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Periodic spin unrestricted, gradient corrected DFT calculations joined with atomistic thermodynamic modeling and experiment were used to study the structure and stability of various reactive oxygen species (ROS) and oxygen vacancies produced on the most stable terminations of the cobalt spinel ( 100) surface. The surface state diagram of oxygen in a wide range of pressures and temperatures was constructed for coverage varying from Θ O = 1.51 atom•nm −2 to Θ O = 6.04 atom•nm −2 . A large variety of the unraveled surface ROS includes diatomic superoxo (Co O −O 2 − −Co O ), peroxo (Co T −O 2 2− −Co O ), and spin paired (Co O −O 2 −Co O ) adducts along with monatomic metal-oxo (Co T −O + , Co O −O 2+ ) species, where Co T and Co O stand for the tetrahedral and octahedral cobalt surface centers, respectively. There are also two kinds of peroxo species associated with surface oxygen ions connected with 3Co O or 2Co O and 1Co T cations ((O 2O,1T −O) 2− and (O 3O −O) 2− ), respectively). The results revealed that in the oxygen pressure range of typical catalytic reactions (p O 2 /p°from ∼0.01 to 1), the most stable stoichiometric (100)-S surface accommodates the Co T −O 2 2− −Co O and Co O −O 2 −Co O adducts at temperatures below 250−300 °C. In the temperature from 250 to 300 °C and from 550 to 700 °C, it is covered by the O species associated with the exposed tetrahedral cobalt sites (Co T − O + ) or remains in a bare state. In more reducing conditions (T > 550−700 °C), the (100)-S facet is readily defected due to trigonal oxygen (O 2O,1T ) release and formation of surface oxygen vacancies. The reactivity of surface ROS was tested in 16 O 2 / 18 O 2 isotopic exchange, N 2 O decomposition, and oxidation of CH 4 and CO model reactions, carried over Co 3 O 4 and Co 3 18 O 4 nanocrystalline samples with the predominant (100) faceting revealed by high angle angular dark field STEM examination. The Co O −O 2+ adducts associated with octahedral cobalt sites, as well as the peroxo (O 2O,1T −O) 2− and (O 3O −O) 2− surface species being thermodynamically unstable are involved in surface oxygen recombination processes, probed by 16 O 2 / 18 O 2 exchange and N 2 O decomposition. It was shown that at low temperatures CO is oxidized by the suprafacial Co O −O 2 −Co O and Co T −O 2 −Co O diatomic oxygen, whereas in CH 4 activation, the highly reactive cobalt-oxo species (Co T −O + ) are involved. Above 600 °C at p O 2 /p°= 0.01, due to the onset of oxygen vacancy formation, the suprafacial methane oxidation gradually changes into the intrafacial Mars-van Krevelen scheme. The constructed surface phase diagram was used for rationalization of the obtained catalytic data, allowing delineation of the specific role of the chemical state of the cobalt spinel surface in the investigated processes, as well as the range of the corresponding temperatures and oxygen pressures. It also provides a convenient background for molecular understanding of remarkable activity of Co 3 O 4 in many other catalytic redox reactions.

