Theory of Adsorption on Metal Substrates

Contents: 5.1 Introduction 5.2 Concepts and definitions 5.3 The tight-binding picture of bonding 5.4 Adsorption of isolated adatoms 5.5 Alkali-metal adsorption: the traditional picture of on-surface adsorption 5.6 Substitutional adsorption and formation of surface alloys 5.7 Adsorption of CO on transition-metal surfaces - a model system for a simple molecular adsorbate 5.8 Co-adsorption [the example CO plus O on Ru(0001)] 5.9 Chemical reactions at metal surfaces 5.10 The catalytic oxidation of CO 5.11 Summary outline of main pointsComment: 73 pages including 44 figures. A version with high-resolution figures and related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

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.Comment: to be published in Surface Reviews and Letters, 18 pages, 18 figure

Coadsorption of CO and O on Ru(0001): A structural analysis by density functional theory

Knowledge of the atomic geometry of a surface is a prerequisite for any detailed understanding of the surface's electronic structure and chemical properties. Previous studies have convincingly demonstrated that density functional theory (DFT) yields accurate surface atomic geometries and that reliable predictions concerning stable and metastable phases can be made on the basis of the calculated energetics. In the present work we use DFT to investigate the atomic structure of four ordered coadsorbate phases of carbon monoxide and oxygen on Ru(0001). All of the structures have a (2x2) periodicity with differing concentrations of CO molecules and O atoms. For two of these phases dynamical low-energy electron diffraction (LEED) intensity analyses have been performed and the agreement between our DFT- and the LEED-determined structures is found to be very good. We predict the atomic geometry of the third phase for which no structural determination based on experiments has been made to date. We also predict the stability of a new ordered mixed phase.Comment: 6 pages, 1 figure, submitted to Israel Journal of Chemistry (June 29, 1998). Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm

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.Comment: 20 pages, 7 figures. J. Vac. Sci. and Techn., in press (submitted September 1996

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.Comment: 13 pages, 4 figures. Surface Science, in press (submitted July 1996

First-principles prediction of phonon-mediated superconductivity in XBC (X= Mg, Ca, Sr, Ba)

From first-principles calculations, we predict four new intercalated hexagonal $X$BC ($X$=Mg, Ca, Sr, Ba) compounds to be dynamically stable and phonon-mediated superconductors. These compounds form a LiBC like structure but are metallic. The calculated superconducting critical temperature, $T{_c}$, of MgBC is 51 K. The strong attractive interaction between $\sigma$-bonding electrons and the B${_{1g}}$ phonon mode gives rise to a larger electron-phonon coupling constant (1.135) and hence high $T_c$; notably, higher than that of MgB$_2$. The other compounds have a low superconducting critical temperature (4-17 K) due to the interaction between $\sigma$-bonding electrons and low energy phonons (E${_{2u}}$ modes). Due to their energetic and dynamic stability, we envisage that these compounds can be synthesized experimentally.Comment: 7 pages, 6 figure

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Towards a first-principles theory of surface thermodynamics and kinetics

Understanding of the complex behavior of particles at surfaces requires detailed knowledge of both macroscopic and microscopic processes that take place; also certain processes depend critically on temperature and gas pressure. To link these processes we combine state-of-the-art microscopic, and macroscopic phenomenological, theories. We apply our theory to the O/Ru(0001) system and calculate thermal desorption spectra, heat of adsorption, and the surface phase diagram. The agreement with experiment provides validity for our approach which thus identifies the way for a predictive simulation of surface thermodynamics and kinetics.Comment: 4 pages including 3 figures. Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

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.Comment: RevTeX, 6 pages, 3 figures, to appear in Phys. Rev. Let

Metastable precursors during the oxidation of the Ru(0001) surface

Using density-functional theory, we predict that the oxidation of the Ru(0001) surface proceeds via the accumulation of sub-surface oxygen in two-dimensional islands between the first and second substrate layer. This leads locally to a decoupling of an O-Ru-O trilayer from the underlying metal. Continued oxidation results in the formation and stacking of more of these trilayers, which unfold into the RuO_2(110) rutile structure once a critical film thickness is exceeded. Along this oxidation pathway, we identify various metastable configurations. These are found to be rather close in energy, indicating a likely lively dynamics between them at elevated temperatures, which will affect the surface chemical and mechanical properties of the material.Comment: 11 pages including 9 figures. Submitted to Phys. Rev. B. Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm