Showing papers by "Fritz Haber Institute of the Max Planck Society published in 2013"
TL;DR: There still is a gap between the model studies conducted with well-defined simplified materials with clean surfaces at low pressures to elaborate the current knowledge of ammonia synthesis and the industrial process, and the question of a dynamical change of the catalyst under true reaction conditions remains to be studied and calls for in situ experimentation.
Abstract: Ammonia synthesis is one of the largest processes in chemical industries. It was first operated at BASF one hundred years ago based on the fundamental work of Fritz Haber and process engineering by Carl Bosch. Haber combined feed gas recycling with application of high pressure (P = 200 bar) and a Ruthenium catalyst to achieve sufficiently high conversions of nitrogen according to N2 + 3 H2 .2 NH3. This success enabled the large scale production of artificial fertilizers, which was a prerequisite to face the world’s increase in population and is known as the “extraction of air from bread” – a term that was coined later by Max von Laue. Today, contrary to the generation of syngas for ammonia, only little has changed in the industrial process for the actual synthesis of ammonia.The process is operated at typical temperatures of 500 °C and pressures around 200 bar, resulting in ammonia concentrations in the exhaust gas of up to 17 vol.%. Approximately 80% of the worldwide ammonia output of 136 Mtons (2011) is used for the production of fertilizers. A key development for the modern Haber-Bosch process, however, has been the catalyst development at BASF that was led by Alwin Mittasch in the early 20 century. After testing 22 000 different formulations in a gigantic effort, the work was concluded in 1922 with the identification of a very unique catalyst synthesis. To achieve a highly active iron catalyst, magnetite, Fe3O4, was promoted by fusing it together with irreducible oxides (K2O, Al2O3, later also CaO) in an oxide melt at temperatures around 1000 °C. The fused magnetite is mechanically granulated and its reduction need to be conducted with great care in the syngas feed to finally give the active α-Fe catalyst. This special synthesis leads to certain crucial properties of the resulting α-Fe phase, which is commonly termed “ammonia iron”. In addition to its outstanding economic relevance, ammonia synthesis acts as a “drosophila reaction” for catalysis research and has always been a test case for the maturity of catalysis science in the context of a technologically mature application. Today, due to the enormous efforts in surface science, physical and theoretical chemistry, and chemical engineering a consistent picture of the reaction mechanism and the role of the Fe catalyst and its promoters has emerged. Key contributions to the modern understanding of the ammonia synthesis reactions came from the teams lead by Gerhard Ertl, Michel Boudart, Gabor Somorjai, Haldor Topsoe and Jens K. Norskov, just to mention a few. However, even after 100 years of application and research there still is scientific interest in the Haber-Bosch process, mainly because of two aspects. Firstly, catalysts with improved lowtemperature activity, higher specific surface area and higher tolerance against poisons and on-off operations are generally desirable. Also the development of a more elegant synthesis route for the Fe-based catalyst without the melting step and the extremely critical activation procedure could foster the potential application of ammonia as an energy storage molecule. Secondly, there still is a gap between the model studies conducted with well-defined simplified materials with clean surfaces at low pressures to elaborate the current knowledge of ammonia synthesis and the industrial process. These so-called pressure and materials gaps often prevent straightforward extrapolation of model studies to real industrial processes. Thus, the question of a dynamical change of the catalyst under true reaction conditions remains to be studied and calls for in situ experimentation. This point requires special attention in case of the ammonia synthesis over iron catalysts, because it is well known and has been studied for decades in the context of steel hardening and catalytic ammonia decomposition that iron can be easily nitrided by ammonia. Ertl and co-workers described the reaction mechanism of ammonia synthesis. 14] He and other authors showed that the reaction is structure sensitive. The dissociative chemisorption of di-nitrogen on the iron surface is the rate limiting step in ammonia synthesis and opens possibilities for sub-surface diffusion of the atomic nitrogen. Ertl et al. proposed the surface dissolution of nitrogen into iron forming a surface nitride of the approximate composition Fe2N and the presence of in-situ formed metastable γFe4N. [6a] Thus, for experimental conditions remote from the HaberBosch process, participation of stoichiometric bulk nitrides like FeN has been excluded. Instead, Herzog et al. proposed formation of [∗] Timur Kandemir, Dr. Manfred.E. Schuster, Dr. Malte Behrens, Prof. Dr. Robert Schlogl Department of Inorganic Chemistry Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4-6, D-14195 Berlin, Germany Fax: (+)49-(0)30-8413-4401 E-mail: behrens@fhi-berlin.mpg.de, acsek@fhi-berlin.mpg.de
428 citations
TL;DR: The results show that the approach employed here can reach the demanding accuracy of crystal-structure prediction and organic material design with minimal empiricism.
