Partial Oxidation of Methanol on Gold: How Selectivity Is Steered by Low-Coordinated Sites.

Partial methanol oxidation proceeds with high selectivity to methyl formate (MeFo) on nanoporous gold (npAu) catalysts. As low-coordinated sites on npAu were suggested to affect the selectivity, we experimentally investigated their role in the isothermal selectivity for flat Au(111) and stepped Au(332) model surfaces using a molecular beam approach under well-defined conditions. Direct comparison shows that steps enhance desired MeFo formation and lower undesired overoxidation. DFT calculations reveal differences in oxygen distribution that enhance the barriers to overoxidation at steps. Thus, these results provide an atomic-level understanding of factors controlling the complex reaction network on gold catalysts, such as npAu.

Kinetic Trapping of Charge-Transfer Molecules at Metal Interfaces.

Despite the common expectation that conjugated organic molecules on metals adsorb in a flat-lying layer, several recent studies have found coverage-dependent transitions to upright-standing phases, which exhibit notably different physical properties. In this work, we argue that from an energetic perspective, thermodynamically stable upright-standing phases may be more common than hitherto thought. However, for kinetic reasons, this phase may often not be observed experimentally. Using first-principles kinetic Monte Carlo simulations, we find that the structure with lower molecular density is (almost) always formed first, reminiscent of Ostwald's rule of stages. The phase transitions to the upright-standing phase are likely to be kinetically hindered under the conditions typically used in surface science. The simulation results are experimentally confirmed for the adsorption of tetracyanoethylene on Cu(111) using infrared and X-ray photoemission spectroscopy. Investigating both the role of the growth conditions and the energetics of the interface, we find that the time for the phase transition is determined mostly by the deposition rate and, thus, is mostly independent of the nature of the molecule.

Interplay of Adsorption Geometry and Work Function Evolution at the TCNE/Cu(111) Interface.

The adsorption of organic electron acceptors on metal surfaces is a powerful way to change the effective work function of the substrate through the formation of charge-transfer-induced dipoles. The work function of the interfaces is hence controlled by the redistribution of charges upon adsorption of the organic layer, which depends not only on the electron affinity of the organic material but also on the adsorption geometry. As shown in this work, the latter dependence controls the work function also in the case of adsorbate layers exhibiting a mixture of various adsorption geometries. Based on a combined experimental (core-level and infrared spectroscopy) and theoretical (density functional theory) study for tetracyanoethylene (TCNE) on Cu(111), we find that TCNE adsorbs in at least three different orientations, depending on TCNE coverage. At low coverage, flat lying TCNE dominates, as it possesses the highest adsorption energy. At a higher coverage, additionally, two different standing orientations are found. This is accompanied by a large increase in the work function of almost 3 eV at full monolayer coverage. Our results suggest that the large increase in work function is mainly due to the surface dipole of the free CN groups of the standing molecules and less dependent on the charge-transfer dipole of the differently oriented and charged molecules. This, in turn, opens new opportunities to control the work function of interfaces, e.g., by synthetic modification of the adsorbates, which may allow one to alter the adsorption geometries of the molecules as well as their contributions to the interface dipoles and, hence, the work function.

New Perspectives for Evaluating the Mass Transport in Porous Catalysts and Unfolding Macro- and Microkinetics.

In this article we shed light on newly emerging perspectives to characterize and understand the interplay of diffusive mass transport and surface catalytic processes in pores of gas phase metal catalysts. As a case study, nanoporous gold, as an interesting example exhibiting a well-defined pore structure and a high activity for total and partial oxidation reactions is considered. PFG NMR (pulsed field gradient nuclear magnetic resonance) measurements allowed here for a quantitative evaluation of gas diffusivities within the material. STEM (scanning transmission electron microscopy) tomography furthermore provided additional insight into the structural details of the pore system, helping to judge which of its features are most decisive for slowing down mass transport. Based on the quantitative knowledge about the diffusion coefficients inside a porous catalyst, it becomes possible to disentangle mass transport contributions form the measured reaction kinetics and to determine the kinetic rate constant of the underlying catalytic surface reaction. In addition, predictions can be made for an improved effectiveness of the catalyst, i.e., optimized conversion rates. This approach will be discussed at the example of low-temperature CO oxidation, efficiently catalysed by npAu at 30 °C. The case study shall reveal that novel porous materials exhibiting well-defined micro- and mesoscopic features and sufficient catalytic activity, in combination with modern techniques to evaluate diffusive transport, offer interesting new opportunities for an integral understanding of catalytic processes.

