Reactive Oxygen Species on the (100) Facet of Cobalt Spinel Nanocatalyst and their Relevance in 16O2/18O2 Isotopic Exchange, deN2O, and deCH4 Processes-A Theoretical and Experimental Account

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 spine] (100) surface. The surface state diagram of oxygen in a wide range of pressures and temperatures was constructed for coverage varying from circle dot(o) = 1.51 atom.nm(-2) to circle dot(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+, CoO-O2+) 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-30(-O))(2-)), respectively). The results revealed that in the oxygen pressure range of typical catalytic reactions (po(2)/p degrees from similar to 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(2-)-Co-O adducts at temperatures below 250-300 degrees C. In the temperature from 250 to 300 degrees C and from 550 to 700 degrees C, it is covered by the O species associated with the exposed tetrahedral cobalt sites (COT-O+) or remains in a bare state. In more reducing conditions (T > 550-700 degrees C), the (100)-S facet is readily defected due to trigonal oxygen (O-20,O-1T) release and formation of surface oxygen vacancies. The reactivity of surface ROS was tested in (O2O2)-O-16-O-/18 isotopic exchange, N2O decomposition, and oxidation of CH, and CO model reactions, carried over Co3O4 and (Co3O4)-O-18 nanocrystalline samples with the predominant (100) faceting revealed by high angle angular dark field STEM examination. The Co-O-O2+ adducts associated with octahedral cobalt sites, as well as the peroxo (O-20,1T(-O))(2-) and (O-30-O)(2-) surface species being thermodynamically unstable are involved in surface oxygen recombination processes, probed by 1602/1802 exchange and N2O 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, activation, the highly reactive cobalt-oxo species (Co-T-O+) are involved. Above 600 degrees C at po(2)/p degrees = 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 Co3O4 in many other catalytic redox reactions.