Adsorption of water on FeO(111) and Fe3O4(111): identification of active sites for dissociation

The adsorption of water on epitaxial FeO(111) and Fe3O4(111) films structurally well characterized by STM and LEED was investigated with photoelectron and thermal desorption spectroscopy. On the FeO(111) surface terminated by a closely packed oxygen layer, monomeric water species are physisorbed. On the Fe3O4(111) surface terminated by 1/4 monolayer of Fe atoms located over a closely packed oxygen layer, water dissociates, resulting in adsorbed OH groups. The OH saturation coverage corresponds to the number of surface Fe atoms, which is much larger than the surface defect concentrations. Therefore, the dissociation takes place at Fe sites exposed on the regular Fe3O4(111) surface, and the FeO(111) surface is chemically inert because no Fe sites exist thereon.