Formation and Desorption of Oxygen Species in Nanoporous Crystal 12CaO·7Al2O3

The formation and desorption mechanisms of oxygen species in nanoporous crystal 12CaO·7Al2O3 (C12A7) are studied by temperature-programmed desorption (TPD), electron-spin resonance (ESR), and Raman spectroscopy. Various oxygen ion species, including O-, O2-, and O22-, are observed to exist in C12A7 at high concentrations (~1020 cm-3). The desorption of these oxygen species in C12A7 is proposed to proceed in three steps: (1) diffusion of oxygen species from bulk to the surface (rate-limiting step), (2) decomposition of oxygen species into oxygen molecules, and (3) release of molecular oxygen into the gas phase. Oxygen species in C12A7 can be regenerated by heating in an O2 atmosphere at higher than 400 C, and the concentrations of O- and O2- increase with temperature. More O2- is formed than O- at low activation temperatures, whereas nearly equal amounts of O- and O2- are formed at high temperatures around 1350 C. At 400-700 C, a considerable amount of molecular oxygen, O2, is suggested to be present together with other oxygen species. The formation of oxygen ion species occurs in a manner reverse to the desorption processes, with the first step involving diffusion of oxygen from the atmosphere into nanocages of C12A7