Showing papers by "Fritz Haber Institute of the Max Planck Society published in 2007"
TL;DR: Progress made in this research area is highlighted by reviewing recent infrared photodissociation (IR-PD) experiments on small and medium sized vanadium oxide ions by using a combination of modern experimental and quantum chemical approaches.
Abstract: A corner stone in the study of the size-dependent properties of cluster ions in the gas phase is their structural characterization. Over the last 10 years, significant progress has been made in this research field because of important advances in the gas phase vibrational spectroscopy of mass-selected ions. Using a combination of modern experimental and quantum chemical approaches, it is now in most cases possible to uniquely identify the geometric structure of cluster ions, based on the comparison of the experimental and simulated infrared spectra. In this article, we highlight the progress made in this research area by reviewing recent infrared photodissociation experiments on small and medium sized vanadium oxide ions. # 2007 Wiley Periodicals, Inc., Mass Spec Rev 26:542–562, 2007
196 citations
TL;DR: In this article, time-dependent density functional theory (TDDFT) is proposed for the calculation of neutral excitations, based on knowledge of the density instead of the many-body wavefunction.
Abstract: For the calculation of neutral excitations, time-dependent density functional theory (TDDFT) is an exact reformulation of the many-body time-dependent Schrodinger equation, based on knowledge of the density instead of the many-body wavefunction. The density can be determined in an efficient scheme by solving one-particle non-interacting Schrodinger equations—the Kohn–Sham equations. The complication of the problem is hidden in the—unknown—time-dependent exchange and correlation potential that appears in the Kohn–Sham equations and for which it is essential to find good approximations. Many approximations have been suggested and tested for finite systems, where even the very simple adiabatic local-density approximation (ALDA) has often proved to be successful. In the case of solids, ALDA fails to reproduce optical absorption spectra, which are instead well described by solving the Bethe–Salpeter equation of many-body perturbation theory (MBPT). On the other hand, ALDA can lead to excellent results for loss functions (at vanishing and finite momentum transfer). In view of this and thanks to recent successful developments of improved linear-response kernels derived from MBPT, TDDFT is today considered a promising alternative to MBPT for the calculation of electronic spectra, even for solids. After reviewing the fundamentals of TDDFT within linear response, we discuss different approaches and a variety of applications to extended systems.
183 citations
TL;DR: In this article, a new concept to circumvent some of the problems that are hindering a rational metallic catalyst development is introduced, which employs well-defined, ordered and in-situ stable unsupported intermetallic compounds.
175 citations
TL;DR: In this article, the Pt−Pd/C catalyst has been studied experimentally and theoretically to gain insight into the promotion effect of PtPd nanoclusters to ORR and to provide guide.
Abstract: ORR (Oxygen Reduction Reaction) has been studied on Pt−Pd/C catalyst both experimentally and theoretically to gain insight into the promotion effect of PtPd nanoclusters to ORR and to provide guide...
168 citations
TL;DR: Electrochemical characterization allowed to estimate the surface composition of Pt-Au alloys, which was close to that of the bulk material Pt(20)Au(80) andStructural characterization was performed using XRD at wide angles (WAXS), which showed that Pt- Au particles exhibited alloy properties.
Abstract: Carbon supported Pt−Au catalysts with different bimetallic compositions were prepared by water in oil (w/o) microemulsion. Carbon Vulcan XC-72 was added during the synthesis of particles in order to obtain their good dispersion and a mean particle size distribution of 5.02 ± 0.56 nm. Structural characterization was performed using XRD at wide angles (WAXS), which showed that Pt−Au particles exhibited alloy properties. Electrochemical characterization allowed to estimate the surface composition of Pt−Au alloys, which was close to that of the bulk material Pt20Au80. This catalyst composition displayed the best catalytic activity in steady-state conditions in comparison with Pt50Au50 or Pt and Au alone. Moreover, a Pt−Au/C catalyst with a metal loading of 40 wt % was immobilized onto a carbon porous tube as anode. A membrane-less biofuel cell was tested using laccase/ABTS biocathode in phosphate buffer (pH 5).
166 citations
TL;DR: Experimental and theoretical infrared spectra for tryptophan-watern complexes suggest that the zwitterionic structure becomes competitive in energy at the high end of the series, and a detailed comparison of the experimental and the theoretical spectra allows us to determine both the preferred solvent binding sites on the amino acid and the evolution of conformational structures of tryPTophan as the number of attached solvent molecules increases.
