Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

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  • 2022 • 173 CALPHAD-informed phase-field model for two-sublattice phases based on chemical potentials: η-phase precipitation in Al-Zn-Mg-Cu alloys
    Liu, C. and Davis, A. and Fellowes, J. and Prangnell, P.B. and Raabe, D. and Shanthraj, P.
    Acta Materialia 226 (2022)
    The electrochemical properties of high strength 7xxx aluminium alloys strongly depend on the substitutional occupancy of Zn by Cu and Al in the strengthening η-phase with the two-sublattice structure, and its microstructural and compositional prediction is the key to design of new generation corrosion resistant alloys. In this work, we have developed a chemical-potential-based phase-field model capable of describing multi-component and two-sublattice ordered phases, during commercial multi-stage artificial ageing treatments, by directly incorporating the compound energy CALPHAD formalism. The model developed has been employed to explore the complex compositional pathway for the formation of the η-phase in Al-Zn-Mg-Cu alloys during heat treatments. In particular, the influence of alloy composition, solute diffusivity, and heat treatment parameters on the microstructural and compositional evolution of η-phase precipitates, was systematically investigated from a thermodynamic and kinetic perspective and compared to electron probe microanalysis validation data. The simulated η-phase growth kinetics and the matrix residual solute evolution in the AA7050 alloy indicates that Zn depletion mainly controlled the η-phase growth process during the early stage of ageing, resulting in fast η-phase growth kinetics, enrichment of Zn in the η-phase, and an excess in residual Cu in the matrix. The gradual substitution of Zn by Cu atoms in the η-phase during the later ageing stage was in principle a kinetically controlled process, owing to the slower diffusivity of Cu relative to Zn in the matrix. It was also found that the higher nominal Zn content in alloys like the AA7085 alloy, compared to the AA7050 alloy, could significantly enhance the chemical potential of Zn, but this had a minor influence on Cu, which essentially led to the higher Zn content (and consequently lower Cu) seen in the η-phase. Finally, substantial depletion of Zn and supersaturation of Cu in the matrix of the AA7050 alloy was predicted after 24 h ageing at 120 ∘C, whereas the second higher-temperature ageing stage at 180 ∘C markedly enhanced the diffusion of Cu from the supersaturated matrix into the η-phase, while the matrix residual Zn content was only slightly affected. © 2021 The Author(s)
    view abstractdoi: 10.1016/j.actamat.2021.117602
  • 2022 • 172 Competing Effects in the Hydration Mechanism of a Garnet-Type Li7La3Zr2O12 Electrolyte
    Arinicheva, Y. and Guo, X. and Gerhards, M.-T. and Tietz, F. and Fattakhova-Rohlfing, D. and Finsterbusch, M. and Navrotsky, A. and Guillon, O.
    Chemistry of Materials 34 1473-1480 (2022)
    Li-ion conducting oxides (Li7La3Zr2O12, LLZO) with a cubic garnet-type structure are among the most promising candidates to be used as solid electrolytes in all-solid-state Li batteries. However, the environmental instability of the electrolyte, induced by interaction between the material and gas molecules commonly found in air, namely, water and carbon dioxide, poses challenges for its manufacture and application. Herein, a combined experimental kinetic and thermodynamic study was performed as a function of temperature to clarify the mechanism of hydration of a garnet-type LLZO electrolyte in moist air. It was found that the kinetics of LLZO hydration is diffusion-limited and the hydration mechanism at room temperature and at higher temperatures differs. The hydration of LLZO increases up to 200 °C. Above this temperature, stagnation of water uptake is observed due to the onset of a competing dehydration process. The dehydration of LLZO takes place up to 400 °C. The partial pressure of water significantly affects the extent of hydration. Expanding this combined kinetic and thermodynamic approach to LLZO materials with a variety of chemical compositions and morphologies would allow prediction of their reactivity in a humid atmosphere and adjustment of the processing conditions accordingly to meet the requirements of technological applications. © 2022 American Chemical Society
    view abstractdoi: 10.1021/acs.chemmater.1c02581
  • 2022 • 171 Dimethyl ether (DME) and dimethoxymethane (DMM) as reaction enhancers for methane: Combining flame experiments with model-assisted exploration of a polygeneration process
    Zhang, H. and Kaczmarek, D. and Rudolph, C. and Schmitt, S. and Gaiser, N. and Oßwald, P. and Bierkandt, T. and Kasper, T. and Atakan, B. and Kohse-Höinghaus, K.