Involvement of suprafacial and intrafacial oxygen species in catalytic combustion of methane over the (100) faceted cobalt spinel was systematically examined as a function of temperature and CH4 conversion (X CH4 ). The clear-cut Co3O4 nanocubes of uniform size were synthesized using a hydrothermal method and characterized with XRD, RS, HR-TEM, XRF, TPSR (CH4/16/18O2), and SSITKA (CH4/16/18O2) techniques. The experimental results were corroborated by first-principles thermodynamic and DFT+U molecular modeling, providing a rational framework for a detailed understanding of the origin of a different redox comportment of the catalyst with the varying temperature and its mechanistic implications. Three temperature/conversion stages of the methane oxidation reaction were distinguished, depending on involvement of the adsorbed or lattice oxygen and the redox state of the catalyst. A stoichiometric (100) surface region (300 °C < T < 450 °C, X CH4 < 25%) is featured by the dominant suprafacial (Langmuir–Hinshelwood) mechanism of methane oxidation. A region of slightly defected surface (450 °C < T < 650 °C, 25% < X CH4 < 80%), in which oxygen vacancies produced upon CO2 and H2O release are virtually refilled by dioxygen, is characterized by coexistence of the suprafacial (Langmuir–Hinshelwood) and intrafacial (Mars–van Krevelen) mechanistic steps. In a nonstoichiometric surface region (T > 650 °C, X CH4 > 80%), the oxygen vacancies are only partially refilled, the catalyst is significantly reduced, and methane is combusted according to the Mars–van Krevelen scheme. Molecular modeling revealed that the suprafacial Co–Oads adoxygen species are more active (ΔE a = 0.83 eV) than the intrafacial Co–Osurf surface sites (ΔE a = 1.11 eV) in the CH4 oxidation. The (100) surface state diagrams for the three distinguished conversion regions were constructed to elucidate the catalyst thermodynamic behavior under those conditions. It was shown that the activity of cobalt spinel is maintained by redox autotuning of the catalyst and dynamic adjustment of uneven participation of the suprafacial and intrafacial oxygen species in methane oxidation to the actual reaction conditions. These factors have important structural and mechanistic consequences for the catalytic CH4 combustion on cobalt spinel and related systems, controlling not only the sustainable versus the stoichiometric turnovers but also for the prevalence or coexistence of the Langmuir–Hinshelwood and the Mars–van Krevelen mechanisms with the reaction progress.

Manganese, iron, and cobalt model spinel catalysts were systematically investigated for understanding the roots of their divergent performance in N2O decomposition. The catalysts were characterized by XRD, RS, N2-BET, SEM, and STEM/EELS techniques before and after the reaction. Their redox properties and the thermodynamic stability range were thoroughly examined by survey and narrow scan TPR/TPO cycles. The results were accounted for by the constructed size-dependent Ellingham diagrams. It was shown that Fe3O4 and Mn3O4 spinels exhibit redox-labile Mn2+/Mn3+ and Fe2+/Fe3+constituents, and under the conditions of the deN2O reaction these catalysts have a pronounced tendency for stoichiometric overoxidation. The redox properties of Co3O4 are highly anisotropic, with Co2+ being reluctant to undergo oxidation but Co3+ being prone to easy reduction. The stability of the Co3O4 catalyst is then controlled by partial reduction of octahedral Co3+ cations, due to the surface oxygen release at elevated temperatures in lean oxygen environments. The N2O decomposition was studied by temperature-programmed surface reaction (TPSR) and pulse experiments using 18O labeling of the catalysts. It was shown that Co3O4 provides a sustainable redox Co3+/Co4+ couple for catalytic decomposition of N2O, which operates along a reversible one-electron process, leading to formation of O– surf intermediates that recombine next into dioxygen. As the reaction temperature increases, the deN2O mechanism evolves from suprafacial to intrafacial recombination of the oxygen intermediates. Fe3O4 decomposes nitrous oxide in a stoichiometric way via irreversible two-electron reduction of oxygen intermediates into O2–, giving rise to lattice expansion and formation of a γ-Fe2O3 shell, as discerned by Raman spectroscopy. Postreaction STEM/EELS imaging confirmed a magnetite-core and a maghemite-shell morphology of the catalyst grains. A similar tendency for autogenous oxidation was observed for Mn3O4, yet a rather weak thermodynamic driving force makes this catalyst kinetically more stable. At higher reaction temperatures, the incipient γ-Mn2O3 layer may be decomposed back to the parent Mn spinel, when oxygen pressure is low. To quantify gradual oxidation of the investigated spinels during the N2O decomposition, size-dependent thermodynamic 3D diagrams were developed and used for rationalization of the experimental observations. The obtained results reveal the dynamic nature of the investigated spinels under varying redox conditions and explain the remarkable performance of Co3O4 in comparison to Fe3O4 and Mn3O4. The catalytic behavior of the latter two spinels is actually governed by a sesquioxide shell, produced spontaneously in the course of the deN2O reaction.

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