Showing papers by "Fritz Haber Institute of the Max Planck Society published in 2012"

The Active Site of Methanol Synthesis over Cu/ZnO/Al2O3 Industrial Catalysts

Abstract: Unlike homogeneous catalysts, heterogeneous catalysts that have been optimized through decades are typically so complex and hard to characterize that the nature of the catalytically active site is not known. This is one of the main stumbling blocks in developing rational catalyst design strategies in heterogeneous catalysis. We show here how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al{sub 2}O{sub 3} methanol synthesis catalyst. Using a combination of experimental evidence from bulk-, surface-sensitive and imaging methods collected on real high-performance catalytic systems in combination with DFT calculations. We show that the active site consists of Cu steps peppered with Zn atoms, all stabilized by a series of well defined bulk defects and surface species that need jointly to be present for the system to work.

Accurate and Efficient Method for Many-Body van der Waals Interactions

Abstract: An efficient method is developed for the microscopic description of the frequency-dependent polarizability of finite-gap molecules and solids. This is achieved by combining the Tkatchenko-Scheffler van der Waals (vdW) method [Phys. Rev. Lett. 102, 073005 (2009)] with the self-consistent screening equation of classical electrodynamics. This leads to a seamless description of polarization and depolarization for the polarizability tensor of molecules and solids. The screened long-range many-body vdW energy is obtained from the solution of the Schrodinger equation for a system of coupled oscillators. We show that the screening and the many-body vdW energy play a significant role even for rather small molecules, becoming crucial for an accurate treatment of conformational energies for biomolecules and binding of molecular crystals. The computational cost of the developed theory is negligible compared to the underlying electronic structure calculation.

Density-Functional Theory with Screened van der Waals Interactions for the Modeling of Hybrid Inorganic-Organic Systems

Abstract: The electronic properties and the function of hybrid inorganic-organic systems (HIOS) are intimately linked to their interface geometry. Here we show that the inclusion of the many-body collective response of the substrate electrons inside the inorganic bulk enables us to reliably predict the HIOS geometries and energies. This is achieved by the combination of dispersion-corrected density-functional theory (the DFTþ van der Waals approach) [Phys. Rev. Lett. 102, 073005 (2009)], with the Lifshitz-Zaremba-Kohn theory for the nonlocal Coulomb screening within the bulk. Our method yields geometries in remarkable agreement (� 0:1 � A) with normal incidence x-ray standing wave measurements for the 3, 4, 9, 10-perylene-tetracarboxylic acid dianhydride (C24O6H8, PTCDA) molecule on Cu(111), Ag(111), and Au(111) surfaces. Similarly accurate results are obtained for xenon and benzene adsorbed on metal surfaces.

Density-Functional Theory with Screened van der Waals Interactions for the Modeling of Hybrid Inorganic/Organic Systems

Abstract: The electronic properties and the function of hybrid inorganic-organic systems (HIOS) are intimately linked to their interface geometry. Here we show that the inclusion of the many-body collective response of the substrate electrons inside the inorganic bulk enables us to reliably predict the HIOS geometries and energies. This is achieved by the combination of dispersion-corrected density-functional theory (the DFTþ van der Waals approach) [Phys. Rev. Lett. 102, 073005 (2009)], with the Lifshitz-Zaremba-Kohn theory for the nonlocal Coulomb screening within the bulk. Our method yields geometries in remarkable agreement (� 0:1 � A) with normal incidence x-ray standing wave measurements for the 3, 4, 9, 10-perylene-tetracarboxylic acid dianhydride (C24O6H8, PTCDA) molecule on Cu(111), Ag(111), and Au(111) surfaces. Similarly accurate results are obtained for xenon and benzene adsorbed on metal surfaces.

Tip-enhanced Raman spectroscopy: near-fields acting on a few molecules.

Abstract: Tip-enhanced Raman spectroscopy (TERS) is a very powerful variant of surface-enhanced Raman spectroscopy (SERS). In a sense, TERS overcomes most of the drawbacks of SERS but keeps its advantages, such as its high sensitivity. TERS offers the additional advantages of high spatial resolution, much beyond the Abbe limit, and the possibility to correlate TER and other scanning probe microscope images, i.e., to correlate topographic and chemical data. TERS finds application in a number of fields, such as surface science, material science, and biology. Single-molecule TERS has been observed even for TERS enhancements of “only” 10 6 –10 7 . In this review, TERS enhancements are discussed in some detail, including a condensed overview of measured contrasts and estimated total enhancements. Finally, recent developments for TERS under ultrahigh vacuum conditions are presented, including TERS on a C60 island with a diameter of a few tens of nanometers, deposited on a smooth Au(111) surface.

