Showing papers by "Fritz Haber Institute of the Max Planck Society published in 1998"

The Hematite ( α- Fe 2 O 3 ) (0001) Surface: Evidence for Domains of Distinct Chemistry

Abstract: Using spin-density functional theory we investigated various possible structures of the hematite (0001) surface. Depending on the ambient oxygen partial pressure, two geometries are found to be particularly stable under thermal equilibrium: one being terminated by iron and the other by oxygen. Both exhibit huge surface relaxations ( $\ensuremath{-}57%$ for the Fe and $\ensuremath{-}79%$ for the O termination) with important consequences for the surface electronic and magnetic properties. With scanning tunneling microscopy we observe two different surface terminations coexisting on single crystalline $\ensuremath{\alpha}$- ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ (0001) films, which were prepared in high oxygen pressures.

Raman spectroscopy of monolayer-type oxide catalysts: Supported Mo oxides

Abstract: Oxides of the group VIb metals (Cr, Mo, W) and oxides of vanadium, rhenium, and niobium supported on a second high-surface-area metal oxide such as Al2O3, TiO2, Si02, ZrO2, and so forth are recognized as industrially important catalysts or catalyst precursors for various reactions [1–11], These materials frequently have been described as so-called monolayer catalysts based on a structural model which assumed spreading of the active oxide over the support surface. These catalysts have been investigated by a variety of techniques, conventional bulk sampling techniques as well as by surface-sensitive electron and ion spectroscopies, in an attempt to elucidate the nature of the catalyst surface species, and to study the coordination environment of the active metal center(s). Electronic spectroscopy gives rise to broad bands and the spectra are less informative than vibrational spectra. In addition, although techniques such as Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS...

Dynamics of Electron-Induced Manipulation of Individual CO Molecules on Cu(111)

Abstract: Electrons tunneling from a scanning tunneling microscope tip to individual CO molecules on Cu(111) can cause their hopping from the surface to the tip if the bias exceeds a threshold of 2.4 V. Polarization- and time-resolved two-photon photoemission identifies the underlying elementary process as intermediate population of a CO $2{\ensuremath{\pi}}^{*}$-derived level, which exhibits an ultrashort lifetime of 0.8--5 fs. From an isotope effect of ${2.7}_{\ensuremath{-}0.5}^{+0.3}$ it can be calculated that $\ensuremath{\approx}0.05%$ of the tunneling current transiently occupies this level while a desorption of the excited molecule occurs only in $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ of the cases.

Growth and structure of ultrathin FeO films on Pt(111) studied by STM and LEED

Abstract: The growth of iron-oxide films on Pt(111) prepared by iron deposition and subsequent oxidation was studied by scanning tunneling microscopy (STM) and high-resolution low-energy electron diffraction (LEED). Despite a 10% lattice mismatch to the substrate, an epitaxial growth of well-ordered films is observed. The oxide starts to grow layer by layer in a (111) orientation of the metastable cubic FeO structure up to a thickness of about 2.2 monolayers (ML). The completion of the second and third FeO layer depends on the precise oxidation temperature, and at coverages of approximately 2 ML three-dimensional ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ islands start to grow. The FeO(111) layers consist of hexagonal close-packed iron-oxygen bilayers that are laterally expanded when compared to bulk FeO and slightly rotated against the platinum substrate. They all exhibit oxygen-terminated unreconstructed $(1\ifmmode\times\else\texttimes\fi{}1)$ surface structures. With increasing coverage several structural film changes occur, and four coincidence structures with slightly different lateral lattice constants and rotation misfit angles against the platinum substrate are formed. In the submonolayer regime an FeO(111) bilayer with a lattice constant of 3.11 \AA{} and rotated by 1.3\ifmmode^\circ\else\textdegree\fi{} against the platinum substrate is observed. Upon completion of the first layer the film gets compressed leading to a lattice constant of 3.09 \AA{} and a rotation misfit angle of 0.6\ifmmode^\circ\else\textdegree\fi{}. Between 1.5 and 2 ML a coincidence structure rotated by 30\ifmmode^\circ\else\textdegree\fi{} against the platinum substrate forms, and at 2 ML a nonrotated coincidence structure with a lattice constant of 3.15 \AA{} evolves. All these coincidence structures exhibit large periodicities between approximately 22 and 38 \AA{} that are visible in the STM images up to the third FeO layer surface. The LEED patterns exhibit characteristic multiple scattering satellite spots. The different coincidence structures reflect lowest-total-energy arrangements, balancing the contributions of substrate-overlayer interface energies and elastic energies within the strained oxide overlayer for each coverage.

