Witold Piskorz, Eko Budiyanto, Zbigniew Sojka, Filip Zasada, Janusz Janas
A unified molecular, thermodynamic, and kinetic picture of dioxygen activation, surface diffusion, and reactivity over stoichiometric and defected cobalt spinel (100) termination was provided by combination of GGA–DFT+U modeling and experimental isotopic exchange investigations using Co3O4 nanocubes. Various diatomic (CoO5c–O2––CoO5c superoxo, CoO5c–O22––CoT2c peroxo, (O–Osurf)2– peroxo) and monatomic (O–CoT2c and O–CoO5c metal-oxo) reactive oxygen species were described in detail regarding their electronic and magnetic structure. The band alignment diagrams between the pDOS of dioxygen and the exposed cobalt cations were constructed and used to rationalize the revealed pronounced speciation of the surface oxygen, depending on the adsorption geometry (monodentate η1, bidentate η2, bridging μ), varying extent of oxygen reduction (one, two, four electrons), and the entailed complex spin relaxation. It was shown that surface cobalt cations work in tandem constituting dual CoO5c–CoO5c and CoO5c–CoT2c sites for O2 activation. The metal-oxo species were formulated in terms of the O– moieties ferromagnetically coupled to Co ions, and orbital overlap type (σ for O–CoT2c or π for O–CoO5c) is mainly responsible for the observed differences in their stabilities and mobilities. A three-dimensional plot of the O/Co ratio as a function of T and pO2 provided a suitable contextual thermodynamic background for understanding the dioxygen/surface interactions, and was used to support the kinetic data of the isotopic 16O2/18O2 exchange reaction. The elaborated molecular mechanism of the dioxygen interaction with the cobalt spinel (100) surface was applied for ab initio microkinetic modeling of the isotopic 16O2/18O2 exchange reaction in the reactant lean and rich conditions, providing a theoretical account for TAP and TPSR experiments. Three stages of the evolution were distinguished: latent (surface accumulation of the dissociated 18O* and 16O* adspecies), transient (gradual development of the 16O18O isotopomer in the gas phase), and equilibrium (equilibration of the isotopic composition). It was also shown that CoT2c2+–O– acts as rigid (Ediff = 1.35 eV) spectator species and only the labile CoO5c3+–O– (Ediff = 0.68 eV) are directly involved in the isotopic exchange. The simulated TPSR curves were confronted with the 16O2/18O2 exchange experiments performed on a cobalt spinel nanocube catalyst, synthesized by the hydrothermal method and characterized by XRD, RS, and HR-TEM techniques. An excellent quantitative agreement between experiment and theory substantiates the developed molecular mechanism, selection of the kinetically relevant steps, and calculation of their energetic and entropic barriers.