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

Abstract

Water dissociative adsorption on the clean and Oprecovered 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 H 2 O species can migrate easily. Considering adsorption and H-bonding, the adsorbed H 2 O molecules can be dispersed or aggregated in close energies at low coverage, while in different aggregations at high coverage, indicating that the adsorbed H 2 O molecules might not have defined structures, as observed experimentally. On the O-precovered surface (n O = 1−8), the first dissociation step, nO + H 2 O = (n − 1)O + 2OH, has a very low barrier and is reversible; and the barriers of the sequential OH dissociation steps, (n − 1)O + 2OH = nO + H + OH and nO + H + OH = (n + 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 H 2 O and H 2 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.

A DFT study of H2O dissociation on metal‐precovered Fe (100) surface

“…The CO molecules was adsorbed at the H n sites, and the E ads was lower than the same sites on the clean surface. The similar results were in agreement with some researchers who suggested that adsorbates always adsorb at the empty H sites of precovered Fe surface and the range of E ads for molecules are from −0.8 eV to −4.1 eV …”

“…Because, for both kinds of metal‐precovered surfaces, most of the H 2 O molecules prefer vertical upward adsorption (Figure A), a little H 2 O molecules adsorb in a parallel (the plane of H 2 O is parallel to the surface, Figure B), vertical (the plane of H 2 O is vertical to the surface, and the center axis of O‐H bonds is parallel to the surface, Figure C), or tilted (the plane of O and H is vertical to the surface, and the center axis of O─H bonds is into a certain angle of surface, Figure Dd) direction related to the surface. Some reports have shown the similar results that the orientation of adsorbates relative to the surface was upward and parallel, vertical, and tilted . We can also see that the values are not obtained in Table for 3 adsorption sites on both surfaces, because the metal atoms assist H 2 O dissociation into the OH and H groups on these metal‐precovered surface.…”

“…In both cases, on surfaces 1 and 2, the top site is slightly less stable than the hollow or bridge site, while the top site is locally unstable if we consider the energies calculated with reference to the molecular species. Most the adsorption energy of H 2 O is higher than that (−0.58 eV; −0.41 eV) on the clean surface, this means that the surface impurity atoms would improve the adsorption energy and promote the dissociation of H 2 O. Until now, there is little work about water dissociative adsorption on these metal‐precovered Fe surface; the reasonable of our data cannot be verified directly.…”

High coverage adsorption and co-adsorption of CO and H2 on Ru(0001) from DFT and thermodynamics

“…As a result, we carried out RPBE single-point energy calculation on the PBE optimized structure; and the computed CO adsorption energy is practically the same as found from RPBE optimization. [46][47][48] However, for the adsorption of H 2 O and H 2 on the Cu(111) surface, the computed adsorption energies are in excellent agreement with the available experimental data. Despite the excellent agreement in CO adsorption energy between experiment and RPBE computation, we are interested in the long-range dispersion corrections for van der Waals interaction.…”

First-Principles Modeling of Direct versus Oxygen-Assisted Water Dissociation on Fe(100) Surfaces

“…Shavorskiy et al [50] also found the formation of OH when water reacted with co-adsorbed O on the missing-row reconstructed Pt{110}(1ˆ2) surface. A similar promotion effect on the Fe(111) surface has been reported [51]. Hung et al [52] found that adsorbed oxygen facilitated water dissociation via the hydrogen transfer process on the pre-oxidized Fe(100) surface.…”