Abstract: The development and application of computational methods for studying molecular crystals, particularly density-functional theory (DFT), is a large and ever-growing field, driven by their numerous applications. Here we expand on our recent study of the importance of many-body van der Waals interactions in molecular crystals [A. M. Reilly and A. Tkatchenko, J. Phys. Chem. Lett. 4, 1028 (2013)], with a larger database of 23 molecular crystals. Particular attention has been paid to the role of the vibrational contributions that are required to compare experiment sublimation enthalpies with calculated lattice energies, employing both phonon calculations and experimental heat-capacity data to provide harmonic and anharmonic estimates of the vibrational contributions. Exact exchange, which is rarely considered in DFT studies of molecular crystals, is shown to have a significant contribution to lattice energies, systematically improving agreement between theory and experiment. When the vibrational and exact-exchange contributions are coupled with a many-body approach to dispersion, DFT yields a mean absolute error (3.92 kJ/mol) within the coveted "chemical accuracy" target (4.2 kJ/mol). The role of many-body dispersion for structures has also been investigated for a subset of the database, showing good performance compared to X-ray and neutron diffraction crystal structures. The results show that the approach employed here can reach the demanding accuracy of crystal-structure prediction and organic material design with minimal empiricism.
268 citations
TL;DR: An efficient microscopic method is utilized to demonstrate that van der Waals interactions in nanomaterials act at distances greater than typically assumed, and can be characterized by different scaling laws depending on the dimensionality and size of the system.
Abstract: Van der Waals interactions have a large influence on phenomena that occur at short-length scales. Gobre et al. demonstrate that van der Waals interactions in low-dimensional materials act at very large distances, and can significantly influence the self-assembly of nanostructured systems.
255 citations
TL;DR: Complementary in situ X-ray photoelectron spectroscopy, XPS, and environmental scanning electron microscopy are used to fingerprint the entire graphene chemical vapor deposition process on technologically important polycrystalline Cu catalysts to address the current lack of understanding of the underlying fundamental growth mechanisms and catalyst interactions.
Abstract: Complementary in situ X-ray photoelectron spectroscopy (XPS), X-ray diffractometry, and environmental scanning electron microscopy are used to fingerprint the entire graphene chemical vapor deposition process on technologically important polycrystalline Cu catalysts to address the current lack of understanding of the underlying fundamental growth mechanisms and catalyst interactions. Graphene forms directly on metallic Cu during the high-temperature hydrocarbon exposure, whereby an upshift in the binding energies of the corresponding C1s XPS core level signatures is indicative of coupling between the Cu catalyst and the growing graphene. Minor carbon uptake into Cu can under certain conditions manifest itself as carbon precipitation upon cooling. Postgrowth, ambient air exposure even at room temperature decouples the graphene from Cu by (reversible) oxygen intercalation. The importance of these dynamic interactions is discussed for graphene growth, processing, and device integration.
224 citations
TL;DR: The relatively high rate of reaction observed for the catalytic decomposition of NH3 is ascribed to highly energetic sites (twin boundaries, stacking faults, steps, and defects) which are observed in both the molybdenum carbide and nitride samples, resulting in a much higher H2 production rate in comparison with that for previously reported Mo-based catalysts.
Abstract: Constant CO(x)-free H2 production from the catalytic decomposition of ammonia could be achieved over a high-surface-area molybdenum carbide catalyst prepared by a temperature-programmed reduction-carburization method. The fresh and used catalyst was characterized by N2 adsorption/desorption, powder X-ray diffraction, scanning and transmission electron microscopy, and electron energy-loss spectroscopy at different stages. Observed deactivation (in the first 15 h) of the high-surface-area carbide during the reaction was ascribed to considerable reduction of the specific surface area due to nitridation of the carbide under the reaction conditions. Theoretical calculations confirm that the N atoms tend to occupy subsurface sites, leading to the formation of nitride under an NH3 atmosphere. The relatively high rate of reaction (30 mmol/((g of cat.) min)) observed for the catalytic decomposition of NH3 is ascribed to highly energetic sites (twin boundaries, stacking faults, steps, and defects) which are observed in both the molybdenum carbide and nitride samples. The prevalence of such sites in the as-synthesized material results in a much higher H2 production rate in comparison with that for previously reported Mo-based catalysts.
