Showing papers by "Fritz Haber Institute of the Max Planck Society published in 2003"
TL;DR: In this paper, the size range of 2-5 nm was synthesized by a yeast strain MKY3, when challenged with 1 mM soluble silver in the log phase of growth and the nanoparticles were separated from dilute suspension by devising a new method based on differential thawing of the sample.
Abstract: Silver nanoparticles in the size range of 2-5 nm were synthesized extracellularly by a silver-tolerant yeast strain MKY3, when challenged with 1 mM soluble silver in the log phase of growth. The nanoparticles were separated from dilute suspension by devising a new method based on differential thawing of the sample. Optical absorption, transmission electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy investigations confirmed that metallic (elemental) silver nanoparticles were formed. Extracellular synthesis of nanoparticles could be highly advantageous from the point of view of synthesis in large quantities and easy downstream processing.
750 citations
TL;DR: Ab initio, atomistic thermodynamics is employed to construct a phase diagram of surface structures in the (T,p) space from ultrahigh vacuum to technically relevant pressures and temperatures.
Abstract: The main prerequisites for reaching a microscopic understanding of heterogeneous catalysis are the identification of the composition and geometry of the catalyst’s surface and the determination of the various chemical reactions that take place under realistic conditions. Unfortunately, most surface science experimental techniques are difficult if not impossible to use at the pressures (of the order of 1 atm) and temperatures (often higher than 300 K) that are typically applied in steadystate catalysis. Therefore, what is considered to be important elementary processes at the catalyst’s surface (e.g., dissociation, diffusion, and chemical reactions) has usually been concluded from chemical intuition and extensive knowledge from ultrahigh vacuum (UHV) experiments. However, several studies revealed the danger of this approach (see, e.g., Ref. [1] and references therein), and the need to reliably bridge the temperature and pressure gap between UHV and ‘‘real life’’ is probably the main challenge in modern surface science. In ab initio theory the consideration of high temperature and high pressure can be achieved by explicitly taking into account the surrounding gas phase in terms of ‘‘ab initio, atomistic thermodynamics’’ (cf., e.g., Refs. [2 ‐6]). This is also an appropriate (first) approach to steady-state catalysis, which is often run close to thermodynamic equilibrium (or a constrained equilibrium) to prevent catalyst degradation. We will qualify this statement below when analyzing our results. In the following we show how the combination of thermodynamics and density-functional theory (DFT) can be applied to obtain the lowest-energy surface structures in a (constrained) equilibrium with the surrounding gas
379 citations
TL;DR: It is demonstrated that a thin metallic film on a semiconductor surface may open an efficient and hitherto not expected diffusion channel for lateral adatom transport: adatoms may prefer diffusion within this metallic layer rather than on top of the surface.
Abstract: Employing density-functional theory in combination with scanning tunneling microscopy, we demonstrate that a thin metallic film on a semiconductor surface may open an efficient and hitherto not expected diffusion channel for lateral adatom transport: adatoms may prefer diffusion within this metallic layer rather than on top of the surface. Based on this concept, we interpret recent experiments: We explain why and when In acts as a surfactant on GaN surfaces, why Ga acts as an autosurfactant, and how this mechanism can be used to optimize group-III nitride growth.
286 citations
TL;DR: Electron spectra of photoexcited Ne clusters are shown to display a signal at low kinetic energies that is neither present in the Ne monomer nor at photon energies below the inner-valence 2s threshold, strong evidence for the existence of interatomic Coulombic decays (ICD).
Abstract: Electron spectra of photoexcited Ne clusters are shown to display a signal at low kinetic energies that is neither present in the Ne monomer nor at photon energies below the inner-valence $2s$ threshold. These findings are strong evidence for the existence of interatomic Coulombic decays (ICD), a mechanism that was recently predicted theoretically [Phys. Rev. Lett. 79, 4778 (1997)]. In ICD, an inner-valence hole state in a weakly bonded system can undergo ultrafast relaxation due to energy transfer to a neighboring atom, followed by electron emission from this neighboring site.