    Combustion and Flame 237 (2022)
    The potential of dimethyl ether (DME) and dimethoxymethane (DMM), representatives of the attractive oxymethylene ether (OME) alternative fuel family, are explored here as reactivity enhancers for methane-fueled polygeneration processes. Typically, such processes that can flexibly generate power, heat, or chemicals, operate under fuel-rich conditions in gas turbines or internal combustion engines. To provide a consistent basis for the underlying reaction mechanisms, it is recognized that speciation data for the DME/CH4 fuel combination are available for such conditions while such information for the DMM/CH4 system is largely lacking. In addition, it should be noted that a detailed speciation study in flames, i.e., combustion systems involving chemistry and transport processes over a large temperature range, is still missing in spite of the potential of such systems to provide extended species information. In a systematic approach using speciation with electron ionization molecular-beam mass spectrometry (EI-MBMS), we thus report, as a first step, investigation of six fuel-rich premixed flames of DME and DMM and their blends with methane with special attention on interesting chemicals. Secondly, a comprehensive but compact DME/DMM/CH4 model (PolyMech2.1) is developed based on these data. This model is then examined against available experimental data under conditions from various facilities, focusing preferentially on elevated pressure and fuel-rich conditions. Comparison with existing literature models is also included in this evaluation. Thirdly, an analysis is given on this basis, via the extensively tested PolyMech2.1 model, for assumed polygeneration conditions in a homogeneous charge compression ignition (HCCI) engine environment. The main interest of this model-assisted exploration is to evaluate whether addition of DME or DMM in a polygeneration process can lead to potentially useful conditions for the production of syngas or other chemicals, along with work and heat. The flame results show that high syngas yields, i.e., up to ∼78% for CO and ∼35% for H2, can be obtained in their burnt gases. From the large number of intermediates detected, predominantly acetylene, ethylene, ethane, and formaldehyde show yields of 2.1−4.4% (C2 hydrocarbons) and 3.4−8.7% (CH2O), respectively. Also, methanol and methyl formate show comparably high yields of up to 0.6−6.7% in the flames with DMM, which is 1–2 orders of magnitude higher than in those with DME as the additive. In the modeling-assisted exploration of the engine process, the PolyMech2.1 model is seen to perform at significantly reduced computational costs compared to a recently validated model without sacrificing the prediction performance. Promising conditions for the assumed polygeneration process using fuel combinations in the DME/DMM/CH4 system are identified with attractive syngas yields of up to 77% together with work and heat output at exergetic efficiencies of up to 89% with DME. © 2021
    view abstractdoi: 10.1016/j.combustflame.2021.111863
  • 2021 • 170 A positivity-preserving and conservative intersection-distribution-based remapping algorithm for staggered ALE hydrodynamics on arbitrary meshes
    Kenamond, M. and Kuzmin, D. and Shashkov, M.
    Journal of Computational Physics 435 (2021)
    We introduce new intersection-distribution-based remapping tools for indirect staggered arbitrary Lagrangian-Eulerian (ALE) simulations of multi-material shock hydrodynamics on arbitrary meshes. In addition to conserving momentum and total energy, the three-stage remapper proposed in this work preserves non-negativity of the internal energy. At the first stage, we construct slope-limited piecewise-linear reconstructions of all conserved quantities on zones of the source mesh and perform intersection-based remap to obtain bound-preserving zonal quantities on the target mesh. At the second stage, we define bound-preserving nodal quantities of the staggered ALE discretization as convex combinations of corner quantities. The nodal internal energy is corrected in a way which keeps it non-negative, while providing exact conservation of total energy. At the final stage, we distribute the non-negative nodal internal energy to corners, zones and materials using non-negative weights. Proofs of positivity preservation are provided for each stage. This work is a natural extension of our paper [14] in which a similar intersection-distribution-based remapping procedure was employed. The original version used a nodal kinetic energy fix which did not provably ensure positivity preservation for the zonal internal energy after the final distribution stage. The new algorithm cures this potential drawback by using ‘coordinated’ limiters for piecewise-linear reconstructions, remapping the internal energy to nodes and correcting it before redistribution. The effectiveness of the new nodal fix is illustrated by numerical examples. © 2021 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcp.2021.110254
  • 2021 • 169 A safety cap protects hydrogenase from oxygen attack
    Winkler, M. and Duan, J. and Rutz, A. and Felbek, C. and Scholtysek, L. and Lampret, O. and Jaenecke, J. and Apfel, U.-P. and Gilardi, G. and Valetti, F. and Fourmond, V. and Hofmann, E. and Léger, C. and Happe, T.
    Nature Communications 12 (2021)
    [FeFe]-hydrogenases are efficient H2-catalysts, yet upon contact with dioxygen their catalytic cofactor (H-cluster) is irreversibly inactivated. Here, we combine X-ray crystallography, rational protein design, direct electrochemistry, and Fourier-transform infrared spectroscopy to describe a protein morphing mechanism that controls the reversible transition between the catalytic Hox-state and the inactive but oxygen-resistant Hinact-state in [FeFe]-hydrogenase CbA5H of Clostridium beijerinckii. The X-ray structure of air-exposed CbA5H reveals that a conserved cysteine residue in the local environment of the active site (H-cluster) directly coordinates the substrate-binding site, providing a safety cap that prevents O2-binding and consequently, cofactor degradation. This protection mechanism depends on three non-conserved amino acids situated approximately 13 Å away from the H-cluster, demonstrating that the 1st coordination sphere chemistry of the H-cluster can be remote-controlled by distant residues. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41467-020-20861-2
  • 2021 • 168 Automated assessment of a kinetic database for fcc Co-Cr-Fe-Mn-Ni high entropy alloys
    Abrahams, K. and Zomorodpoosh, S. and Khorasgani, A.R. and Roslyakova, I. and Steinbach, I. and Kundin, J.