Quantum coherence controls the charge separation in a prototypical artificial light harvesting system

Abstract: Summary form only given. In artificial light harvesting systems the conversion of light into charges or chemical energy happens on the femtosecond time scale and is thought to involve the incoherent jump of an electron from the optical absorber to an electron acceptor. Here we investigate the primary process of electronic charge transfer dynamics in a carotene-porphyrin-fullerene triad, a prototypical elementary component for an artificial light harvesting system combining coherent femtosecond spectroscopy and first-principles quantum dynamics simulations. Our experimental and theoretical results provide strong evidence that the driving mechanism of the photoinduced current generation cycle is a quantum-correlated wavelike motion of electrons and nuclei on a timescale of few tens of femtoseconds. We furthermore highlight the fundamental role played by the interface between the light-absorbing chromophore and the charge acceptor in triggering the coherent wavelike electron-hole splitting.

Nanoplasma Dynamics of Single Large Xenon Clusters Irradiated with Superintense X-Ray Pulses from the Linac Coherent Light Source Free-Electron Laser

Abstract: The plasma dynamics of single mesoscopic Xe particles irradiated with intense femtosecond x-ray pulses exceeding ${10}^{16}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ from the Linac Coherent Light Source free-electron laser are investigated. Simultaneous recording of diffraction patterns and ion spectra allows eliminating the influence of the laser focal volume intensity and particle size distribution. The data show that for clusters illuminated with intense x-ray pulses, highly charged ionization fragments in a narrow distribution are created and that the nanoplasma recombination is efficiently suppressed.

Ultrafast changes in lattice symmetry probed by coherent phonons

Abstract: The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO(2) and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.

Size dependence of the adsorption energy of CO on metal nanoparticles: a DFT search for the minimum value.

Abstract: With a density functional theory method, we studied computationally the size dependence of adsorption properties of metal nanoparticles for CO as a probe on Pdn clusters with n = 13–116 atoms. For large particles, the values slowly decrease with cluster size from the asymptotic value for an (ideal) infinite surface. For clusters of 13–25 atoms, starting well above the asymptotic value, the adsorption energies drop quite steeply with increasing cluster size. These opposite trends meet in an intermediate size range, for clusters of 30–50 atoms, yielding the lowest adsorption energies. These computational results help to resolve a controversy on the size-dependent behavior of adsorption energies of metal nanoparticles.

Effect of particle size on the activity and durability of the Pt/C electrocatalyst for proton exchange membrane fuel cells

Abstract: Carbon supported Pt (Pt/C) with various average particle sizes ranging from sub 3 nm to 65 nm were in situ prepared and characterized at the cathode of proton exchange membrane fuel cells (PEMFCs) A clear Pt particle size effect on both the catalytic activity for oxygen reduction reaction (ORR) and the durability of the electrocatalyst was revealed With the Pt particle size increase, both the surface specific activity and the electrochemical stability of Pt/C improved; however, the mass specific activity of Pt/C is balanced by the electrochemical surface area loss The reduced occupation of corner and edge atoms on the Pt surface during the Pt particle size increase is believed to weaken the adsorption of the oxygenated species on Pt, and thereafter releases more available active sites for ORR and also renders the Pt surface a stronger resistance against potential cycling It is therefore proposed that by designing the Pt microstructure with more face atoms on the surface, cathode electrocatalyst with both improved activity and enhanced durability would be developed for PEMFCs (C) 2011 Elsevier BV All rights reserved

Water oxidation by electrodeposited cobalt oxides--role of anions and redox-inert cations in structure and function of the amorphous catalyst.

