Electronic theory of the chemisorption effect on surface segregation in transition-metal alloys

The tight-binding Ising model generalized for a system consisting of a transition-metal alloy with chemisorbed adatoms is used to calculate the effect of hydrogen chemisorption on surface segregation in a Cu---Ni alloy. The simple model of chemisorption and of the band structure of the alloy is assumed. The charge-neutrality condition is incorporated into the model.     

Tight-binding study of ammonia and hydrogen adsorption on magnetic cobalt systems

The semi-empirical self-consistent tight-binding model of ammonia and hydrogen adsorption at Co(0001) and small Co clusters is used to study the chemisorption role in surface magnetism. The adsorbate choice has been suggested by recent experiments. At the Co(0001) surface the atomic magnetization is predicted to diminish locally by 0.26 μ_B due to an isolated hydrogen atom adsorption; for Co₁₃ clusters the change is somewhat smaller but less localized. At H(1×1)–Co(0001) the magnetization of surface Co atoms drops to 0.88 μ_B. The hydrogen magnetic moment is very small and couples antiferromagnetically to Co. Ammonia adsorption is found to reduce the Co atom magnetization locally by 0.1 μ_B or less. We discuss the possibility of adsorbate–metal antiferromagnetic coupling in more detail.     

Field adsorption of helium and neon on metals: an integrated theory

Density functional theory has been used to carry out self-consistent calculations relating to the field adsorption of helium and neon on metal surfaces. The transition from physisorption in low electric fields to field-induced chemisorption in high electric fields is explicitly demonstrated with electron density maps. The higher binding energies of rare gases above protruding metal surface atoms are explained as resulting from local field enhancement, and an approximate formula for the helium-on-tungsten system is derived that gives the total short-range field-adsorption binding energy as a function of the external field and a local field-enhancement factor. A survey of earlier theoretical approaches to rare-gas adsorption in electric fields is included. The present treatment can be regarded as an integrated theory in which both covalent and polarisation effects have their place; it brings out the role of polarisation contributions to the total binding energy, and allows us to clarify the relationship between the earlier classical models and the present quantum theory of field-induced chemisorption.     

Pd deposits on Ni(111): a theoretical study

The ASED-MO method has been used to gather electronic and energetic information on Pd deposits on Ni(111) and Pd atom inclusion in the first Ni layer since these model catalysts exhibit a striking catalytic efficiency towards butadiene hydrogenation. The electronic structure of Pd atoms is strongly altered compared with pure Pd. A Pd(4d)→Pd(5s) electronic transfer occurs in the case of the deposit when a slight similar transfer and a charge transfer from Pd to surrounding Ni takes place in the case of the inclusion. Those results are consistent with XPS experimental data. A low density of states, near the Fermi level, is also observed. The optimal geometrical situation for Pd deposits is found to be 2D-aggregates (in pseudoepitaxy or pseudomorphy with the underlying Ni surface, depending on the aggregate size). Small aggregates (part of the first Ni layer) are found to be the most stable in the case of a Pd inclusion in the Ni with a Pd---Pd distance of 2.64 Å, in agreement with available EXAFS experimental data.     

Evaluation of different models for the dissociation of silane on the Si(1 1 1)7 × 7 surface using the extended Brenner empirical potential

Several models have been proposed in the literature for the initial stages of the dissociative chemisorption of silane (SiH₄) on the Si(1 1 1)7 × 7 surface. In this paper, geometry optimisation calculations using the extended Brenner empirical potential have been performed to determine which of these models yields the minimum energy structure. The lowest energy configurations are found to correspond to the dissociation of silane into SiH₂ and two hydrogen atoms. The minimum energy structure involves the adsorption of the two hydrogen atoms onto the dangling bonds of an adjacent adatom and rest atom, and the insertion of the remaining SiH₂ fragment into one of the adatom backbonds. These results are discussed in the light of the existing experimental data.     

A first-principles study of surface and subsurface H on and in Ni(1 1 1): diffusional properties and coverage-dependent behavior

Periodic, self-consistent, density functional theory (GGA-PW91) calculations are performed for both surface and subsurface atomic hydrogen on and in Ni(1 1 1). At a low coverage (θ=0.25 ML), the binding energies (BEs) of a hydrogen atom in surface fcc, subsurface octahedral (first layer), and subsurface octahedral (second layer) sites are −2.89, −2.18, and −2.11 eV, respectively. The activation energy barriers for hydrogen diffusion from the surface to the first subsurface layer and from the first to the second subsurface layer are estimated to be 0.88 and 0.52 eV, respectively. In the entire coverage range studied, hydrogen occupies surface fcc and hcp sites and subsurface octahedral sites. In addition, the magnitude of the BE per hydrogen atom and the magnetization of the nickel slabs both decrease as hydrogen coverage increases. Vibrational frequencies of hydrogen at various surface and subsurface sites are calculated and are in reasonable agreement with experimental data. A phase stability calculation with a 2 × 2 surface unit cell shows that a p(2 × 2)-2H overlayer structure (θ=0.5 ML) and a p(1 × 1)-1H structure (θ=1.0 ML) are stable at low hydrogen pressures, in agreement with numerous experimental results. A very large increase in pressure is required to populate subsurface sites. After such an increase occurs, the first subsurface layer is filled completely.     

