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 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.