193 citations
Max Planck Society1, Stanford University2, Technical University of Berlin3, Pierre-and-Marie-Curie University4, Fritz Haber Institute of the Max Planck Society5, Aarhus University6, Saha Institute of Nuclear Physics7, University of Hamburg8, Hiroshima University9, Tohoku University10, German National Metrology Institute11, Kansas State University12
TL;DR: Signs of ultrafast charge redistribution from the inner-shell ionized Se atom to its molecular partners are found, and significant displacement of the atomic constituents in the course of multiple ionization is observed.
Abstract: Ionization and fragmentation of methylselenol (${\mathrm{CH}}_{3}\mathrm{SeH}$) molecules by intense ($g{10}^{17}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$) 5 fs x-ray pulses ($\ensuremath{\hbar}\ensuremath{\omega}=2\text{ }\text{ }\mathrm{keV}$) are studied by coincident ion momentum spectroscopy. We contrast the measured charge state distribution with data on atomic Kr, determine kinetic energies of resulting ionic fragments, and compare them to the outcome of a Coulomb explosion model. We find signatures of ultrafast charge redistribution from the inner-shell ionized Se atom to its molecular partners, and observe significant displacement of the atomic constituents in the course of multiple ionization.
175 citations
166 citations
TL;DR: Using the Perdew-Burke-Ernzerhof (PBE), PBE+vdW (van der Waals) and the recently developed PBE +vdW surf (density-functional theory with vdW interactions that include the collective electronic response of the substrate) methods, this article calculated the structures and energy properties for benzene on transition-metal surfaces: Cu, Ag, Au, Pd, Pt, Rh and Ir.
Abstract: The adsorption of benzene on metal surfaces is an important bench- mark system for hybrid inorganic/organic interfaces. The reliable determination of the interface geometry and binding energy presents a significant challenge for both theory and experiment. Using the Perdew-Burke-Ernzerhof (PBE), PBE+vdW (van der Waals) and the recently developed PBE+vdW surf (density- functional theory with vdW interactions that include the collective electronic response of the substrate) methods, we calculated the structures and energet- ics for benzene on transition-metal surfaces: Cu, Ag, Au, Pd, Pt, Rh and Ir. Our calculations demonstrate that vdW interactions increase the binding en- ergy by more than 0.70eV for physisorbed systems (Cu, Ag and Au) and by an even larger amount for strongly bound systems (Pd, Pt, Rh and Ir). The collective response of the substrate electrons captured via the vdW surf method plays a significant role for most substrates, shortening the equilibrium dis- tance by 0.25A for Cu and decreasing the binding energy by 0.27eV for Rh. The reliability of our results is assessed by comparison with calculations using the random-phase approximation including renormalized single excitations,
157 citations
TL;DR: It is found that the substitution of a single atom leads to strong variations of the adsorption height, as predicted by state-of-the-art density-functional theory, including van der Waals interactions with collective substrate response effects.
Abstract: We measured the adsorption geometry of single molecules with intramolecular resolution using noncontact atomic force microscopy with functionalized tips. The lateral adsorption position was determined with atomic resolution, adsorption height differences with a precision of 3 pm, and tilts of the molecular plane within 0.2 � . The method was applied to five � -conjugated molecules, including three molecules from the olympicene family, adsorbed on Cu(111). For the olympicenes, we found that the substitution of a single atom leads to strong variations of the adsorption height, as predicted by state-ofthe-art density-functional theory, including van der Waals interactions with collective substrate response effects.
152 citations
TL;DR: For the first time excellent agreement between theoretically predicted and experimentally measured vanadium L-edge NEXAFS spectra of V2O5 was achieved, indicating that the covalency of the metal-ligand bonds is correctly described by the calculations.
