“…In particular, the mechanism of the reaction of water with oxide surfaces has been a subject of intensive study because of the widespread use of oxides as catalysts, catalyst supports, and promoters. , Much of the study has been focused on the surfaces of bulk oxides for understanding the effect of surface structures and surface defects on the dissociation of H 2 O molecules. Generally, H 2 O adsorbs molecularly at a perfect oxide surface while dissociatively at a nonperfect oxide surface (i.e., surface defect sites). − More recently, there have been extensive studies of H 2 O adsorption on ultrathin aluminum oxide films formed on Al or NiAl surfaces under ultrahigh vacuum (UHV) and non-UHV conditions, − where the oxide films serve as a surface-science substitute for macroscopically sized single crystals of alumina that typically prohibit the application of surface sensitive tools based on the interactions of matter with charged particles (e.g., electrons and ions). Ultrathin oxide films on metal supports, which can be considered as an “electrically conductive” counterpart of insulating bulk oxides, represent a unique combination of material systems with distinct properties that rely critically on the thickness of the oxide films.…”