Showing papers by "Fritz Haber Institute of the Max Planck Society published in 2000"

Atomic-scale structure and catalytic reactivity of the RuO(2)(110) surface

Abstract: The structure of RuO(2)(110) and the mechanism for catalytic carbon monoxide oxidation on this surface were studied by low-energy electron diffraction, scanning tunneling microscopy, and density-functional calculations. The RuO(2)(110) surface exposes bridging oxygen atoms and ruthenium atoms not capped by oxygen. The latter act as coordinatively unsaturated sites-a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces-onto which carbon monoxide can chemisorb and from where it can react with neighboring lattice-oxygen to carbon dioxide. Under steady-state conditions, the consumed lattice-oxygen is continuously restored by oxygen uptake from the gas phase. The results provide atomic-scale verification of a general mechanism originally proposed by Mars and van Krevelen in 1954 and are likely to be of general relevance for the mechanism of catalytic reactions at oxide surfaces.

Effect of the environment on alpha-Al2O3 (0001) surface structures

Abstract: We report that calculating the Gibbs free energy of the alpha-Al2O3 (0001) surfaces in equilibrium with a realistic environment containing both oxygen and hydrogen species is essential for obtaining theoretical predictions consistent with experimental observations. Using density-functional theory we find that even under conditions of high oxygen partial pressure the metal-terminated surface is surprisingly stable. An oxygen-terminated alpha-Al2O3 (0001) surface becomes stable only if hydrogen is present on the surface. In addition, including hydrogen on the surface resolves discrepancies between previous theoretical work and experimental results with respect to the magnitude and direction of surface relaxations.

Quantitative speciation of Mn-bearing particulates emitted from Autos burning (methylcyclopentadienyl)manganese tricarbonyl-added gasolines using XANES spectroscopy

Abstract: The chemical nature of Mn-containing particulates emitted from (methylcyclopentadienyl)manganese tricarbonyl-added gasoline engines has been elucidated using Mn K-edge X-ray absorption fine structure (XAFS) spectroscopy. Edge shift data from the X-ray absorption near-edge structure (XANES) spectra showed that the average Mn valence in these particulates is ∼2.2. Using a principal component analysis (PCA) algorithm, the number and type of probable species contained in these particulates were determined to be three, consisting of Mn3O4, MnSO4·H2O, and a divalent manganese phosphate, Mn5(PO4)[PO3(OH)]2·4H2O. The proportions of these Mn phases in each particulate sample were evaluated quantitatively using least-squares fitting (LSF) of the experimental XANES spectra with linear combinations of these principal component (model compound) spectra. Two groups of Mn-bearing particulates may be distinguished: group I having 4−9 wt % of Mn3O4 and exhibiting a single intense first major absorption maximum at the Mn ...

Ultrafast electron dynamics at metal surfaces: Competition between electron-phonon coupling and hot-electron transport

Abstract: An experimental scheme (double pump/reflectivity probe using femtosecond laser pulses) enables the investigation of nonequilibrium electron dynamics at metal surfaces by measuring the equilibrated surface temperature. The competition between electron-phonon coupling and hot-electron transport gives rise to a reduced equilibrated temperature when the two pump pulses overlap in time, and provides a way of accurately determining the electron-phonon coupling constant. These observations have important consequences for femtosecond photochemical investigations.

Photon emission spectroscopy of individual oxide-supported silver clusters in a scanning tunneling microscope

Abstract: Photon emission spectra of individual alumina-supported silver clusters have been measured for the first time. The light emission stimulated by electron injection from the tip of a scanning tunneling microscope can be assigned to the $(1,0)$ mode of the Mie-plasmon resonance in small silver particles. As cluster sizes decrease, the resonance position shifts to higher energies and the linewidth increases. In the size range examined (1.5--12 nm), intrinsic size effects are discussed as possible origins for the observed size dependence of the Mie resonance.

