High Coverage Water Aggregation and Dissociation on Fe(100): A Computational Analysis

Water adsorption and dissociation on the Fe(100) surface at different coverages have been calculated using density functional theory methods and ab initio thermodynamics. For the adsorption of (H2O)(n) clusters on the (3 x 4) Fe(100) surface, the adsorption energy is contributed by direct H2OFe interaction and hydrogen bonding. For n = 13, direct H2OFe interaction is dominant, and hydrogen bonding becomes more important for n = 45. For n = 68 and 12, structurally different adsorption configurations have very close energies. Monomeric H2O dissociation is more favored on the clean Fe(100) surface than that on H2O or OH precovered surfaces. O-assisted H2O dissociation is favorable kinetically (O + H2O = 2OH), and further OH dissociation is roughly thermo-neutral. With the increase of surface O coverage (nO, n = 27), further H2O dissociation has similar potential energy surfaces, and H-2 formation from surface adsorbed H atoms becomes easy, while the desorption energy is close to zero for n = 7. The calculated thermal desorption temperatures of H2O and H-2 on clean surface agree well with the available experiment data. The characteristic desorption temperatures of H2O and H-2 coincided at 310 K are controlled by the kinetics of disproportionation (2OH -> O + H2O) and dissociation (2OH -> 2O + H-2) of surface OH groups. The dispersion corrections (PBE-D2) overestimate slightly the adsorption energies and temperatures of H2O and H-2 on iron surface. At 0.5 ML coverage (6 x OH), the adsorbed OH groups at the bridge sites do not share surface iron atoms and form two well-ordered parallel lines, and each OH group acting as donor and acceptor forms hydrogen bonding with the adjacent OH groups, in agreement with the experimentally observed surface structures. At 1 ML coverage of OH (12 x OH) and O (12 x O), the adsorbed OH groups at the bridge sites share surface iron atoms and form four well-ordered parallel lines; and the adsorbed O atoms are located at the hollow sites. Energetic analysis reveals that 1 ML OH coverage is accessible both kinetically and thermodynamically, while the formation of 1 ML O coverage is hindered kinetically since the OH dissociation barrier increases strongly with the increase of O pro-covered coverage. All these results provide insights into water-involved reactions catalyzed by iron and broaden our fundamental understanding into water interaction with metal surfaces.