Abstract: A detailed study of the electronic and geometric structure of V2O5 and its X-ray spectroscopic properties is presented. Cluster models of increasing size were constructed in order to represent the surface and the bulk environment of V2O5. The models were terminated with hydrogen atoms at the edges or embedded in a Madelung field. The structure and interlayer binding energies were studied with dispersion-corrected local, hybrid and double hybrid density functional theory as well as the local pair natural orbital coupled cluster method (LPNO-CCSD). Convergence of the results with respect to cluster size was achieved by extending the model to up to 20 vanadium centers. The O K-edge and the V L2,3-edge NEXAFS spectra of V2O5 were calculated on the basis of the newly developed Restricted Open shell Configuration Interaction with Singles (DFT-ROCIS) method. In this study the applicability of the method is extended to the field of solid-state catalysis. For the first time excellent agreement between theoretically predicted and experimentally measured vanadium L-edge NEXAFS spectra of V2O5 was achieved. At the same time the agreement between experimental and theoretical oxygen K-edge spectra is also excellent. Importantly, the intensity distribution between the oxygen K-edge and vanadium L-edge spectra is correctly reproduced, thus indicating that the covalency of the metal–ligand bonds is correctly described by the calculations. The origin of the spectral features is discussed in terms of the electronic structure using both quasi-atomic jj coupling and molecular LS coupling schemes. The effects of the bulk environment driven by weak interlayer interactions were also studied, demonstrating that large clusters are important in order to correctly calculate core level absorption spectra in solids.
126 citations
TL;DR: When a paramagnetic molecule is placed on a superconducting surface the lifetime of its spin excitations increases dramatically as discussed by the authors, caused by the depletion of the electronic states within the energy gap at the Fermi level.
Abstract: When a paramagnetic molecule is placed on a superconducting surface the lifetime of its spin excitations increases dramatically. This effect, caused by the depletion of the electronic states within the energy gap at the Fermi level, could find application in coherent spin manipulation.
TL;DR: Higher reducibility at loadings close to the monolayer measured by temperature-programmed reduction and a steep increase in the catalytic activity observed in metathesis of propene and oxidative dehydrogenation of propane at 8 % of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions.
Abstract: Highly dispersed molybdenum oxide supported on mesoporous silica SBA-15 has been prepared by anion exchange resulting in a series of catalysts with changing Mo densities (0.2–2.5 Mo atoms nm−2). X-ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO4 units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temperature-programmed reduction and a steep increase in the catalytic activity observed in metathesis of propene and oxidative dehydrogenation of propane at 8 % of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calculations, NEXAFS spectra at the O-K-edge at high Mo loadings are explained by distorted MoO4 complexes. Limited availability of anchor silanol groups at high loadings forces the MoO4 groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.
TL;DR: In this article, a hybrid density functional theory (DFT) based approach is proposed to obtain a system-dependent fraction of exact exchange (α) within the framework of hybrid DFT, which is consistent with the G0W0 approach.
Abstract: We propose a scheme to obtain a system-dependent fraction of exact exchange (α) within the framework of hybrid density functional theory (DFT) that is consistent with the G0W0 approach, where G0 is the noninteracting Green function of the system and W0 the screened Coulomb interaction. We exploit the formally exact condition of exact DFT that the energy of the highest occupied molecular orbital corresponds to the ionization potential of a finite system. We identify the optimal α value for which this statement is obeyed as closely as possible and thereby remove the starting point dependence from the G0W0 method. This combined approach is essential for describing electron transfer (as exemplified by the TTF/TCNQ dimer) and yields the vertical ionization potentials of the G2 benchmark set with a mean absolute percentage error of only ≈3%.
TL;DR: The direct observation of intramolecular hydrogen atom transfer reactions (tautomerization) within a single porphycene molecule on a Cu(110) surface by scanning tunneling microscopy is reported and it is found that the tautomersization can be induced via inelastic electron tunneling at 5 K.
Abstract: We report the direct observation of intramolecular hydrogen atom transfer reactions (tautomerization) within a single porphycene molecule on a Cu(110) surface by scanning tunneling microscopy. It is found that the tautomerization can be induced via inelastic electron tunneling at 5 K. By measuring the bias-dependent tautomerization rate of isotope-substituted molecules, we can assign the scanning tunneling microscopy-induced tautomerization to the excitation of specific molecular vibrations. Furthermore, these vibrations appear as characteristic features in the dI/dV spectra measured over individual molecules. The vibrational modes that are associated with the tautomerization are identified by density functional theory calculations. At higher temperatures above ∼75 K, tautomerization is induced thermally and an activation barrier of about 168 meV is determined from an Arrhenius plot.
TL;DR: In this paper, the authors propose an effective range-separation of the coupling between the atomic response functions that extends the already broad applicability of the MBD method to non-metallic materials with highly anisotropic responses, such as layered nanostructures.