253 citations
TL;DR: The interaction of C60 with single-wall carbon nanotubes (SWNTs) and graphite is studied experimentally by thermal desorption spectroscopy and theoretically by molecular-mechanics and molecular-dynamics calculations to compare the efficiency of different mechanisms by which C60 can be encapsulated into SWNTs.
Abstract: The interaction of C60 with single-wall carbon nanotubes (SWNTs) and graphite is studied experimentally by thermal desorption spectroscopy and theoretically by molecular-mechanics and molecular-dynamics calculations. The van der Waals parameters and force field for C60-graphene and C60-SWNT interactions are derived from the low-coverage C60 binding energy to the graphite surface. We use these to compare the efficiency of different mechanisms by which C60 can be encapsulated into SWNTs.
200 citations
TL;DR: For the conditions of ethylene epoxidation, a thin oxidelike structure is most stable, suggesting that such atomic O species are actuating the catalysis, in contrast to hitherto proposed molecularlike species.
Abstract: Extensive density-functional theory calculations, and taking into account temperature and pressure, afford a comprehensive picture of the behavior and interaction of oxygen and Ag(111), and provides valuable insight into the function of silver as an oxidation catalyst. The obtained phase diagram reveals the most stable species present in a given environment and thus identifies (and excludes) possibly active oxygen species. In particular, for the conditions of ethylene epoxidation, a thin oxidelike structure is most stable, suggesting that such atomic O species are actuating the catalysis, in contrast to hitherto proposed molecularlike species.
184 citations
TL;DR: In this article, the mass selected ion yield as a function of IR laser frequency yields the IR-REMPI (infrared resonance enhanced multiphoton ionization) spectrum of the molecule or cluster.
Abstract: Resonant IR excitation of gas-phase molecules and clusters can lead to superhot species that thermally emit an electron. Monitoring the mass selected ion yield as a function of IR laser frequency yields the IR-REMPI (infrared resonance enhanced multiphoton ionization) spectrum of the molecule or cluster. Although this IR-REMPI spectrum is not the same as the linear absorption spectrum, it can be quite similar and it yields valuable information on the IR optical properties of the species investigated. In this article, the method and the necessary tools are presented. Results from experiments on fullerenes, metal carbide, metal oxide, and metal nitride clusters are shown.
117 citations
TL;DR: Line defects of a thin alumina film on NiAl(110) have been studied on the atomic level with scanning tunneling microscopy at 4 K while boundaries between two reflection domains do not expose a characteristic structure, antiphase domain boundaries are well ordered.
Abstract: Line defects of a thin alumina film on NiAl(110) have been studied on the atomic level with scanning tunneling microscopy at 4 K. While boundaries between two reflection domains do not expose a characteristic structure, antiphase domain boundaries are well ordered. The latter boundaries result from the insertion of a row of O atoms, as atomically resolved images of the topmost oxygen layer show. The insertion occurs only in two of the three characteristic directions of the quasihexagonal O lattice. Depending on the direction, either straight or zigzagged boundaries form. An atomic characterization of line defects on the oxide surface is a first step to correlate their topographic structure and chemical activity.
110 citations
TL;DR: In this article, the energy of polyalanine chains in the α-helical and extended structure was studied by employing density-functional theory, where the full peptide−peptide connectivity (backbone) of proteins was considered.
Abstract: We studied the energetics of finite and infinite polyalanine chains in the α-helical and extended structure by employing density-functional theory. On the basis of these results we extracted the energy of hydrogen bonds (hb's) and their interactions by taking the full peptide−peptide connectivity (backbone) of proteins into account. We focus on two limiting cases: an isolated hb and one within an infinite α-helical chain. In the infinite chain the cooperativity within an infinite network of hb's strengthens each individual bond by more than a factor of 2. This effect has important consequences for the stability of α-helices.
103 citations
TL;DR: In this article, the structure of metal particles on a thin alumina film grown on a NiAl(1/1/0) substrate were studied as a function of annealing temperature.