Abstract: How many solvent molecules are required to solvate an amino acid? This apparently simple question, which relates to the number of solvent molecules necessary to change the amino acid from its gas-phase neutral structure to the zwitterionic solvated structure, remains unanswered to date. Here we present experimental and theoretical (density functional theory: B3LYP/6-31+G**) infrared spectra for tryptophan−watern complexes where n = 1−6, which suggest that the zwitterionic structure becomes competitive in energy at the high end of the series. Compelling evidence for a gradual transition to zwitterionic structures comes from tryptophan−methanol complexes up to n = 9. Starting from n = 5, the infrared spectra show increasing intensity in the diagnostic asymmetric COO- stretch and in the weaker NH3+ bending modes as the cluster size increases. Moreover, convergence toward the Fourier transform infrared spectrum of a solution of tryptophan in methanol is clearly observed. For small solvent complexes (n = 1−4)...
117 citations
TL;DR: In situ band gap mapping of the V2O5(001) crystal surface revealed a reversible metal-to-insulator transition at 350-400 K, which occurs inhomogeneously across the surface and expands preferentially in the direction of the vanadyl (V=O) double rows.
Abstract: In situ band gap mapping of the ${\mathrm{V}}_{2}{\mathrm{O}}_{5}(001)$ crystal surface revealed a reversible metal-to-insulator transition at 350--400 K, which occurs inhomogeneously across the surface and expands preferentially in the direction of the vanadyl ($\mathrm{V}=\mathrm{O}$) double rows. Supported by density functional theory and Monte Carlo simulations, the results are rationalized on the basis of the anisotropic growth of vanadyl-oxygen vacancies and a concomitant oxygen loss driven metal-to-insulator transition at the surface. At elevated temperatures irreversible surface reduction proceeds sequentially as ${\mathrm{V}}_{2}{\mathrm{O}}_{5}(001)\ensuremath{\rightarrow}{\mathrm{V}}_{6}{\mathrm{O}}_{13}(001)\ensuremath{\rightarrow}{\mathrm{V}}_{2}{\mathrm{O}}_{3}(0001)$.
112 citations
TL;DR: The vibrational lifetime in the frequency range of 3200 to 3500 cm(-1) is found to closely resemble that of bulk water, indicating ultrafast exchange of vibrational energy between surface water molecules and those in the bulk.
Abstract: (Received 11 September 2006; published 1 March 2007)We report a femtosecond time-resolved study of water at the neat water-air interface. The O-H stretchvibrational lifetime of hydrogen-bonded interfacial water is measured using surface-specific 4th-ordernonlinear optical spectroscopy with femtosecond infrared pulses. The vibrational lifetime in the frequencyrange of 3200 to 3500cm
104 citations
TL;DR: In this article, it is shown that the errors of present-day exchange-correlation functionals are rather short-range and that the XC correction rapidly approaches a universal dependence with cluster size.
Abstract: It is shown that the errors of present-day exchange-correlation (XC) functionals are rather short ranged. For extended systems, the correction can therefore be evaluated by analyzing properly chosen clusters and employing highest-quality quantum chemistry methods. The XC correction rapidly approaches a universal dependence with cluster size. The method is applicable to bulk systems as well as to defects in the bulk and at surfaces. It is demonstrated here for CO adsorption at transition-metal surfaces, where present-day XC functionals dramatically fail to predict the correct adsorption site, and for the crystal bulk cohesive energy.
102 citations
TL;DR: In this paper, the reactivity of several Pd-O species toward CO oxidation was compared experimentally, making use of chemically, structurally and morphologically different model systems such as single-crystalline Pd(111) covered by adsorbed oxygen or a Pd5)O(4) surface oxide layer.
