Activation of Oxygen in Ce2Zr2O7+x across Pyrochlore to Fluorite Structural Transformation: First-Principles Analysis

Cationic substitution of zirconium in ceria (CeO2) greatly improves its oxygen storage capacity (OSC) and thermal stability. Although the fluorite structure of Ce0.5Zr0.5O2 (Ce2Zr2O8) exhibits good oxygen storage and release properties, its reduced counterpart Ce2Zr2O7 in the pyrochlore structure derived from the fluorite structure does, not. Here, we present an analysis of the structural evolution of Ce2Zr2O7+z from pyrochlore-Ce2Zr2O7 (z = 0) to fluorite-Ce2Zr2O8 (z = 1) using first-principles density functional theoretical calculations and bond-valence theory and correlate the consequent activation of oxygen to the observed oxygen storage capacity. The gradual addition of oxygen atoms to the otherwise ordered vacant tetrahedral sites for anions in pyrochlore-Ce2Zr2O7 structure will lead to a transition to fluorite-Ce2Zr2O8 structure, and we demonstrate,that this transition involves an increase in the number of weakly bonded, activated oxygen sites that areliertinent to higher OSC observed for the fluorite-Ce0.5Zr0.5O2 structure. The structural descriptors of OSC demonstrated:here will facilitate the understanding and rational design of oxide materials with improved OSC, which is key to catalyzing a number of reactions for Various applications.