Abstract: An accurate determination of the electron correlation energy is essential for describing the structure, stability, and function in a wide variety of systems, ranging from gas-phase molecular assemblies to condensed matter and organic/inorganic interfaces. Even small errors in the correlation energy can have a large impact on the description of chemical and physical properties in the systems of interest. In this context, the development of efficient approaches for the accurate calculation of the long-range correlation energy (and hence dispersion) is the main challenge. In the last years a number of methods have been developed to augment density functional approximations via dispersion energy corrections, but most of these approaches ignore the intrinsic many-body nature of correlation effects, leading to inconsistent and sometimes even qualitatively incorrect predictions. Here we build upon the recent many-body dispersion (MBD) framework, which is intimately linked to the random-phase approximation for the correlation energy. We separate the correlation energy into short-range contributions that are modeled by semi-local functionals and long-range contributions that are calculated by mapping the complex all-electron problem onto a set of atomic response functions coupled in the dipole approximation. We propose an effective range-separation of the coupling between the atomic response functions that extends the already broad applicability of the MBD method to non-metallic materials with highly anisotropic responses, such as layered nanostructures. Application to a variety of high-quality benchmark datasets illustrates the accuracy and applicability of the improved MBD approach, which offers the prospect of first-principles modeling of large structurally complex systems with an accurate description of the long-range correlation energy.
TL;DR: By applying a genetic algorithm and ab initio atomistic thermodynamics, the stable and metastable compositions and structures of MgMOx clusters at realistic temperatures and oxygen pressures are identified and density-functional theory with a hybrid exchange-correlation functional is needed for predicting accurate phase diagrams of metal-oxide clusters.
Abstract: By applying a genetic algorithm and ab initio atomistic thermodynamics, we identify the stable and metastable compositions and structures of ${\mathrm{Mg}}_{M}{\mathrm{O}}_{x}$ clusters at realistic temperatures and oxygen pressures. We find that small clusters ($M\ensuremath{\lesssim}5$) are in thermodynamic equilibrium when $xgM$. The nonstoichiometric clusters exhibit peculiar magnetic behavior, suggesting the possibility of tuning magnetic properties by changing environmental pressure and temperature conditions. Furthermore, we show that density-functional theory with a hybrid exchange-correlation functional is needed for predicting accurate phase diagrams of metal-oxide clusters. Neither a (sophisticated) force field nor density-functional theory with (semi)local exchange-correlation functionals is sufficient for even a qualitative prediction.
TL;DR: First-principles calculations are employed to systematically explore the interaction mechanism for benzene derivatives on metal surfaces, enabling us to selectively tune the stability and the barrier between two metastable adsorption states for tetrachloropyrazine molecule.
Abstract: Transient precursor states are often experimentally observed for molecules adsorbing on surfaces. However, such precursor states are typically rather short-lived, quickly yielding to more stable adsorption configurations. Here we employ first-principles calculations to systematically explore the interaction mechanism for benzene derivatives on metal surfaces, enabling us to selectively tune the stability and the barrier between two metastable adsorption states. In particular, in the case of the tetrachloropyrazine molecule, two equally stable adsorption states are identified with a moderate and conceivably reversible barrier between them. We address the feasibility of experimentally detecting the predicted bistable behaviour and discuss its potential usefulness in a molecular switch.
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TL;DR: In this article, the authors investigated the effect of doping on formation energy and concentration of oxygen vacancies at a metal oxide surface, using MgO (100) as an example and employed density-functional theory, where the performance of the exchange-correlation functional is carefully analyzed, and the functional is chosen according to a fundamental condition on DFT ionization energies.
Abstract: We investigate effects of doping on formation energy and concentration of oxygen vacancies at a metal oxide surface, using MgO (100) as an example. Our approach employs density-functional theory, where the performance of the exchange-correlation functional is carefully analyzed, and the functional is chosen according to a fundamental condition on DFT ionization energies. The approach is further validated by CCSD(T) calculations for embedded clusters. We demonstrate that the concentration of oxygen vacancies at a doped oxide surface is largely determined by formation of a macroscopically extended space charge region.
TL;DR: The complexes of a DNA base bound to graphitic systems are studied and the optB86b nonlocal functional and the Tkatchenko-Scheffler functional are used to study the binding energies of nucleobases on graphene.
