Showing papers by "Fritz Haber Institute of the Max Planck Society published in 2015"
TL;DR: In this article, the authors define requirements for a suitable descriptor and demonstrate how a meaningful descriptor can be found systematically, for a classic example, the energy difference of zinc blende or wurtzite and rocksalt semiconductors.
Abstract: Statistical learning of materials properties or functions so far starts with a largely silent, nonchallenged step: the choice of the set of descriptive parameters (termed descriptor). However, when the scientific connection between the descriptor and the actuating mechanisms is unclear, the causality of the learned descriptor-property relation is uncertain. Thus, a trustful prediction of new promising materials, identification of anomalies, and scientific advancement are doubtful. We analyze this issue and define requirements for a suitable descriptor. For a classic example, the energy difference of zinc blende or wurtzite and rocksalt semiconductors, we demonstrate how a meaningful descriptor can be found systematically.
641 citations
TL;DR: In this article, the authors show how the presence or absence of the Zn promoter dramatically changes not only the activity, but also unexpectedly the reaction mechanism itself, and propose two different sites for methanol synthesis, Zn-promoted and unpromoted.
Abstract: Methanol, an important chemical, fuel additive, and precursor for clean fuels, is produced by hydrogenation of carbon oxides over Cu-based catalysts. Despite the technological maturity of this process, the understanding of this apparently simple reaction is still incomplete with regard to the reaction mechanism and the active sites. Regarding the latter, recent progress has shown that stepped and ZnOx-decorated Cu surfaces are crucial for the performance of industrial catalysts. Herein, we integrate this insight with additional experiments into a full microkinetic description of methanol synthesis. In particular, we show how the presence or absence of the Zn promoter dramatically changes not only the activity, but unexpectedly the reaction mechanism itself. The Janus-faced character of Cu with two different sites for methanol synthesis, Zn-promoted and unpromoted, resolves the long-standing controversy regarding the Cu/Zn synergy and adds methanol synthesis to the few major industrial catalytic processes that are described on an atomic level.
388 citations
TL;DR: For the first time, clear evidence for the formation of metastable "graphite-like" ZnO layers during reductive activation is provided and might contribute to the understanding of synergistic effects between the components of the Cu/ZnO/Al2 O3 catalysts.
Abstract: In industrially relevant Cu/ZnO/Al2 O3 catalysts for methanol synthesis, the strong metal support interaction between Cu and ZnO is known to play a key role. Here we report a detailed chemical transmission electron microscopy study on the nanostructural consequences of the strong metal support interaction in an activated high-performance catalyst. For the first time, clear evidence for the formation of metastable "graphite-like" ZnO layers during reductive activation is provided. The description of this metastable layer might contribute to the understanding of synergistic effects between the components of the Cu/ZnO/Al2 O3 catalysts.
383 citations
TL;DR: Graphene growth inside the chamber of a modified environmental scanning electron microscope under relevant low-pressure CVD conditions allows visualizing structural dynamics of the active catalyst simultaneously with graphene nucleation and growth in an unparalleled way.
Abstract: This work highlights the importance of in situ experiments for an improved understanding of graphene growth on copper via metal-catalyzed chemical vapor deposition (CVD). Graphene growth inside the chamber of a modified environmental scanning electron microscope under relevant low-pressure CVD conditions allows visualizing structural dynamics of the active catalyst simultaneously with graphene nucleation and growth in an unparalleled way. It enables the observation of a complete CVD process from substrate annealing through graphene nucleation and growth and, finally, substrate cooling in real time and nanometer-scale resolution without the need of sample transfer. A strong dependence of surface dynamics such as sublimation and surface premelting on grain orientation is demonstrated, and the influence of substrate dynamics on graphene nucleation and growth is presented. Insights on the growth mechanism are provided by a simultaneous observation of the growth front propagation and nucleation rate. Furthermo...
189 citations
TL;DR: In this perspective, recent advances in the understanding of collective and many-body van der Waals interactions and their role and impact for molecular materials are discussed.
