Mono- and Diatomic Reactive Oxygen Species Produced upon O2 Interaction with the (111) Facet of Cobalt Spinel at Various Conditions—Molecular DFT and Atomistic Thermodynamic Investigations

Periodic spin unrestricted DFT + U calculations joined with atomistic thermodynamics were used to study the location, structure, and stability along with the electronic and magnetic properties of various surface oxygen species and oxygen vacancies, produced under different thermodynamic conditions on the (111) surface of the cobalt spinel nano-octahedra. The density of state alignment diagrams between dioxygen and cobalt centers were used to rationalize speciation of the surface oxygen varieties into diatomic superoxo (μ-Co O 3c –O 2 –Co T 3c, η 2 -O 2 –Co O 3c ) or intrafacial peroxo ([O–O surf. ] 2– ) and monoatomic metal–oxo (Co T 3c –O, Co O 3c –O) entities. It was shown that the surface cobalt cations work in tandem constituting dual Co O 3c –Co T 3c sites for O 2 adsorption, where the Co O 3c d xz (β) and d yz (β) states act as spin-polarized electron donor centers, producing the most stable bridging μ-Co O 3c –O 2 –Co T 3c adducts (Δ E a = −1.86 eV). The single site mono- and bidentate binding modes η 1 -O 2 –Co O 3c (Δ E a = −1.66 eV) and η 2 -O 2 –Co O 3c (Δ E a = −1.12 eV) are less stable. These results imply that the most probable pathway of dioxygen activation involves an η 1 -O 2 –Co O 3c → η 2 -O 2 –Co O 3c → μ-Co O 3c –O 2 –Co T 3c sequence. Subsequent dissociation of the O–O bond in the bridging peroxide moiety leads to the formation of ferromagnetically coupled [↑Co O 3c IV O↑] 3+ and [↑↑↑Co T 3c II O↑] + species, characterized by a large difference of their stability (Δ E a = −1.24 and −0.43 eV respectively). First principles thermodynamic modeling revealed that in typical catalytic pressures of dioxygen ( p O 2 / p ° ∼ 0.01 ÷ 1), the most stable bridging μ-Co O 3c –O 2 –Co T 3c species persist on the surface below 375 °C, whereas above this temperature the surface is covered with monoatomic species (Co O 3c –O stable up to 475 °C). At T > 475 °C, bare and then oxygen vacancy bearing ( T > 575 °C) surfaces are thermodynamically preferred. The cobalt–oxo species may diffuse across the surface with the involvement of the thermodynamically metastable O 2O,1T –O and O 3O –O peroxy species. The obtained results provide a convenient conceptual background for the interpretation of the redox catalytic and electrocatalytic processes over cobalt spinel with dioxygen participation.