Abstract: The complexes of a DNA base bound to graphitic systems are studied. Considering naphthalene as the simplest graphitic system, DNA base−naphthalene complexes are scrutinized at high levels of ab initio theory including coupled cluster theory with singles, doubles, and perturbative triples excitations (CCSD(T)) at the complete basis set (CBS) limit. The stacked configurations are the most stable, where the CCSD(T)/CBS binding energies of guanine, adenine, thymine, and cytosine are 9.31, 8.48, 8.53, 7.30 kcal/mol, respectively. The energy components are investigated using symmetry-adapted perturbation theory based on density functional theory including the dispersion energy. We compared the CCSD(T)/CBS results with several density functional methods applicable to periodic systems. Considering accuracy and availability, the optB86b nonlocal functional and the Tkatchenko−Scheffler functional are used to study the binding energies of nucleobases on graphene. The predicted values are 18−24 kcal/mol, though many-body effects on screening and energy need to be further considered.
TL;DR: In this article, the role of surface orientation on reactivity was investigated with two different orientations of the CeO2 films, i.e. CeO 2(100) and Ce2(111) surfaces.
Abstract: Cerium oxide is a principal component in many heterogeneous catalytic processes. One of its key characteristics is the ability to provide or remove oxygen in chemical reactions. The different crystallographic faces of ceria present significantly different surface structures and compositions that may alter the catalytic reactivity. The structure and composition determine the number of coordination vacancies surrounding surface atoms, the availability of adsorption sites, the spacing between adsorption sites and the ability to remove O from the surface. To investigate the role of surface orientation on reactivity, CeO2 films were grown with two different orientations. CeO2(100) films were grown ex situ by pulsed laser deposition on Nb-doped SrTiO3(100). CeO2(111) films were grown in situ by thermal deposition of Ce metal onto Ru(0001) in an oxygen atmosphere. The chemical reactivity was characterized by the adsorption and decomposition of various molecules such as alcohols, aldehydes and organic acids. In general the CeO2(100) surface was found to be more active, i.e. molecules adsorbed more readily and reacted to form new products, especially on a fully oxidized substrate. However the CeO2(100) surface was less selective with a greater propensity to produce CO, CO2 and water as products. The differences in chemical reactivity are discussed in light of possible structural terminations of the two surfaces. Recently nanocubes and nano-octahedra have been synthesized that display CeO2(100) and CeO2(111) faces, respectively. These nanoparticles enable us to correlate reactions on high surface area model catalysts at atmospheric pressure with model single crystal films in a UHV environment.
TL;DR: The first direct calorimetric measurement of oxygen binding energies on Pd nanoparticles investigated as a function of particle size and with the reference to a Pd(111) single crystal is reported on.
Abstract: How strongly does a molecule or an atom bind to a metal nanoparticle and how does this binding energy change with changing particle size? These questions are at the heart of many fundamental and practical problems, ranging from heterogeneous catalysis to important applied processes connected to materials science. In particular the interaction of oxygen with transition-metal nanoparticles is of pivotal importance for a variety of industrially and environmentally relevant processes such as CO oxidation in exhaust catalytic converters and methane combustion. Understanding the effect of a nanometer-scale confinement of matter on the binding strength of gaseous adsorbates is a current scientific challenge targeting the rational design of new catalytic and functional materials. Studies in this area provide a basis for the fundamental understanding of how the surface binds reactants and guides them through various elementary steps of a reaction to the products. The interaction of oxygen with palladium surfaces has been the subject of numerous studies, performed both on single-crystal surfaces and well-defined model systems consisting of Pd nanoclusters supported on thin oxide films. Presently, a very detailed microscopic-level understanding the interaction of oxygen with palladium is available, which proves to be a complex interplay between chemisorption, diffusion of oxygen into the subsurface region and bulk, 5, 10] formation of surface oxide layers, refaceting, particle reconstruction, and bulk oxide formation. The processes related to subsurface diffusion, refaceting, reconstruction, and oxidation are typically observed beyond a critical coverage of surface-adsorbed oxygen and temperatures above 300 K. Despite this comprehensive understanding and general agreement on the surface chemistry of the oxygen–palladium system, quantitative information on binding energies of oxygen on Pd nanoparticles is still missing, which is precisely because of the richness of the surface chemistry. When the binding strength is probed by a traditional desorption-based method, such as temperature-programed desorption (TPD), the O–Pd system