    Modelling and Simulation in Materials Science and Engineering 29 (2021)
    The development of accurate kinetic databases by parametrizing the composition and temperature dependence of elemental atomic mobilities, is essential for correct multicomponent calculations and simulations. In this work the automated assessment procedure for the establishment of CALPHAD-type kinetic databases is proposed, including the storage of raw data and assessment results, automatic weighting of data, parameter selection and automated reassessments. This allows the establishment of reproducible up-to-date databases. The proposed software, written in python, is applied to the assessment of a kinetic database for the fcc Co-Cr-Fe-Mn-Ni high entropy alloy using only tracer diffusion data for a sharp separation of thermodynamic and kinetic data. The established database is valid for the whole composition range of the five-component high entropy alloy. © 2021 The Author(s). Published by IOP Publishing Ltd Printed in the UK
    view abstractdoi: 10.1088/1361-651X/abf62b
  • 2021 • 167 CALPHAD-informed phase-field modeling of grain boundary microchemistry and precipitation in Al-Zn-Mg-Cu alloys
    Liu, C. and Garner, A. and Zhao, H. and Prangnell, P.B. and Gault, B. and Raabe, D. and Shanthraj, P.
    Acta Materialia 214 (2021)
    The grain boundary (GB) microchemistry and precipitation behaviour in high-strength Al-Zn-Mg-Cu alloys has an important influence on their mechanical and electrochemical properties. Simulation of the GB segregation, precipitation, and solute distribution in these alloys requires an accurate description of the thermodynamics and kinetics of this multi-component system. CALPHAD databases have been successfully developed for equilibrium thermodynamic calculations in complex multi-component systems, and in recent years have been combined with diffusion simulations. In this work, we have directly incorporated a CALPHAD database into a phase-field framework, to simulate, with high fidelity, the complex kinetics of the non-equilibrium GB microstructures that develop in these important commercial alloys during heat treatment. In particular, the influence of GB solute segregation, GB diffusion, precipitate number density, and far-field matrix composition, on the growth of a population of GB η-precipitates, was systematically investigated in a model Al-Zn-Mg-Cu alloy of near AA7050 composition. It is shown that the GB solute distribution in the early stages of ageing was highly heterogeneous and strongly affected by the distribution of GB η-precipitates. Significant Mg and Cu GB segregation was predicted to remain during overageing, while Zn was rapidly depleted. This non-trivial GB segregation behaviour markedly influenced the resulting precipitate morphologies, but the overall precipitate transformation kinetics on a GB were relatively unaffected. Furthermore, solute depletion adjacent to the GB was largely determined by Zn and Mg diffusion, which will affect the development of precipitate free zones during the early stages of ageing. The simulation results were compared with scanning transmission electron microscopy and atom probe tomography characterisation of alloys of the similar composition, with good agreement. © 2021
    view abstractdoi: 10.1016/j.actamat.2021.116966
  • 2021 • 166 Catalytic influence of mineral compounds on the reactivity of cellulose-derived char in O2-, CO2-, and H2O-containing atmospheres
    Pflieger, C. and Lotz, K. and Hilse, N. and Berger, C.M. and Schiemann, M. and Debiagi, P. and Hasse, C. and Scherer, V. and Muhler, M.
    Fuel 287 (2021)
    The catalytic effects of mineral compounds on the conversion of a biomass-derived char in air- and oxyfuel-related atmospheres were investigated by thermogravimetric analysis at atmospheric pressure. The applied char originated from the hydrothermal carbonization (HTC) of cellulose followed by pyrolysis at 1073 K and subsequent mixing with 20 wt% of minerals by grinding to achieve tight contact. The reactivities of the mineral-loaded HTC chars were evaluated based on isothermal experiments in O2-, CO2-, and H2O-containing atmospheres as a function of their composition applying a magnetic suspension balance. The reactivity sequence K2CO3 > Na2CO3 ≫ Fe2O3 > CaO > MgO ≥ mineral-free was derived consistently for char oxidation in O2/inert as well as for char gasification in diluted H2O and CO2 mixtures. In addition to this qualitative assessment, the kinetic experiments were first modelled based on a simple global nth-order power-law rate expression. Then, the more complex Carbon Burnout Kinetics (CBK/G) model and the PoliMi model were applied. All three modeling approaches enabled a systematic quantification of the catalytic effects and led to a comparable lowering in the apparent activation energy. In combination with the kinetic parameters determined for the mineral-free char, the lowered apparent activation energies specific for the applied mineral and atmosphere facilitate the implementation of catalytic effects on the conversion of biomass-derived char into combustion models. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2020.119584
  • 2021 • 165 Cross-section-phenomena in rotary drums with sectional internals
    Priessen, J. and Kawka, T. and Behrens, M. and Schultz, H.J.
    Powder Technology 381 229-244 (2021)
    The effect of sectional internals on the solid bed cross section in rotary drums is investigated with a novel, fully optical accessible rotary drum apparatus for different internal configurations. The impact on mixing behavior is analyzed for different section numbers and filling degrees as well as material configurations. The mixing kinetics is affected positively by the sections, particularly for lower rotational speeds and filling degrees. The goodness of mixing in segregating particulate systems can be improved by sectional internals. Additionally to mixing effects, the impact of sectional internals on phase interfaces of the cross section is analyzed. A mathematical model description of the solid bed cross section is created and validated with the experimental data. The gas/solid interface is increasing logarithmically compared to the bare drum. The solid/inner wall interface is increasing linearly with the number of sections and the solid/outer wall interface is not affected significantly by sectional internals. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2020.11.048
  • 2021 • 164 Crystalline ytterbium disilicate environmental barrier coatings made by high velocity oxygen fuel spraying
    Wolf, M. and Mack, D.E. and Mauer, G. and Guillon, O. and Vaßen, R.
    International Journal of Applied Ceramic Technology (2021)
    Dense environmental barrier coatings (EBCs) are an essential prerequisite to exploit the advantages offered by SiC-based fiber reinforced ceramic matrix composites (CMCs) to increase efficiency in gas turbines. Today's state-of-the art materials for application as EBCs are rare-earth (RE) silicates which, however, form amorphous phases upon rapid quenching from the melt. This makes their processing by thermal spray a challenge. Recently, high velocity oxygen fuel (HVOF) spraying was proposed as potential solution since the melting degree of the feedstock can be controlled effectively. This work studies the deposition of ytterbium disilicate (YbDS) at short stand-off distances and variant total feed rates and oxy-fuel ratios of the working gas. It was found that the overall degree of crystallinity could be kept at high level above 90%. The kinetic energy transferred by impinging particles was found to be an effective parameter to control the densification of the coatings. Porosities well below 10% were achieved while fully dense coatings were impeded due to the progressive accumulation of stresses in the coatings. © 2021 The Authors. International Journal of Applied Ceramic Technology published by Wiley Periodicals LLC on behalf of American Ceramics Society (ACERS)
    view abstractdoi: 10.1111/ijac.13829
  • 2021 • 163 Experimental determination of walnut shell pyrolysis kinetics in N2 and CO2 via thermogravimetric analysis, fluidized bed and drop tube reactors
    Ontyd, C. and Pielsticker, S. and Yildiz, C. and Schiemann, M. and Hatzfeld, O. and Ströhle, J. and Epple, B. and Kneer, R. and Scherer, V.
    Fuel 287 (2021)
    A thermogravimetric analyzer (TGA), a fluidized bed reactor (FBR) and a drop tube reactor (DTR) are used to study the effect of reactor type, heating rate and temperature on the pyrolysis of pulverized walnut shell particles in N2 and in CO2. These setups cover a temperature range of 400–1300 K with heating rates of 10−1 to 105 K s−1. The single first-order model in combination with an Arrhenius approach is used to describe the pyrolysis reaction. Derived activation energies for all setups show similar values (Ea,TGA = 71.96 kJ mol−1, Ea,FBR = 68.60 kJ mol−1 and Ea,DTR = 60.83 kJ mol−1), while an increase in the reactor temperature tend to lower the activation energy. Pyrolysis gas compositions in FBR and DTR reveal consistent trends towards lower H2O and higher CO contents with increasing reactor temperature. To evaluate the impact of CO2 on the solid conversion, TGA measurements in CO2 are used to determine gasification kinetics (Ea,g = 214.1 kJ mol−1, Ag = 71.96 s−1). CFD simulations using these kinetics in CO2 drop tube experiments let assume that the changed thermophysical properties of the gas and not the gasification reaction lead to the observed stronger conversion in CO2 compared to N2. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2020.119313
  • 2021 • 162 General Efficacy of Atomically Dispersed Pt Catalysts for the Chlorine Evolution Reaction: Potential-Dependent Switching of the Kinetics and Mechanism
    Lim, T. and Kim, J.H. and Kim, J. and Baek, D.S. and Shin, T.J. and Jeong, H.Y. and Lee, K.-S. and Exner, K.S. and Joo, S.H.
    ACS Catalysis 11 12232-12246 (2021)
    The electrochemical chlorine evolution reaction (CER) is a key anodic reaction in the chlor-alkali process for Cl2production, on-site generation of ClO-, and Cl2-mediated electrosynthesis. Although Ru-based mixed metal oxides have long been used as CER catalysts, they suffer from a selectivity problem due to the competing oxygen evolution reaction. To overcome this shortcoming, we have developed a new CER catalyst composed of atomically dispersed Pt-N4sites on carbon nanotubes (Pt1/CNT). In this study, we demonstrate that the catalytically active Pt-N4sites can be constructed from H2PtCl6·6H2O and an ionic liquid via a bottom-up approach and a Pt-porphyrin-driven top-down method. Both catalysts exhibit excellent CER activity and remarkable selectivity, demonstrating the general efficacy of Pt1/CNT for the CER. The electrochemical and in situ X-ray absorption spectroscopy analyses reveal that Pt1/CNT catalysts show a reaction order of ∼1.8 in the low overpotential regime, where the Volmer step is reconciled with the rate-determining step (RDS). Interestingly, in the high overpotential region, the CER over Pt1/CNT proceeds with a lower reaction order and the RDS switches to the Heyrovský step. These unprecedented kinetic insights are clearly distinguished from the oxide-based CER catalysts with the opposite sequence of the RDS. © 2021 American Chemical Society
    view abstractdoi: 10.1021/acscatal.1c03893
  • 2021 • 161 In situ correlation between metastable phase-transformation mechanism and kinetics in a metallic glass
    Orava, J. and Balachandran, S. and Han, X. and Shuleshova, O. and Nurouzi, E. and Soldatov, I. and Oswald, S. and Gutowski, O. and Ivashko, O. and Dippel, A.-C. and Zimmermann, M. and Ivanov, Y.P. and Greer, A.L. and Raabe, D. and...