Abstract: For the production of nonfossil fuels, water oxidation by inexpensive cobalt-based catalysts is of high interest. Films for the electrocatalysis of water oxidation were obtained by oxidative self-assembly (electrodeposition) from aqueous solutions containing, apart from Co, either K, Li or Ca with either a phosphate, acetate or chloride anion. X-ray absorption spectroscopy (XAS) at the Co K-edge revealed clusters of edge-sharing CoO(6) octahedra in all films, but the size or structural disorder of the Co-oxido clusters differed. Whereas potassium binding is largely unspecific, CaCo(3) O(4) cubanes, which resemble the CaMn(3) O(4) cubane of the biological catalyst in oxygenic photosynthesis, may form, as suggested by XAS at the Ca K-edge. Cyclic voltammograms in a potassium phosphate buffer at pH 7 revealed that no specific combination of anions and redox-inactive cations is required for catalytic water oxidation. However, the anion type modulates not only the size (or order) of the Co-oxido clusters, but also electrodeposition rates, redox potentials, the capacity for oxidative charging, and catalytic currents. On these grounds, structure-activity relations are discussed.

Learning Invariant Representations of Molecules for Atomization Energy Prediction

Abstract: The accurate prediction of molecular energetics in chemical compound space is a crucial ingredient for rational compound design. The inherently graph-like, non-vectorial nature of molecular data gives rise to a unique and difficult machine learning problem. In this paper, we adopt a learning-from-scratch approach where quantum-mechanical molecular energies are predicted directly from the raw molecular geometry. The study suggests a benefit from setting flexible priors and enforcing invariance stochastically rather than structurally. Our results improve the state-of-the-art by a factor of almost three, bringing statistical methods one step closer to chemical accuracy.

Toward a Light-Driven Motorized Nanocar: Synthesis and Initial Imaging of Single Molecules

Abstract: A second generation motorized nanocar was designed, synthesized, and imaged. To verify structural integrity, NMR-based COSY, NOESY, DEPT, HSQC, and HMBC experiments were conducted on the intermediate motor. All signals in 1H NMR were unambiguously assigned, and the results were consistent with the helical structure of the motor. The nanocar was deposited on a Cu(111) surface, and single intact molecules were imaged by scanning tunneling microscopy (STM) at 5.7 K, thereby paving the way for future single-molecule studies of this motorized nanocar atop planar substrates.

Thin silica films on Ru(0001): monolayer, bilayer and three-dimensional networks of [SiO4] tetrahedra.

Abstract: The atomic structure of thin silica films grown over a Ru(0001) substrate was studied by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, low energy electron diffraction, helium ion scattering spectroscopy, CO temperature programmed desorption, and scanning tunneling microscopy in combination with density functional theory calculations. The films were prepared by Si vapor deposition and subsequent oxidation at high temperatures. The silica film first grows as a monolayer of corner-sharing [SiO4] tetrahedra strongly bonded to the Ru(0001) surface through the Si–O–Ru linkages. At increasing amounts of Si, the film forms a bilayer of corner-sharing [SiO4] tetrahedra which is weakly bonded to Ru(0001). The bilayer film can be grown in either the crystalline or vitreous state, or both coexisting. Further increasing the film thickness leads to the formation of vitreous silica exhibiting a three-dimensional network of [SiO4]. The principal structure of the films can be monitored by infrared spectroscopy, as each structure shows a characteristic vibrational band, i.e., ∼1135 cm−1 for a monolayer film, ∼1300 cm−1 for the bilayer structures, and ∼1250 cm−1 for the bulk-like vitreous silica.

Crystalline-vitreous interface in two dimensional silica.

Abstract: The interface between a crystalline and a vitreous phase of a thin metal supported silica film was studied by low temperature scanning tunneling microscopy. The locally resolved evolution of Si-Si nearest neighbor distances and characteristic angles was evaluated across the border. Furthermore, we investigated the behavior of the ring size distribution close to the crystalline-vitreous transition. The crystalline order was found to decay gradually within about 1.6 nm into the vitreous state.

Structure determination of neutral MgO clusters--hexagonal nanotubes and cages.

Abstract: Structural information for neutral magnesium oxide clusters has been obtained by a comparison of their experimental vibrational spectra with predictions from theory. (MgO)n clusters with n = 3–16 have been studied in the gas phase with a tunable IR-UV two-color ionization scheme and size-selective infrared spectra have been measured. These IR spectra are compared to the calculated spectra of the global minimum structures predicted by a hybrid ab initio genetic algorithm. The comparison shows clear evidence that clusters of the composition (MgO)3k (k = 1–5) form hexagonal tubes, which confirm previous theoretical predictions. For the intermediate sizes (n a 3k) cage-like structures containing hexagonal (MgO)3 rings are identified. Except for the cubic (MgO)4 no evidence for bulk like structures is found.

