Showing papers by "Fritz Haber Institute of the Max Planck Society published in 2010"

Covalent Triazine Framework as Catalytic Support for Liquid Phase Reaction

Abstract: An important goal in the preparation of highly active supported metal particles is the enhancement of the metal support interaction, providing a more stable catalyst, especially for liquid phase reactions as the leaching and reconstruction of the active phase causes deactivation. In this work, a covalent triazine framework (CTF) as support for Pd nanoparticles is compared to activated carbon (AC), the typical support used in liquid phase reactions. The results indicate that the presence of the N-heterocyclic moieties on the surface of the frameworks is beneficial for improving the stability of Pd nanoparticles during the liquid phase glycerol oxidation. Pd/CTF showed better activity and in particular better stability when compared to Pd supported on activated carbon (AC).

Stacking and Registry Effects in Layered Materials: The Case of Hexagonal Boron Nitride

Abstract: The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is investigated via a van der Waals corrected density functional theory approach. It is found that the main role of the van der Waals forces is to anchor the layers at a fixed distance, whereas the electrostatic forces dictate the optimal stacking mode and the interlayer sliding energy. A nearly free-sliding path is identified, along which band gap modulations of � 0: 6e Vare obtained. We propose a simple geometric model that quantifies the registry matching between the layers and captures the essence of the corrugated h-BN interlayer energy landscape. The simplicity of this phenomenological model opens the way to the modeling of complex layered structures, such as carbon and boron nitride nanotubes.

A hitherto unrecognized source of low-energy electrons in water

Abstract: Most of the low-energy electrons emitted from a material when it is subjected to ionization radiation are believed to be directly ionized secondary electrons. Coincidence measurements of the electrons ejected from water clusters suggests many are produced by a quantitatively new mechanism, known as intermolecular Coulombic decay. Low-energy electrons are the most abundant product of ionizing radiation in condensed matter. The origin of these electrons is most commonly understood to be secondary electrons1 ionized from core or valence levels by incident radiation and slowed by multiple inelastic scattering events. Here, we investigate the production of low-energy electrons in amorphous medium-sized water clusters, which simulate water molecules in an aqueous environment. We identify a hitherto unrecognized extra source of low-energy electrons produced by a non-local autoionization process called intermolecular coulombic decay2 (ICD). The unequivocal signature of this process is observed in coincidence measurements of low-energy electrons and photoelectrons generated from inner-valence states with vacuum-ultraviolet light. As ICD is expected to take place universally in weakly bound aggregates containing light atoms between carbon and neon in the periodic table2,3, these results could have implications for our understanding of ionization damage in living tissues.

The physical and chemical properties of heteronanotubes

Abstract: Carbon nanotubes undoubtedly take a leading position in nanotechnology research owing to their well-known outstanding structural and electronic properties. Inspired by this, hybrid and functionalized tubular structures have been constructed via several modification paths that involve the presence of molecules, generation of defects, and partial or full replacement of the carbon atoms, always maintaining a nanotube structure. The possibilities are countless. However, this review is mainly dedicated to giving a fundamental insight into the concepts behind wall modification, doping, and formation of a carbon nanotube structure. Theoretical concepts and experimental achievements ranging from carbon nanotubes with low B or N doping to the new physics behind boron nitride nanotubes are covered. Furthermore, special attention is devoted to the bulk and local characterization tools employed with these materials, their suitability and limitations. The theoretical approaches to describing the physical and chemical properties of heteronanotubes are objectively analyzed versus the materials available at this moment.

Structure and energetics of azobenzene on Ag(111): benchmarking semiempirical dispersion correction approaches.

Abstract: We employ normal-incidence x-ray standing wave and temperature programed desorption spectroscopy to derive the adsorption geometry and energetics of the prototypical molecular switch azobenzene at Ag(111). This allows us to assess the accuracy of semiempirical correction schemes as a computationally efficient means to overcome the deficiency of semilocal density-functional theory with respect to long-range van der Waals (vdW) interactions. The obtained agreement underscores the significant improvement provided by the account of vdW interactions, with remaining differences mainly attributed to the neglect of electronic screening at the metallic surface.