    Nature Communications 12 (2021)
    A combination of complementary high-energy X-ray diffraction, containerless solidification during electromagnetic levitation and transmission electron microscopy is used to map in situ the phase evolution in a prototype Cu-Zr-Al glass during flash-annealing imposed at a rate ranging from 102 to 103 K s−1 and during cooling from the liquid state. Such a combination of experimental techniques provides hitherto inaccessible insight into the phase-transformation mechanism and its kinetics with high temporal resolution over the entire temperature range of the existence of the supercooled liquid. On flash-annealing, most of the formed phases represent transient (metastable) states – they crystallographically conform to their equilibrium phases but the compositions, revealed by atom probe tomography, are different. It is only the B2 CuZr phase which is represented by its equilibrium composition, and its growth is facilitated by a kinetic mechanism of Al partitioning; Al-rich precipitates of less than 10 nm in a diameter are revealed. In this work, the kinetic and chemical conditions of the high propensity of the glass for the B2 phase formation are formulated, and the multi-technique approach can be applied to map phase transformations in other metallic-glass-forming systems. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41467-021-23028-9
  • 2021 • 160 Intersection-distribution-based remapping between arbitrary meshes for staggered multi-material arbitrary Lagrangian-Eulerian hydrodynamics
    Kenamond, M. and Kuzmin, D. and Shashkov, M.
    Journal of Computational Physics 429 (2021)
    We present a new intersection-distribution-based remapping method between arbitrary polygonal meshes for indirect staggered multi-material arbitrary Lagrangian-Eulerian hydrodynamics. All cell-centered material quantities are conservatively remapped using intersections between the Lagrangian (old, source) mesh and the rezoned (new, target) mesh. The new nodal masses are obtained by conservative distribution of all material masses in each new cell to the cell's corners and then collecting those corner masses at new nodes. This distribution is done using a local constrained optimization approach for each cell in the new mesh. In order to remap nodal momentum we first define cell-centered momentum for each cell in the old mesh, conservatively remap this to the new mesh and then conservatively distribute the new zonal momentum to each cell's bounding nodes, again using local constrained optimization. Our method also conserves total energy by applying a new nodal kinetic energy correction that relies on a process similar to that used for remapping nodal mass and momentum. Cell-centered kinetic energy is computed, conservatively remapped and then distributed to nodes. The discrepancy between this conservatively remapped and actual nodal kinetic energy is then conservatively distributed to the internal energies of the materials in the cells surrounding each node. Unlike conventional cell-based corrections of this type, this new nodal kinetic energy correction has not been observed to drive material internal energy negative in any of our testing. Unlike flux based remapping, our new intersection-distribution method can be applied to remapping between source and target meshes that are arbitrarily different, which provides superior flexibility in the rezoning strategy. Our method is accurate, essentially conservative and essentially bounds preserving. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcp.2020.110014
  • 2021 • 159 Investigation of Turbulent Pulverized Solid Fuel Combustion with Detailed Homogeneous and Heterogeneous Kinetics
    Wang, B. and Shamooni, A. and Stein, O.T. and Kronenburg, A. and Kempf, A.M. and Debiagi, P. and Hasse, C.
    Energy and Fuels (2021)
    A comprehensive Euler-Lagrange framework for pulverized coal combustion using detailed multi-step heterogeneous kinetics is presented. The heterogeneous kinetics employ the POLIMI model that involves 37 species (22 solid species and 15 gas species) and 49 reactions to describe detailed pyrolysis as well as char oxidation, gasification, and annealing for a wide range of coals. The porous structure of the coal particles is considered, and the heterogeneous reactions are assumed to occur throughout the entire particle in a volume-based approach. The ordinary differential equations of the heterogeneous kinetics are integrated on each Lagrangian coal particle and predict the conversion of the raw coal components to light volatile hydrocarbons, heavy tar species, and char off-gases. Hence, the composition of the solid fuel components and the released gas changes dynamically in space and time, providing high-fidelity predictions of solid fuel combustion. The chemical conversion of the released species in the gas phase is described by a homogeneous kinetic mechanism with 76 species and 973 reactions that was reduced from the comprehensive CRECK-G-1407 kinetic mechanism. The new modeling framework is employed within carrier-phase direct numerical simulations (CP-DNS) of pulverized coal combustion in a three-dimensional turbulent mixing layer. This configuration includes the additional physics of turbulence and particle group combustion by mixing solid fuel particles suspended in a primary oxidizer stream with the products from lean volatile combustion in a secondary stream. The CP-DNS results are analyzed with and without the available set of 14 char conversion reactions, and a low degree of char conversion indicated by an increased rate of CO production is captured for particles with temperatures higher than 1800 K. The CP-DNS results from the detailed POLIMI approach feature a distinct bimodal shape of the volatile release curve and multi-regime combustion. The POLIMI data are used to evaluate the predictive capability of simpler pyrolysis models. The original competing two-step model (C2SM) by Kobayashi is investigated and shown to predict heavily delayed ignition. A new competing two-step devolatilization approach is proposed as an alternative model reduction suitable for fitting bimodal volatile release rates, such as that predicted by POLIMI. The CP-DNS using the alternative pyrolysis model faithfully captures the onset of ignition and multi-regime flame branches. Differences arise in the local tar species compositions in the gas phase as a result of the time-varying (POLIMI) and fixed (new C2SM) volatile compositions for the respective models. The flame structure is further analyzed by chemical explosive mode analysis (CEMA), and the occurrence of premixed and non-premixed flames zones is confirmed, whereas a simpler flame index analysis fails to correctly indicate the multi-regime nature of the flame. This recognition of multi-regime combustion serves as a guidance for selecting suitable conditioning variables for flamelet and other combustion submodels in large eddy simulation. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.energyfuels.0c03479
  • 2021 • 158 Single-Particle Hyperspectral Imaging Reveals Kinetics of Silver Ion Leaching from Alloy Nanoparticles
    Al-Zubeidi, A. and Stein, F. and Flatebo, C. and Rehbock, C. and Hosseini Jebeli, S.A. and Landes, C.F. and Barcikowski, S. and Link, S.