84 citations
TL;DR: In this article, ZnS:Mn was stabilized using thioglycerol [HSCH2CH(OH)CH2OH] molecules and tetraethylorthosilicate (TEOS) in an aqueous medium, yielding either a disordered silica matrix or spherical core-shell particles of up to ∼900 nm size.
Abstract: Zinc sulphide nanoparticles doped with manganese (ZnS:Mn) have been stabilized using thioglycerol [HSCH2CH(OH)CH2OH] molecules. The nanoparticles (∼1.7 nm) are highly stable and exhibit photoluminescence at ∼600 nm when excited with ultraviolet light. For increasing luminescence and stability the particles are further treated with tetraethylorthosilicate (TEOS)[Si(C2H5O)4] in an aqueous medium, yielding either a disordered silica matrix or spherical core-shell particles of up to ∼900 nm size with strongly enhanced luminescence under certain conditions. Photoluminescence, excitation spectroscopy, transmission electron microscopy, energy dispersive analysis of x-rays, x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy measurements have been performed for the characterization of the ZnS:Mn nanoparticles alone, in the silica matrix as well as in spherical silica shells. Among other things, the analysis indicates that the thioglycerol capping has been affected by the coating neither in...
TL;DR: Using a synchrotron infrared light source, IR images of protein structure were obtained at a subcellular resolution, revealing regions of decreased alpha-helical content and elevated beta-sheet structure in and around infected neurons in the 263 K scrapie hamster model.
17 Feb 2003
TL;DR: In this article, the substitutional Co ions incorporated into polycrystalline Co-doped ferromagnetic rutile tend to cluster, and then the neighboring interstitial sites become energetically favorable for Co to reside.
Abstract: The local environment around magnetic impurity atoms was reported to be unchanged in a structural phase transition of nanocrystalline Co-doped ferromagnetic ${\mathrm{TiO}}_{2}.$ Our ab initio density functional theory investigations show that the substitutional Co ions incorporated into ${\mathrm{TiO}}_{2}$ rutile tend to cluster, and then the neighboring interstitial sites become energetically favorable for Co to reside. This suggests that a Co-doped rutile containing only substitutional Co may not be an appropriate reference bulk system in determining the local environment of Co in polycrystalline $(\mathrm{T}\mathrm{i},\mathrm{C}\mathrm{o}){\mathrm{O}}_{2}$ rutile. We also find that the interstitial Co is in the low spin state and destroys the spin polarization of the surrounding substitutional Co, and hence reduces the average magnetic moment of impurity atoms.
TL;DR: In this article, the active surface of vanadium phosphorus oxide (VPO) catalysts in the course of the oxidation of n-butane to maleic anhydride (MA) was investigated.
Abstract: In situ X-ray absorption spectroscopy (XAS) and in situ X-ray photoelectron spectroscopy (XPS) have been applied to study the active surface of vanadium phosphorus oxide (VPO) catalysts in the course of the oxidation of n-butane to maleic anhydride (MA). The V L3 near edge X-ray absorption fine structure (NEXAFS) of VPO is related to the details of the bonding between the central vanadium atom and the surrounding oxygen atoms. Reversible changes of the NEXAFS were observed when going from room temperature to the reaction conditions. These changes are interpreted as dynamic rearrangements of the VPO surface, and the structural rearrangements are related to the catalytic activity of the material that was verified by proton-transfer reaction mass spectrometry (PTR-MS). The physical origin of the variation of the NEXAFS is discussed and a tentative assignment to specific V−O bonds in the VPO structure is given. In situ XPS investigations were used to elucidate the surface electronic conductivity and to probe ...
TL;DR: In this paper, a series of AlO·(Al2O3)n clusters, n≤34, were obtained by tuning the IR wavelength, pointing to the γ-Al 2O3 structure.
