Formation of oxygen radicals in 12CaO·7Al₂O₃: Instability of extraframework oxide ions and uptake of oxygen gas

The thermodynamics and kinetics of oxygen radical formation in cages of a nanoporous crystal, 12CaO·7Al₂O₃, were examined during isothermal annealing in a dry oxygen atmosphere. The formation of oxygen radicals, O₂^- and O^-, involves the outward diffusion of extraframework oxide ions, their oxidation by absorbed oxygen molecules to form O^- and O₂^- at the surface, and subsequent inward diffusion of the oxygen radicals. The instability of the extraframework oxide ions provides the driving force for this process. Evaluating the radical formation enthalpy and entropy enabled us to determine the temperature and oxygen partial pressure dependences of equilibrium oxygen radical concentration. The rate-limiting process for the radical formation is not the surface reaction, but the total ionic diffusion process, in which the smaller diffusivity of O₂^- likely dominates the process.