must be heated to about 900–1000 K to desorb chemisorbed oxygen; this is often accompanied by subsurface O diffusion, surface oxide formation, and particle restructuring. These side processes together with the restrictions imposed by the kinetic modeling of the TPD spectra strongly limit the quantitative determination of binding energies of oxygen on Pd nanoparticles by traditional desorption-based methods, which results in a strong scatter of data available in literature. A strategy to overcome those shortcomings is a direct calorimetric measurement of adsorption enthalpies under isothermal conditions. At present, such fundamental information on the correlation between oxygen binding energies and the exact nature of the adsorption site as well as the size of the metal nanoparticles is not available. Herein we report on the first direct calorimetric measurement of oxygen binding energies on Pd nanoparticles investigated as a function of particle size and with the reference to a Pd(111) single crystal. The binding energies were obtained on well-defined Pd nanoparticles supported on thin oxide films prepared under ultra-high-vacuum (UHV) conditions. We apply a newly developed UHV single-crystal adsorption calorimeter (SCAC) based on molecular beam techniques in combination with infrared reflection adsorption spectroscopy (IRAS) to investigate the effect that the reduced dimensionality of metallic particles has on the interaction strength with oxygen. Complementary TPD experiments were performed to provide a link between the direct isothermal calorimetric studies and the traditional desorption-based approach. We show that there are two major structural factors determining the oxygen binding energy on Pd: the local configuration of the adsorption site, and the particle size. We provide direct experimental evidence that the change of the local adsorption environment from a multifold-bound position on the extended singlecrystal surface to an edge site of Pd nanoparticles results in a strong increase of the oxygen binding energy. On the other hand, if the local environment of the adsorbate is kept [*] Dipl.-Chem. M. Peter, Dr. J. M. Flores Camacho, Dr. S. Adamovski, Dipl.-Chem. K.-H. Dostert, Dr. C. P. O’Brien, Dr. S. Schauermann, Prof. Dr. H.-J. Freund Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4–6, 14195 Berlin (Germany) E-mail: schauermann@fhi-berlin.mpg.de
TL;DR: In this article, the interaction of probe molecules with bridging hydroxyls was studied on a novel two-dimensional zeolite model system consisting of an aluminosilicate forming a planar sheet of polygonal prisms, supported on a Ru(0001) surface.
Abstract: Bridging hydroxyls (Si–OH–Al) in zeolites are catalytically active for a multitude of important reactions, including the catalytic cracking of crude oil, oligomerization of olefins, conversion of methanol to hydrocarbons, and the selective catalytic reduction of NOx. The interaction of probe molecules with bridging hydroxyls was studied here on a novel two-dimensional zeolite model system consisting of an aluminosilicate forming a planar sheet of polygonal prisms, supported on a Ru(0001) surface. These bridging hydroxyls are strong Bronsted acid sites and can interact with both weak and strong bases. This interaction is studied here for two weak bases (CO and C2H4) and two strong bases (NH3 and pyridine), by infrared reflection absorption spectroscopy, in comparison with density functional theory calculations. Additionally, ethene is the reactant in the simplest case of the olefin oligomerization reaction which is also catalyzed by bridging hydroxyls, making the study of this adsorbed precursor state part...
TL;DR: The package FHI-gap is presented, an all-electron G W implementation based on the full-potential linearized augmented planewave plus local orbital (LAPW) method, which can handle core, semicore, and valence states on the same footing, which allows for a correct treatment of core–valence interaction.
TL;DR: The heteronuclear oxo-cluster [VPO4](•+) is generated via electrospray ionization and investigated with respect to both its electronic structure as well as its gas-phase reactivity toward small hydrocarbons, thus permitting a comparison to the well-known vanadium-oxide cation [V2O4?]
Abstract: The heteronuclear oxo-cluster [VPO4](•+) is generated via electrospray ionization and investigated with respect to both its electronic structure as well as its gas-phase reactivity toward small hydrocarbons, thus permitting a comparison to the well-known vanadium-oxide cation [V2O4](•+). As described in previous studies, the latter oxide exhibits no or just minor reactivity toward small hydrocarbons, such as CH4, C2H6, C3H8, n-C4H10, and C2H4, while substitution of one vanadium by a phosphorus atom yields the reactive [VPO4](•+) ion; the latter brings about oxidative dehydrogenation (ODH) of saturated hydrocarbons, e.g., propane and butane as well as oxygen-atom transfer (OAT) to unsaturated hydrocarbons, e.g. ethene, at thermal conditions. Further, the gas-phase structure of [VPO4](•+) is determined by IR photodissociation spectroscopy and compared to that of [V2O4](•+). DFT calculations help to elucidate the reaction mechanism. The results underline the crucial role of phosphorus in terms of C-H bond activation of hydrocarbons by mixed VPO clusters.