Abstract: Neutral aluminum oxide clusters are produced in a molecular beam by laser vaporization in a pulsed-nozzle cluster source. These clusters are ionized via
(multi-) photon absorption from either an ultraviolet excimer laser or from a far-infrared free electron laser. Ultraviolet (multi-) photon ionization produces sparse mass spectra with only relatively light aluminum oxide clusters, while infrared ionization produces a smooth distribution of higher molecular weight ions from the same nascent source distribution. Tuning the IR wavelength, multiphoton infrared spectra are recorded pointing to the γ-Al2O3 structure for a whole series of AlO·(Al2O3)n clusters, n≤34.
TL;DR: It is found that the ethene molecule binds directly to a vanadium atom in the cluster, although this it is not in all cases the most stable arrangement.
Abstract: Infrared spectra of complexes of small vanadium oxide clusters with ethene are determined using infrared multiple photon dissociation (IR-MPD) spectroscopy in the range of 550−1850 cm-1. The structures of the complexes have been identified by comparison of the experimental spectra with the harmonic vibrational frequencies and corresponding IR intensities of possible isomers calculated with DFT methods. We find that the ethene molecule binds directly to a vanadium atom in the cluster, although this it is not in all cases the most stable arrangement.
TL;DR: In this paper, a spectroscopic study of two forms of adsorbed atomic oxygen on a silver surface, which participate in ethylene epoxidation reaction, is presented.
Abstract: Results of a spectroscopic study of two forms of adsorbed atomic oxygen on a silver surface, which participate in ethylene epoxidation reaction, are presented. The possibility of the combined use of the methods of photoelectron spectroscopy and X-ray absorption for a detailed analysis of adsorbate electron structure on solid surfaces is demonstrated. It is found that a significant difference in the position of O 1s lines for nucleophilic (528.3 eV) and electrophilic (530.4 eV) oxygen is determined by the effects of the initial state, that is, by the difference in the charge state of oxygen anions. The use of the well-know correlation of the Auger line splitting with a Pauling charge at an oxygen atom showed a substantial difference (∼1 electron charge unit) in charge transfer from metal to the nucleophilic or electrophilic adsorbed oxygen atom. Based on the X-ray absorption data of the oxygen K-edge, it is found that there is a substantial overlap of the 4d- and 5sp orbitals of silver with oxygen 2p orbitals in the nucleophilic state in the formation of an Ag–O bond and there is only an overlap of 5sp orbitals of silver with oxygen 2p orbitals in the electrophilic state. Structural models of the adsorption site are presented for both states. The conclusion is drawn that the charge state of oxygen in oxide systems may depend substantially on its binding to metal atoms.
TL;DR: In this paper, the authors focus on the evidence about the existence of Keggin ions under various reactive conditions, including the stability of the hydrated parent heteropoly acid (HPA) phases in water, by thermal methods in the gas phase, by in situ X-ray diffraction and in situ EXAFS.
Abstract: The present study focuses on the evidence about the existence of Keggin ions under various reactive conditions. The stability of the hydrated parent heteropoly acid (HPA) phases is probed in water, by thermal methods in the gas phase, by in situ X-ray diffraction and in situ EXAFS. An extensive analysis of the in situ optical spectra as UV-Vis-near-IR (NIR) in diffuse reflectance yields detailed information about the activated species that are clearly different from Keggin ions but are also clearly no fragments of binary oxides in crystalline or amorphous form. Infrared spectroscopy with CO as probe molecule is used to investigate active sites for their acidity. Besides OH groups evidence for electron-rich Lewis acid sites was found in activated HPA. All information fit into a picture of a metastable defective polyoxometallate anion that is oligomerised to prevent crystallisation of binary oxides as the true nature of the “active HPA” catalyst. The as-synthesized HPA crystal is thus a precatalyst and the precursor oxide mixture is the final deactivated state of the catalyst.
TL;DR: Infrared spectra of isolated unsaturated rhodium cluster-CO complexes in the region of the CO stretching vibration, nu( CO), are measured using a molecular beam depletion technique to provide benchmarks for interpreting values of nu(CO) that are found when CO is used to probe Rh surfaces and supported Rh nanoparticles.
