Dissociative Adsorption and Aggregation of Water on the Fe(100) Surface: A DFT Study

Abstract

Aggregation, molecular adsorption, and dissociation of water on the Fe(100) surface were investigated using spin-polarized density functional calculations. The preferential sites for H2O, HO, O, and H were carefully investigated on this surface. Also, the dissociation of H2O into H + OH species, and further OH into O + H species, was examined. The charge transfer mechanism during these dissociation processes, as well as of small water aggregates at different orientations on the Fe(100) surface, was studied within the Bader charge analysis. The coverage dependence on the adsorption properties was examined by comparing the results of a (2 × 2) with a (3 × 3) supercell. These calculations predicted that H2O is weakly adsorbed (physisorption) on hollow, bridge, and on-top sites, with the on-top site being slightly preferred for both coverages of 0.11 and 0.25 monolayer. As expected, OH was predicted to be strongly adsorbed (chemisorption) on the Fe(100) sites, producing a large charge transfer from the surface to p-orbitals of the O atom. A dissociation barrier of about 1.0 eV, for the dissociation H2O → OH + H, was calculated from the on-top site to the next most stable bridge and hollow sites, respectively. In contrast, a smaller barrier of ca. 0.8 eV was calculated for the dissociation of OH. Regarding the adsorption of small water aggregates on Fe(100), the present study has demonstrated that they are strongly reoriented on the surface in comparison to the isolated structures, leading to stable adsorbates. Most interestingly, the dissociation of a water molecule, after the dimer formation, leads to an energy barrier of 1.25 eV, about 25% higher than the corresponding value of an adsorbed single water molecule.