Abstract: The reactivity of several Pd-O species toward CO oxidation was compared experimentally, making use of chemically, structurally and morphologically different model systems such as single-crystalline Pd(111) covered by adsorbed oxygen or a Pd(5)O(4) surface oxide layer, an oriented Pd(111) thin film on NiAl oxidized toward PdO(x) suboxide and silica-supported uniform Pd nanoparticles oxidized to PdO. The oxygen reactivity decreased with increasing oxidation state: O(ad) on metallic Pd(111) exhibited the highest reactivity and could be reduced within a few minutes already at 223 K, using low CO beam fluxes around 0.02 ML s(-1). The Pd(5)O(4) surface oxide on Pd(111) could be reacted by CO at a comparable rate above 330 K using the same low CO beam flux. The more deeply oxidized Pd(111) thin film supported on NiAl was already much less reactive, and reduction in 10(-6) mbar CO at T > 500 K led only to partial reduction toward PdO(x) suboxide, and the metallic state of Pd could not be re-established under these conditions. The fully oxidized PdO nanoparticles required even rougher reaction conditions such as 10 mbar CO for 15 min at 523 K in order to re-establish the metallic state. As a general explanation for the observed activity trends we propose kinetic long-range transport limitations for the formation of an extended, crystalline metal phase. These mass-transport limitations are not involved in the reduction of O(ad), and less demanding in case of the 2-D Pd(5)O(4) surface oxide conversion back to metallic Pd(111). They presumably become rate-limiting in the complex separation process from an extended 3-D bulk oxide state toward a well ordered 3-D metallic phase.
94 citations
TL;DR: In this article, a locally constrained density-functional theory approach is presented to confine electrons to subspaces of the Hilbert space, e.g., to selected atoms or groups of atoms.
Abstract: Nonadiabatic effects play an important role in many chemical processes. In order to study the underlying nonadiabatic potential-energy surfaces (PESs), we present a locally constrained density-functional theory approach, which enables us to confine electrons to subspaces of the Hilbert space, e.g., to selected atoms or groups of atoms. This allows one to calculate nonadiabatic PESs for defined charge and spin states of the chosen subsystems. The capability of the method is demonstrated by calculating nonadiabatic PESs for the scattering of a sodium and a chlorine atom, for the interaction of a chlorine molecule with a small metal cluster, and for the dissociation of an oxygen molecule at the Al(111) surface.
TL;DR: In this article, the reaction between CH4 and O2 (1:5) was studied by in situ XPS during heating and cooling in a 0.33 mbar reaction mixture.
Abstract: The reaction between CH4 and O2 (1:5) was studied by in situ XPS during heating and cooling in a 0.33 mbar reaction mixture. During heating, the reaction rate exhibited an activity maximum at 650 K, whereas no activity maximum was found during the subsequent cooling ramp. This kinetic hysteresis was assigned to the spectroscopically observed difference in the surface oxidation state. During heating, the reaction rate approached the 650 K maximum in the stability range of bulk PdO seeds among the otherwise Pd5O4 2D oxide covered surface. On the other hand, no PdO seeds were formed during cooling, most likely due to kinetic limitations of PdO nucleation on a passivating surface oxide layer containing less oxygen than Pd5O4.
TL;DR: The observed critical size for doubly doped silicon clusters indicates that larger caged molecules can be formed, eventually leading to the growth of metal-doped silicon nanorods.
Abstract: We report on an element-dependent critical size for argon physisorption at 80 K on transition-metal-doped silicon clusters. Argon does not attach to elemental silicon clusters but only to surface-located transition-metal atoms. Thus physisorption provides structural information. Specifically, the minimal cluster size for the formation of endohedral singly metal-doped silicon cages has been determined. The observed critical size for doubly doped silicon clusters indicates that larger caged molecules can be formed, eventually leading to the growth of metal-doped silicon nanorods.
TL;DR: It was found that very small Pd particles on Fe(3)O(4) show a strongly modified adsorption behavior, characterized by atypically weak CO adsorptive behavior and a characteristic CO stretching frequency around 2130 cm(-1).