Growth and structure of crystalline silica sheet on Ru(0001)

Abstract: Thin SiO₂ films were grown on a Ru(0001) single crystal and studied by photoelectron spectroscopy, infrared spectroscopy and scanning probe microscopy. The experimental results in combination with density functional theory calculations provide compelling evidence for the formation of crystalline, double-layer sheet silica weakly bound to a metal substrate.

Role of Ceria in Oxidative Dehydrogenation on Supported Vanadia Catalysts

Abstract: The effect of the suppport on oxidative dehydrogenation activity for vanadia/ceria systems is examined for the oxidation of methanol to formaldehyde by use of well-defined VO(x)/CeO(2)(111) model catalysts. Temperature-programmed desorption at low vanadia loadings revealed reactivity at much lower temperature (370 K) as compared to pure ceria and vanadia on inert supports such as silica. Density functional theory is applied and the energies of hydrogenation and oxygen vacancy formation also predict an enhanced reactivity of the vanadia/ceria system. At the origin of this support effect is the ability of ceria to stabilize reduced states by accommodating electrons in localized f-states.

Infrared spectroscopy of hydrated bicarbonate anion clusters: HCO3(-)(H2O)(1-10).

Abstract: Infrared multiple photon dissociation spectra are reported for HCO3−(H2O)1−10 clusters in the spectral range of 600−1800 cm−1. In addition, electronic structure calculations at the MP2/6-311+G(d,p) level have been performed on the n = 1−8 clusters to identify the structure of the low-lying isomers and to assign the observed spectral features. General trends in the stepwise solvation motifs of the bicarbonate anion can be deduced from the overall agreement between the calculated and experimental spectra. The most important of these is the strong preference of the water molecules to bind to the negatively charged CO2 moiety of the HCO3− anion. However, a maximum of four water molecules interact directly with this site. The binding motif in the most stable isomer of the n = 4 cluster, a four-membered ring with each water forming a single H-bond with the CO2 moiety, is retained in all of the lowest-energy isomers of the larger clusters. Starting at n = 6, additional solvent molecules are found to form a secon...

Describing Both Dispersion Interactions and Electronic Structure Using Density Functional Theory: The Case of Metal-Phthalocyanine Dimers

Abstract: Noncovalent interactions, of which London dispersion is an important special case, are essential to many fields of chemistry. However, treatment of London dispersion is inherently outside the reach of (semi)local approximations to the exchange-correlation functional as well as of conventional hybrid density functionals based on semilocal correlation. Here, we offer an approach that provides a treatment of both dispersive interactions and the electronic structure within a computationally tractable scheme. The approach is based on adding the leading interatomic London dispersion term via pairwise ion-ion interactions to a suitably chosen nonempirical hybrid functional, with the dispersion coefficients and van der Waals radii determined from first-principles using the recently proposed "TS-vdW" scheme (Tkatchenko, A.; Scheffler, M. Phys. Rev. Lett. 2009, 102, 073005). This is demonstrated via the important special case of weakly bound metal-phthalocyanine dimers. The performance of our approach is additionally compared to that of the semiempirical M06 functional. We find that both the PBE-hybrid+vdW functional and the M06 functional predict the electronic structure and the equilibrium geometry well, but with significant differences in the binding energy and in their asymptotic behavior.

Structure and excitonic coupling in self-assembled monolayers of azobenzene-functionalized alkanethiols.

Abstract: Optical properties and the geometric structure of self-assembled monolayers of azobenzene-functionalized alkanethiols have been investigated by UV/visible and near edge X-ray absorption fine structure spectroscopy in combination with density-functional theory. By attaching a trifluoro-methyl end group to the chromophore both the molecular tilt and twist angle of the azobenzene moiety are accessible. Based on this detailed structural analysis the energetic shifts observed in optical reflection spectroscopy can be qualitatively described within an extended dipole model. This substantiates sizable excitonic coupling among the azobenzene chromophores as an important mechanism that hinders trans to cis isomerization in densely packed self-assembled monolayers.