C---H bond activation of formate on clean and K-covered Ru(001): a theoretical study

An extended Hückel (EH) study is performed on the clusters representing a formate adsorbed on Ru(001) and on in order to investigate the effect of potassium on the selectivity of decomposition of formate on Ru(001). The adsorption geometry and the VSIP (valence state ionization potential) values of EH parameters are determined from ab initio calculations on small clusters. The EH calculation reproduces well the site preference of formate on each surface suggested from experiments. The C---H bond of formate, which is our focus in this study, is calculated to be almost the same on the two surfaces when the molecular plane of formate is perpendicular to the surface; but when the plane is tilted from the surface normal and thus the C---H bond approaches the surface, the C---H bond is weakened to a much higher extent on clean Ru(001) than on K-covered Ru(001). This is in good agreement with the experimental result that the presence of potassium changes the reaction pathway of the decomposition of formate on Ru(001) by suppressing C---H bond cleavage.     

Local adsorption sites and bondlength changes in Ni(1 0 0)/H/CO and Ni(1 0 0)/CO

The local adsorption geometry of CO adsorbed in different states on Ni(1 0 0) and on Ni(1 0 0) precovered with atomic hydrogen has been determined by C 1s (and O 1s) scanned-energy mode photoelectron diffraction, using the photoelectron binding energy changes to characterise the different states. The results confirm previous spectroscopic assignments of local atop and bridge sites both with and without coadsorbed hydrogen. The measured Ni–C bondlengths for the Ni(1 0 0)/CO states show an increase of 0.16 ± 0.04 Å in going from atop to bridge sites, while comparison with similar results for Ni(1 1 1)/CO for threefold coordinated adsorption sites show a further lengthening of the bond by 0.05 ± 0.04 Å. These changes in the Ni–CO chemisorption bondlength with bond order (for approximately constant adsorption energy) are consistent with the standard Pauling rules. However, comparison of CO adsorbed in the atop geometry with and without coadsorbed hydrogen shows that the coadsorption increases the Ni–C bondlength by only 0.06 ± 0.04 Å, despite the decrease in adsorption energy of a factor of 2 or more. This result is also reproduced by density functional theory slab calculations. The results of both the experiments and the density functional theory calculations show that CO adsorption onto the Ni(1 0 0)/H surface is accompanied by significant structural modification; the low desorption energy may then be attributed to the energy cost of this restructuring rather than weak local bonding.     

Quantum states of hydrogen (H, D, T) atoms on Cu(1 0 0) and (1 1 0) surfaces

We investigate the quantum mechanical behavior of adsorbed hydrogen (H, D, T) on Cu(1 0 0) and (1 1 0) surfaces. We construct potential energy surfaces (PESs) for the motion of the hydrogen H atom on Cu(1 0 0) and (1 1 0) surfaces within the framework of density functional theory. The potential energy takes a minimum value on the hollow site of Cu(1 0 0) and on the short bridge site of Cu(1 1 0). Moreover, we calculate the quantum states of hydrogen atom motion on these calculated PESs. The ground state wave function of the hydrogen atom motion is strongly localized around the hollow site on the Cu(1 0 0) surface. On the other hand, the ground state wave function of the hydrogen atom motion on Cu(1 1 0) is distributed from the short bridge site to two neighboring pseudo-threefold sites. We finally show isotope effects on the quantum states of the motion of hydrogen on both surfaces.     

First-principles studies of the oxygen adsorption on unreconstructed and reconstructed Ni(1 1 0) surfaces

First-principles calculations have been performed to investigate the adsorption of oxygen on unreconstructed and reconstructed Ni(1 1 0) surfaces. The energetics, structural, electronic and magnetic properties are given in detail. For oxygen adsorption on unreconstructed surface, (n×1)(n=2,3) substrate with oxygen atom on short-bridge site is found to be the most stable adsorption configuration. Whereas energetically most favorable adsorption phase of reconstructed surface is p(n×1) substrate with oxygen atom located at long-bridge site. Our calculations suggest that the surface reconstruction is induced by the oxygen adsorption. We also find there are redistributions of electronic structure and electron transfer from the substrate to adsorbate. Our calculations also indicate surface magnetic moment is enhanced on clean surfaces and oxygen atoms are magnetized weakly after oxygen adsorption. Interestingly, adsorption on unreconstructed surface does not change surface magnetic moment. However, adsorbate leads to reduction of surface magnetic moment in reconstructed system remarkably.     

