO 2 did not translate into higher VOC oxidation activity. However, other factors to be considered are the presence of active sites, such as Co 2+ and Co 3+ ions, strong interaction between the oxide phases and oxygen vacancies on the Co 20 /γ-Al 2 O 3 -CeO 2 catalyst surface as confirmed by the characterization results discussed above, which is related to the oxidation of hydrocarbons (Cordatos et al, 1996;Lefez et al, 1996;Putna et al, 1999;Volta and Portefaix, 1985;Todorova et al, 2009;Todorova et al, 2011). The cobalt enrichment on the catalyst surface (as evidenced by the XPS data) may have contributed to the higher activity observed for the Co 20 /γ-Al 2 O 3 -CeO 2 catalyst, since cobalt is a very active species in oxidation reactions.…”

The Water–Gas-Shift Reaction on Pd/Ceria–Praseodymia: The Effect of Redox Thermodynamics

“…This may indicate that the Pr ?3 ions form preferentially at the surface of the catalyst. Indeed, calculations with ceria clusters have suggested that ?3 ions should form at the surface first in order to maximize Coulombic interactions [35]. Whether this could be changed by different synthesis conditions is unclear.…”

The effect of oxide dopants in ceria on n-butane oxidation

“…In addition to the XPS results shown here, there are some indications that enrichment should be expected for at least some of these dopants. For example, energy-minimization calculations of CeO 2 reduction indicate that Ce +3 ions should form most easily at the surface because this will minimize the penalty associated with the lower Coulombic attraction of +3 ions with lattice-oxygen anions [37]. There are also indications from Low Energy Ion Scattering studies with Gd 0.2 Ce 0.8 O 1.9 that the surface is enriched to a Ce:Gd ratio of 1 [38].…”