Steffen Wilke

Six-dimensional quantum dynamics of adsorption and desorption of H_2 at Pd(100): Steering and steric effects

We report the first six-dimensional quantum dynamical calculations of dissociative adsorption and associative desorption. Using a potential energy surface obtained by density functional theory calculations, we show that the initial decrease of the sticking probability with increasing kinetic energy in the system H_2/Pd(100), which is usually attributed to the existence of a molecular adsorption state, is due to dynamical steering. In addition, we examine the influence of rotational motion and orientation of the hydrogen molecule on adsorption and desorption.

Force calculation and atomic-structure optimization for the full-potential linearized augmented plane-wave code WIEN

Abstract Following the approach of Yu, Singh, and Krakauer [Phys. Rev. B 43 (1991) 641], we extended the linearized augmented plane wave code WIEN of Blaha, Schwarz, and coworkers by the evaluation of forces. In this paper we describe the approach, demonstrate the high accuracy of the force calculation, and use them for an efficient geometry optimization of poly-atomic systems.

Mechanism of Poisoning the Catalytic Activity of Pd(100) by a Sulfur Adlayer

The modification of the potential-energy surface (PES) of H2 dissociation over Pd(100) as induced by the presence of a (2×2) S adlayer is investigated by density-functional theory and the linear augmented plane wave method. It is shown that the poisoning effect of S originates from the formation of energy barriers hindering the dissociation of H2. The barriers are in the entrance channel of the PES and their magnitude strongly depends on the lateral distance of the H2 molecule from the S adatoms.

Poisoning of Pd(100) for the dissociation of H2 : a theoretical study of co-adsorption of hydrogen and sulphur

Abstract The presence of sulphur adatoms on Pd(100) is known to hinder the dissociative adsorption of hydrogen. Using density-functional theory we studied the adsorption of hydrogen on clean and sulphur precovered Pd(100) surfaces. The results show that the poisoning effect of sulphur is not caused by a strict blocking of hydrogen adsorption sites in the vicinity of S adatoms. For a (2 × 2) sulphur adlayer (coverage θ s = 1 4 ) hydrogen adsorption remains an exothermic process for all surface hollow sites not occupied by sulphur. The blocking of hydrogen adsorption happens only for higher sulphur coverages. We conclude that the poisoning of Pd(100) is a combined effect of the formation of energy barriers hampering the H2 dissociation and a modest decrease of the adsorption energy in the vicinity of the sulphur adatoms.

Frustrated H-induced instability of Mo(110).

The large interest in the properties of transition metal surfaces has been recently fostered by inelastic helium atom scattering experiments carried out by Hulpke and Luedecke. Sharp and giant anomalies in the surface phonon dispersion curves along Gamma-H and Gamma-S have been detected on W(110) and Mo(110) at a coverage of one monolayer of hydrogen. At the same critical wave vectors a smaller second indentation is present in the experimental phonon branches. Recently we have proposed a possible interpretation which is able to explain this and other experiments. In this paper we discuss results of our recent ab initio calculations of the atomic and electronic properties of the clean and H-covered Mo(110) surface in more details. For the full monolayer coverage the calculated Fermi-surface contours are characterized by strong nesting features which originate the observed anomalies.

Theoretical investigation of water formation on Rh and Pt Surfaces

Catalytic water formation from adsorbed H and O adatoms is a fundamental reaction step in a variety of technologically important reactions involving organic molecules. In particular, the water-formation rate determines the selectivity of the catalytic partial oxidation of methane to syngas. In this report we present a theoretical investigation of the potential-energy diagram for water formation from adsorbed O and H species on Rh(111) and Pt(111) surfaces. The study is based on accurate first-principles calculations applying density-functional theory. Our results are compared to the potential-energy diagram for this reaction inferred from experimental data by Hickman and Schmidt [AIChE. J. 39, 1164 (1993)]. The calculations essentially reproduce the scheme of Hickman and Schmidt for water formation on Rh(111) with the important difference that the OH molecule is significantly more stable than assumed by Hickman and Schmidt. On Pt(111) surfaces, however, the calculations predict a barrier to OH formation v...

