Cobalt Spinel (111) Facets of Various Stoichiometry—DFT+U and Ab Initio Thermodynamic Investigations

A combination of periodic, spin unrestricted DFT calculations and first principle thermodynamic modeling was used to evaluate the structure and stability of nine different pristine and eight defected terminations of the (111) surface of cobalt spinel under various redox conditions (T, pO2). The surface redox state diagram of possible spinel (111) terminations in the wide stoichiometry range from Co2.62O4 to Co3O3.75 was constructed and thoroughly discussed, revealing that three regions of spinel surface redox behavior may be distinguished. The region of temperatures and pressures of typical catalytic processes (T ∼200 to ∼500 °C, pO2/p° ∼0.001 to ∼1) corresponds to an energetically well separated stoichiometric (111)-S facet exposing both truncated tetrahedral CoT3c and octahedral CoO3c cations. They preserve their bulk-like divalent (qB = 1.29 |e|) and trivalent (qB = 1.45 |e|) state, respectively, in contrast to the spin state of CoO3c (S = 1, μ = 1.9 μB) comparing to diamagnetic bulk CoO6c. Under more reducing conditions (T > 600 °C and pO2/p° < 0.0001), several terminations of different stoichiometry induced by the formation of oxygen vacancies may coexist, providing a thermodynamic background for enhanced redox activity. Redox properties are associated with extended electronic and spin relaxations that entail several octahedral surface (CoO3c, qB = 1.29) and subsurface (CoO6c, qB = 0.01) cations which are reduced (ΔqB = −0.24 |e|, Δμ = 0.87 μB) upon oxygen release (formation of redox active ensemble). Under the oxygen rich conditions (T < 200 °C and pO2/p° > 10), the oxidized (111)-O termination, exposing the CoT3c ions only, is the most stable, and with the rising oxygen pressure, it becomes defected due to the presence of the cationic vacancies in the tetrahedral sites. The sub-stoichiometry (Co deficiency) of the (111)-O surface is reflected in the trivalent state of half of the surface CoT3c cations, which are characterized by larger charge (ΔqB = 0.15 |e|) and lower magnetization (Δμ = −0.7 μB) in line with the (dz2, dz2–y2)4(dxy)1(dxz)1 configuration. The shape of the Co3O4 nanograins was modeled by means of the Wulff construction for oxidizing, ambient, and reducing redox conditions. It was shown that rhombicubooctahedral morphology is preserved regardless of the oxygen chemical potential and the (111) termination is more abundant in an oxygen rich environment.