Abstract: van der Waals (vdW) dispersion interactions are a key ingredient in the structure, stability, and response properties of many molecular materials and essential for us to be able to understand and design novel intricate molecular systems. Pairwise-additive models of vdW interactions are ubiquitous, but neglect their true quantum-mechanical many-body nature. In this perspective we focus on recent developments and applications of methods that can capture collective and many-body effects in vdW interactions. Highlighting a number of recent studies in this area, we demonstrate both the need for and usefulness of explicit many-body treatments for obtaining qualitative and quantitative accuracy for modelling molecular materials, with applications presented for small-molecule dimers, supramolecular host–guest complexes, and finally stability and polymorphism in molecular crystals.
177 citations
TL;DR: In this article, a femtosecond x-ray laser pulses are used to probe the carbon monoxide (CO) oxidation reaction on ruthenium (Ru) initiated by an optical laser pulse.
Abstract: Femtosecond x-ray laser pulses are used to probe the carbon monoxide (CO) oxidation reaction on ruthenium (Ru) initiated by an optical laser pulse. On a time scale of a few hundred femtoseconds, the optical laser pulse excites motions of CO and oxygen (O) on the surface, allowing the reactants to collide, and, with a transient close to a picosecond (ps), new electronic states appear in the OK-edge x-ray absorption spectrum. Density functional theory calculations indicate that these result from changes in the adsorption site and bond formation between CO and O with a distribution of OC-O bond lengths close to the transition state (TS). After 1 ps, 10% of the CO populate the TS region, which is consistent with predictions based on a quantum oscillator model.
176 citations
TL;DR: It is shown that attosecond metrology (1 as = 10−18 seconds) now enables quantitative insight into weakly disturbed electron wave packet propagation on the atomic length scale without being hampered by scattering effects, which inevitably occur over macroscopic propagation length scales.
Abstract: Attosecond light pulses are now available experimentally, enabling ultrafast processes on the atomic scale to be probed; here the free-electron-like propagation of electrons through ultrathin layers of magnesium is observed in real time. The recent availability of attosecond light pulses means that it is now possible to observe ultrafast processes at an atomic scale. So far, such measurements have been carried out in gases, but now Reinhard Kienberger and colleagues use attosecond pulses to probe a fundamental process in the solid state, namely the transport of electrons through a crystal. They use attosecond pulses to launch photoelectron wavepackets inside a tungsten crystal that is covered by a controllable number of magnesium layers. Measuring the time of arrival of the wavepackets at the surface as a function of the number of layers reveals free-electron-like propagation of electrons inside the magnesium layers. The study demonstrates that real-time access to atomic-scale electron transport on the surface is possible. The propagation and transport of electrons in crystals is a fundamental process pertaining to the functioning of most electronic devices. Microscopic theories describe this phenomenon as being based on the motion of Bloch wave packets1. These wave packets are superpositions of individual Bloch states with the group velocity determined by the dispersion of the electronic band structure near the central wavevector in momentum space1. This concept has been verified experimentally in artificial superlattices by the observation of Bloch oscillations2—periodic oscillations of electrons in real and momentum space. Here we present a direct observation of electron wave packet motion in a real-space and real-time experiment, on length and time scales shorter than the Bloch oscillation amplitude and period. We show that attosecond metrology3 (1 as = 10−18 seconds) now enables quantitative insight into weakly disturbed electron wave packet propagation on the atomic length scale without being hampered by scattering effects, which inevitably occur over macroscopic propagation length scales. We use sub-femtosecond (less than 10−15 seconds) extreme-ultraviolet light pulses3 to launch photoelectron wave packets inside a tungsten crystal that is covered by magnesium films of varied, well-defined thicknesses of a few angstroms4. Probing the moment of arrival of the wave packets at the surface with attosecond precision reveals free-electron-like, ballistic propagation behaviour inside the magnesium adlayer—constituting the semi-classical limit of Bloch wave packet motion. Real-time access to electron transport through atomic layers and interfaces promises unprecedented insight into phenomena that may enable the scaling of electronic and photonic circuits to atomic dimensions. In addition, this experiment allows us to determine the penetration depth of electrical fields at optical frequencies at solid interfaces on the atomic scale.
157 citations
TL;DR: It is found that cobalt is anchored on graphene via carbonyl-like species, namely Co(CO)x , promoting the reduction of Co(3+) to Co(2+), which is believed to be the active site of the catalyst.