Abstract: Combining scanning tunneling microscopy (STM), IR reflection absorption spectroscopy (IRAS) and molecular beam (MB) techniques, we have investigated particle size effects on a Pd/Fe3O4 model catalyst. We focus on the particle size dependence of (i) CO adsorption, (ii) oxygen adsorption and (iii) Pd nanoparticle oxidation/reduction. The model system, which is based on Pd nanoparticles supported on an ordered Fe3O4 film on Pt(111), is characterized in detail with respect to particle morphology, nucleation, growth and coalescence behavior of the Pd particles. Morphological changes upon stabilization by thermal treatment in oxygen atmosphere are also considered. The size of the Pd particles can be varied roughly between 1 and 100 nm. The growth and morphology of the Pd particles on the Fe3O4/Pt(111) film were characterized by STM and IRAS of adsorbed CO as a probe molecule. It was found that very small Pd particles on Fe3O4 show a strongly modified adsorption behavior, characterized by atypically weak CO adsorption and a characteristic CO stretching frequency around 2130 cm−1. This modification is attributed to a strong interaction with the support. Additionally, the kinetics of CO adsorption was studied by sticking coefficient experiments as a function of particle size. For small particles it is shown that the CO adsorption rate is significantly enhanced by the capture zone effect. The absolute size of the capture zone was quantified on the basis of the STM and sticking coefficient data. Finally, oxygen adsorption was studied by means of MB CO titration experiments. Pure chemisorption of oxygen is observed at 400 K, whereas at 500 K partial oxidation of the particles occurs. The oxidation behavior reveals strong kinetic hindrances to oxidation for larger particles, whereas facile oxidation and reduction are observed for smaller particles. For the latter, estimates point to the formation of oxide layers which, on average, are thicker than the surface oxides on corresponding single crystal surfaces.
TL;DR: In this article, a Pt/CeO2 catalyst was used to remove carbon dioxide from hydrogen feed using in situ experimentation with time-resolved and temperature-programmed diffuse reflectance infrared spectroscopy.
Abstract: The production of clean hydrogen is a key requirement for a future hydrogen economy, in general, and, specifically, for the application of proton exchange membrane fuel cells (PEMFC). Here, we focus on one of the essential purification methods, the so-called “PROX” reaction, the preferential oxidation of traces of CO in a large hydrogen excess. Small platinum particles on a reducible support like ceria are effective to remove CO from hydrogen feed. The paper specifically addresses the mechanism of the PROX reaction on a Pt/CeO2 catalyst using in situ experimentation with time-resolved and temperature-programmed diffuse reflectance infrared spectroscopy. Surface species (carbonates, formates, carbonyls, hydroxyls, and adsorbed water) present under reaction conditions are identified, and correlations of their abundance with catalytic performance allow the discrimination between mechanistically relevant species (intermediates) and spectator species. The following scenario is proposed: hydrogen initially ads...
TL;DR: In this paper, a detailed analysis of the dynamics of the metal sulphide aqueous electrolyte interface reveals that protons are involved in the mechanism on which bacterial activity is based.
Abstract: Thiobacillus ferrooxidans has been cultivated successfully on synthetic metal sulphides with large energy gaps (CdS, ZnS) as the only energy source for many culture generations during a period of 4 years. The results obtained, which were quantitatively evaluated by calculations of electron transfer probabilities, show that a direct electron transfer from the metal sulphide valence band to the bacterial metabolic system (hole injection into the valence band of the sulphide) has to be excluded as the cause for the enhanced oxidative dissolution for energetic reasons. A detailed analysis of the dynamics of the metal sulphide aqueous electrolyte interface reveals that protons are involved in the mechanism on which bacterial activity is based. By reacting chemically with the metal sulphide surface they break chemical bonds and shift electronic states energetically into the forbidden energy gap to produce surface states which can be chemically described as −SH∂ groups. These control the rate of dissolution of the metal sulphide and are removed by bacterial activity. In this way the proton is recycled and its action can be considered catalytic. For sulphides in which the valence band of the semiconductor is derived from metal orbitals instead of from sulphur orbitals, this mechanism is bound to fail. MoS2 and WS2 are discussed as examples of such metal sulphides which are not a suitable energy source for bacteria. Some kinetic aspects of the bacterial surface reaction are discussed.
TL;DR: In this paper, the suitability of metal sulphides for bacterial oxidation was investigated with respect to their suitability as an energy source for Thiobacillus ferrooxidans.
