arXiv subjects

M. Scheffler

Publications and source records attributed to M. Scheffler.

65 records · Page 4Linked to original sources

arXiv1996

Mechanism of efficient carbon monoxide oxidation at Ru(0001)

We performed density-functional theory calculations using the generalized gradient approximation for the exhange-correlation functional to investigate the unusual catalytic behavior of Ru under elevated gas pressure conditions for the carbon monoxide oxidation reaction, which includes a particularly high CO_2 turnover. Our calculations indicate that a full monolayer of adsorbed oxygen actuates the high rate, enabling CO_2 formation via both scattering of gas-phase CO molecules as well as by CO molecules adsorbed at oxygen vacancies in the adlayer, where the latter mechanism is expected to be very efficient due to the relatively weak adsorption energy of both CO and O, as well as the close proximity of these reactants. In the present paper we analyse the bonding and electronic properties associated with the reaction pathway for CO_2 production via the scattering reaction. We find that the identified ``bent'' transition state is due to electron transfer into the unoccupied 2 pi orbitals of the CO molecule which reduces the Pauli repulsion between the impinging CO and the O-covered surface. Bond formation to CO_2 then proceeds by electron transfer back from the CO 2 pi orbitals into the bonding region between CO and the adsorbed O atom.

cond-mat.mtrl-sci↗

arXiv1996

Study of CO Oxidation over Ru(0001) at High Gas Pressures

Experiments performed at high gas partial pressures have demonstrated that the kinetics of the CO oxidation reaction at Ru(0001) is different and somewhat anomalous compared to that over other transition metal surfaces and, in particular, the turnover rate is exceptionally high. In order to gain insight into the underlying reasons for this behavior, we performed density functional theory calculations using the generalized gradient approximation for the exchange-correlation functional. We find that the high rate is due to a weakly, but nevertheless well bound, (1x1) oxygen adsorbate layer which may form for high O_2 pressures but not under usual ultra high vacuum conditions. The calculations indicate that reaction to CO_2 occurs both via scattering of gas-phase CO molecules as well as by CO molecules weakly adsorbed at vacancies in the oxygen adlayer, where the latter mechanism dominates the rate.

cond-mat↗

arXiv1996

Diffusivity of Ga and Al adatoms on GaAs(001)

The diffusivity of Ga and Al adatoms on the (2x4) reconstructed GaAs(001) surface are evaluated using detailed ab initio total energy calculations of the potential energy surface together with transition state theory. A strong diffusion anisotropy is found, with the direction of fastest diffusion being parallel to the surface As-dimer orientation. In contrast to previous calculations we identify a short--bridge position between the two As atoms of a surface dimer as the adsorption site for Al and Ga adatoms.

mtrl-th↗

arXiv1996

Strained tetragonal states and Bain paths in metals

Paths of tetragonal states between two phases of a material, such as bcc and fcc, are called Bain paths. Two simple Bain paths can be defined in terms of special imposed stresses, one of which applies directly to strained epitaxial films. Each path goes far into the range of nonlinear elasticity and reaches a range of structural parameters in which the structure is inherently unstable. In this paper we identify and analyze the general properties of these paths by density functional theory. Special examples include vanadium, cobalt and copper, and the epitaxial path is used to identify an epitaxial film as related uniquely to a bulk phase.

mtrl-th↗

arXiv1996

Structure and stability of a high-coverage (1x1) oxygen phase on Ru(0001)

The formation of chemisorbed O-phases on Ru(0001) by exposure to O_2 at low pressures is apparently limited to coverages Theta <= 0.5. Using low-energy electron diffraction and density functional theory we show that this restriction is caused by kinetic hindering and that a dense O overlayer (Theta = 1) can be formed with a (1x1) periodicity. The structural and energetic properties of this new adsorbate phase are analyzed and discussed in view of attempts to bridge the so-called "pressure gap" in heterogeneous catalysis. It is argued that the identified system actuates the unusually high rate of oxidizing reactions at Ru surfaces under high oxygen pressure conditions.

mtrl-th↗

arXiv1996

The GaAs Equilibrium Crystal Shape from First-Principles

Surface energies for different GaAs surface orientations have been calculated as a function of the chemical potential. We use an energy density formalism within the first-principles pseudopotential density-functional approach. The equilibrium crystal shape (ECS) has been derived from the surface energies for the (110), (100), (111), and (-1-1-1) orientations. Under As-rich conditions all four considered surface orientations exist in thermodynamic equilibrium, in agreement with experimental observations. Moreover, our calculations allow us to decide on previous contradictory theoretical values for the surface energies of the (111) and (-1-1-1) facets.

mtrl-th↗

arXiv1995

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.

mtrl-th↗

arXiv1995

Theory of alkali metal adsorption on close-packed metal surfaces

Results of recent density functional theory calculations for alkali metal adsorbates on close-packed metal surfaces are discussed. Single adatoms on the (111) surface of Al and Cu are studied with the self-consistent surface Green-function method by which the pure adsorbate-substrate interaction may be analyzed. Higher coverage ordered adlayers of K on Al(111), Na on Al(111), and Na on Al(001) are treated using the ab-initio pseudopotential plane wave method which affords the prediction of coverage dependent stable and metastable adsorbate geometries and phase transitions of the adsorbate layers. Together, these studies give insight and understanding into current key issues in alkali metal adsorption, namely, the nature of the adsorbate-substrate bond at low coverage and the occurrence of hitherto unanticipated adsorbate geometries, and the associated electronic properties.

mtrl-th↗

arXiv1994

Theory of adsorption and desorption of H_2/Si(001)

While the small sticking coefficient for molecular hydrogen on the Si(001) surface apparently requires a large energy barrier of adsorption, no such barrier is observed in desorption experiments. We have calculated the potential-energy surface of an H_2 molecule in front of a Si(001) surface. If we relax the Si substrate, we find an optimum desorption path with a low (\lesssim 0.3 eV) adsorption energy barrier. While molecules impinging on the surface will mostly be reflected at the larger barrier of some frozen-substrate, molecules adsorbed on the surface can desorb along the low-barrier path.

mtrl-th↗

arXiv1994

High-dimensional quantum dynamics of adsorption and desorption of H$_2$ at Cu(111)

We performed high-dimensional quantum dynamical calculations of the dissociative adsorption and associative desorption of hydrogen on Cu(111). The potential energy surface (PES) is obtained from density functional theory calculations. Two regimes of dynamics are found, at low energies sticking is determined by the minimum energy barrier, at high energies by the distribution of barrier heights. Experimental results are well-reproduced qualitatively, but some quantitative discrepancies are identified as well.

mtrl-th↗

arXiv1994

Frustrated H-Induced Instability of Mo(110)

Using helium atom scattering Hulpke and L"udecke recently observed a giant phonon anomaly for the hydrogen covered W(110) and Mo(110) surfaces. An explanation which is able to account for this and other experiments is still lacking. Below we present density-functional theory calculations of the atomic and electronic structure of the clean and hydrogen-covered Mo(110) surfaces. For the full adsorbate monolayer the calculations provide evidence for a strong Fermi surface nesting instability. This explains the observed anomalies and resolves the apparent inconsistencies of different experiments.

mtrl-th↗