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry.

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

On the Importance of Nanoparticle Necks and Carbon Impurities for Charge Trapping in TiO2.

Particle attachment and neck formation inside TiO2 nanoparticle networks determine materials performance in sensing, photo-electrochemistry, and catalysis. Nanoparticle necks can feature point defects with potential impact on the separation and recombination of photogenerated charges. Here, we investigated with electron paramagnetic resonance a point defect that traps electrons and predominantly forms in aggregated TiO2 nanoparticle systems. The associated paramagnetic center resonates in the g factor range between g = 2.0018 and 2.0028. Structure characterization and electron paramagnetic resonance data suggest that during materials processing, the paramagnetic electron center accumulates in the region of nanoparticle necks, where O2 adsorption and condensation can occur at cryogenic temperatures. Complementary density functional theory calculations reveal that residual carbon atoms, which potentially originate from synthesis, can substitute oxygen ions in the anionic sublattice, where they trap one or two electrons that mainly localize at the carbon. Their emergence upon particle neck formation is explained by the synthesis- and/or processing-induced particle attachment and aggregation facilitating carbon atom incorporation into the lattice. This study represents a substantial advance in linking dopants, point defects, and their spectroscopic fingerprints to microstructural features of oxide nanomaterials.

Current Research on Theory and Practice of Digital Libraries: Best Papers from TPDL 2019 & 2020.

This volume presents a special issue on selected papers from the 2019 & 2020 editions of the International Conference on Theory and Practice of Digital Libraries (TPDL). They cover different research areas within Digital Libraries, from Ontology and Linked Data to quality in Web Archives and Topic Detection. We first provide a brief overview of both TPDL editions, and we introduce the selected papers.

Heterogeneity of oxygen reactivity: key for selectivity of partial methanol oxidation on gold surfaces.

Recent evidence for low-temperature oxidation of methyl formate on Au(332) may affect the selectivity of gold catalysts during partial oxidation of methanol. Under isothermal conditions, overoxidation of methyl formate is significantly slower than methanol oxidation which can be attributed to special oxygen species required for overoxidation.

Defects of thin CaO(001) on Mo(001): an EPR spectroscopic perspective.

Paramagnetic defects of thin CaO(001) films grown on Mo(001) are characterized using electron paramagnetic resonance (EPR) spectroscopy under ultrahigh vacuum conditions. A variety of paramagnetic centers located in the volume of the films are identified whose speciation as well as relative abundance was found to depend on the growth rate of the films. Pristine films prepared at a lower growth rate exhibited a larger number and a different speciation of paramagnetic defects than films grown at a higher rate. Annealing of the films to 1030 K, which improves their long-range order, results in quenching of most of the paramagnetic species observed for the pristine film; however, films prepared at a lower growth rate exhibit new paramagnetic signals upon annealing, which are absent in films prepared at a higher growth rate. The signals can be assigned to paramagnetic Mo ions previously shown to diffuse into these films. These results indicate that the amount and speciation of the transition metal ions depend on the preparation conditions which in turn can also affect the surface chemistry of these systems.

Transition-Metal-Doping of CaO as Catalyst for the OCM Reaction, a Reality Check.