The role of Cu on the reduction behavior and surface properties of Fe-based Fischer–Tropsch catalysts

Abstract: The effect of Cu on the reduction behavior and surface properties of supported and unsupported Fe-based Fischer–Tropsch synthesis (FTS) catalysts was investigated using in situ X-ray photoelectron spectroscopy (XPS) and in situ X-ray absorption spectroscopy (XAS) in combination with ex situ bulk characterization. During exposure to 0.4 mbar CO–H2 above 180 °C, the reduction of CuO to Cu0 marked the onset of the reduction of Fe3O4 to α-Fe. The promotion effects of Cu are explained by a combination of spillover of H2 and/or CO molecules from metallic Cu0 nuclei to closely associated iron oxide species and textural promotion. XAS showed that in the supported catalyst, Cu+ and Fe2+ species were stabilized by SiO2 and, as a result, Fe species were not reduced significantly beyond Fe3O4 and Fe2+, even after treatment at 350 °C. After the reduction treatment, XPS showed that the concentration of oxygen and carbon surface species was higher in the presence of Cu. Furthermore, it was observed that the unsupported, Cu-containing catalyst showed higher CO2 concentration in the product gas stream during and after reduction and Fe surface species were slightly oxidized after prolonged exposure to CO–H2. These observations suggest that, in addition to facilitating the reduction of the iron oxide phase, Cu also plays a direct role in altering the surface chemistry of Fe-based FTS catalysts.

Semiconductor-half metal transition at the Fe3O4(001) surface upon hydrogen adsorption

Abstract: The adsorption of H on the magnetite (001) surface was studied with photoemission spectroscopies, scanning tunneling microscopy, and density-functional theory. At saturation coverage the insulating $(\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})R45\ifmmode^\circ\else\textdegree\fi{}$ reconstruction is lifted and the surface undergoes a semiconductor--half metal transition. This transition involves subtle changes in the local geometric structure linked to an enrichment of ${\text{Fe}}^{2+}$ cations at the surface. The ability to manipulate the electronic properties by surface engineering has important implications for magnetite-based spintronic devices.

Microwave-Assisted Preparation of Mo2C/CNTs Nanocomposites as Efficient Electrocatalyst Supports for Oxygen Reduction Reaction

Abstract: Nanostructured Mo2C/CNTs composites have been synthesized by using a novel methodology of microwave-assisted thermolytic molecular precursor with Mo(CO)6 as single source precursor. Pt electrocatalysts supported on the Mo2C/CNTs composites were prepared by using the modified ethylene glycol method. The resulting Mo2C/CNTs and Pt−Mo2C/CNTs were characterized by inductively coupled plasma-optical emission spectroscopy, thermogravimetric analysis, X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The rotating disk electrode experiments were used to measure electrocatalytic activity for oxygen reduction reaction. The results showed highly dispersed sphere-like Mo2C and Pt particles with 3−6 nm can be prepared upon CNTs by the above-mentioned methods. The formation process of Mo2C includes the following steps: decomposition of Mo(CO)6 precursor to the metallic Mo and CO, CO dismutation reaction, formation of the MoOxCy by the metallic Mo and CO, the MoOxCy carburiza...

Oxidative Purification of Carbon Nanotubes and Its Impact on Catalytic Performance in Oxidative Dehydrogenation Reactions

Abstract: Oxidative purification with mild diluted HNO(3) followed by NaOH washing lowers the amount of amorphous carbon attached to multiwalled carbon nanotubes (CNTs). The graphitic structure improves remarkably by further annealing in argon at elevated temperatures, that is, 1173, 1573, and 1973 K. The influence of the purification treatment on the catalytic activity of the CNTs is investigated for the oxidative dehydrogenation (ODH) of ethylbenzene and propane as probe reactions. All samples tend to approach an appropriately ordered structure and Raman analysis of the used samples displays a D/G band ratio of 0 95-1 42. Oxygen functionalities are partly removed by the annealing treatment and can be rebuilt to some extent by oxygen molecules in the ODH reactant flow. The presence of amorphous carbon is detrimental to the catalytic performance as it allows for unwanted functional groups occurring in parallel with the formation of the selective (di)ketonic active sites

