Mono- and Diatomic Reactive Oxygen Species Produced
upon O2 Interaction with the (111) Facet of Cobalt Spinel
at Various ConditionsMolecular DFT and Atomistic Thermodynamic
Investigations
posted on 2018-11-12, 00:00authored byFilip Zasada, Joanna Gryboś, Witold Piskorz, Zbigniew Sojka
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-CoO3c–O2–CoT3c, η2-O2–CoO3c) or intrafacial peroxo ([O–Osurf.]2–) and monoatomic metal–oxo (CoT3c–O, CoO3c–O) entities.
It was shown that the surface cobalt cations work in tandem constituting
dual CoO3c–CoT3c sites for O2 adsorption, where the CoO3c dxz(β) and dyz(β) states act as spin-polarized electron
donor centers, producing the most stable bridging μ-CoO3c–O2–CoT3c adducts (ΔEa = −1.86 eV).
The single site mono- and bidentate binding modes η1-O2–CoO3c (ΔEa = −1.66 eV) and η2-O2–CoO3c (ΔEa = −1.12 eV) are less stable. These
results imply that the most probable pathway of dioxygen activation
involves an η1-O2–CoO3c → η2-O2–CoO3c → μ-CoO3c–O2–CoT3c sequence.
Subsequent dissociation of the O–O bond in the bridging peroxide
moiety leads to the formation of ferromagnetically coupled [↑CoO3cIVO↑]3+ and [↑↑↑CoT3cIIO↑]+ species,
characterized by a large difference of their stability (ΔEa = −1.24 and −0.43 eV respectively).
First principles thermodynamic modeling revealed that in typical catalytic
pressures of dioxygen (pO2/p° ∼ 0.01 ÷ 1), the most stable
bridging μ-CoO3c–O2–CoT3c species persist on the surface below 375 °C,
whereas above this temperature the surface is covered with monoatomic
species (CoO3c–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 O2O,1T–O and O3O–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.