Adsorption Structures and Energies of Cu_<i>n</i> Clusters on the Fe(110) and Fe₃C(001) Surfaces

Spin-polarized density functional theory computations have been carried out to investigate the adsorption configurations of Cu_<i>n</i> (<i>n</i> = 1–7, 13) on the most stable Fe(110) and Fe₃C­(001) surfaces. On both surfaces the adsorbed Cu_<i>n</i> clusters favor aggregation over dispersion, and monolayer adsorption configurations are more favored thermodynamically than the two-layer adsorbed structures because of the stronger Fe–Cu interaction over the Cu–Cu bonding. On the basis of the computed adsorption energies the Fe(110) surface has stronger Cu affinity than the Fe₃C­(001) surface, in agreement with the experimental results. The Fe(110) surface also has stronger Cu_<i>n</i> aggregation energies and more pronounced charge transfer from surface to adsorbed Cu_<i>n</i> clusters than the Fe₃C­(001) surface. Different Cu_<i>n</i> growth modes have been discussed accordingly

Copper Promotion in CO Adsorption and Dissociation on the Fe(100) Surface

Spin-polarized density functional theory computations have been carried out to study the adsorption and dissociation of CO on clean as well as <i>n</i>Cu-adsorbed and <i>n</i>Cu-substituted Fe(100) surfaces (<i>n</i> = 1–3) at different coverage to explore the Cu promotion effect in CO activation. Increasing Cu content not only lowers CO dissociation energies but also increases CO dissociation barriers as well as making CO dissociation thermodynamically less favorable, and the clean Fe(100) surface is most active in CO adsorption and dissociation. The <i>n</i>Cu-substituted Fe(100) surface can suppress CO adsorption and dissociation more strongly than the <i>n</i>Cu-adsorbed Fe(100) surface. CO stretching frequencies at different coverages have been computed for assisting experimental investigations

High Coverage CO Activation Mechanisms on Fe(100) from Computations

CO activation on Fe(100) at different coverage was systematically computed on the basis of spin-polarized density functional theory. At the saturated coverage (11CO) on a <i>p</i>(3 × 4) surface size (24 exposed Fe atoms), top (1CO), bridge (3CO) and 4-fold hollow (7CO) adsorption configurations coexist. The stepwise adsorption energies and dissociation barriers at different coverage reveal equilibriums between desorption and dissociation of adsorbed CO molecules. It is found that only molecular adsorption is likely for <i>n</i>_CO = 8–11, and mixed molecular and dissociative adsorption becomes possible for <i>n</i>_CO = 5–7, while only dissociative adsorption is favorable for <i>n</i>_CO = 1–4. The computed CO adsorption configurations and stretching frequencies as well as desorption temperatures from ab initio thermodynamic analysis agree well with the available experimental data

Surface Morphology of Cu Adsorption on Different Terminations of the Hägg Iron Carbide (χ-Fe₅C₂) Phase