Abstract: Electrochemically grown cobalt on graphene exhibits exceptional performance as a catalyst for the oxygen evolution reaction (OER) and provides the possibility of controlling the morphology and the chemical properties during deposition. However, the detailed atomic structure of this hybrid material is not well understood. To elucidate the Co/graphene electronic structure, we have developed a flow cell closed by a graphene membrane that provides electronic and chemical information on the active surfaces under atmospheric pressure and in the presence of liquids by means of X-ray photoelectron spectroscopy (XPS). We found that cobalt is anchored on graphene via carbonyl-like species, namely Co(CO)x , promoting the reduction of Co(3+) to Co(2+), which is believed to be the active site of the catalyst.
126 citations
TL;DR: A framework to evaluate the Hartree–Fock exchange operator for periodic electronic-structure calculations based on general, localized atom-centered basis functions and linear scaling of the implementation with system size is demonstrated.
124 citations
TL;DR: Sliding pathways are identified which rationalize the observed reduced friction for thicker multilayers and provide quantitative explanation for the anisotropy of the friction force.
Abstract: The interlayer sliding potential of multilayered hexagonal boron nitride (h-BN) and graphene is investigated using density-functional theory including many-body van der Waals (vdW) interactions. We find that interlayer sliding constraints can be employed to tune the contribution of electrostatic interactions and dispersive forces to the sliding energy profile, ultimately leading to different sliding pathways in these two materials. In this context, vdW interactions are found to contribute more to the interlayer sliding potential of polar h-BN than they do in nonpolar graphene. In particular, the binding energy, the interlayer distance, and the friction force are found to depend sensitively on the number of layers. By comparing with the experimental findings, we identify sliding pathways which rationalize the observed reduced friction for thicker multilayers and provide quantitative explanation for the anisotropy of the friction force.
120 citations
TL;DR: A fully self-consistent implementation of the density-dependent interatomic vdW functional of Tkatchenko and Scheffler is derived, revealing a nontrivial connection between electrostatics and long-range electron correlation effects.
Abstract: How strong is the effect of van der Waals (vdW) interactions on the electronic properties of molecules and extended systems? To answer this question, we derived a fully self-consistent implementation of the density-dependent interatomic vdW functional of Tkatchenko and Scheffler [Phys. Rev. Lett. 102, 073005 (2009)]. Not surprisingly, vdW self-consistency leads to tiny modifications of the structure, stability, and electronic properties of molecular dimers and crystals. However, unexpectedly large effects were found in the binding energies, distances, and electrostatic moments of highly polarizable alkali-metal dimers. Most importantly, vdW interactions induced complex and sizable electronic charge redistribution in the vicinity of metallic surfaces and at organic-metal interfaces. As a result, a substantial influence on the computed work functions was found, revealing a nontrivial connection between electrostatics and long-range electron correlation effects.
TL;DR: This work indicates that strong metal-support interactions can be achieved on a non-reducible support, as exemplified for nanocarbon, by carefully tuning the surface structure of the support, thus providing a good example for designing a high-performance nanostructured catalyst.
Abstract: Nanodiamond-graphene core-shell materials have several unique properties compared with purely sp(2) -bonded nanocarbons and perform remarkably well as metal-free catalysts. In this work, we report that palladium nanoparticles supported on nanodiamond-graphene core-shell materials (Pd/ND@G) exhibit superior catalytic activity in CO oxidation compared to Pd NPs supported on an sp(2) -bonded onion-like carbon (Pd/OLC) material. Characterization revealed that the Pd NPs in Pd/ND@G have a special morphology with reduced crystallinity and are more stable towards sintering at high temperature than the Pd NPs in Pd/OLC. The electronic structure of Pd is changed in Pd/ND@G, resulting in weak CO chemisorption on the Pd NPs. Our work indicates that strong metal-support interactions can be achieved on a non-reducible support, as exemplified for nanocarbon, by carefully tuning the surface structure of the support, thus providing a good example for designing a high-performance nanostructured catalyst.
TL;DR: A model system has been created to shuttle electrons through a metal-insulator-metal (MIM) structure to induce the formation of a CO2 anion radical from adsorbed gas-phase carbon dioxide that subsequently reacts to form an oxalate species.
Abstract: A model system has been created to shuttle electrons through a metal-insulator-metal (MIM) structure to induce the formation of a CO2 anion radical from adsorbed gas-phase carbon dioxide that subsequently reacts to form an oxalate species. The process is completely reversible, and thus allows the elementary steps involved to be studied at the atomic level. The oxalate species at the MIM interface have been identified locally by scanning tunneling microscopy, chemically by IR spectroscopy, and their formation verified by density functional calculations.