Adsorption of SH and OH and coadsorption of S, O and H on Ni(111)

The adsorption of SH and OH radicals on Ni(111) is treated using an ab initio embedding theory. The Ni(111) surface is modeled as a three-layer, 28-atom cluster with the Ni atoms fixed at bulk lattice sites. The Ni(111) energy surface is very flat for SH adsorption if the H tilt angle is allowed to vary. At both atop and bridge sites, the S---H axis is tilted away from the surface normal by 70°, resulting in the sulfur atom being sp³-hybridized and the adsorption energy being 59 kcal mol⁻¹. For SH at the three-fold site, the S---H axis is normal to the surface, the sulfur is sp-hybridized, and the adsorption energy is 58 kcal mol⁻¹. OH is preferentially adsorbed at the three-fold site. The calculated adsorption energy is 90 kcal mol⁻¹ and the O---H axis is perpendicular to the surface. OH adsorption at the atop and bridge sites is 16 and 5 kcal mol⁻¹ less stable than at the three-fold site, respectively. Atomic H, O and S are preferentially adsorbed at the three-fold site. The calculated adsorption energies are 62, 92 and 87 kcal mol⁻¹, for H, O and S, respectively. The calculated adsorbate---Ni bond distances of 1.86 Å for H, 1.86 Å for O and 2.29 Å for S are in good agreement with experimental data. SH and OH bonding to the surface involves a combination of ionic and covalent contributions and substantial mixing with the Ni 3d orbitals. Dipole-moment calculations indicate strong ionic bonding for the atomic O/Ni system and ionic plus covalent character for the atomic S/Ni interactions. Adsorption of S and O at the three-fold site blocks H adsorption at the nearby surface. Moving H away from the S or O adatom reduces the repulsion. The dissociation of SH_ad → S_ad + H_ad is calculated to be exothermic by 5 kcal mol⁻¹ and OH_ad → O_ad + H_ad to be endothermic by 30 kcal mol⁻¹ for infinite separation between S, O and H.     

Theoretical study of the Ni growth on Pt stepped surfaces

We have investigated the growth of Ni on Pt stepped surfaces with (1 1 1) terraces by means of potentials derived from the second moment approximation in a tight-binding model. The activation energies associated to these processes are determined. The Schwoebel barriers of Ni atoms descending steps of Pt stepped surfaces are calculated for different kinds of straight steps (A and B steps) differing by the orientation of the ledge. In addition, we study the diffusion of Ni adatoms at fcc or hcp sites in the presence of small adislands on the terraces, in the vicinity of the A and B steps. We show that a good estimate of the potential wells and diffusion barriers could be given by introducing a lateral effective pair interaction model, the interactions extending up to the next nearest neighbors. Finally, we have carried out Kinetic Monte-Carlo simulations to investigate the Ni wire formation at Pt step edges and the influence of the exchange processes in the alloy formation.     

Construction of modified embedded atom method potentials for Cu, Pt and Cu-Pt and modelling surface segregation in Cu3Pt alloys

In this work, surface segregation to Cu₃Pt surfaces is studied with the modified embedded atom method (MEAM). This work is triggered by the catalytic importance of Cu-Pt alloys, together with the contradictory experimental results for the surface segregation in Cu₃Pt(1 1 1) alloys based on low energy ion scattering (LEIS) [Y.G. Shen, D.J. O’Connor, K. Wandelt, R.J. MacDonald, Surf. Sci. 328 (1995) 21] and low energy electron diffraction (LEED) [Y. Gauthier, A. Senhaji, B. Legrand, G. Tréglia, C. Becker, K. Wandelt, Surf. Sci. 527 (2003) 71]. In order to accurately describe the segregation behaviour in the Cu₃Pt system, a reliable potential, that is also applicable to surface phenomena, is indispensable. Therefore, first, new MEAM parameters are derived, consistently based on ab initio density functional theory (DFT) calculations, according to a method that is a modification of previous work [P. van Beurden, G.J. Kramer, Phys. Rev. B 63 (2001) 165106]. Upon testing, these parameters prove to reproduce very well various surface properties of this system. Next, Monte Carlo (MC) simulations combined with the newly derived MEAM potentials are set up to investigate surface segregation to low index single crystal surfaces. For the Cu₃Pt(1 1 1) surface, our MC/MEAM simulations agree completely with the available LEIS evidence and contradict the unusual depth profile based on LEED. However, the slight Pt enrichment observed in the LEED experiments can be reproduced by assuming a slight Pt excess in the bulk of the sample. The simulated composition depth profile, on the other hand, does not agree with the LEED evidence. Also, for the Cu₃Pt(1 0 0) surface, the MC/MEAM results agree completely with LEIS experiments. For the Cu₃Pt(1 1 0) surface, finally, the MC/MEAM simulations show a somewhat deviating behaviour with respect to the experimental LEIS evidence. The possibility of a missing-row reconstruction is evaluated, but cannot explain the discrepancy for the Cu₃Pt(1 1 0) system. In order to further investigate the deviation from the experiments, additional DFT and MEAM calculations are performed in search of the preferred surface termination for Cu₃Pt(1 1 0). Both DFT and MEAM calculations agree on the pure Cu layer as the most stable surface termination. Although the experiment was extensively tested for reproducibility, it possibly reflects a metastable state. Finally, in view of the importance of small and less orderly particles in catalysis, the newly derived MEAM parameters are used in order to study the segregation to Cu₃Pt vicinal surfaces with {1 1 1} terraces, for which no experimental information is available yet.     