Vibration–rotation-tunneling states of the benzene dimer: an ab initio study

Abstract: An improved intermolecular potential surface for the benzene dimer is constructed from interaction energies computed by symmetry-adapted perturbation theory, SAPT(DFT), with the inclusion of third-order contributions. Twelve characteristic points on the surface have been investigated also using the coupled-cluster method with single, double, and perturbative triple excitations, CCSD(T), and triple-zeta quality basis sets with midbond functions. The SAPT and CCSD(T) results are in close agreement and provide the best representation of these points to date. The potential was used in calculations of vibration–rotation-tunneling (VRT) levels of the dimer by a method appropriate for large amplitude intermolecular motions and tunneling between multiple equivalent minima in the potential. The resulting VRT levels were analyzed with the use of the permutation-inversion full cluster tunneling (FCT) group G576 and a chain of subgroups that starts from the molecular symmetry group Cs(M) of the rigid dimer at its equilibrium Cs geometry and leads to G576 if all possible intermolecular tunneling mechanisms are feasible. Further information was extracted from the calculated wave functions. It was found, in agreement with the experimental data, that for all of the 54 G576 symmetry species (with different nuclear spin statistical weights) the lower VRT states have a tilted T-shape (TT) structure; states with the parallel-displaced structure are higher in energy than the ground state of A+1 symmetry by at least 30 cm−1. The dissociation energy D0 equals 870 cm−1, while the depth De of the TT minimum in the potential is 975 cm−1. Hindered rotation of the cap in the TT structure and tilt tunneling lead to level splittings on the order of 1 cm−1. Also intermolecular vibrations with excitation energies starting at a few cm−1 were identified. A further small, but probably significant, level splitting was assigned to cap turnover, although in scans of the potential surface we could not find a plausible ‘reaction path’ for this process. Rotational constants were extracted from energy levels calculated for total angular momentum J = 0 and 1, and from expectation values of the inertia tensor. Although the end-over-end rotational constant B + C agrees well with the measured microwave spectra, there is disagreement with the measurements concerning the (a)symmetric rotor character of the benzene dimer. It is concluded from calculations for the 54 nuclear spin species that the microwave spectrum should show overlapping contributions from many different species. Another interesting conclusion regards the role of the quantum number K, for a prolate near-symmetric rotor the projection of the total angular momentum on the prolate axis. For the benzene dimer, K has a substantial effect on the energy levels associated with the intermolecular motions of the complex.

Role of hydrogen species in palladium-catalyzed alkyne hydrogenation

Abstract: Selective alkyne hydrogenation in the presence of carbon−carbon double bond compounds, for which Pd is an excellent catalyst, is a strategically important large-scale industrial process. Although in palladium, functionality and structure are closely interrelated, knowledge of the structure of Pd is insufficient as the interaction with the chemical environment causes drastic compositional changes near the subsurface region: while unselective hydrogenation proceeds in the presence of a β-hydride phase, selective hydrogenation can be achieved only in the presence of a near-surface Pd−C phase. Here, we show from a combination of in situ prompt gamma activation analysis and ab initio simulations based on density functional theory that (i) the presence of the Pd−C phase created under selective conditions implies a strong change in the surface and subsurface stability of hydrogen, (ii) there is still a slower exchange of bulk and surface hydrogen, and (iii) the reaction rate for selective hydrogenation is indepe...

Infrared induced reactivity on the surface of isolated size-selected clusters: dissociation of N2O on rhodium clusters.

Abstract: Multiple photon infrared excitation of size-selected Rh6N2O+ clusters drives surface chemistry resulting in partially oxidized clusters.

CO adsorption on monometallic and bimetallic Au-Pd nanoparticles supported on oxide thin films

Abstract: Supported Au−Pd catalysts have been shown to exhibit superior catalytic performances when compared to their monometallic counterparts in a variety of reactions. In addition, the nature of the support often plays a critical role in reactivity. To gain a deeper understanding of the structure−reactivity relationship of the Au−Pd catalysts, here we have employed model systems where monometallic and bimetallic Au−Pd nanoparticles are deposited on well-ordered thin films of reducible and irreducible oxides (i.e., Fe3O4(111), MgO(100), and CeO2(111)). Surface structures of the model systems were characterized by temperature-programmed desorption, sum frequency generation, and infrared reflection absorption spectroscopy of CO as a probe molecule. In agreement with previous studies, the results show segregation of gold to the surface. Density functional theory calculations confirm that Au prefers to be at the edges of AuPd alloy particles under vacuum conditions. Strong similarities between the spectral features o...

Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

Abstract: The geometric structures of small cationic rhodium clusters Rhn+ (n = 6–12) are investigated by comparison of experimental far-infrared multiple photon dissociation spectra with spectra calculated using density functional theory. The clusters are found to favor structures based on octahedral and tetrahedral motifs for most of the sizes considered, in contrast to previous theoretical predictions that rhodium clusters should favor cubic motifs. Our findings highlight the need for further development of theoretical and computational methods to treat these high-spin transition metal clusters.

Amide-I and -II Vibrations of the Cyclic β-Sheet Model Peptide Gramicidin S in the Gas Phase

Abstract: In the condensed phase, the peptide gramicidin S is often considered as a model system for a β-sheet structure. Here, we investigate gramicidin S free of any influences of the environment by measuring the mid-IR spectra of doubly protonated (deuterated) gramicidin S in the gas phase. In the amide I (i.e., C═O stretch) region, the spectra show a broad split peak between 1580 and 1720 cm−1. To deduce structural information, the conformational space has been searched using molecular dynamics methods and several structural candidates have been further investigated at the density functional level. The calculations show the importance of the interactions of the charged side-chains with the backbone, which is responsible for the lower frequency part of the amide I peak. When this interaction is inhibited via complexation with two 18-crown-6 molecules, the amide I peak narrows and shows two maxima at 1653 and 1680 cm−1. A comparison to calculations shows that for this complexed ion, four C═O groups are in an anti...

State-to-state inelastic scattering of Stark-decelerated OH radicals with Ar atoms.

Abstract: The Stark deceleration method exploits the concepts of charged particle accelerator physics to produce molecular beams with a tunable velocity. These tamed molecular beams offer interesting perspectives for precise crossed beam scattering studies as a function of the collision energy. The method has advanced sufficiently to compete with state-of-the-art beam methods that are used for scattering studies throughout. This is demonstrated here for the scattering of OH radicals (X2Π3/2, J = 3/2, f) with Ar atoms, a benchmark system for the scattering of open-shell molecules with atoms. Parity-resolved integral state-to-state inelastic scattering cross sections are measured at collision energies between 80 and 800 cm−1. The threshold behavior and collision energy dependence of 13 inelastic scattering channels is accurately determined. Excellent agreement is obtained with the cross sections predicted by close-coupling scattering calculations based on the most accurate ab initio OH + Ar potential energy surfaces to date.

Temperature-Induced Modifications of PdZn Layers on Pd(111)

Abstract: Ultrathin PdZn surface alloys on Pd(111) are model systems well-suited for obtaining a microscopic understanding of the mechanisms of Pd/Zn-based catalysis for methanol steam reforming. The temperature-induced compositional and structural changes of these alloy films are investigated in the catalytically relevant temperature range. Heating of multilayer Zn films to 500 K results in the formation of multilayer PdZn alloy films with surface and near-surface composition close to 1:1. In the temperature regime above 550 K the subsurface layers deplete quickly in Zn due to diffusion of Zn atoms into the Pd bulk. In contrast, the composition of the surface layer changes only slightly, indicating formation of a PdZn film with strong monolayer character. This change in subsurface composition triggers a change of the original Zn-out/Pd-in surface corrugation, leading ultimately to a Pd-out/Zn-in situation for annealing temperatures beyond 700 K. The altered corrugation pattern is also obtained when submonolayer am...

Au NPs on anionic-exchange resin as catalyst for polyols oxidation in batch and fixed bed reactor

Abstract: Commercial weak basic anion resin (Dowex M-43) was used as support for gold nanoparticles (Au NPs). Au NPs stabilized by tetrakishydroxypropylphosphonium chloride (THPC) were immobilized on the support (AuTHPC/Dowex M-43). In alternative, Au NPs were generated by adsorbing NaAuCl4 on the resin and then reduced by NaBH4 (Auads/Dowex M-43). These catalysts were tested in the liquid phase oxidation of glycerol in a glass batch reactor: AuTHPC/Dowex M-43 showed better activity than Auads/Dowex M-43. The higher activity can be attributed to the smaller Au nanoparticles in AuTHPC/Dowex M-43 than Auads/Dowex M-43. Moreover AuTHPC/Dowex M-43 was tested in the glycerol oxidation in a fixed bed reactor showing good stability during the reaction time. All the catalysts have been characterised by TEM, FT-IR and XPS.