Abstract: Sixteen metal sulphides with widely differing solid state, chemical and electrochemical properties have been investigated with respect to their suitability as an energy source for Thiobacillus ferrooxidans. The most critical rate-determining parameter for bacterial oxidation (in the absence of any electron acceptors, e.g. Fe3+) was the solubility product of the sulphide. Deviations from the systematic dependence are, however, observed when a large concentration of the holes are present in the semiconducting sulphide (p-type conduction), or when holes are generated through electron extraction from the sulphide surface (e.g. by Fe3+). The experimental results are consistent with a mechanism in which Th. ferrooxidans utilises H+ and Fe3+ as catalytic agents which break surface bonds by chemical and electrochemical mechanisms, respectively. They are subsequently recycled while the bacterium oxidises the generated surface products, — SHδ and So, etc., to sulphate. On the basis of the derived mechanism it should be possible to estimate and predict the suitability of sulphides for bacterial oxidation and to analyse the influence of crystalline quality and impurities, as well as the composition of the solution, on the rate of oxidation.
TL;DR: The mid-IR spectra of 9-ethyl guanine, guanosine, and 2-deoxyguanosine measured by IR-UV double-resonance spectroscopy confirm the enol form for all structures and demonstrate the efficacy of a new approach to DFT calculations that includes dispersion interactions.
Abstract: We present the mid-IR (400-1800 cm -1 ) spectra of 9-ethyl guanine, guanosine, and 2-deoxyguanosine measured by IR-UV double-resonance spectroscopy. We compare the recorded mid-IR spectra with the spectra of the most stable structures obtained from RI-MP2 and RI-DFT-D calculations. The results confirm the enol form for all structures and demonstrate the efficacy of a new approach to DFT calculations that includes dispersion interactions.
TL;DR: It is found that the CO molecule dissociates on bare vanadium clusters, while it adsorbs intact on all saturated hydrogen covered V5-20+ clusters, with the distinctive exceptions of V5+, V9+, V11+, and V19+.
Abstract: We report on the size-dependent interaction of carbon monoxide molecules with hydrogen covered vanadium clusters containing between 5 and 20 atoms. Structural information on these hydrogen covered vanadium clusters and their complexes with CO is obtained from infrared multiple photon dissociation spectroscopy, complemented with density functional theory calculations for the V5 to V9 cluster sizes. The non-dissociative or dissociative binding of CO on the metal clusters is detected by the presence or absence of the ν(CO) stretching band in the infrared spectra. It is found that the CO molecule dissociates on bare vanadium clusters, while it adsorbs intact on all saturated hydrogen covered V5-20+ clusters, with the distinctive exceptions of V5+, V9+, V11+, and V19+. We show that dissociative chemisorption is prevented when the potential binding sites of atomic C and O atoms are blocked by H atoms.
TL;DR: A reactivity screening revealed that only high-valent vanadium oxide clusters are reactive with respect to H2 uptake, oxygen transfer, and elimination of H2O, whereas smaller and low- valent cluster cations are capable of dehydrogenating methanol via elimination ofH2.
Abstract: Bare vanadium oxide and hydroxide cluster cations, VmOn+ and VmOn-1(OH)+ (m = 1−4, n = 1−10), generated by electrospray ionization, were investigated with respect to their reactivity toward methanol using mass spectrometric techniques. Several reaction channels were observed, such as abstraction of a hydrogen atom, a methyl radical, or a hydroxymethyl radical, elimination of methane, and adduct formation. Moreover, dehydrogenation of methanol to generate formaldehyde was found to occur via four different pathways. Formaldehyde was released as a free molecule either upon transfer of two hydrogen atoms to the cluster or upon transfer of an oxygen atom from the cluster to the neutral alcohol concomitant with elimination of water. Further, formaldehyde was attached to VmOn+ upon loss of H2 or neutral water to produce the cation VmOn(OCH2)+ or VmOn-1(OCH2)+, respectively. A reactivity screening revealed that only high-valent vanadium oxide clusters are reactive with respect to H2 uptake, oxygen transfer, and e...
TL;DR: In this paper, a comparison of X-ray photoelectron spectroscopy (XPS), temperature-programmed CO desorption (TPD), ferromagnetic resonance (FMR), and IRAS in ultrahigh vacuum was made.