H-atom relay reactions in real space

Abstract: Hydrogen bonds are the path through which protons and hydrogen atoms can be transferred between molecules. The relay mechanism, in which H-atom transfer occurs in a sequential fashion along hydrogen bonds, plays an essential role in many functional compounds. Here we use the scanning tunnelling microscope to construct and operate a test-bed for real-space observation of H-atom relay reactions at a single-molecule level. We demonstrate that the transfer of H-atoms along hydrogen-bonded chains assembled on a Cu(110) surface is controllable and reversible, and is triggered by excitation of molecular vibrations induced by inelastic tunnelling electrons. The experimental findings are rationalized by ab initio calculations for adsorption geometry, active vibrational modes and reaction pathway, to reach a detailed microscopic picture of the elementary processes. The relay mechanism in which hydrogen atom transfer occurs along hydrogen bonds plays a crucial role in many functional compounds. Using a scanning tunnelling microscope, the transfer of hydrogen atoms along hydrogen-bonded chains assembled on a Cu(110) surface is shown to be controllable and reversible.

Modeling Zeolites with Metal‐Supported Two‐Dimensional Aluminosilicate Films

Abstract: Zeolites are one of the most widely used materials in heterogeneous catalysis. However, the current understanding of the relation between structure and reactivity of these complex and highly porous materials mostly comes from studies employing bulk-sensitive techniques and from theoretical calculations based on educated assumptions about the inner surface within the pores present in the framework. Zeolite frameworks are formed by ordered arrangements of [SiO4/2] and [AlO4/2 ] tetrahedra, conferring the characteristic negative charge to the system, which is typically compensated by extra-framework metal cations M or H. Modeling such materials under controlled conditions, and taking advantage of the analytical tools commonly used in surface science, would provide a new playground for exploring structures and chemical reactions on zeolites. The preparation of welldefined aluminosilicate thin films was first reported using a Mo(112) substrate. It was shown that this film consists of a single layer network of corner-sharing [SiO4/2] tetrahedra and [AlO3/2] units, and the film is strongly bound to the Mo(112) surface by Si-O-Mo linkages (Figure 1a). Certainly, for those monolayer films the metal support has to be explicitly included in the proper description of the system. Furthermore, this film lacks the negative framework charge present in zeolites, which is responsible for the presence of acidic OH groups. To create a more adequate model system, herein we present the preparation of aluminosilicate films that a) are constituted of tetrahedral [SiO4] and [AlO4 ] building blocks, b) are weakly bound to the underlying metal support, and c) expose highly acidic OH species. Our results open up an avenue for experimental and theoretical modeling of zeolite surfaces that is aimed at a fundamental understanding of structure–reactivity relationships in those materials. As a starting point for the preparation of the aluminosilicate films, we used the recently reported preparation of a silica bilayer film weakly bound to a metal, in this case, Ru(0001) (see Figure 1b). The structure allows oxygen atoms to reversibly adsorb directly on the metal surface underneath the silica film, which can be grown either in the crystalline or vitreous state. 6] We will refer to all these films as silica films. For aluminosilicate films, reported here (see Experimental Section), the sum of the molar amounts of Si and Al was equal to the amounts of Si necessary to prepare the bilayer silica film. The structural characterization was performed by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and infrared reflection–absorption spectroscopy (IRAS), in combination with density functional theory (DFT) calculations. Using a silica film as a reference sample, the XPS results show that both Si and Al are in the highest oxidation states. For the O1s core level, a signal at about 530.7 eV develops as a shoulder to the main peak at 531.7 eV that originates from the O atoms surrounded solely by Si in the silica films (Supporting Information, Figure S1). This shoulder becomes more prominent at increasing Al/Si ratios and it has previously been assigned to the Al-O-Si linkage. The fact that the integral O1s signal intensity remains practically constant upon Al doping is consistent with Al substituting Si in the [SiO4] tetrahedra, giving an AlxSi(1 x)O2 composition, where x is the Al molar fraction. At low Al/Si ratios, the resulting aluminosilicate surfaces show only (2 2) LEED patterns and are nearly atomically flat. STM images of an Al0.12Si0.88O2 film, revealed irregularly shaped areas (marked A in Figure 2) with a slightly different Figure 1. Structural models of a) an AlSi7O19 film on Mo(112); b) a HAlSi7O16 film on O(2 1)/Ru(0001); and c) chabasite (H-CHA) with the proton on O1. Top and cross views are shown in (a) and (b), adsorbed CO are shown in (b) and (c). One of the surface O atoms on Ru(0001) underneath the film is not seen in the top view. Si yellow, O red, Al dark gray, C black, H white.