Electron-phonon interaction in single-wall carbon nanotubes: A time-domain study

Abstract: We investigate the electron-phonon $(e\ensuremath{-}\mathrm{ph})$ interaction in single-wall carbon nanotube samples at room temperature using femtosecond time-resolved photoemission. By probing electrons from the vicinity of the Fermi level we are able to study the $e\ensuremath{-}\mathrm{ph}$ interaction in the metallic nanotube species only. The observed electron dynamics can be used to calculate $e\ensuremath{-}\mathrm{ph}$ scattering matrix elements for two likely scattering scenarios: forward scattering from twistons and backscattering by longitudinal acoustic phonons. The corresponding matrix elements reveal an intrinsically weak $e\ensuremath{-}\mathrm{ph}$ interaction approximately $50%$ smaller than predicted by tight-binding calculations.

Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling.

Abstract: We demonstrate a simple scheme for capturing the temporal waveforms of a freely propagating terahertz electromagnetic transient in a single shot. The method relies on electro-optic sampling in a noncollinear geometry for the terahertz radiation and the visible probe beam, coupled with multichannel detection. The approach provides time resolution that is comparable to that of conventional electro-optic sampling measurements.

GaAs(001) surface under conditions of low As pressure: evidence for a novel surface geometry

Abstract: Using density-functional theory we identify a new low-energy structure for GaAs(001) in an As-poor environment. The discovered geometry is qualitatively different from the usual surface-dimer based reconstructions of III-V semiconductor (001) surfaces. The stability of the new structure, which has a c(8x2) periodicity, is explained in terms of bond saturation and favorable electrostatic interactions between surface atoms. Simulated scanning tunneling microscopy images are in good agreement with experimental data, and a low-energy electron diffraction analysis supports the theoretical prediction.

From atoms to crystallites: adsorption on oxide-supported metal particles

Abstract: The properties of metal clusters and nanoparticles are attracting more and more attention in fundamental and applied research. The application of such structures in heterogeneous catalysis is a particularly important area. Nevertheless, there is only limited fundamental knowledge about the dependence of adsorption behaviour and catalytic activity on particle size. In this article, we describe an approach to tackle such questions on the basis of model systems which are prepared by metal vapour deposition under ultrahigh vacuum conditions. A thin alumina film grown on a metal substrate is used as support. This has the advantage that scanning tunneling microscopy and photoelectron spectroscopy can be applied without any charging problems. Firstly, structural data covering palladium, rhodium and iridium deposits demonstrate that a wide spectrum of particle sizes and morphologies may be obtained by taking advantage of kinetically controlled nucleation and growth processes. Subsequently, we discuss the adsorption of simple molecules on the particles, such as CO and ethene. These studies, which have been carried out using infrared and photoemission spectroscopy, illustrate possible variations in the adsorption and reaction behaviour as a function of particle size. Aspects which will be considered include: adsorption at facets and defects, size-dependent variation of adsorption sites, formation of surface complexes, decomposition of molecules as well as co-adsorption phenomena.

Femtosecond Surface Vibrational Spectroscopy of CO Adsorbed on Ru(001) during Desorption

Abstract: The dynamics of the interaction between molecules and metal surfaces is of fundamental importance in surface science, since these determine key physical and chemical properties —essential in, e.g., catalysis —such as energy transfer and molecular reactivity [1]. Surface chemical dynamics are determined by the time scales of various competing relaxation channels [2]: For a complete understanding of surface reactions, it is essential to know how different vibrational modes of the reactant are coupled to each other and to the substrate. Time-resolved surface vibrational infrared (IR) spectroscopy [3] offers a powerful means of obtaining such insights. Using picosecond (ps) techniques, information about the dynamics of energy exchange between adsorbed carbon monoxide (CO) and metal substrates has been obtained [4], by monitoring transient band shifts of the C— O vibration after optical excitation. The technique has further been used to observe vibrational energy relaxation [5] and reversible transient chemical transformations [6] at surfaces. These experiments were limited to temperatures below those at which desorption of the adsorbate occurs. However, when one is interested in surface reactions, the more relevant situation occurs at higher temperatures, where higher-lying vibrational modes get thermally occupied that play an important role in the surface chemistry. We present here femtosecond time-resolved vibrational sum-frequency generation (fs-SFG) spectra of CO molecules adsorbed on a Ru(001) surface, taken while a significant number (50%) of these molecules is desorbing due to fs laser excitation. With this fs-SFG method, snapshots of the (C— O) stretch vibration can be taken, under conditions where desorption is occurring (at lattice temperatures transiently exceeding the desorption temperature by over 500 K), shedding new light on the dynamics of the desorption process and the coupling of

