Kinetics of electron decay in hydride ion-doped mayenite

Persistent electrons are generated in hydride (H^-) ion-doped 12CaO·7Al₂O₃ (C12A7) by UV irradiation at room temperature. The kinetics of thermal electron decay is examined using experimental data collected at 200-295 °C. The observed reaction order together with activation energy shifts approximately from one-half to first depending on the electron concentration and temperature. The data is explained by a two-electron transfer reaction (H^- + O^2- ↔ OH^- + 2e^-) with H⁰ and H⁺ as reaction intermediates and diffusion of these hydrogen species. The observed one-half-order kinetics originates from a zero-order reaction influenced by inhomogeneity of the electron concentration. The electron decay rate is principally determined by the supply of H⁺ ions liberated from OH^- ions. The large activation energy (1.9 ± 0.3 eV) of this process is responsible for the persistence of electrons at room temperature. As the residual electron concentration decreases, the contribution of electron trapping by the intermediate hydrogen species becomes dominant for the rate-determination, shifting the reaction order and activation energy.