Tuning the electronic structure of ultrathin crystalline silica films on Ru(0001)

Abstract: A combination of density functional theory calculations and photoelectron spectroscopy provides new insights into the atomistic picture of ultrathin silica films grown on Ru(0001). The silica film features a double-layer silicate sheet formed by corner-sharing [SiO${}_{4}$] tetrahedra and is weakly bound to the Ru(0001) substrate. This allows oxygen atoms to reversibly adsorb directly on the metal surface underneath the silica film. We demonstrate that the amount of adsorbed oxygen can be reversibly varied by vacuum annealing and oxidation, which in turn result in gradual changes of the silica/Ru electronic states. This finding opens the possibility for tuning the electronic properties of oxide/metal systems without altering the thickness or the structure of an oxide overlayer.

In situ surface coverage analysis of RuO2-catalysed HCl oxidation reveals the entropic origin of compensation in heterogeneous catalysis.

Abstract: In heterogeneous catalytic processes the Arrhenius parameters are often found to be interrelated (compensation phenomenon). Using state-of-the-art experiments and density functional theory, the origin of compensation is studied. A similar dependence on the rate-limiting surface-coverage term is found for both apparent activation energy and prefactor terms, which can be translated into surface configurational entropy contributions.

Gating the Charge State of Single Molecules by Local Electric Fields

Abstract: The electron-acceptor molecule TCNQ is found in either of two distinct integer charge states when embedded into a monolayer of a charge transfer complex on a gold surface. Scanning tunneling spectroscopy measurements identify these states through the presence or absence of a zero-bias Kondo resonance. Increasing the (tip-induced) electric field allows us to reversibly induce the oxidation or reduction of TCNQ species from their anionic or neutral ground state, respectively. We show that the different ground states arise from slight variations in the underlying surface potential, pictured here as the gate of a three-terminal device.

Polymerization on Stepped Surfaces: Alignment of Polymers and Identification of Catalytic Sites

Abstract: Stepped surfaces have widely been used as model catalysts as they provide active sites which trigger many catalytic processes. Molecular-dissociation processes on surfaces are expected to be sensitive to the surface structure and occur preferentially at defects, such as step edges and kinks. Various surface-science techniques that average over the entire surface have been employed for the investigation of catalytic properties. Although such studies demonstrate the catalytic activity of a surface in general, they do not offer sitespecific information about the exact location of the reaction. Scanning tunneling microscopy (STM) provides a local probe, allowing site-specific information to be obtained about the structure and location of active sites on a catalytically active surface with atomic resolution. STM studies have allowed the direct observation of the active sites of heterogeneous catalysts, as shown for the dissociation of nitric oxide on a ruthenium (0001) surface, where the dissociation process was found to occur along steps, but the difference in reactivity between step edges and kinks is not clear. The majority of STM studies in catalysis have focused on small molecules, such as diatomic species, with relatively few studies investigating the reactivity of larger organic molecules. The largest molecule investigated to date is thiophene on MoS2 nanoclusters. The deposition of complex molecules, potentially carrying an intrinsic function, onto stepped surfaces has attracted much attention, especially with regard to templated supramolecular structures. Laterally ordered structures have been studied on various stepped gold surfaces with (111) terraces, but oligomers or polymers have not been investigated to date. Herein we study the adsorption of a,wdibromoterfluorene (DBTF) molecules on a stepped gold surface. Low-temperature STM under ultrahigh vacuum (UHV) conditions is used to locally determine both the molecular adsorption geometry and the site-specific catalytic activity of the step edges. We identify the step-edge kinks as the active sites in the catalytic process, because these defects promote the selective C Br bond dissociation at specific locations within the DBTF molecule. Thermally induced polymerization leads to the formation of chains, which align along the step edges in a predefined orientation. This system thus provides catalytically active sites as well as an anisotropic structure for the alignment of both the monomer precursors and the polymer products. For our studies, a gold surface with (10,7,7) orientation was used (experimental details in Supporting Information). This vicinal Au(111) surface has a misorientation angle of approximately 98 and {100}-orientated microfacets. It provides straight step edges along the [011] direction, separate flat terraces with (111) orientation, terrace widths (Wt) of around 1.4 nm [18] (Figure 1a), and is thus suitable for the adsorption of one row of molecules along each terrace. STM images of the clean surface show these terraces with an average width of 1.48 0.07 nm (Figure 1b). In agreement

