title: First-principles local density approximation (generalized gradient approximation) +U study of catalytic CenOm clusters: U value differs from bulk.
abstract: Ceria possesses strong catalytic properties for CONO(x) removal and H(2) production. Clusters often show more intriguing functionalities than their bulk counterparts. Here, the geometric and electronic structures of Ce(n)O(m) (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 Ce(n)O(2n-1) clusters, though less so on the stoichiometric Ce(n)O(2n). 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 Ce(n)O(m) 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 Ce(n)O(m) clusters are far greater than bulk CeO(2).
authors.affiliation: School of Physics and Engineering, Zhengzhou University, Zhengzhou 450001, People's Republic of China.
authors.forename: S F
authors.initials: SF
authors.lastname: Li
authors.forename: Heqiang
authors.initials: H
authors.lastname: Lu
authors.forename: Pinglin
authors.initials: P
authors.lastname: Li
authors.forename: Zongxian
authors.initials: Z
authors.lastname: Yang
authors.forename: Z X
authors.initials: ZX
authors.lastname: Guo