Abstract: Cobalt and bimetallic Co-Pd systems are well-known Fischer-Tropsch catalysts. Compared to Co, the bimetallic systems exhibit an increased activity toward CO hydrogenation and methane conversion, attributed to resistance against oxidation. To study the oxidation behavior, model catalysts have been generated by depositing either Co or first Co and subsequently Pd onto a thin epitaxial alumina film grown on NiAl(110). Pure Co particles and bimetallic particles with a Co core and a Pd shell have been studied before and after exposure to oxygen and after thermal treatments, using X-ray photoelectron spectroscopy (XPS), temperatureprogrammed CO desorption (TPD), ferromagnetic resonance (FMR), and infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum. Large doses of O2 (1000 langmuirs; 1 langmuir ) 10 -6 Torr‚s) at 300 K lead to complete oxidation of Co particles. Upon annealing to temperatures above 530 K, XPS indicates that the cobalt oxide is mostly reduced by transfer of oxygen to the alumina support, resulting in its thickening. TPD, however, indicates the existence of persistent surface oxygen species, reducing the CO adsorption energy on the particles. Exposures to small doses of O2 (30-50 langmuirs) were also studied by a careful comparison of XPS, TPD, and FMR data. In this case, XPS indicates Co in a metallic state, whereas TPD and FMR indicate oxidic behavior. We conclude that small amounts of non-stoichiometric subsurface oxygen or subsurface and surface oxygen are present which are not detectable in the Co 2p XPS signal but have a pronounced effect on the surface chemistry and the magnetism, i.e., on certain bulk properties. In the case of bimetallic Co/Pd particles, an incomplete Pd shell on the Co particles even promotes oxygen uptake, while only a complete Pd shell inhibits oxidation.
TL;DR: In this article, a scanning tunneling microscope (STM) was used to study the growth of Au particles on thin MgO films prepared on Mo(001) and showed that with increasing Au coverage, rapid quenching of the characteristic, defect-related mgO emission is observed, while a new peak emerges in the optical spectra that is attributed to Mie plasmon resonances of the Au particles.
Abstract: Electroluminescence measurements have been performed with a scanning tunneling microscope (STM) to study the growth of Au particles on thin MgO films prepared on Mo(001). With increasing Au coverage, rapid quenching of the characteristic, defect-related MgO emission is observed, while a new peak emerges in the optical spectra that is attributed to Mie plasmon resonances of the Au particles. The fast decline of the MgO signal with metal exposure indicates that Au nucleation preferentially takes place at the optically active centers in the MgO surface. The nucleation sites have been identified in STM measurements as being located along the step edges of the MgO surface. A sputter-deposition of Au results in a significantly slower decline of the MgO optical signal, manifesting the delayed blocking of optically active MgO centers due to Au nucleation on artificial surface defects.
TL;DR: With the aid of GC/MS analysis of organic products in the liquid phase, it is shown that the controlled hydrolysis of TiCl4 with water, which was in situ generated from theTiCl4-catalyzed aldol condensation reactions of acetones, played an important role in the formation of the titania nanomaterials.
Abstract: Single-phase titania nanomaterials were prepared by autoclaving titanium tetrachloride in acetone at 80−140 °C. Depending on the molar ratio of TiCl4 to acetone (TiCl4/Ac), TiO2 materials with different phases and morphologies were obtained. When the TiCl4 concentration was no higher than TiCl4/Ac = 1/15, single-phase anatase TiO2 nanocrystals in sizes ranging from 4 to 10 nm were prepared by tuning TiCl4/Ac ratios from 1/90 to 1/15. However, when the TiCl4 concentration was high enough (e.g., TiCl4/Ac ≥ 1/10), single-phase rutile TiO2 nanofibers were obtained selectively. The materials were characterized comprehensively using X-ray diffraction, transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis, and nitrogen adsorption measurements. With the aid of GC/MS analysis of organic products in the liquid phase, it is shown that the controlled hydrolysis of TiCl4 with water, which was in situ generated from the TiCl4-catalyzed aldol condensation reactions of acetones, played an impor...
TL;DR: In this paper, a zirconium oxynitride (ZrON) catalyst was used for the decomposition of ammonia to hydrogen and nitrogen, and the onset of catalytic activity at ∼550°C coincided with the onset nitrogen ion mobility in the material.
TL;DR: It turns out that the (4 x 4)O structure only forms under extremely clean conditions, indicating that the(4 x4)O phase and similar oxygen-induced reconstructions of the Ag(111) surface are chemically unstable.
