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

The formation and desorption mechanisms of oxygen species in nanoporous crystal 12CaO·7Al 2 O 3 (C 12 A 7 ) are studied by temperature-programmed desorption (TPD), electron-spin resonance (ESR), and Raman spectroscopy. Various oxygen ion species, including O -, O 2 -, and O 2 2 -, are observed to exist in C 12 A 7 at high concentrations (∼10 20 cm - 3 ). The desorption of these oxygen species in C 12 A 7 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 C 12 A 7 can be regenerated by heating in an O 2 atmosphere at higher than 400 °C, and the concentrations of O - and O 2 - increase with temperature. More O 2 - is formed than O - at low activation temperatures, whereas nearly equal amounts of O - and O 2 - are formed at high temperatures around 1350 °C. At 400−700 °C, a considerable amount of molecular oxygen, O 2, 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 C 12 A 7 .