Abstract: Infrared spectra of isolated unsaturated rhodium cluster−CO complexes in the region of the CO stretching vibration, ν(CO), are measured using a molecular beam depletion technique. These spectra provide benchmarks for interpreting values of ν(CO) that are found when CO is used to probe Rh surfaces and supported Rh nanoparticles. Supported nanoparticles have shifts of ν(CO) of as much as +100 cm-1 compared to the free clusters measured here, indicative of significant charge transfer to the support.
TL;DR: Ghenciu et al. as discussed by the authors used X-ray photo-electron spectroscopy to characterize sulfated zirconia catalysts and found that they showed activity in n-hexane conversion between 300 and 473 K. This observation pointed to a limited amount of active sites able to start the reaction.
Abstract: Sulfated zirconia catalysts were prepared and characterized by X-ray photoelectron spectroscopy taken in the dried state (fresh) and after calcination at 900 K (calc.). A maximum activity was observed as a function of the calcination temperature. The Zr 3d region showed that any Zr hydroxide in the dried catalyst transformed into zirconium oxide upon calcination. The O 1s peak could be fitted by two components corresponding to ZrO 2 and sulfate, respectively. Sulfur was present as sulfate. Both catalysts showed activity in n -hexane conversion (including isomerization) between 300 and 473 K. The activity of the calcined catalyst was much higher. The main products were isopentane and isobutane, along with 2-methyl- and 3-methylpentane. The activity was not stable and only a limited amount of n -hexane transformed before final deactivation. This observation pointed to a limited amount of active sites able to start the reaction. The activity could be fully regenerated by oxygen treatment. Thus, the “oxidative” start of the reaction [ A. Ghenciu, D. Farcasiu, Catal. Lett. 44 (1997) 29] may have also played a role apart from those on strong acid sites. Deactivation may have been due to a partial reduction of sulfate groups rather than to carbon accumulation, as shown also by the minor amounts of S 4+ detected by XPS. Parallel isomerization and splitting of hexane into two C 3 units may occur, followed by the formation of surface C 9 units, the latter being intermediate of larger fragments.
TL;DR: A microscopic model for the excitation and relaxation processes in photochemistry at surfaces is developed based on ab initio calculations and the surrogate Hamiltonian method treating surface electron-hole pairs as a bath of two-level systems.
Abstract: A microscopic model for the excitation and relaxation processes in photochemistry at surfaces is developed. Our study is based on ab initio calculations and the surrogate Hamiltonian method treating surface electron-hole pairs as a bath of two-level systems. Desorption probabilities and velocities in the experimentally observed range are obtained. The excited state lifetime is calculated, and a dependence of observables on pulse length is predicted.
TL;DR: In-situ soft X-ray absorption spectroscopy (XAS) has been applied to study the iron redox behavior in over-exchanged Fe/ZSM5.
Abstract: In-situ soft X-ray absorption spectroscopy (XAS) has been applied to study the iron redox behavior in over-exchanged Fe/ZSM5. The Fe L2,3 XAS and O K spectral shapes of the Fe/ZSM5 surface have been measured during heat treatments and reduction/oxidation cycles. Charge-transfer multiplet calculations provide a detailed understanding of the L2,3 spectra of iron in Fe/ZSM5. The oxidized form of Fe/ZSM5 contains FeIII ions in an octahedral surrounding, with a total crystal field splitting of ∼1.0 eV. This value is significantly smaller than that for Fe2O3, which is indicative of a much weaker Fe−O bonding. The reduced form of Fe/ZSM5 has FeII ions in a tetrahedral oxygen surrounding. The Fe L2,3 spectra show that iron in calcined Fe/ZSM5 is reduced in 15 min to an average valence state of 2.65, under 10 mbar of pure helium at room temperature. This value has a relative uncertainty on the order of 0.01. Heating in helium up to 350 °C under the same pressure further reduces the iron valence to 2.15. The oxygen...
TL;DR: In this article, the Stillinger-Weber empirical potential was modified and its parameters were determined to achieve a realistic description of the microscopic structure and the energetics of different planar defects and their interactions in wurtzite GaN.
TL;DR: In this paper, a model of interacting motile chaotic elements is proposed, where chaotic elements are distributed in space and interact with each other through interactions depending on their positions and their internal states.