TL;DR: Intense terahertz electromagnetic pulses are applied to window and substrate materials commonly used in THz spectroscopy and induced optical birefringence is observed in all samples, except in quartz and Si, where, respectively, a linear electrooptic signal and a response beyond the perturbative regime are found.
Abstract: We apply intense terahertz (THz) electromagnetic pulses with field strengths exceeding 2 MV cm−1 at ~1 THz to window and substrate materials commonly used in THz spectroscopy and determine the induced optical birefringence. Materials studied are diamond, sapphire, magnesium oxide (MgO), polymethylpentene (TPX), low-density polyethylene (LDPE), silicon nitride membrane (SiN) and crystalline quartz. We observe a Kerr-effect-type transient birefringence in all samples, except in quartz and Si, where, respectively, a linear electrooptic signal and a response beyond the perturbative regime are found. We extract the nonlinear refractive indices and the electrooptic coefficient (in the case of quartz) of these materials and discuss implications for their use as windows or substrates in THz pump-optical probe spectroscopy.
Journal Article•
TL;DR: In this paper, the Coulomb potential is treated either in reciprocal space or in real space, where the sparsity of the density matrix can be exploited for computational efficiency, and linear scaling of the implementation with system size is demonstrated by calculating the electronic structure of a bulk semiconductor with up to 1,024 atoms per unit cell without compromising the accuracy.
Abstract: We describe a framework to evaluate the Hartree–Fock exchange operator for periodic electronic-structure calculations based on general, localized atom-centered basis functions. The functionality is demonstrated by hybrid-functional calculations of properties for several semiconductors. In our implementation of the Fock operator, the Coulomb potential is treated either in reciprocal space or in real space, where the sparsity of the density matrix can be exploited for computational efficiency. Computational aspects, such as the rigorous avoidance of on-the-fly disk storage, and a load-balanced parallel implementation, are also discussed. We demonstrate linear scaling of our implementation with system size by calculating the electronic structure of a bulk semiconductor (GaAs) with up to 1,024 atoms per unit cell without compromising the accuracy.
TL;DR: Tkatchenko et al. as mentioned in this paper demonstrate the importance of collective van der Waals dispersion effects beyond the pairwise approximation for organic-inorganic systems on the example of atoms, molecules, and nanostructures adsorbed on metals.
Abstract: A correct description of electronic exchange and correlation effects for molecules in contact with extended (metal) surfaces is a challenging task for first-principles modeling In this work, we demonstrate the importance of collective van der Waals dispersion effects beyond the pairwise approximation for organic–inorganic systems on the example of atoms, molecules, and nanostructures adsorbed on metals We use the recently developed many-body dispersion (MBD) approach in the context of density-functional theory [Tkatchenko et al, Phys Rev Lett 108, 236402 (2012) and Ambrosetti et al, J Chem Phys 140, 18A508 (2014)] and assess its ability to correctly describe the binding of adsorbates on metal surfaces We briefly review the MBD method and highlight its similarities to quantum-chemical approaches to electron correlation in a quasiparticle picture In particular, we study the binding properties of xenon, 3,4,9,10-perylene-tetracarboxylic acid, and a graphene sheet adsorbed on the Ag(111) surface Accounting for MBD effects, we are able to describe changes in the anisotropic polarizability tensor, improve the description of adsorbate vibrations, and correctly capture the adsorbate–surface interaction screening Comparison to other methods and experiment reveals that inclusion of MBD effects improves adsorption energies and geometries, by reducing the overbinding typically found in pairwise additive dispersion-correction approaches
TL;DR: It is shown for the first time that selective hydrogenation of the C=O bond in acrolein to form an unsaturated alcohol is possible over Pd(111) with nearly 100% selectivity.