Bridging the Pressure and Materials Gaps: High Pressure Sum Frequency Generation Study on Supported Pd Nanoparticles

Abstract: Infrared-visible sum frequency generation vibrational spectroscopy is applied for the first time to monitor CO stretching vibrations on alumina supported Pd nanoparticles in a pressure range from 10(-7) to 200 mbar. The adsorption behavior of Pd aggregates with 3 and 6 nm mean size is dominated by surface defects and two different adsorption sites (twofold bridging and on-top) were identified. The CO adsorption site occupancy on Pd nanocrystals is mainly governed by the gas phase pressure while the structure of the particles and their temperature have a smaller influence.

Structure and Stability of Monodisperse 1.4-nm ZnS Particles Stabilized by Mercaptoethanol

Abstract: Extremely small 1.4-nm size mercaptoethanol-stabilized ZnS clusters have been synthesized with narrow size distribution. The structure of these clusters was studied by wide-angle X-ray scattering. The scattering curves were compared with the calculated scattered intensity of a variety of model clusters (ZnS)N and different defect types via Debye functions. In the as-received state the pattern is best described by a fragment of the zinc blende lattice, with N ≈ 30, and a defective stacking of three to four (111) planes. A large improvement of the simulation is gained by introducing liquidlike disorder to the model structure. This raises the unanswered question of a “real” liquid state of these small clusters at room temperature. The cluster matrix is thermally stable to 583 K. Above this temperature the primary cluster coalesce to form larger particles. Annealed at 1013 K the particles grow to >4.0 nm with a highly defective zinc blende structure.

EPITAXIAL GROWTH OF SiO2 ON Mo(112)

Abstract: A new preparation is reported which, for the first time, results in a thin, crystalline SiO2 film on a Mo(112) single crystal. The procedure consists of repeated cycles of silicon deposition and subsequent oxidation, followed by a final annealing procedure. AES and XPS have been used to control film stoichiometry. LEED pictures of high contrast show a hexagonal, crystalline SiO2 overlayer with a commensurate relationship to the Mo(112) substrate. The wetting of the substrate by the film has been investigated by LEED, XPS and TDS, revealing that the film covers the substrate completely.

Spontaneous formation of indium-rich nanostructures on InGaN(0001) surfaces

Abstract: InGaN(0001) surfaces prepared by molecular beam epitaxy have been studied using scanning tunneling microscopy and first-principles total energy calculations. Nanometer-size surface structures are observed consisting of either vacancy islands or ordered vacancy rows. The spontaneous formation of these structures is shown to be driven by significant strain in the surface layers and by the relative weakness of the In-N bond compared to Ga-N. Theory indicates that In will preferentially bind at the edges and interior of the structures, thereby giving rise to an inhomogeneous In distribution at the surface.