TL;DR: In this article, a model consisting of ⩾2 monolayers of Ga terminating the GaN surface was proposed, and a surface reconstruction with approximate symmetry of 4×5 was found.
Abstract: GaN is grown by plasma-assisted molecular-beam epitaxy on ZnO(1100) substrates. Well-oriented (1100) GaN surfaces are obtained, and (1101) oriented facets are also observed. On the GaN(1100) surfaces under Ga-rich conditions, a surface reconstruction with approximate symmetry of “4×5” is found. A model is proposed in which this reconstruction consists of ⩾2 monolayers of Ga terminating the GaN surface.
TL;DR: The gas-phase infrared absorption spectra of neutral benzyl and tropyl, isomers of formula C7H7, have been measured and the theory shows satisfactory overlap with the experiment, although vibrations involving the CH2 group might be anharmonic.
Abstract: The gas-phase infrared absorption spectra of neutral benzyl and tropyl, isomers of formula C7H7, have been measured in the 400−1800 cm-1 spectral region In addition, a quantum chemical calculation has been performed to model the infrared spectra For the benzyl radical, the theory shows satisfactory overlap with the experiment, although vibrations involving the CH2 group might be anharmonic The tropyl radical, which is subject to the Jahn−Teller effect, seems well modeled for the out-of-plane vibrational modes, but less so for the in-plane vibrational modes
TL;DR: In this paper, the dimensionality of the (1)-derived surface state changes from two dimensions to one dimension when the electron energy and wave vector comply with the lateral confinement conditions of the individual terrace and an alternative explanation can be given for the gradual changeover from terrace to step modulation with an increasing miscut angle reported in earlier photoemission experiments.
Abstract: Electronic structure investigations of Cu(554) using low-temperature scanning tunneling microscopy/spectroscopy indicate that the dimensionality of the (111)-derived surface state changes from two dimensions to one dimension when the electron energy and wave vector comply with the lateral confinement conditions of the individual terrace. Moreover, an alternative explanation can be given for the gradual changeover from terrace to step modulation with an increasing miscut angle reported in earlier photoemission experiments.
TL;DR: In this paper, the convergence of chemisorption properties of H on Cu(0 0 1) with respect to the cluster size is discussed. But, for the fourfold site, it is shown that even for very large clusters, convergence is not achieved.
TL;DR: The question of the stability of a laterally uniform electrode surface during bulk CO electrooxidation is addressed, motivated by a recent observation of a dynamic instability of the uniform system, namely bistable reaction rates during CO oxidation on rotating Pt electrodes.
Abstract: The oxidation of CO on Pt electrodes serves as a prototype system in studies on the basic understanding of different perspectives of electrocatalysis, such as chemisorption phenomena, and electron, proton and oxygen transfer processes. Understanding CO electrooxidation is also of particular technological importance. For example, in low-temperature H2/O2 and direct methanol fuel cells, site-blocking of CO on Pt is a major source of voltage losses at the anodes (for a review see, for example, ref. [2]). So far, all studies have implicitly assumed that the electrode surface reacts spatially uniformly, at least when averaged over mesoscopic or macroscopic areas. Herein, we address the question of the stability of a laterally uniform electrode surface during bulk CO electrooxidation. Our studies are motivated by a recent observation of a dynamic instability of the uniform system, namely bistable reaction rates during CO oxidation on rotating Pt electrodes. The bistability manifested itself in an S-shaped stationary current ± potential characteristic under potential control, and could be traced back to an autocatalytic reaction cycle that is caused by the competitive Langmuir ±Hinshelwood mechanism between surface-bonded CO and OH. 4] The interaction between an autocatalytic reaction cycle and transport processes in the electrolyte, especially migration, however, is very likely to destabilize the uniform state and give rise to self-sustained reaction patterns on the electrode surface. The experimental setup is shown in Figure 1. The working electrode (WE) consisted of a polycrystalline Pt ring of 1 mm width and 85 mm mean circumference, embedded in cylindrical