Abstract: We present a mechanistic study on selective hydrogenation of acrolein over model Pd surfaces—both single crystal Pd(111) and Pd nanoparticles supported on a model oxide support. We show for the first time that selective hydrogenation of the C═O bond in acrolein to form an unsaturated alcohol is possible over Pd(111) with nearly 100% selectivity. However, this process requires a very distinct modification of the Pd(111) surface with an overlayer of oxopropyl spectator species that are formed from acrolein during the initial stages of reaction and turn the metal surface selective toward propenol formation. By applying pulsed multimolecular beam experiments and in situ infrared reflection–absorption spectroscopy, we identified the chemical nature of the spectator and the reactive surface intermediate (propenoxy species) and experimentally followed the simultaneous evolution of the reactive intermediate on the surface and formation of the product in the gas phase.
TL;DR: It is proved that the generally accepted simple model of water dissociation to form two individual OH groups per water molecule is not correct and formation of a dimer is suggested, which consists of one water molecule dissociated into two OH groups and another non-dissociated water molecule creating a thermodynamically very stable dimer-like complex.
Abstract: We present a mechanistic study on the interaction of water with a well-defined model Fe3O4(111) surface that was investigated by a combination of direct calorimetric measurements of adsorption energies, infrared vibrational spectroscopy, and calculations bases on density functional theory (DFT). We show that the adsorption energy of water (101 kJ mol(-1)) is considerably higher than all previously reported values obtained by indirect desorption-based methods. By employing (18)O-labeled water molecules and an Fe3 O4 substrate, we proved that the generally accepted simple model of water dissociation to form two individual OH groups per water molecule is not correct. DFT calculations suggest formation of a dimer, which consists of one water molecule dissociated into two OH groups and another non-dissociated water molecule creating a thermodynamically very stable dimer-like complex.
TL;DR: Ionic liquids have attracted continuous interest because of their remarkable physicochemical properties, including high thermal and chemical stability, nonflammability, negligible vapor pressure, designable cation/anion pairs, electrical and ionic conductivity, low melting points, and affinity towards many compounds as discussed by the authors.
Abstract: Ionic liquids (ILs) have attracted continuous interest because of their remarkable physicochemical properties, including high thermal and chemical stability, nonflammability, negligible vapor pressure, designable cation/anion pairs, electrical and ionic conductivity, low melting points, and affinity towards many compounds. These properties make ionic liquids valuable in the creation of new materials and new processes throughout almost the entire field of materials chemistry. An emerging field is the use of ionic liquids in carbon-based nanomaterials. Initially, ionic liquids were used as self-templating carbon sources for the generation of unusual carbon materials in which homogeneous heteroatom doping (e.g., N, B, S) can be accomplished fairly easily. Later, ionic liquids were recognized as suitable for use in the conversion of biomass into porous carbon materials, acting as both reaction media and porosity-directing regulator. Such applications open the door towards the synthesis and accurate tuning of carbon nanostructures, for example, pore structure, morphology, heteroatom doping, and surface functionality. In addition, the hybridization of ionic liquids and nanocarbons enables the development of composites by combining the properties of the ionic liquid (e.g., ionic conductivity or catalytic activity) and those of a host (e.g., chemical or mechanical stability). Currently, although the research of this topic is rapidly expanding, the rational design and synthesis of carbon-based materials, particularly their applications in energy storage and transformation, is still in its infancy. In this review, we focus on several aspects of ionic liquid derived carbons with the aim of shedding light on this new topic: 1) Ionic liquids as an advanced medium for carbon synthesis, 2) ionogels derived from nanocarbon, 3) ionic liquids as fluid precursors for functional carbons, and 4) ionic liquid derived carbons for heterogeneous catalysis.
TL;DR: This work explores the energy landscapes of two unsolvated 20-residue peptides that result from a shift of the position of one amino acid in otherwise the same sequence, and assesses the performance of current state-of-the-art density-functional theory for predicting the structure of such large and complex systems.