In this study, first-row transition metal-doped calcium oxide materials (Mn, Ni, Cr, Co., and Zn) were synthesized, characterized, and tested for the OCM reaction. Doped carbonate precursors were prepared by a co-precipitation method. The synthesis parameters were optimized to yield materials with a pure calcite phase, which was verified by XRD. EPR measurements on the doped CaO materials indicate a successful substitution of Ca2+ with transition metal ions in the CaO lattice. The materials were tested for their performance in the OCM reaction, where a beneficial effect towards selectivity and activity effect could be observed for Mn, Ni, and Zn-doped samples, where the selectivity of Co- and Cr-doped CaO was strongly reduced. The optimum doping concentration could be identified in the range of 0.04-0.10 atom%, showing the strongest decrease in the apparent activation energy, as well as the maximum increase in selectivity.

Methanol oxidation on Au(332): an isothermal pulsed molecular beam study.

Isothermal molecular beam experiments on the methanol oxidation over the stepped Au(332) surface were conducted under well-defined ultra-high vacuum conditions. In the measurements, a continuous flux of methanol at excess in the gas phase and pulses of atomic oxygen were provided to the surface kept at 230 K. The formation of the partial oxidation product methyl formate under the applied conditions was evidenced by time-resolved mass spectrometry, and accumulation of formate species, which resulted in a deactivation of the surface for methyl formate formation, was followed by in situ Infrared Reflection Absorption Spectroscopy measurements. The results suggest a different reactivity of oxygen accumulated during the oxygen pulses and atomic oxygen for the competing reaction pathways in the oxidation of methanol to the desired partial and the unwanted overoxidation products.

Controlling the Interparticular Distances of Extended Non-Close-Packed Colloidal Monolayers.

A versatile method for the preparation extended, well-ordered, non-close-packed monolayers of silica nanoparticles (137 ± 4 nm diameter) with adjustable interparticle distances is presented, which is based on a simple self-assembly procedure using aqueous dispersion with different ionic strengths. It is shown that these structures can be successfully transferred to air without suffering from aggregation. Scanning electron microscopy (SEM) is used to characterize the structures after transfer into the atmosphere. These investigations were combined with a quartz crystal microbalance with dissipation (QCM-D) experiments to follow the self-assembly process in solution. The nearest-neighbor distance distribution reveals a monotonous decrease of the average nearest-neighbor distance from 290 to 200 nm with increasing ionic strength from 0.05 to 1 mM, which indicates an increased shielding of the electrostatic interaction with increasing ionic strength. The observed saturation coverages for all studied ionic strengths are well explained with an effective hard-sphere model in which the saturation coverage is limited by Coulomb repulsion. However, at ionic strengths above 1 mM, significant amounts of aggregates are found in the dried samples, suggesting that the observed aggregates at high ionic strengths are formed during the drying process caused by capillary forces between the particles. Tuning the barrier for lateral diffusion, e.g., by changing the surface morphology or functionalization of the particles will offer a route to further extend the range of particle distances. The present approach can be easily expanded to a broad range of colloidal materials on surfaces, while it only requires low-cost laboratory equipment.

F-doping of nanostructured ZnO: a way to modify structural, electronic, and surface properties.

Polycrystalline ZnO is a material often used in heterogeneous catalysis. Its properties can be altered by the addition of dopants. We used gaseous fluorine (F2(g)) as direct way to incorporate fluoride in ZnO as anionic dopants. Here, the consequences of this treatment on the structural and electronic properties, as well as on the acidic/basic sites of the surface, are investigated. It is shown that the amount of F incorporation into the structure can be controlled by the synthesis parameters (t, T, p). While the surface of ZnO was altered as shown by, e.g., IR spectroscopy, XPS, and STEM/EDX measurements, the F2 treatment also influenced the electronic properties (optical band gap, conductivity) of ZnO. Furthermore, the Lewis acidity/basicity of the surface was affected which is evidenced by using, e.g., different probe molecules (CO2, NH3). In situ investigations of the fluorination process offer valuable insights on the fluorination process itself.

Combining EPR spectroscopy and X-ray crystallography to elucidate the structure and dynamics of conformationally constrained spin labels in T4 lysozyme single crystals.