    ACS Nano 15 8363-8375 (2021)
    Gold-silver alloy nanoparticles are interesting for multiple applications, including heterogeneous catalysis, optical sensing, and antimicrobial properties. The inert element gold acts as a stabilizer for silver to prevent particle corrosion, or conversely, to control the release kinetics of antimicrobial silver ions for long-term efficiency at minimum cytotoxicity. However, little is known about the kinetics of silver ion leaching from bimetallic nanoparticles and how it is correlated with silver content, especially not on a single-particle level. To characterize the kinetics of silver ion release from gold-silver alloy nanoparticles, we employed a combination of electron microscopy and single-particle hyperspectral imaging with an acquisition speed fast enough to capture the irreversible silver ion leaching. Single-particle leaching profiles revealed a reduction in silver ion leaching rate due to the alloying with gold as well as two leaching stages, with a large heterogeneity in rate constants. We modeled the initial leaching stage as a shrinking-particle with a rate constant that exponentially depends on the silver content. The second, slower leaching stage is controlled by the electrochemical oxidation potential of the alloy being steadily increased by the change in relative gold content and diffusion of silver atoms through the lattice. Interestingly, individual nanoparticles with similar sizes and compositions exhibited completely different silver ion leaching yields. Most nanoparticles released silver completely, but 25% of them appeared to arrest leaching. Additionally, nanoparticles became slightly porous. Alloy nanoparticles, produced by scalable laser ablation in liquid, together with kinetic studies of silver ion leaching, provide an approach to design the durability or bioactivity of alloy nanoparticles. ©
    view abstractdoi: 10.1021/acsnano.0c10150
  • 2021 • 157 Solvent effect on the kinetics of the hydrogenation of n-butyl levulinate to γ-valerolactone
    Capecci, S. and Wang, Y. and Casson Moreno, V. and Held, C. and Leveneur, S.
    Chemical Engineering Science 231 (2021)
    The use of lignocellulosic biomass in the chemical industry can significantly contribute to respect the various international agreements on climate change. One of the most promising platform molecules issued from the lignocellulosic biomass hydrolysis is γ-valerolactone (GVL). GVL can be upgraded to valuable chemicals and produced by the hydrogenation of alkyl levulinates. Although these reactions are widely studied, seldom research focused on the solvent effect. To fill this gap, the effect of three different reaction mixtures with an excess of butyl levulinate (BL), of butanol and GVL was studied on the kinetics of BL hydrogenation to GVL over Ru/C. PC-SAFT (Perturbed-Chain Statistical Associating Fluid Theory) shows that the solubility of hydrogen is not constant during the reaction progress, and it was taken into account. To allow a fair comparison, kinetic models were developed using Bayesian statistics for each reaction mixture. The best performances were obtained when the reaction mixture has an excess of GVL. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.ces.2020.116315
  • 2021 • 156 Sustainable steel through hydrogen plasma reduction of iron ore: Process, kinetics, microstructure, chemistry
    Souza Filho, I.R. and Ma, Y. and Kulse, M. and Ponge, D. and Gault, B. and Springer, H. and Raabe, D.
    Acta Materialia 213 (2021)
    Iron- and steelmaking is the largest single industrial CO2 emitter, accounting for 6.5% of all CO2 emissions on the planet. This fact challenges the current technologies to achieve carbon-lean steel production and to align with the requirement of a drastic reduction of 80% in all CO2 emissions by around 2050. Thus, alternative reduction technologies have to be implemented for extracting iron from its ores. The hydrogen-based direct reduction has been explored as a sustainable route to mitigate CO2 emissions, where the reduction kinetics of the intermediate oxide product FexO (wüstite) into iron is the rate-limiting step of the process. The total reaction has an endothermic net energy balance. Reduction based on a hydrogen plasma may offer an attractive alternative. Here, we present a study about the reduction of hematite using hydrogen plasma. The evolution of both, chemical composition and phase transformations was investigated in several intermediate states. We found that hematite reduction kinetics depends on the balance between the initial input mass and the arc power. For an optimized input mass-arc power ratio, complete reduction was obtained within 15 min of exposure to the hydrogen plasma. In such a process, the wüstite reduction is also the rate-limiting step towards complete reduction. Nonetheless, the reduction reaction is exothermic, and its rates are comparable with those found in hydrogen-based direct reduction. Micro- and nanoscale chemical and microstructure analysis revealed that the gangue elements partition to the remaining oxide regions, probed by energy dispersive spectroscopy (EDS) and atom probe tomography (APT). Si-enrichment was observed in the interdendritic fayalite domains, at the wüstite/iron hetero-interfaces and in the oxide particles inside iron. With proceeding reduction, however, such elements are gradually removed from the samples so that the final iron product is nearly free of gangue-related impurities. Our findings provide microstructural and atomic-scale insights into the composition and phase transformations occurring during iron ore reduction by hydrogen plasma, propelling better understanding of the underlying thermodynamics and kinetic barriers of this essential process. © 2021
    view abstractdoi: 10.1016/j.actamat.2021.116971
  • 2021 • 155 Towards the Suitability of Information Entropy as an LES Quality Indicator
    Engelmann, L. and Ihme, M. and Wlokas, I. and Kempf, A.