Azobenzene versus 3,3′,5,5′-tetra-tert-butyl-azobenzene (TBA) at Au(111): characterizing the role of spacer groups

Abstract: We present large-scale density-functional theory (DFT) calculations and temperature programmed desorption measurements to characterize the structural, energetic and vibrational properties of the functionalized molecular switch 3,3′,5,5′-tetra-tert-butyl-azobenzene (TBA) adsorbed at Au(111). Particular emphasis is placed on exploring the accuracy of the semi-empirical dispersion correction approach to semi-local DFT (DFT-D) in accounting for the substantial van der Waals component in the surface bonding. In line with previous findings for benzene and pure azobenzene at coinage metal surfaces, DFT-D significantly overbinds the molecule, but seems to yield an accurate adsorption geometry as far as can be judged from the experimental data. Comparing the trans adsorption geometry of TBA and azobenzene at Au(111) reveals a remarkable insensitivity of the structural and vibrational properties of the –NN– moiety. This questions the established view of the role of the bulky tert-butyl-spacer groups for the switching of TBA in terms of a mere geometric decoupling of the photochemically active diazo-bridge from the gold substrate.

Bulk and Surface Structure and High-Temperature Thermoelectric Properties of Inverse Clathrate-III in the Si-P-Te System

Abstract: The creation of thermoelectric materials for waste heat recovery and direct solar energy conversion is a challenge that forces the development of compounds that combine appreciable thermoelectric figure-of-merit with high thermal and chemical stability. Here we propose a new candidate for high-temperature thermoelectric materials, the type-III Si 172―x P x Te y cationic clathrate, in which the framework is composed of partially ordered silicon and phosphorus atoms, whereas tellurium atoms occupy guest positions. We show that the utmost stability of this clathrate (up to 1500 K) in air is ensured by the formation of a nanosized layer of phosphorus-doped silica on the surface, which prevents further oxidation and degradation. As-cast (non-optimized) Si-P-Te clathrates display rather high values of the thermoelectric figure-of-merit (ZT=0.24―0.36) in the temperature range of 700―1100 K. These ZT values are comparable to the best values achieved for the properly doped transition-metal-oxide materials. The methods of the thermoelectric efficiency optimization are discussed.

Observation of electronic energy bands in argon clusters

Abstract: The 3p valence region of argon clusters has been investigated with photoemission near the photoionization threshold. A strong feature between 14.6 and 15.3 eV binding energy shows a photon-energy dependence indicative of electronic-energy band dispersion. A similar feature at approximately the same binding energy and with a similar dispersion occurs in photoemission spectra of both the ordered and disordered solids. The effect is already fully developed for scaling-law mean cluster sizes of approximately 200 atoms, thus showing an early onset of bulklike electronic properties.

Synthesis of zeolite crystals with unusual morphology: Application in acid catalysis

Abstract: ZSM-5 zeolite nanocrystals having a rectangular morphology (French fries-like) and a size comprised between 50 and 100 nm were prepared using cheap sugar cane bagasse as a sacrificial template. Two sugar cane biomass types, un-treated and acid hydrolyzed bagasse, were used. A relationship has been established between the chemical composition of the sacrificial vegetal and the morphology of grown zeolite crystals. Aligned zeolite nanocrystals were allowed to grow on non hydrolyzed sugar cane substrate, using a combination of supramolecular templating and conventional self-assembly of template cations and silica species. The latter material allowed enhanced diffusional properties in comparison with purely microporous zeolite, as well as ZSM-5 crystals grown on hydrolyzed bagasse. Their catalytic behavior in n-hexane cracking reaction led to a higher selectivity toward light olefins. The double novelty consists in the manufacture of hierarchical zeolite microspheres (diameter about 1 μm) formed by the self-aggregation of rectangular subunits. In addition, a renewable vegetal source can be used as a supramolecular template in replacement of expensive polymers.