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 ...