Electron paramagnetic resonance (EPR) spectroscopy in combination with site-directed spin labeling is used to investigate the structure and dynamics of conformationally constrained spin labels in T4 lysozyme single crystals. Within a single crystal, the oriented ensemble of spin bearing moieties results in a strong angle dependence of the EPR spectra. A quantitative description of the EPR spectra requires the determination of the unit cell orientation with respect to the sample tube and the orientation of the spin bearing moieties within the crystal lattice. Angle dependent EPR spectra were analyzed by line shape simulations using the stochastic Liouville equation approach developed by Freed and co-workers and an effective Hamiltonian approach. The gain in spectral information obtained from the EPR spectra of single crystalline samples taken at different frequencies, namely the X-band and Q-band, allows us to discriminate between motional models describing the spectra of isotropic solutions similarly well. In addition, it is shown that the angle dependent single crystal spectra allow us to identify two spin label rotamers with very similar side chain dynamics. These results demonstrate the utility of single crystal EPR spectroscopy in combination with spectral line shape simulation techniques to extract valuable dynamic information not readily available from the analysis of isotropic systems. In addition, it will be shown that the loss of electron density in high resolution diffraction experiments at room temperature does not allow us to conclude that there is significant structural disorder in the system.

Structural Characterization of Ordered, Non-Close-Packed Functionalized Silica Nanoparticles on Gold Surfaces.

Nanostructured surfaces play an important role in modern science and technology. In particular, ordered arrangements of nonclose-packed nanoparticles created by self-assembly offer a versatile route to prepare systems, which can be used in various applications such as sensing, plasmonic devices or antireflection coatings. Self-assembly based systems are particularly appealing as preparation is rather simple. The ability of nanoparticle systems to form nonclosed packed monolayers by self-assembly depends on the balance of various energetic contributions in particular the adsorption energy, the lateral barrier for diffusion and the repulsion between particles. Even for simple model systems such as the monodispersed silica particles adsorbed on a bare gold surface investigated here, none of these quantities is easy to determine experimentally. To this end, we will report on a detailed characterization of the adsorption in particular with respect to the structural properties of the above-mentioned model system. Based on experimental results obtained by using quartz crystal microbalance with dissipation monitoring (QCM-D) as well as scanning electron microscopy (SEM) it is possible to determine the electrostatic pair potential from the lateral arrangement of the nano particles in the limit of low coverage.

Internal Dynamics of the 3-Pyrroline-N-Oxide Ring in Spin-Labeled Proteins.

Site-directed spin labeling is a versatile tool to study structure as well as dynamics of proteins using EPR spectroscopy. Methanethiosulfonate (MTS) spin labels tethered through a disulfide linkage to an engineered cysteine residue were used in a large number of studies to extract structural as well as dynamic information on the protein from the rotational dynamics of the nitroxide moiety. The ring itself was always considered to be a rigid body. In this contribution, we present a combination of high-resolution X-ray crystallography and EPR spectroscopy of spin-labeled protein single crystals demonstrating that the nitroxide ring inverts fast at ambient temperature while exhibiting nonplanar conformations at low temperature. We have used quantum chemical calculations to explore the potential energy that determines the ring dynamics as well as the impact of the geometry on the magnetic parameters probed by EPR spectroscopy.

Tracking Transient Conformational States of T4 Lysozyme at Room Temperature Combining X-ray Crystallography and Site-Directed Spin Labeling.

Proteins are dynamic molecules that can transiently adopt different conformational states. As the function of the system often depends critically on its conformational state a rigorous understanding of the correlation between structure, energetics and dynamics of the different accessible states is crucial. The biophysical characterization of such processes is, however, challenging as the excited states are often only marginally populated. We show that a combination of X-ray crystallography performed at 100 K as well as at room temperature and EPR spectroscopy on a spin-labeled single crystal allows to correlate the structures of the ground state and a thermally excited state with their thermodynamics using the variant 118R1 of T4 lysozyme as an example. In addition, it is shown that the surrounding solvent can significantly alter the energetic as well as the entropic contribution to the Gibbs free energy without major impact on the structure of both states.