The Binding of Sodium Dodecyl Sulfate to the ABA Block Copolymer Pluronic F127 (EO97PO69EO97): An Electromotive Force, Microcalorimetry, and Light Scattering Investigation

Abstract: The binding of SDS to pluronic F127 was studied using a SDS surfactant selective electrode via electromotive force, isothermal titration calorimetry, and light scattering. At a concentration of 0.5% w/v the block copolymer F127 exists as an equilibrium mixture of micelles and monomers at 35 °C. When SDS is gradually added to this solution, binding of SDS to the F127 micelles takes place even at the lowest measured SDS concentration (1 × 10-5 mol dm-3). Initially F127/SDS mixed micelles are formed and the size of these micelles remains constant until ∼5 × 10-5 mol dm-3 of SDS has been reached. At a total SDS concentration of 5 × 10-5 mol dm-3 there are ∼6 SDS monomers in an aggregate containing 69 F127 monomers. Further addition of SDS results in a dramatic breakdown of the F127 rich mixed micelles into smaller aggregates. During this process SDS continually binds to the F127 micelles forming mixed micelles which simultaneously break down to smaller aggregates and also become richer in SDS. This process co...

Thermal and chemical stability and adhesion strength of Pt nanoparticle arrays supported on silica studied by transmission electron microscopy and atomic force microscopy

Abstract: The thermal, chemical, and mechanical stability of Pt nanoparticles supported on silica has been measured with transmission electron microscopy (TEM) and atomic force microscopy (AFM). The nanoparticle arrays were fabricated using electron beam lithography, which produced uniform particle sizes (20 ± 1 nm) and uniform interparticle distances (150 ± 1 nm). TEM studies provided information about the array periodicity, particle dimensions, and crystallinity of individual particles. Before heat treatments, individual Pt nanoparticles were found to be polycrystalline with crystalline domain sizes of 4−8 nm. After heating to 1000 K in high vacuum (10-7 Torr) and 1 atm H2, the crystalline domain sizes within individual particles grew larger, without noticeable deformation of the array. A similar enlargement of crystalline domains was seen in 1 atm O2 at a lower temperature of 700 K. Using contact mode AFM, the height, periodicity, and adhesion of the particles were determined. On a newly prepared sample, Pt part...

Dynamics of reactions at surfaces

Abstract: The rate of a catalytic reaction is determined by the dynamics of the individual steps involved, which may be classified according to a rough hierarchy: The quantum level concerns the energy exchange between the different degrees of freedom. If these are in thermal equilibrium at all stages, the concepts of transition state theory may be applied to formulate the rates of the elementary processes constituting the reaction mechanism. Combination of these steps eventually enables rationalization of the macroscopic kinetics. The contributions of surface science to the current knowledge of the various levels of these complex phenomena are illustrated by selected examples.

Binding of Sodium Dodecyl Sulfate to Some Polyethyleneimines and Their Ethoxylated Derivatives at Different pH Values. Electromotive Force and Microcalorimetry Studies

Abstract: Electromotive force and isothermal titration microcalorimetry measurements have been carried out to study the interactions of sodium dodecyl sulfate (SDS) with polyethyleneimines (PEI's) and some ethoxylated PEI's at different pH's. In all cases the polymers show a remarkable affinity toward SDS. The SDS concentration at the onset of binding decreases with decreasing pH. At low pH's phase separation similar to that observed for strong polyelectrolyte/oppositely charged surfactant systems occurs presumably as a consequence of the protonation of the N atoms in the polymer to form a polycation. The SDS range over which phase separation occurs decreases as the size of the ethoxylated chains increases, and for the polymer with the longest ethoxylated chain no precipitation occurs. In many cases where phase separation occurs, as more SDS is added the polymer/surfactant complex often resolubilizes and binding proceeds until the polymer becomes saturated with bound SDS. For any given polymer at this limiting stag...

