509 research outputs found
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
Anomalous Behavior of Ru for Catalytic Oxidation: A Theoretical Study of the Catalytic Reaction CO + 1/2 O_2 --> CO_2
Recent experiments revealed an anomalous dependence of carbon monoxide
oxidation at Ru(0001) on oxygen pressure and a particularly high reaction rate.
Below we report density functional theory calculations of the energetics and
reaction pathways of the speculated mechanism. We will show that the
exceptionally high rate is actuated by a weakly but nevertheless well bound
(1x1) oxygen adsorbate layer. Furthermore it is found that reactions via
scattering of gas-phase CO at the oxygen covered surface may play an important
role. Our analysis reveals, however, that reactions via adsorbed CO molecules
(the so-called Langmuir-Hinshelwood mechanism) dominate.Comment: 5 pages, 4 figures, Phys. Rev. Letters, Feb. 1997, in prin
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
Trends in adsorption of noble gases He, Ne, Ar, Kr, and Xe on Pd(111)(√3 x √3)R30<sup>o</sup>: All-electron density-functional calculations
It was recently found from ab initio investigations [J. L. F. Da Silva et al., Phys. Rev. Lett. 90, 066104 (2003)] that polarization effects and the site dependence of the Pauli repulsion largely dictate the nature of the interaction and the site preference of Xe adatoms on close-packed metal surfaces. It is unclear if the same interaction mechanism occurs for all rare-gas atoms adsorbed on such surfaces. To address this question, we perform all-electron density-functional theory calculations with the local-density approximation (LDA) and generalized gradient approximations (GGA) for [He, Ne, Ar, Kr, and Xe]/Pd(111) in the )-(√3 x √3)R30° structure. Our results confirm that polarization effects of the rare-gas adatoms and Pd atoms in the topmost surface layer, together with the site-dependent Pauli repulsion, largely determine the interaction between rare-gas atoms and the Pd(111) surface. Similar to the earlier ab initio study, the on-top site preference is obtained by the LDA for all rare-gas adatoms, while the GGA functionals yield the on-top site preference for Xe, Kr, and He adatoms, but the fcc site for Ne and Ar
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