Location of Trapped Electron Centers in the Bulk of Epitaxial MgO(001) Films Grown on Mo(001) Using in situ W-band Electron Paramagnetic Resonance Spectroscopy.

We present the first in situ W-band (94-GHz) electron paramagnetic resonance (EPR) study of a trapped electron center in thin MgO(001) films. The improved resolution of the high-field EPR experiments proves that the signal originate from a well-defined species present in the bulk of the films, whose projection of the principal g-tensor components onto the (001) plane are oriented along the [110] direction of the MgO lattice. Based on a comparison between the structural properties of the films, knowledge of the ability of bulk defects to trap electrons, and the properties of the EPR signal, it is possible to propose that the paramagnetic species are located at the origin of a screw dislocation in the bulk of the film.

Controlling the Interaction and Non-Close-Packed Arrangement of Nanoparticles on Large Areas.

In light of the importance of nanostructured surfaces for a variety of technological applications, the quest for simple and reliable preparation methods of ordered, nanometer ranged structures is ongoing. Herein, a versatile method to prepare ordered, non-close-packed arrangements of nanoparticles on centimeter sized surfaces by self-assembly is described using monodisperse (118-162 nm Ø), amino-functionalized silica nanoparticles as an exploratory example. It is shown that the arrangement of the particles is governed by the interplay between the electrostatic repulsion between the particles and the interaction between particles and surfaces. The latter is tuned by the properties of the particles such as their surface roughness as well as the chemistry of the linkage. Weak dispersive interactions between amino groups and gold surfaces are compared to a covalent amide linkage of the amino groups with carboxylic acid functionalized self-assembled monolayers. It was shown that the order of the former systems may suffer from capillary forces between particles during the drying process, while the covalently bonded systems do not. In turn, covalently bonded systems can be dried quickly, while the van der Waals bonded systems require a slow drying process to minimize aggregation. These highly ordered structures can be used as templates for the formation of a second, ordered, non-close-packed layer of nanoparticles exemplified for larger polystyrene particles (Ø 368 ± 14 nm), which highlights the prospect of this approach as a simple preparation method for ordered arrays of nanoparticles with tunable properties.

Ordered Structures of Functionalized Silica Nanoparticles on Gold Surfaces: Correlation of Quartz Crystal Microbalance with Structural Characterization.

Quartz crystal microbalance (QCM) is frequently used to investigate adsorption of nanometer-sized objects such as proteins, viruses, or organic as well as inorganic nanoparticles from solution. The interpretation of the data obtained for heterogeneous adsorbate layers is not straightforward in particular if the systems exhibit sizable amounts of dissipation. In this study we investigate the deposition of monodisperse, amine functionalized silica nanoparticles on gold surfaces using QCM with dissipation (QCM-D) to obtain frequency and dissipation changes during adsorption from the liquid phase. These investigations are combined with ex situ scanning electron microscopy (SEM) measurements to study both coverage as well as lateral arrangement of the particles. An ordered layer of particles is found at saturation coverage due to the charged particle surface resulting in a repulsive interaction between the particles. The repulsion ensures a minimal distance between the particles, which leads to a saturation coverage of 15% for particles of 137 nm diameter. The frequency shift is shown to be a linear function of coverage which is a behavior expected for an elastic medium according to the Sauerbrey equation. However, the system shows a strong dependence of the normalized frequency shift on the overtones as well as a large dissipation, which is a clear indication for a system with viscoelastic properties. The analysis of the data show that a reliable determination of the adsorbed mass solely on the basis of QCM-D results is not possible, but additional information as determined by SEM in the present case is required to determine the coverage. From a correlation of the QCM-D results with the structural characterization it is possible to infer that the dissipation is a long ranged phenomenon. A lower boundary of the interaction length could be derived being twice the particle diameter for the particles studied here. In contrast to that the frequency response behaves like local phenomenon.