    Flow, Turbulence and Combustion (2021)
    The Shannon entropy is a rigorous measure to evaluate the complexity in dynamical systems. Shannon entropy can be directly calculated from any set of experimental or numerical data and yields the uncertainty of a given dataset. Originating from information theory, the concept can be generalized from assessing the uncertainty in a message to any dynamical system. Following the concept of ergodicity, turbulence forms another class of dynamical systems, which is generally assessed using statistical measures. The quantification of resolution quality is a crucial aspect in assessing turbulent-flow simulations. While a vast variety of statistical measures for the evaluation of resolution is available, measures closer representing the dynamics of a turbulent systems, such as the Wasserstein metric or the Ljapunov exponent become popular. This study investigates how the Shannon entropy can lead to useful insights in the quality of turbulent-flow simulations. The Shannon entropy is calculated based on distributions, which enables the direct evaluation from unsteady flow simulations or by post-processing. A turbulent channel flow and a planar turbulent jet are used as validation tests. The Shannon entropy is calculated for turbulent velocity- and scalar-fields and correlations with physical quantities, such as turbulent kinetic energy and passive scalars, are investigated. It is shown that the spatial structure of the Shannon entropy can be related to flow phenomena. This is illustrated by the investigation of the entropy of the velocity fluctuations, passive scalars and turbulent kinetic energy. Grid studies reveal the Shannon entropy as a converging measure. It is demonstrated, that classical turbulent-kinetic-energy-based quality measures struggle with the identification of insufficient resolution, while the Shannon entropy has demonstrated potential to form a solid basis for LES quality assessment. © 2021, The Author(s).
    view abstractdoi: 10.1007/s10494-021-00277-8
  • 2020 • 154 A six-compound, high performance gasoline surrogate for internal combustion engines: Experimental and numerical study of autoignition using high-pressure shock tubes
    Cancino, L.R. and da Silva, A., Jr. and De Toni, A.R. and Fikri, M. and Oliveira, A.A.M. and Schulz, C. and Curran, H.J.
    Fuel 261 (2020)
    This paper presents experimental and modeling data for the autoignition of a novel, six-component, high performance gasoline surrogate fuel comprising ethanol, n-heptane, i-octane, 1-hexene, methylcyclohexane, and toluene (AL-P-I-O-N-A). Experimental tests are conducted in two high-pressure shock tubes to determine the ignition delay time as a function of pressure, temperature and equivalence ratio. Ignition delay times were measured at 10 and 30 bar in the temperature range from 749 to 1204 K and equivalence ratios ranging from 0.35 to 1.30. A modified Arrhenius equation is defined to mathematically describe the ignition delay time of the proposed surrogate. For experimental data with temperatures higher than 900 K, a multiple linear regression identified the pressure dependence exponent of 0.72 and stoichiometry dependence exponent of 0.62, as well as a global activation energy of ≈109 kJ/mol. A simplistic approach to mechanism reduction based on the elimination of reactions with no relevant rate of progress was used in order to reduce an extensive detailed kinetics model (hierarchically constructed with more than 17800 reactions). The reduced detailed kinetics model with 4885 elementary reactions among 326 chemical species was used for numerical simulations. Comparisons between the experimental and numerical data are favorable, with the predictions using the reduced kinetics model differing by less than 0.056% when compared to the complete mechanism. It was observed that for low temperatures the proposed reduced kinetics model agrees only qualitatively with the measurements. In order to understand the likely cause of this discrepancy a brute force sensitivity analysis on IDT was performed, elucidating the more influencing reactions on the ignition delay times. The experimental data obtained in this research was compared to available data in the literature in terms of anti-knock index (AKI) and for a scaled pressure of 30 bar (τ30) at a stoichiometric composition. A modified Arrhenius equation was then fitted and an AKI dependence exponent of -1.11 was obtained, inferring that the higher the AKI the higher the IDT, independent of fuel composition at temperatures lower than the NTC region. This trend should be confirmed by further studies. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2019.116439
  • 2020 • 153 Acidity enhancement through synergy of penta- and tetra-coordinated aluminum species in amorphous silica networks
    Wang, Z. and Li, T. and Jiang, Y. and Lafon, O. and Liu, Z. and Trébosc, J. and Baiker, A. and Amoureux, J.-P. and Huang, J.
    Nature Communications 11 (2020)
    Amorphous silica-aluminas (ASAs) are widely used in acid-catalyzed C-H activation reactions and biomass conversions in large scale, which can be promoted by increasing the strength of surface Brønsted acid sites (BAS). Here, we demonstrate the first observation on a synergistic effect caused by two neighboring Al centers interacting with the same silanol group in flame-made ASAs with high Al content. The two close Al centers decrease the electron density on the silanol oxygen and thereby enhance its acidity, which is comparable to that of dealuminated zeolites, while ASAs with small or moderate Al contents provide mainly moderate acidity, much lower than that of zeolites. The ASAs with enhanced acidity exhibit outstanding performances in C–H bond activation of benzene and glucose dehydration to 5-hydroxymethylfurfural, simultaneously with an excellent calcination stability and resistance to leaching, and they offer an interesting potential for a wide range of acid and multifunctional catalysis. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-019-13907-7
  • 2020 • 152 Beyond thermodynamic-based material-screening concepts: Kinetic scaling relations exemplified by the chlorine evolution reaction over transition-metal oxides
    Exner, K.S.