Abstract: Reliable, quantitative predictions of the structure of peptides based on their amino-acid sequence information are an ongoing challenge. We here explore the energy landscapes of two unsolvated 20-residue peptides that result from a shift of the position of one amino acid in otherwise the same sequence. Our main goal is to assess the performance of current state-of-the-art density-functional theory for predicting the structure of such large and complex systems, where weak interactions such as dispersion or hydrogen bonds play a crucial role. For validation of the theoretical results, we employ experimental gas-phase ion mobility-mass spectrometry and IR spectroscopy. While unsolvated Ac-Ala19-Lys + H+ will be shown to be a clear helix seeker, the structure space of Ac-Lys-Ala19 + H+ is more complicated. Our first-principles structure-screening strategy using the dispersion-corrected PBE functional (PBE + vdWTS) identifies six distinctly different structure types competing in the low-energy regime (≈16 kJ mol−1). For these structure types, we analyze the influence of the PBE and the hybrid PBE0 functional coupled with either a pairwise dispersion correction (PBE + vdWTS, PBE0 + vdWTS) or a many-body dispersion correction (PBE + MBD*, PBE0 + MBD*). We also take harmonic vibrational and rotational free energy into account. Including this, the PBE0 + MBD* functional predicts only one unique conformer to be present at 300 K. We show that this scenario is consistent with both experiments.
01 Oct 2015
TL;DR: In this article, the authors highlight approaches and developing ideas on the adaptation of photoelectron, X-ray absorption, vibrational spectroscopy, nuclear magnetic resonance, and Xray and neutron reflectometry in electrochemical studies.
Abstract: The operation of all electrochemical energy-related systems, such as supercapacitors, batteries, fuel cells, etc. depends largely on the processes occurring at electrochemical interfaces at which charge separation and chemical reactions occur. Evolution of structure and composition at the interface between electrodes and electrolytes affects all the device′s functional parameters including power and long-term performance stability. The analytical techniques capable of exploring the interfaces are still very limited, and more often only ex situ studies are performed. This sometimes leads to a loss of important pieces of the puzzle, hindering the development of novel technologies, as in many cases intermediates and electrochemical reaction products cannot be “quenched” for post-process analyses. Techniques capable of operando probing of electrochemical interfaces by photons and neutrons have become an extensively growing field of research. This review aims at highlighting approaches and developing ideas on the adaptation of photoelectron, X-ray absorption, vibrational spectroscopy, nuclear magnetic resonance, and X-ray and neutron reflectometry in electrochemical studies.
TL;DR: IR spectra of the protonated five amino acid peptide leu-enkephalin embedded in superfluid helium droplets show resolved spectra, which are in good agreement with theoretical calculations, as well as with the available gas-phase data indicating that the helium environment does not induce a significant matrix-shift.
Abstract: Ultracold IR spectra of the protonated five amino acid peptide leu-enkephalin (Tyr-Gly-Gly-Phe-Leu) embedded in superfluid helium droplets have been recorded using a free-electron laser as radiation source. The results show resolved spectra, which are in good agreement with theoretical calculations, as well as with the available gas-phase data indicating that the helium environment does not induce a significant matrix-shift. In addition, the effect of the interaction between the charge and the peptide backbone has been further investigated by complexing protonated leu-enkephalin with one 18-crown-6 molecule. Good agreement between the experimental and theoretical results allow for an assignment of a preferred molecular structure.
TL;DR: Conformer‐selective UVPD data for 7+ ubiquitin ions are provided, showing that certain conformers fall into groups with similar UVPD fragmentation pattern, however, the conformers within each group can differ tremendously in their collision cross‐section.
Abstract: The top-down approach in protein sequencing requires simple methods in which the analyte can be readily dissociated at every position along the backbone. In this context, ultraviolet photodissociation (UVPD) recently emerged as a promising tool because, in contrast to slow heating techniques such as CID, the absorption of UV light is followed by a rather statistically distributed cleavage of backbone bonds. As a result, nearly complete sequence coverage can be obtained. It is well known, however, that gas-phase proteins can adopt a variety of different, sometimes coexisting conformations and the influence of this structural diversity on the UVPD fragmentation behavior is not clear. Using ion mobility-UVPD-MS, we recently showed that UVPD is sensitive to the higher order structure of gas-phase proteins. In particular, the cis/trans isomerization of certain proline peptide bonds was shown to significantly influence the UVPD fragmentation pattern of two extended conformers of 11+ ubiquitin. Building on these results, we here provide conformer-selective UVPD data for 7+ ubiquitin ions, which are known to be present in a much more diverse and wider ensemble of different structures, ranging from very compact to highly extended species. Our data show that certain conformers fall into groups with similar UVPD fragmentation pattern. Surprisingly, however, the conformers within each group can differ tremendously in their collision cross-section. This indicates that the multiple coexisting conformations typically observed for 7+ ubiquitin are caused by a few, not easily interconvertible, subpopulations.
