Coverage Dependent Water Dissociative Adsorption on the Clean and O‑Precovered Fe(111) Surfaces

Water dissociative adsorption on the clean and O-precovered Fe(111) surfaces at different coverage have been studied using the density functional theory method (GGA-PBE) and ab initio atomistic thermodynamics. On the clean p(3 × 3) Fe(111) surface, surface H, O, OH, and H₂O species can migrate easily. Considering adsorption and H-bonding, the adsorbed H₂O molecules can be dispersed or aggregated in close energies at low coverage, while in different aggregations at high coverage, indicating that the adsorbed H₂O molecules might not have defined structures, as observed experimentally. On the O-precovered surface (<i>n</i>_O = 1–8), the first dissociation step, <i>n</i>O + H₂O = (<i>n</i> – 1)O + 2OH, has a very low barrier and is reversible; and the barriers of the sequential OH dissociation steps, (<i>n</i> – 1)O + 2OH = <i>n</i>O + H + OH and <i>n</i>O + H + OH = (<i>n</i> + 1)O + 2H, are close (0.9–1.2 eV). All of these barriers are coverage independent. For OH and H adsorption at 1/3 ML coverage, surface OH forms a trimer (OH)₃ unit, and surface O forms a regular linear pattern. At one ML coverage, there are three dispersed (OH)₃ units for OH adsorption and three well-ordered parallel lines for O adsorption. The average adsorption energies for OH and O adsorption are coverage independent. Desorption temperatures of H₂O and H₂ under ultrahigh vacuum conditions are computed. Systematic comparison among the Fe(110), Fe(100), and Fe(111) surfaces reveal their intrinsic differences in water dissociative adsorption and provide a basic understanding of water-involved reactions catalyzed by iron and interaction mechanisms of water interaction with metal surfaces