A DFT investigation of potential energy surface and vibrational properties of hydrogen adsorbed on the Rh(1 1 1) surface

Vibrational properties of hydrogen on the Rh(1 1 1) surface have been investigated theoretically. The potential energy surface for this system has been calculated within the density functional theory. The potential is found to be very anharmonic. The wave functions and their energies for the hydrogen motion on the potential energy surface (PES) have been calculated and assigned by using discrete variable representation. It was found that the vibrational wave function is localized at hollow site in the ground state for hydrogen on Rh(1 1 1). Higher excited states are of delocalized nature and mixed parallel and perpendicular character. Our results are in good agreement with the observed vibrational spectra of hydrogen on the Rh(1 1 1) surface.     

Dissociative chemisorption of H2 on Pt(1 1 1): isotope effect and effects of the rotational distribution and energy dispersion

Six-dimensional quantum dynamics calculations on dissociative scattering of H₂ and D₂ from Pt(1 1 1) are performed. The six-dimensional potential energy surface used was generated using density functional theory employing the generalized gradient approximation. The isotope effect, the effect of widening the rotational distribution on the dissociation probability and the effect of the energy dispersion are investigated, as they are possible reasons for a discrepancy between previous theoretical work and molecular beam experiments. It was found that these effects cannot explain the differences between the theoretical and experimental results.     

Adsorption and diffusion of a Si adatom on the H/Si(1 1 1) surface: comparison with the H/Si(0 0 1) surface

Using a first-principles method, we investigate the adsorption and diffusion of a Si adatom on the H-terminated Si(1 1 1) substrate, which would be useful in understanding the initial stages of Si homoepitaxy using a H surfactant. The adatom substitutes H atom(s) to form a monohydride structure or a dihydride structure. In forming the monohydride structure, the energy barrier for H substitution is absent. The adatom migrates on the surface with alternating its chemical state between monohydride and dihydride. These behaviors of the adatom are quite similar to those on the H/Si(0 0 1)2 × 1 surface, despite the significant difference in the substrate structure between both orientations. The resulting diffusion barrier is 1.30 eV, which is also comparable to that on the H/Si(0 0 1)2 × 1 surface.     

An angle-scanned photoelectron diffraction (XPD) study of the growth and structure of ultrathin Fe films on Au(001)

We report a study of the growth and structure of Fe films on Au(001) at room temperature using angle-resolved photoelectron spectroscopy (ARXPS, AlK) and polar-scan photoelectron diffraction (XPD, AlK), exploiting the forward scattering (FS) enhancement of photoelectrons along atomic chains. The structure of the Fe 3p and 2p XPD polar diagrams and the development of the FS features with film growth evidence that Fe grows pseudomorphically in a nearly perfect layer-by-layer mode with bcc (001) structure rotated by 45° about the surface normal. At least up to 4 and probably up to 6 monolayers Fe, a segregated Au monolayer (surfactant layer) exists on top of the Fe film. This follows from the comparison of a simple model for the development of the substrate and film FS enhancements with the experimental data. By using angular shifts of the Fe 3p and Fe 2p bcc-[111] and bcc-[101] FS peaks we could determine the Au(on top)---Fe and Fe---Fe interlayer distances for 1 and 2 ML thick films to be 1.71(0.04) Å and 1.48(0.08) Å, respectively, showing that very thin films have a slightly expanded bcc structure (bct). The regular bcc angle positions are observed above 4–6 ML.