Tailoring the morphology of Pd nanoparticles on CNTs by nitrogen and oxygen functionalization

Abstract: The influence of N and O functionalization of CNT on the morphology of supported Pd-PVA nanoparticles is studied with respect to the catalytic activity in the liquid phase oxidation of benzyl alcohol to benzaldehyde. The impact of specific N and O sites on the carbon surface induced by the high temperature N-functionalization in the temperature range 673–873 K was observed by HRTEM as increased nanoparticles dispersion and enhanced metal wetting at the carbon surface. Those small nanoparticles that stabilized at the N-CNTs surface are beneficial for improving catalytic performance. The interaction of O2 with the metal surface was studied by microcalorimetry. At 353 K, the PVA shell hinders the dissociative oxygen chemisorption at the surface of the fresh catalyst. Differently, a very high (maximum for Pd/N-CNT873K 750 kJ mol 1 ) and oscillating exothermic differential heat is registered for the washed samples. Such high differential heat on the ‘‘washed’’ sample is due to the sum of oxygen chemisorption and PVA oxidation. Thereby, it is demonstrated that the PVA overlayer suppresses the total combustion reaction pathway. This contribution has highlighted the impact of the dynamic change of morphology of these Pd nanoparticles under the reaction conditions on the catalytic performance and how this is modulated by the nature of the support as well as the PVA. The support with its varying ability to strongly bind Pd regulates the morphology of the nanoparticles on which the sub-surface penetration of O, H, C from the reactants depends, all modulating the electronic structure and thus the reactivity.

Gas-phase structures of neutral silicon clusters.

Abstract: Vibrational spectra of neutral silicon clusters Si(n), in the size range of n = 6-10 and for n = 15, have been measured in the gas phase by two fundamentally different IR spectroscopic methods. Silicon clusters composed of 8, 9, and 15 atoms have been studied by IR multiple photon dissociation spectroscopy of a cluster-xenon complex, while clusters containing 6, 7, 9, and 10 atoms have been studied by a tunable IR-UV two-color ionization scheme. Comparison of both methods is possible for the Si(9) cluster. By using density functional theory, an identification of the experimentally observed neutral cluster structures is possible, and the effect of charge on the structure of neutrals and cations, which have been previously studied via IR multiple photon dissociation, can be investigated. Whereas the structures of small clusters are based on bipyramidal motifs, a trigonal prism as central unit is found in larger clusters. Bond weakening due to the loss of an electron leads to a major structural change between neutral and cationic Si(8).

Ga-Pd/Ga2O3 Catalysts: The Role of Gallia Polymorphs, Intermetallic Compounds, and Pretreatment Conditions on Selectivity and Stability in Different Reactions

Abstract: Pd is widely applied in heterogeneous catalysis. It is active in various hydrogenation and reforming reactions. It is known that the catalytic properties of Pd metal can be modified by alloying or formation of intermetallic compounds (IMCs). The latter case is particularly interesting if the selectivity toward a desired product can be improved without losing the typical high activity of Pd. Alteration of the catalytic properties can be understood either in terms of a modified electronic structure or by a geometrical site isolation at the IMC’s surface. In the case of Pd-based IMCs, unsupported Ga-Pd IMCs have been investigated regarding their intrinsic catalytic properties and dramatic differences compared to monometallic Pd catalysts have been observed. The selectivity toward ethylene in the semi-hydrogenation of acetylene was reported to be higher and more stable under conditions in which a pure Pd catalyst produced mainly ethane. This effect has been attributed to site-isolation of the active Pd atoms by increasing Pd-Pd distances and reducing Pd-Pd coordination numbers on the catalyst surface, which affects their adsorbing abilities, as well as a modification of the electronic structure of the material. The Ga-Pd IMCs studied in the aforementioned publications are prepared by melting corresponding amounts of Pd and Ga in a high frequency induction furnace and subsequently annealing at high temperature in an inert atmosphere to obtain single-phase Ga-Pd IMCs. This approach is very useful for the study of the intrinsic properties of these model materials, but it sacrifices surface area. Chemical etching has been proposed as a method to increase the surface area, but lead to partial

