First-principles local density approximation (generalized gradient approximation) +U study of catalytic Ce<inf>n</inf>O<inf>m</inf> clusters: U value differs from bulk

Abstract

Ceria possesses strong catalytic properties for CON Ox removal and H2 production. Clusters often show more intriguing functionalities than their bulk counterparts. Here, the geometric and electronic structures of Cen Om (n=1-4, m=2n-1, 2n) clusters are studied for the first time using the projected augmented wave method in density functional theory with detailed assessment of the exchange-correlation functional and the Hubbard parameter U. We note that the U value strongly affects the electronic structures of the oxygen-deficient Cen O2n-1 clusters, though less so on the stoichiometric Cen O2n. Furthermore, the local density approximation (LDA) +U method is more accurate than the generalized gradient approximation+U in describing the localization of the 4f electrons of the Cen Om clusters. The calculated vibration frequency of the CeO molecule with the LDA+U (U=4 eV) is 818.4 cm-1, in close agreement with experimental values of 820-825 cm-1 for the low lying states. Different optimal U values were noted for the ceria cluster (4 eV) and its bulk (6 eV), due to quantum-size and geometric effects. The largely reduced formation energy of an oxygen vacancy indicates that the catalytic effect of the Cen Om clusters are far greater than bulk Ce O2. © 2008 American Institute of Physics.