TL;DR: It is believed that the results are transferrable to realistic conditions and doping might develop into a powerful method to improve the performance of nonreducible oxides in surface-catalyzed reactions.
Abstract: Nonreducible oxides are characterized by large band gaps and are therefore unable to exchange electrons or to form bonds with surface species, explaining their chemical inertness. The insertion of aliovalent dopants alters this situation, as new electronic states become available in the gap that may be involved in charge-transfer processes. Consequently, the adsorption and reactivity pattern of doped oxides changes with respect to their nondoped counterparts. This Account describes scanning tunneling microscopy (STM) and photoelectron spectroscopy (XPS) experiments that demonstrate the impact of dopants on the physical and chemical properties of well-defined crystalline oxide films. For this purpose, MgO and CaO as archetypical rocksalt oxides have been loaded either with high-valence (Mo, Cr) or low-valence dopants (Li). While the former generate filled states in the oxide band gap and serve as electron donors, the latter produce valence-band holes and give rise to an acceptor response. The dopant-related electronic states and their polarization effect on the surrounding host material are explored with XPS and STM spectroscopy on nonlocal and local scales. Moreover, charge-compensating defects were found to develop in the oxide lattice, such as Ca and O vacancies in Mo- and Li-doped CaO films, respectively. These native defects are able to trap the excess charges of the impurities and therefore diminish the desired doping effect. If noncompensated dopants reside in the host lattice, electron exchange with surface species is observed. Mo ions in CaO, for example, were found to donate electrons to surface Au atoms. The anionic Au strongly binds to the CaO surface and nucleates in the form of monolayer islands, in contrast to the 3D growth prevailing on pristine oxides. Charge transfer is also revealed for surface O2 that traps one Mo electron by forming a superoxo-species. The activated oxygen is characterized by a reinforced binding to the surface, an elongated O-O bond length, and a reduced barrier for dissociation, and represents an important intermediate for oxidation reactions. The charge-transfer processes described here are quenched if Li is inserted into the oxide lattice, neutralizing the effect of the extra electrons. The specific behavior of doped oxides has been explored on a mechanistic level, i.e. on thin-film model systems at ultrahigh vacuum and low temperature. We believe, however, that our results are transferrable to realistic conditions and doping might thus develop into a powerful method to improve the performance of nonreducible oxides in surface-catalyzed reactions.
TL;DR: It was found that in feed of 1 part O2 to 2500 parts C2H4 (PO2 = 1.2 × 10(-4) mbar) the copper surface becomes O-terminated, and the O- terminated surface was found to exhibit partial-oxidation selectivity similar to that of Cu2O.
Abstract: The oxidation of copper catalysts during ethylene epoxidation was characterized using in situ photoemission spectroscopy and electron microscopy. Gas chromatography, proton-transfer reaction mass spectrometry and electron-ionization mass spectrometry were used to characterize the catalytic properties of the oxidized copper. We find that copper corrodes during epoxidation in a 1 : 1 mixture of oxygen and ethylene. The catalyst corrosion passes through several stages, beginning with the formation of an O-terminated surface, followed by the formation of Cu2O scale and eventually a CuO scale. The oxidized catalyst exhibits measurable activity for ethylene epoxidation, but with a low selectivity of 8/2500) Cu2O forms and eventually covers the surface.
TL;DR: In this paper, the authors examined surface hydroxylation as another possible stabilization mechanism for thin (0001)-oriented films of ZnO on metals, which may exhibit interlayer relaxations, resulting in the hexagonal boron nitride-like crystal structure.
Abstract: Thin (0001)-oriented films of ZnO on metals may exhibit interlayer relaxations, resulting in the hexagonal boron nitride-like crystal structure. The driving force for such reconstruction is the polar instability of either Zn- or O- terminated surfaces of ZnO(0001). Here, we examined surface hydroxylation as another possible stabilization mechanism. Zinc oxide films grown on Pt(111) were studied by infrared reflection–absorption spectroscopy (IRAS) as a function of film thickness and morphology as imaged by scanning tunneling microscopy. Despite prepared in pure oxygen ambient, the “as grown” films on Pt(111) expose hydroxyl groups. In contrast, the bilayer films on Ag(111) do not exhibit OH species, not even upon dosing of hydrogen or water. The results show that hydrogen may efficiently be provided by a Pt support, even for the multilayer films, via hydrogen dissociation and subsequent diffusion of H atoms through the film. Thermal stability of the OH-terminated surfaces depends on the film thickness, wi...