Spin-polarized density functional theory computations have been carried out to investigate the surface morphology of Cu_<i>n</i> adsorption on the Fe₅C₂(100), Fe₅C₂(111), Fe₅C₂(510), Fe₅C₂(001), and Fe₅C₂(010) surface terminations in different surface Fe and C ratios. On the Fe₅C₂(100), and Fe₅C₂(510) surfaces, aggregation is thermodynamically more favored than dispersion, while dispersion is more favored than aggregation on the Fe₅C₂(111) surface for <i>n</i> = 2–4, on the Fe₅C₂(010) surface for <i>n</i> = 2 and on the Fe₅C₂(001) surface for <i>n</i> = 2–4. The difference in structures and stability at low coverage depends on the stronger Cu–Fe interaction over the Cu–Cu interaction as well as the location of the adsorption sites. The adsorption energies do not correlate with the surface Fe and C ratios. Comparison among the most stable Fe(110), Fe₃C­(001), and Fe₅C₂(100) surfaces reveals that the Fe(110) surface has higher Cu affinity than the Fe₃C­(001) and Fe₅C₂(100) surfaces; and the carbide surfaces have close Cu affinities; in agreement with the experimental observations. On all these iron and carbide surfaces, two-dimensional monolayer surface adsorption configurations are energetically more favored than the adsorption of three-dimensional Cu_<i>n</i> clusters, and it can be expected that the adsorbed Cu atoms should grow epitaxially as a layer-by-layer mode at the initial stage. On the metallic Fe(110), Fe(100), Fe(111), and Fe₃C­(010) surfaces, the adsorbed Cu atoms are negatively charged; while on the Fe₃C­(100), Fe₅C₂(100), Fe₅C₂(111), Fe₅C₂(010), and Fe₅C₂(001) surfaces, the adsorbed Cu atoms are positively charged. On the Fe₃C­(001) and Fe₅C₂(510) surfaces, the adsorbed Cu atoms mainly interacting with surface Fe atoms are very slightly negatively charged. This trend is in line with their difference in electronegativity. Our results build the foundation for further study of the Cu-promotion effect in Fe-based FTS in particular and for metal-doped heterogeneous catalysis in general

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)_<i>n</i> clusters on the (3 × 4) Fe(100) surface, the adsorption energy is contributed by direct H₂O–Fe interaction and hydrogen bonding. For <i>n</i> = 1–3, direct H₂O–Fe interaction is dominant, and hydrogen bonding becomes more important for <i>n</i> = 4–5. For <i>n</i> = 6–8 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 (<i>n</i>O, <i>n</i> = 2–7), further H₂O dissociation has similar potential energy surfaces, and H₂ formation from surface adsorbed H atoms becomes easy, while the desorption energy is close to zero for <i>n</i> = 7. The calculated thermal desorption temperatures of H₂O and H₂ on clean surface agree well with the available experiment data. The characteristic desorption temperatures of H₂O and H₂ coincided at 310 K are controlled by the kinetics of disproportionation (2OH → O + H₂O) and dissociation (2OH → 2O + H₂) of surface OH groups. The dispersion corrections (PBE-D2) overestimate slightly the adsorption energies and temperatures of H₂O and H₂ on iron surface. At 0.5 ML coverage (6 × 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 × OH) and O (12 × 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

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

Coverage-Dependent N₂ Adsorption and Its Modification of Iron Surfaces Structures

Spin-polarized density functional theory calculations were performed to investigate N₂ dissociative adsorption on iron (100), (110), (111), (210), (211), (310), and (321) surfaces. An ordered <i>c</i>(2 × 2) structure was found on Fe(100) at 0.5 monolayer coverage, which is in excellent agreement with the experiment; and a <i>c</i>(4 × 2) ordered structure is also found at 0.75 monolayer saturation coverage. Strong surface reconstruction is found on Fe(110) upon nitrogen adsorption, where the densely packed (110) is reconstructed into (100)-alike. Under the consideration of temperature and N₂ partial pressure, the estimated N₂ desorption temperature on Fe(100) at 925 K agrees with the experimentally detected 920–950 K. In addition, N₂ pretreatment results in Fe(100) to be mostly exposed, while that of H₂ pretreatment favors Fe(110). Further direct comparison of N₂ and H₂ adsorptions has been made to show their differences and similarities

Results of the combination of cytology and hrHPV testing within the year prior to the histological diagnosis of invasive cervical cancer.

<p>Results of the combination of cytology and hrHPV testing within the year prior to the histological diagnosis of invasive cervical cancer.</p

Comparison of the results of cytology, hrHPV testing, and their combination within the year prior to the histological diagnosis of invasive cervical cancer.

<p>Comparison of the results of cytology, hrHPV testing, and their combination within the year prior to the histological diagnosis of invasive cervical cancer.</p

Cervical cancer histological subtypes in the current study.

<p>Cervical cancer histological subtypes in the current study.</p