Oxygen species stabilized on the cobalt spinel nano-octahedra at various reaction conditions and their role in catalytic CO and CH4 oxidation, N2O decomposition and oxygen isotopic exchange

Periodic spin unrestricted DFT+U calculations joined with atomistic thermodynamics and model catalytic experiments (TPD-O-2, O-18(2)/O-16(4) exchange, N2O decomposition, CO and CH4 oxidation) were used to study the structure, stability and reactivity of various surface oxygen species and oxygen vacancies, produced under different thermodynamic conditions on the (1 1 1) surface exposed by the cobalt spinel nanooctahedra. We thoroughly analyzed the stability of differently oxygen covered terminations starting from Theta(O) = 0.86 nm(-2) (isolated monoatomic species) up to Theta(O) = 5.21 nm(-2) (full oxygen monolayer). The constructed 3-dimensional thermodynamic (gamma, T, p(O2)) surface redox state diagram revealed that in typical catalytic pressures of O-2 (P-O2/P-o similar to 0.01- 1) three principal states of the spinel surface, including a region associated with the presence of mu-superoxo Ca-3c(O)-O-2-Co-3c(T) and metal-oxo Ca-3c(O)-0 species (T < 450- 500 degrees C), a bare surface region (450- 500 degrees C to 550-600 degrees C), and a region were surface oxygen vacancies appear (T > 600 degrees C) may be distinguished. This diagram was used for quantitative assignment of the experimental TPD-O-2 desorption peaks, and also, for interpretation of the role that various adoxygen and lattice oxygen species play in the investigated model catalytic reactions. It was shown that CO is primarily oxidized by the suprafacial Co-3c(O)-O-2-Co-3c(T) diatomic oxygen and/or cobalt-oxo Co-3c(O)-O species, whereas in low temperature activation of CH4 only the Co-3c(O)-0 adducts are involved. With the increasing temperature, the suprafacial methane oxidation route gradually changes into the interfacial Mars van Krevelen scheme, and the later pathway is accelerated above 600 degrees C, due to the onset of oxygen vacancy formation (in line with O-18(2)/(Co3O6)-O-16 isotopic exchange experiments). In the case of N2O decomposition the reaction occurs preferably on a bare surface, and the favorable thermodynamics provides a steady driving force for efficient removal of the recombined adoxygen intermediates in the form of gaseous O-2. The phase diagram of the alterable surface oxygen states on the (1 1 1) termination was compared to the corresponding diagram for the (1 0 0) facet, to explain the nature and differences in catalytic redox behavior of the cobalt spinel nano octahedra and nano-cubes. This allows also for prediction of oxygen speciation anisotropy on cubooctahedral nanocrystals at various temperatures. (C) 2019 Elsevier Inc. All rights reserved.