TL;DR: In this paper, the adsorption structure of the molecular switch azobenzene on Ag(111) was investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory.
Abstract: The adsorption structure of the molecular switch azobenzene on Ag(111) is investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory. The inclusion of nonlocal collective substrate response (screening) in the dispersion correction improves the description of dense monolayers of azobenzene, which exhibit a substantial torsion of the molecule. Nevertheless, for a quantitative agreement with experiment explicit consideration of the effect of vibrational mode anharmonicity on the adsorption geometry is crucial.
TL;DR: It is found that all of these quantities change dramatically with distance from the interface, which opens the way to analogous studies on other heterostructures, which are traditionally a forbidden field for ARPES.
Abstract: We report the momentum-resolved measurement of a two-dimensional electron gas at the LaTiO3=SrTiO3 interface by angle-resolved photoemission spectroscopy (ARPES). Thanks to an advanced sample preparation technique, the orbital character of the conduction electrons and the electronic correlations can be accessed quantitatively as each unit cell layer is added. We find that all of these quantities change dramatically with distance from the interface. These findings open the way to analogous studies on other heterostructures, which are traditionally a forbidden field for ARPES.
TL;DR: In this article, it was shown that collective response effects play a substantial role not only for optical excitations, but also for cohesive properties of noncovalently bound molecular crystals.
Abstract: We show that electrodynamic dipolar interactions, responsible for long-range fluctuations in matter, play a significant role in the stability of molecular crystals. Density functional theory calculations with van der Waals interactions determined from a semilocal “atom-in-a-molecule” model result in a large overestimation of the dielectric constants and sublimation enthalpies for polyacene crystals from naphthalene to pentacene, whereas an accurate treatment of nonlocal electrodynamic response leads to an agreement with the measured values for both quantities. Our findings suggest that collective response effects play a substantial role not only for optical excitations, but also for cohesive properties of noncovalently bound molecular crystals.
TL;DR: Light is shed on the nature of the underlying interactions, especially regarding effects on the polyamide backbone structure, by comparing the conformational ensembles of model peptides in isolation and in the presence of either Li(+) or Na(+) by using state-of-the-art density-functional theory and gas-phase infrared spectroscopy.
Abstract: Specific interactions between cations and proteins have a strong impact on peptide and protein structure. Herein, we shed light on the nature of the underlying interactions, especially regarding effects on the polyamide backbone structure. This was done by comparing the conformational ensembles of model peptides in isolation and in the presence of either Li(+) or Na(+) by using state-of-the-art density-functional theory (including van der Waals effects) and gas-phase infrared spectroscopy. These monovalent cations have a drastic effect on the local backbone conformation of turn-forming peptides, by disruption of the hydrogen-bonding networks, thus resulting in severe distortion of the backbone conformations. In fact, Li(+) and Na(+) can even have different conformational effects on the same peptide. We also assess the predictive power of current approximate density functionals for peptide-cation systems and compare to results with those of established protein force fields as well as high-level quantum chemistry calculations (CCSD(T)).
TL;DR: Evidence is obtained that the dynamic formation of surface carbon species under a reducing atmosphere strongly shifts the product spectrum to the C3-alkene at the expense of hydrogenolysis products, and a structure-activity correlation as a map for catalyst optimization is proposed.
Abstract: The effect of the gas-phase chemical potential on surface chemistry and reactivity of molybdenum carbide has been investigated in catalytic reactions of propane in oxidizing and reducing reactant mixtures by adding H2, O2, H2O, and CO2 to a C3H8/N2 feed. The balance between surface oxidation state, phase stability, carbon deposition, and the complex reaction network involving dehydrogenation reactions, hydrogenolysis, metathesis, water-gas shift reaction, hydrogenation, and steam reforming is discussed. Raman spectroscopy and a surface-sensitive study by means of in situ X-ray photoelectron spectroscopy evidence that the dynamic formation of surface carbon species under a reducing atmosphere strongly shifts the product spectrum to the C3-alkene at the expense of hydrogenolysis products. A similar response of selectivity, which is accompanied by a boost of activity, is observed by tuning the oxidation state of Mo in the presence of mild oxidants, such as H2O and CO2, in the feed as well as by V doping. The results obtained allow us to draw a picture of the active catalyst surface and to propose a structure–activity correlation as a map for catalyst optimization.