Theory of adsorption on metal substrates

Abstract: 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 points

Electrochemical versus gas-phase oxidation of ru single-crystal surfaces

Abstract: The electrochemical uptake of oxygen on a Ru(0001) electrode was investigated by electron diffraction, Auger spectroscopy, and cyclic voltammetry. An ordered (2 × 2)-O overlayer forms at a potential close to the hydrogen region. At +0.42 and +1.12 V vs Ag/AgCl, a (3 × 1) phase and a (1 × 1)-O phase, respectively, emerge. When the Ru electrode potential is maintained at +1.12 V for 2 min, RuO2 grows epitaxially with its (100) plane parallel to the Ru(0001) surface. In contrast to the RuO2 domains, the non-oxidized regions of the Ru electrode surface are flat. If, however, the electrode potential is increased to +1.98 V for 2 min, the remaining non-oxidized Ru area also becomes rough. These findings are compared with O overlayers and oxides on the Ru(0001) and Ru(1010) surfaces created by exposure to gaseous O2 under UHV conditions. On the other hand, gas-phase oxidation of the Ru(1010) surface leads to the formation of RuO2 with a (100) orientation. It is concluded that the difference in surface energy b...

Quasicrystalline valence bands in decagonal AlNiCo

Abstract: Quasicrystals are metallic alloys that possess perfect long-range structural order, in spite of the fact that their rotational symmetries are incompatible with long-range periodicity. The exotic structural properties of this class of materials1 are accompanied by physical properties that are unexpected for metallic alloys. Considerable progress in resolving the geometric structures of quasicrystals has been made using X-ray and neutron diffraction, and concepts such as the quasi-unit-cell model2 have provided theoretical insights. But the basic properties of the valence electronic states—whether they are extended as in periodic crystals or localized as in amorphous materials—are still largely unresolved3. Here we investigate the electronic bandstructure of quasicrystals through angle-resolved photoemission experiments on decagonal Al71.8Ni14.8Co13.4. We find that the s-p and d states exhibit band-like behaviour with the symmetry of the quasiperiodic lattice, and that the Fermi level is crossed by dispersing d-bands. The observation of free-electron-like bands, distributed in momentum space according to the surface diffraction pattern, suggests that the electronic states are not dominated by localization.

The influence of OH groups on the growth of rhodium on alumina: a model study

Abstract: In order to investigate how the presence of surface hydroxyl groups on oxide surfaces affects the interaction with the supported metal, we have modified a well-ordered alumina film on NiAl(110) by Al deposition and subsequent exposure to water. This procedure yields a hydroxylated alumina surface as revealed by infrared and high-resolution electron energy loss spectroscopy. By means of scanning tunneling microscopy, we have studied the growth of rhodium on the modified film at 300 K. Clear differences in the particle distribution and density are observed in comparison to the clean substrate. While, in the latter case, decoration of domain boundaries as typical defects of the oxide film governs the growth mode, a more isotropic island distribution and a drastically increased particle density is found on the hydroxylated surface. From infrared data, it can be deduced that the growth is connected with the consumption of the hydroxyl groups due to the interaction between the metal deposit and the hydroxylated areas. This finding is in line with photoemission results published earlier.

Catalysis and surface science: What do we learn from studies of oxide-supported cluster model systems?

Abstract: Models of supported metal catalysts have been prepared by deposition of transition metal vapor onto thin, well-ordered oxide layers, particularly alumina. The average sizes of the metal particles and their size distributions are determined by nucleation and growth. Here, we review the morphology and structure of the oxide layers and the metal particles dispersed on them, as measured with a variety of methods including low-energy electron diffraction and scanning tunneling microscopy. Electronic and magnetic structure as a function of particle size, adsorption properties, as well as reactivity with varying particle size are reviewed. The electronic structure of supported palladium particles has been studied and indicates that a nonmetal to metal transition occurs for particles exceeding 70–80 atoms per aggregate. Adsorption of CO has been studied in detail by Fourier transform infrared spectroscopy. Interesting variations in the spectra when very small particles consisting of only a few metal atoms are investigated compared with larger particles and single-crystal surfaces are observed. CO dissociation has been studied on rhodium aggregates and a maximal dissociation rate has been found for aggregates containing several hundred metal atoms. The presence of defects on the particles is deduced to be origin of this behavior.