Role of hydrogen atoms in the photoinduced formation of stable electron centers in H-doped 12CaO·7 Al2 O3

In this work we investigate a variety of chemical and photoinduced processes in which different hydrogenous species including H2 molecules, H- ions, and H0 atoms interact with the bulk of a complex nanoporous oxide 12CaO·7 Al2 O3. Our results provide a detailed and consistent explanation of the recently observed phenomenon of photoinduced conversion of the insulating H-doped 12CaO·7 Al2 O3 to a conductor [K. Hayashi, Nature (London) 419, 462 (2002)]. The formation of a large and thermally stable concentration of electron centers in this process is facilitated by a large concentration (up to 1020 cm-3) of extraframework O2- naturally present in this material and homogeneously distributed in its bulk. We show that these species are able to split H2 molecules into pairs of H+ and H- ions and convert H0 atoms into H+ and e- promoting the photoinduced conversion process. The similarity of the mechanisms described in this work to those known for low-coordinated sites at MgO surfaces indicates that the formation of electronic centers in oxides interacting with hydrogenous species could be a generic feature.