Abstract: To identify surface phases that could play a role for the epoxidation of ethylene on Ag catalysts we have studied the interaction of Ag(111) with O2 at elevated pressures. Experiments were performed using high-pressure scanning tunneling microscopy (STM) at temperatures between 450 and 480 K and O2 pressures in the mbar range. Below p(O2) ∼1 mbar the surface largely showed the structure of bare Ag(111). At p(O2) above ∼1 mbar the (4 × 4)O structure and the closely related (4 × 5√3)rect structure were observed. The findings confirm theoretical predictions that the (4 × 4)O structure is thermodynamically stable at the oxygen partial pressure of the industrial ethylene oxide synthesis. However, in other experiments only a rough, disordered structure was observed. The difference is caused by the chemical state of the STM cell that depends on the pretreatment and on previous experiments. The surface was further analyzed by X-ray photoelectron spectroscopy (XPS). Although these measurements were performed after sample transfer to ultra-high vacuum (UHV), so that the surface composition was modified, the two surface states could still be identified by the presence of carbonate or a carbonaceous species, and by the absence or presence of a high-binding energy oxygen species, respectively. It turns out that the (4 × 4)O structure only forms under extremely clean conditions, indicating that the (4 × 4)O phase and similar oxygen-induced reconstructions of the Ag(111) surface are chemically unstable. Chemical reactions at the inner surfaces of the STM cell also complicate the detection of the catalytic formation of ethylene oxide.
TL;DR: The paracrystalline distortions arise, for instance, in solid-solution crystals as mentioned in this paper, and they are experimentally observed in polyethylene and polyethylenes oxide, where values of about 2% for the mosaic blocks in single crystals and of 3% for ultrafibrils in bulk material are found.
Abstract: The paracrystalline state is a quite common state for all kinds of matter. Paracrystalline distortions arise, for instance, in solid-solution crystals. From x-ray scattering they are experimentally observed in polyethylene and polyethylene oxide. Values of g of about 2% for the mosaic blocks in single crystals and of 3% for the ultrafibrils in bulk material are found. The ultrafibrils of Lupolen 6001 have a very uniform thickness of n X 90.7 A., where n = 1, 2, 3, and build up manyfold superstructures. The (hh0) “netplanes” at the boundaries of all observed mosaic blocks both in single PE crystals and in hot-stretched material fluctuate statistically by 0.6 A., hence are no longer planes. The structure in the grain boundaries is discussed. Measurements of the diamagnetic susceptibility give evidence of three types of conformation, α, β, and γ, which play an important role in the different superstructures.
TL;DR: In this article, the formation of sulphur or sulphate as the final state of oxidation is determined by consecutive reactions which are critically dependent on the catalytical properties of the surface and the solution for hydrogen peroxide decomposition.
Abstract: Experiments with a representative number of technically interesting metal sulphides trace out hydrogen peroxide as the reactive intermediate of sulphide oxidation. It is formed through an electrochemical reduction of molecular oxygen. The formation of sulphur or sulphate as the final state of oxidation is determined by consecutive reactions which are critically dependent on the catalytical properties of the surface and the solution for hydrogen peroxide decomposition. The individual steps of the reaction are discussed in some detail. Conclusions with respect to a possible control of the autogeneous heating are drawn.
TL;DR: This study provides clear spectroscopic fingerprints for amine and imide functional end groups, which drive the self-organization process in a number of planar, pi-conjugated molecular structures.
Abstract: The electronic structures of naphthalene tetracarboxylic diimide (NTCDI) and 1,4-bis(4,6-diamino-1,3,5-triazin-2-yl)benzene (BDG) monolayer assemblies grown on Au(111) are investigated by photoemission spectroscopy, X-ray absorption, and density functional theory. The different spectroscopic features in the absorption and core-level photoemission spectra are understood in terms of contributions from different core and molecular levels at N- and O-atom sites. This study provides clear spectroscopic fingerprints for amine and imide functional end groups, which drive the self-organization process in a number of planar, pi-conjugated molecular structures.
TL;DR: In this paper, it was shown that the quantities of covalently bound enzymes as well as the properties of the enzymes themselves are dependent on the structure and chemical composition of the polymer matrices.
Abstract: It was investigated how polymers which carry 2,4-dinitro-fluorobenzene or isothiocyanate groups could be used for preparing protein and enzyme resins. Furthermore it is demonstrated that the quantities of covalently bound enzymes as well as the properties of the enzymes themselves are dependent on the structure and chemical composition of the polymer matrices.