Making the best of mixed-field orientation of polar molecules: a recipe for achieving adiabatic dynamics in an electrostatic field combined with laser pulses.

Abstract: We have experimentally and theoretically investigated the mixed-field orientation of rotational-state-selected OCS molecules and achieved strong degrees of alignment and orientation. The applied moderately intense nanosecond laser pulses are long enough to adiabatically align molecules. However, in combination with a weak dc electric field, the same laser pulses result in nonadiabatic dynamics of the mixed-field orientation. These observations are fully explained by calculations employing both adiabatic and nonadiabatic (time-dependent) models.

Metal‐Free Photocatalytic Graphitic Carbon Nitride on p‐Type Chalcopyrite as a Composite Photocathode for Light‐Induced Hydrogen Evolution

Abstract: Recently, it has been shown that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor. We present herein the preparation and characterization of graphitic carbon nitride (g-C(3)N(4)) films on p-type semiconducting CuGaSe(2) chalcopyrite thin-film substrates by thermal condensation of a dicyandiamide precursor under inert-gas conditions. Structural and surface morphological studies of the carbon nitride films suggest a high porosity of g-C(3)N(4) thin films consisting of a network of nanocrystallites. Photoelectrochemical investigations show light-induced hydrogen evolution upon cathodic polarization for a wide range of proton concentrations in the aqueous electrolyte. Additionally, synchrotron radiation-based photoelectron spectroscopy has been applied to study the surface/near-surface chemical composition of the utilized g-C(3)N(4) film photocathodes. For the first time, it has been shown that g-C(3)N(4) films coated on p-type CuGaSe(2) thin films can be successfully applied as new photoelectrochemical composite photocathodes for light-induced hydrogen evolution.

Crystal-Phase- and Morphology-Controlled Synthesis of Fe2O3 Nanomaterials

Abstract: α- and γ-Fe2O3 nanorods have been prepared from a β-FeOOH precursor that was obtained by aqueous-phase precipitation of ferric chloride. The oxyhydroxide precursor had a rodlike shape with a diameter of 30–40 nm and a length of 400–500 nm. Calcination at 500 °C of the rod-shaped oxyhydroxide in air yielded α-Fe2O3 nanorods, whereas heating to reflux in polyethylene glycol (PEG) at 200 °C resulted in the formation of γ-Fe2O3 nanorods. Both oxides inherited the rodlike morphology of the precursor but exposed different crystalline facets. When being used to catalyze NO reduction by CO, an environmentally important reaction in NO abatement, the γ-Fe2O3 nanorods were much more active than the α-Fe2O3 nanorods and showed an apparent crystal-phase effect. This was because the γ-Fe2O3 nanorods simultaneously exposed iron and oxygen ions on their surfaces, which facilitated the adsorption and activation of NO and CO molecules.

Traveling and pinned fronts in bistable reaction-diffusion systems on networks.

Abstract: Traveling fronts and stationary localized patterns in bistable reaction-diffusion systems have been broadly studied for classical continuous media and regular lattices. Analogs of such non-equilibrium patterns are also possible in networks. Here, we consider traveling and stationary patterns in bistable one-component systems on random Erdos-Renyi, scale-free and hierarchical tree networks. As revealed through numerical simulations, traveling fronts exist in network-organized systems. They represent waves of transition from one stable state into another, spreading over the entire network. The fronts can furthermore be pinned, thus forming stationary structures. While pinning of fronts has previously been considered for chains of diffusively coupled bistable elements, the network architecture brings about significant differences. An important role is played by the degree (the number of connections) of a node. For regular trees with a fixed branching factor, the pinning conditions are analytically determined. For large Erdos-Renyi and scale-free networks, the mean-field theory for stationary patterns is constructed.