Scattering of Rare-Gas Atoms at a Metal Surface: Evidence of Anticorrugation of the Helium-Atom Potential Energy Surface and the Surface Electron Density

Recent measurements of the scattering of He and Ne atoms at Rh(110) suggest that these two rare-gas atoms measure a qualitatively different surface corrugation: While Ne atom scattering seemingly reflects the electron-density undulation of the substrate surface, the scattering potential of He atoms appears to be anticorrugated. An understanding of this perplexing result is lacking. In this paper we present density functional theory calculations of the interaction potentials of He and Ne with Rh(110). We find that, and explain why, the nature of the interaction of the two probe particles is qualitatively different, which implies that the topographies of their scattering potentials are indeed anticorrugated.

Ab initio study of hydrogen adsorption on Pd(100)

Abstract We report an all-electron density-functional theory study of the adsorption of hydrogen at Pd(100). We use the local-density approximation for the exchange-correlation energy functional and the full-potential linear muffintin orbital method (FP-LMTO) to calculate adsorption energies, stable adsorption sites, adsorption-induced surface relaxations, and the work-function changes. It is found that (as expected) for coverages θ ⩽ 1 the surface fourfold hollow site has the largest adsorption energy. For coverages θ > 1 it is predicted that additional hydrogen is incorporated below the surface. The work function increases with hydrogen coverage up to θ = 1 but for additional hydrogen adsorption we find that ΔΦ remains roughly constant. The theoretical results are compared with available experimental data.

Six-dimensional quantum dynamics of adsorption and desorption of H2 at Pd(100): no need for a molecular precursor adsorption state

Abstract We report six-dimensional quantum dynamical calculations of dissociative adsorption and associative desorption of the system H 2 Pd (100) using an ab initio potential energy surface. We focus on rotational effects in the steering mechanism, which is responsible for the initial decrease of the sticking probability with kinetic energy. In addition, steric effects are briefly discussed.

LOCAL ISOELECTRONIC REACTIVITY OF SOLID SURFACES

Density-functional calculations of chemisorption processes and of potential energy surfaces of the dissociation of simple molecules over surfaces, which have become available in the last few years, have greatly improved the fundamental understanding of reactions at solid surfaces. However, those extensive computations are limited to a restricted number of model systems. It remains an important task to develop a methodology that allows the prediction and interpretation of reactions at surfaces in terms of the properties of the noninteracting systems. Such concepts, well known in molecular chemistry, are the essence of “reactivity theory.” They are based on low-order perturbation theory and aim at a description of the early stages of chemical interactions. The ensuing response functions are called “reactivity indices” and characterize the changes of the electronic structure of one reactant as stimulated by the presence of the other or vice versa. In an early contribution to this field, Fukui et al. [1,2] established the correlation between the frontier-orbital density, i.e., the density of the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO), and the reactivity of a system toward electron donation or acceptance. Pearson [3] introduced the electronic “softness,” the magnitude of the change of the electronic structure due to a change of the number of electrons in the system, as a measure of the reactivity. Species are classified as “soft” if only a small energy is required to change their electronic configuration, i.e., if the valence electrons are easily distorted, polarized, removed or added. A “hard” species has the opposite properties, holding its valence electrons more tightly [3,4]. The utility of the hardnesssoftness concept is based on the so called hard and soft acid and base (HSAB) principle formulated by Pearson [3] which states that hard-hard (soft-soft) interactions are preferred. In the case of polyatomic or extended systems, the HSAB principle is used in a local version: The soft (hard) parts of one reactant prefer to interact with soft (hard) areas of the other [5]. Parr and collaborators [5] gave a foundation in density functional theory to those mostly