  Recently, the hydrogen adsorption and diffusion on the CeO2 (111) surface
have been a vividly discussed topic because of its outstanding catalytic
activity and selectivity in hydrogenation reactions. However, for the strongly
reduced CeO2 (111) surface, the structure as well as the stability of hydride
anions are still unclear. By virtue of density functional theory (DFT)
calculations, we show that in the strongly reduced CeO2 (111) surface, the
hydride can be stabilized on the largely metallic surface consisting of Ce3+
when O vacancies are mainly in the outmost layers. For the surface with O
vacancies in deeper layers, the hydride locates in the center of three Ce3+
ions. However, the hydridic phase is not the most stable state
thermodynamically. With the increase of the temperature, the hydride will
transfer from the vacancies to the surface and form hydroxyl group on the
outmost layer.
