Dioxygen Activation Pathways over Cobalt Spinel Nanocubes-From Molecular Mechanism into Ab Initio Thermodynamics and 16O2/18O2 Exchange Microkinetics

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-DFTI+U modeling and experimental isotopic exchange investigations using Co3O4 nanocubes. Various diatomic (Co-5c(O)-O-2(-)-Co-5c(O) superoxo, Co-5c(o)-O-2(2-)-Co-2c(T), peroxo, (O-O-surf)(2-) peroxo) and monatomic (O-Co-2c(T) and O-Co-5c(O) 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 eta(1), bidentate eta(2), bridging mu), 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 Co-5c(O)-Co-5c(O) and Co-5c(O)-Co-2c(T) sites for O-2 activation. The metal-oxo species were formulated in terms of the O- moieties ferromagnetically coupled to Co ions, and orbital overlap type (sigma for O-Co-2c(T) or pi for O-Co-5c(O)) 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 po(2) provided a suitable contextual thermodynamic background for understanding the dioxygen/surface interactions, and was used to support the kinetic data of the isotopic O-16(2)/O-18(2) 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 O-16(2)/O-18(2) 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 O-18* and O-16* adspecies), transient (gradual development of the (OO)-O-16-O-18 isotopomer in the gas phase), and equilibrium (equilibration of the isotopic composition). It was also shown that Co-2c(T) (2+)-O- acts as rigid (E-diff = 1.35 eV) spectator species and only the labile Co-5c(O) (3+)-O- (E-diff = 0.68 eV) are directly involved in the isotopic exchange. The simulated TPSR curves were confronted with the O-16(2)/O-18(2) 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.