On Identification of Labile Oxygen in Ceria-Based Solid Solutions: Which Oxygen Leaves the Lattice?

Abstract

Solid solutions of ceria with other transition metal cations have been known to exhibit enhanced reducibility and oxygen storage capacity. Formation of differential bond lengths in the lattice of solid solutions has been proposed to be responsible for the improvement in their catalytic activity. We have carried out a density functional theory analysis of ceria-based solid solutions (Ce1–xMxO2−δ (M = Zr/Mo/Pd)) to identify the role of cation substitution and vacancy formation in bond length distribution. Different anionic vacancies in close proximity of the substituted cation were identified and the effects of location of the vacancy on bond length distribution and vacancy formation energy were studied. A large improvement in the net oxygen activation was observed with the introduction of a cation in the lattice. Further, nearly 17 kcal/mol reduction in vacancy formation energy was observed for Pd-substituted compound when compared to CeO2 when the vacancy was induced at a site directly coordinated with Pd. This successfully explained reported high activity of Ce1–xPdxO2−δ for redox reactions following dual site mechanism allowing the interaction of surface species in close proximity.