Ab initio quantum and molecular dynamics of the dissociative adsorption of hydrogen on Pd(100)

Abstract: The dissociative adsorption of hydrogen on Pd(100) has been studied by ab initio quantum dynamics and ab initio molecular-dynamics calculations. Treating all hydrogen degrees of freedom as dynamical coordinates implies a high dimensionality and requires statistical averages over thousands of trajectories. An efficient and accurate treatment of such extensive statistics is achieved in a three-step approach: In a first step we evaluate the ab initio potential-energy surface (PES) for a number of appropriate points in configuration space. Then (as step 2) we determine an analytical representation that serves as an interpolation between the actually calculated points. In an independent third step dynamical calculations are performed on the analytical representation of the PES. Thus the dissociation dynamics is investigated without any crucial assumption except for the Born-Oppenheimer approximation which is anyhow employed when density-functional-theory calculations are performed. The ab initio molecular dynamics is compared to detailed quantum-dynamical calculations on exactly the same ab initio PES. The occurence of quantum oscillations in the sticking probability as a function of kinetic energy is addressed. They turn out to be very sensitive to the symmetry of the initial conditions. At low kinetic energies sticking is dominated by the steering effect, which is illustrated using classical trajectories. The steering effect depends on the kinetic energy, but not on the mass of the molecules, as long as no energy transfer to the substrate atoms is considered. The comparison between quantum and classical calculations of the sticking probability shows the importance of zero-point effects in the hydrogen dynamics. The dependence of the sticking probability on the angle of incidence is analyzed; it is found to be in good agreement with experimental data. The results show that the determination of the potential-energy surface combined with high-dimensional dynamical calculations, in which all relevant degrees of freedom are taken into account, leads to a detailed understanding of the dissociation dynamics of hydrogen at a transition metal surface.

Direct imaging of the two-dimensional Fermi contour: Fourier-transform STM

Abstract: Direct images of the two-dimensional Fermi contour at a surface can be generated by a Fourier transform (FT) of scanning tunneling microscopy (STM) images taken at low-bias voltages. The origins of the Fermi contour in the FT are the standing waves of electrons near the Fermi energy caused by defects in the surface. Several examples of FT-STM are presented to illustrate the simplicity of this technique. The advantages and limitations of this Fermi contour imaging technique are discussed.

On the nature of the active site for the ethylbenzene dehydrogenation over iron oxide catalysts

Abstract: The dehydrogenation of ethylbenzene to styrene was studied over single-crystalline iron oxide model catalyst films grown epitaxially onto Pt(111) substrates. The role of the iron oxide stoichiometry and of atomic surface defects for the catalytic activity was investigated by preparing single-phased Fe3O4(111) and α-Fe2O3(0001) films with defined surface structures and varying concentrations of atomic surface defects. The structure and composition of the iron oxide films were controlled by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), the surface defect concentrations were determined from the diffuse background intensities in the LEED patterns. These ultrahigh vacuum experiments were combined with batch reactor experiments performed in water–ethylbenzene mixtures with a total gas pressure of 0.6 mbar. No styrene formation is observed on the Fe3O4 films. The α-Fe2O3 films are catalytically active, and the styrene formation rate increases with increasing surface defect concentration on these films. This reveals atomic surface defects as active sites for the ethylbenzene dehydrogenation over unpromoted α-Fe2O3. After 30 min reaction time, the films were deactivated by hydrocarbon surface deposits. The deactivation process was monitored by imaging the surface deposits with a photoelectron emission microscope (PEEM). It starts at extended defects and exhibits a pattern formation after further growth. This indicates that the deactivation is a site-selective process. Post-reaction LEED and AES analysis reveals partly reduced Fe2O3 films, which shows that a reduction process takes place during the reaction which also deactivates the Fe2O3 films.

Combinatorial Chemistry in Heterogeneous Catalysis: A New Scientific Approach or “the King's New Clothes”?

Abstract: The spectacular success in materials science of the application of combinatorial chemistry has raised the hope that it may eventually lead to a new scientific approach to catalyst development. This method is, within the constraints of heterogeneous catalysis, merely a potentially efficient tool to be used in rational catalyst development and should not be considered an independent novel strategy towards rational catalyst design.