TL;DR: Collision cross sections of high-mannose N-glycans measured by drift tube ion mobility-mass spectrometry in helium and nitrogen gases reveal the existence of distinct conformers exclusive to [M - H]- ions.
Abstract: We report collision cross sections (CCS) of high-mannose N-glycans as [M + Na]+, [M + K]+, [M + H]+, [M + Cl]−, [M + H2PO4]− and [M − H]− ions, measured by drift tube (DT) ion mobility-mass spectrometry (IM-MS) in helium and nitrogen gases. Further analysis using traveling wave (TW) IM-MS reveal the existence of distinct conformers exclusive to [M − H]− ions.
TL;DR: The strong temperature dependence of the I(-)·(H2O)2 vibrational predissociation spectrum is traced to the intracluster dissociation of the ion-bound water dimer into independent water monomers that remain tethered to the ion.
Abstract: The strong temperature dependence of the I(-)·(H2O)2 vibrational predissociation spectrum is traced to the intracluster dissociation of the ion-bound water dimer into independent water monomers that remain tethered to the ion. The thermodynamics of this process is determined using van't Hoff analysis of key features that quantify the relative populations of H-bonded and independent water molecules. The dissociation enthalpy of the isolated water dimer is thus observed to be reduced by roughly a factor of three upon attachment to the ion. The cause of this reduction is explored with electronic structure calculations of the potential energy profile for dissociation of the dimer, which suggest that both reduction of the intrinsic binding energy and vibrational zero-point effects act to weaken the intermolecular interaction between the water molecules in the first hydration shell. Additional insights are obtained by analyzing how classical trajectories of the I(-)·(H2O)2 system sample the extended potential energy surface with increasing temperature.
TL;DR: In this paper, a series of MoOx/SBA-15 catalysts were used for the synthesis of propene to ethene and 2-butenes using ion exchange technique.
Abstract: Metathesis of propene to ethene and 2-butenes was studied over a series of MoOx/SBA-15 catalysts (molybdenum oxide supported on mesoporous silica SBA-15; Mo loading 2.1–13.3 wt %, apparent Mo surface density 0.2–2.5 nm−2). The catalysts have been prepared by an ion exchange technique. Nitrogen adsorption, 1H MAS-NMR, Raman, and FTIR spectroscopies were applied to characterize the catalysts. Adsorption of the reactant propene and the probe molecule NH3 was studied by in situ FTIR spectrometry microcalorimetry and temperature-programmed desorption. Irrespective of the loading, only ≈1 % of the Mo atoms in the MoOx/SiO2 catalysts transform into active carbene (Mo=CHR) sites catalyzing propene metathesis. Isolated, distorted molybdenum di-oxo species in close vicinity to two silanol groups have been shown to be the precursor of the active site. Targeted active site creation by pretreatment with methanol resulted in an increase in initial catalytic activity by a factor of 800.
TL;DR: An experimental and theoretical investigation of rotationally inelastic transitions of OH, prepared in the X(2)Π, v = 0, j = 3/2 F1f level, in collisions with molecular hydrogen, found good agreement between the experimental and computed relative cross sections.
Abstract: We present an experimental and theoretical investigation of rotationally inelastic transitions of OH, prepared in the X2Π, v = 0, j = 3/2 F1f level, in collisions with molecular hydrogen (H2 and D2). In a crossed beam experiment, the OH radicals were state selected and velocity tuned over the collision energy range 75–155 cm−1 using a Stark decelerator. Relative parity-resolved state-to-state integral cross sections were determined for collisions with normal and para converted H2. These cross sections, as well as previous OH–H2 measurements at 595 cm−1 collision energy by Schreel and ter Meulen [J. Chem. Phys. 105, 4522 (1996)], and OH–D2 measurements for collision energies 100–500 cm−1 by Kirste et al. [Phys. Rev. A 82, 042717 (2010)], were compared with the results of quantum scattering calculations using recently determined ab initio potential energy surfaces [Ma et al., J. Chem. Phys. 141, 174309 (2014)]. Good agreement between the experimental and computed relative cross sections was found, although ...