Activation of Molecular O2 on CoFe2O4 (001) Surfaces: An Embedded Cluster Study

Dioxygen activation pathways on the (001) surfaces of cobalt ferrite, CoFe2O4, were investigated computationally using density functional theory and the hybrid Perdew-Burke-Ernzerhof exchange-correlation functional (PBE0) within the periodic electrostatic embedded cluster model. We considered two terminations: the A-layer exposing Fe2+ and Co2+ metal sites in tetrahedral and octahedral positions, respectively, and the B-layer exposing octahedrally coordinated Co3+. On the A-layer, molecular oxygen is chemisorbed as a superoxide on the Fe monocenter or bridging a Fe-Co cation pair, whereas on the B-layer it is adsorbed at the most stable anionic vacancy. Activation is promoted by transfer of electrons provided by the d metal centers onto the adsorbed oxygen. The subsequent dissociation of dioxygen into monoatomic species and surface reoxidation have been identified as the most critical steps that may limit the rate of the oxidation processes. Of the reactive metal-O species, [Fe-III-O](2+) is thermodynamically most stable, while the oxygen of the Co-O species may easily migrate across the A-layer with barriers smaller than the associative desorption.