Nonequilibrium stationary microstructures in surface chemical reactions

Abstract: Adsorbates with attractive lateral interactions between molecules in the presence of adsorption and thermal desorption are considered We show that a sufficiently strong nonequilibrium reaction or the process of photoinduced desorption can destabilize uniform stationary phases of the adsorbates and lead to nonequilibrium microstructures whose wavelength is typically shorter than the diffusion length and may hence be on the micrometer to nanometer scale Despite their small sizes, such microstructures are not destroyed by internal statistical fluctuations

Oxygen adsorption on the ru(1010) surface : anomalous coverage dependence

Abstract: Oxygen adsorption on to Ru(101\ifmmode\bar\else\textasciimacron\fi{}0) results in the formation of two ordered overlayers, i.e., a $c(2\ifmmode\times\else\texttimes\fi{}4)\ensuremath{-}2\mathrm{O}$ and a $(2\ifmmode\times\else\texttimes\fi{}1)p2mg\ensuremath{-}2\mathrm{O}$ phase, which were analyzed by low-energy electron diffraction (LEED) and density functional theory (DFT) calculation. In addition, the vibrational properties of these overlayers were studied by high-resolution electron loss spectroscopy. In both phases, oxygen occupies the threefold coordinated hcp site along the densely packed rows on an otherwise unreconstructed surface. The O atoms are attached to two atoms in the first Ru layer Ru(1) and to one Ru atom in the second layer Ru(2), forming zigzag chains along the troughs. While in the low-coverage $c(2\ifmmode\times\else\texttimes\fi{}4)\ensuremath{-}\mathrm{O}$ phase, the bond lengths of O to Ru(1) and Ru(2) are 2.08 and 2.03 \AA{}, respectively, corresponding bond lengths in the high-coverage $(2\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}p2mg\ensuremath{-}2\mathrm{O}$ phase are 2.01 and 2.04 \AA{} (LEED). Although the adsorption energy decreases by 220 meV with O coverage (DFT calculations), we observe experimentally a shortening of the Ru(1)-O bond length with O coverage. The $v(\mathrm{R}\mathrm{u}\ensuremath{-}\mathrm{O})$ stretch mode is found at 67 meV $[c(2\ifmmode\times\else\texttimes\fi{}4)\ensuremath{-}2\mathrm{O}]$ and 64 meV $[(2\ifmmode\times\else\texttimes\fi{}1)p2mg\ensuremath{-}2\mathrm{O}].$

Condensation in globally coupled populations of chaotic dynamical systems

Abstract: The condensation transition, leading to complete mutual synchronization in large populations of globally coupled chaotic R\"ossler oscillators, is investigated. Statistical properties of this transition and the cluster structure of partially condensed states are analyzed.

Transition from Five-Fold Symmetric to Twinned FCC Gold Particles by Thermally Induced Growth

Abstract: Stable gold clusters around 1 nm in size have been prepared by evaporation of the metal into an organosilicon polymer solution. A controlled particle growth of the entrapped gold particles was achieved by annealing in helium at ∼410 °C. Above 460 °C the growth rate is dramatically increased, concurrent with the onset of pyrolysis of polysilazane to form a porous solid. X-ray diffraction and simulation calculations using Debye functions of model clusters of different sizes and both crystallographic and noncrystallographic symmetry were used to characterize these particles. The exclusive presence of noncrystallographic (decahedral and icosahedral) multiply twinned particles (MTPs) in the as-prepared material is confirmed by EXAFS. The high precision of the XRD experiments and theoretical data fits gives detailed insight into the thermally induced growth process, during which the MTPs progressively disappear and are replaced by singly twinned fcc particles of larger size. This transition occurs in the range ...

X‐Ray Diffraction from Clusters

Abstract: X-ray diffraction (XRD) studies on metal clusters are reported and results for several systems are presented. Special emphasis is devoted to the use of the so called Debye Function Analysis (DFA technique) as a tool for numerical simulation of the diffracted intensity of polydispersed nanocrystalline systems This method is most commonly applied in the field of highly dispersed supported catalysts. For small particles. XRD become increasingly surface sensitive. By analysing in situ one can then observe structure-related reactions taking place at the surface. This is examplified by proof of the Sales-Turuer-Maple mechanism for CO oxidation over supported platinum. Refined experimental and numerical methods are described which give reliable data despite a high back-ground from the support. Of further interest is the observation of clusters in the sub-nanometer size range, that are difficult to analyse by HRTEM, and the characterisation of bimetallic colloids.

Hydrogen dissociation on metal surfaces - a model system for reactions on surfaces

Abstract: Reactions on surfaces play an important role in many technological applications. Since these processes are often rather complex, one tries to understand single steps of these complicated reactions by investigating simpler systems. In particular the hydrogen dissociation on surfaces serves as such a model system. There has been much progress in recent years in the theoretical description of reactions on surfaces by high-dimensional dynamics simulations on potential energy surfaces which are derived from ab initio total energy calculations. In this brief review I will focus on the hydrogen dissociation on the clean and sulfur-covered Pd(100) surface. These calculations established the importance of dynamical concepts such as the steering effect. Furthermore the electronic structure calculations allow the determination of the factors that determine the reactivity of a particular surface. This will be demonstrated for the poisoning of hydrogen dissociation by sulfur adsorption on the Pd(100) surface. In addition, quantum effects in the dynamics can be assessed by comparing the results of classical with quantum dynamical calculations on the same potential energy surface.