    Electrochimica Acta 334 (2020)
    State-of-the-art material screening in the field of electrocatalysis mainly uses the concept of linear scaling relationships in order to express the (free) adsorption energies of different reaction intermediates, adsorbed on the surface of a solid-state electrocatalyst, as function of a descriptor. This thermodynamic analysis, based on the application of the computational hydrogen electrode approach (CHE), ultimately results in the construction of a Volcano plot, which facilitates identifying promising catalysts within a class of materials. The conventional ab initio Volcano concept, however, lacks of two critical aspects: on the one hand the applied overpotential, which constitutes the driving force of an electrocatalytic reaction, is not included in the underlying approach, since the thermodynamic analysis refers to the standard equilibrium potential of the electrocatalytic process; on the other hand, the kinetics is not accounted for. Herein, an alternate material-screening concept is presented, which promotes a discussion of the catalytic performance within a class of materials by explicitly including both the kinetic description and applied overpotential: kinetic scaling relations enable resolving the rate-determining reaction step in a homologous series of single-crystalline electrocatalysts in the overpotential regime of interest for practical applications. The proposed methodology is exemplified by the chlorine evolution reaction over transition-metal oxides, which corresponds to the anode reaction in the industrially relevant chlor-alkali process for the production of gaseous chlorine as basic chemical. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2019.135555
  • 2020 • 151 Effect of temperature on the kinetics of electrochemical insertion of li-ions into a graphite electrode studied by kinetic Monte Carlo
    Gavilán-Arriazu, E.M. and Mercer, M.P. and Pinto, O.A. and Oviedo, O.A. and Barraco, D.E. and Hoster, H.E. and Leiva, E.P.M.
    Journal of the Electrochemical Society 167 (2020)
    The effect of temperature on the kinetics of electrochemical insertion/removal of lithium in graphite is analyzed by kinetic Monte Carlo methods. Different electrochemical techniques are simulated at different temperatures and responses are compared with experimental results. Simulated voltammograms show, similarly to experiment, how the behavior of the system becomes closer to equilibrium as temperature increases. Calculated chronoamperometric profiles show a different qualitative behavior in the current at different temperatures, especially in the Cottrell representation peaks, explained in terms of the relative importance of diffusive versus charge transfer processes at different temperatures. Results at room temperature are in good agreement with experiment, and we further evaluate trends at elevated temperature that have not yet been described in experimental or theoretical works. Exchange current densities for different degrees of lithium intercalation at different temperatures are predicted using potentiostatic simulations, showing an Arrhenius-type relationship. The dependence of the exchange current on electrolyte composition is simulated by investigating the effect of different activation energy barriers at different temperatures. The influence of temperature on diffusion coefficients as a function of lithiation fraction in graphite is simulated and related to Arrhenius plots, explaining the experimentally observed changes in diffusion phenomena with lithium composition and temperature. © The Author(s) 2019. Published by ECS.
    view abstractdoi: 10.1149/2.0332001JES
  • 2020 • 150 Experimental flat flame study of monoterpenes: Insights into the combustion kinetics of α-pinene, β-pinene, and myrcene
    Bierkandt, T. and Hoener, M. and Gaiser, N. and Hansen, N. and Köhler, M. and Kasper, T.
    Proceedings of the Combustion Institute (2020)
    Pinenes and pinene dimers have similar energy densities to petroleum-based fuels and are regarded as alternative fuels. The pyrolysis of the pinenes is well studied, but information on their combustion kinetics is limited. Three stoichiometric, flat premixed flames of the C10H16 monoterpenes α-pinene, β-pinene, and myrcene were investigated by synchrotron-based photoionization molecular-beam mass spectrometry (PI-MBMS) at the Advanced Light Source (ALS). This technique allows isomer-resolved identification and quantification of the flame species formed during the combustion process. Flame-sampling molecular-beam mass spectrometry even enables the detection of very reactive radical species. Myrcene was chosen because of its known formation during β-pinene pyrolysis. The quantitative speciation data and the discussed decomposition steps of the fuels provide important information for the development of future chemical kinetic reaction mechanisms concerning pinene combustion. The main decomposition of myrcene starts with the unimolecular cleavage of the carbon-carbon single bond between the two allylic carbon atoms. Further decompositions by β-scission to stable combustion intermediates such as isoprene (C5H8), 1,2,3-butatriene (C4H4) or allene (aC3H4) are consistent with the observed species pool. Concentrations of all detected hydrocarbons in the β-pinene flame are closer to the myrcene flame than to the α-pinene flame. These similarities and the direct identification of myrcene by its photoionization efficiency spectrum during β-pinene combustion indicate that β-pinene undergoes isomerization to myrcene under the studied flame conditions. Aromatic species such as phenylacetylene (C8H6), styrene (C8H8), xylenes (C8H10), and indene (C9H8) could be clearly identified and have higher concentrations in the α-pinene flame. Consequently, a higher sooting tendency can generally be expected for this monoterpene. The presented quantitative speciation data of flat premixed flames of the three monoterpenes α-pinene, β-pinene, and myrcene give insights into their combustion kinetics. © 2020 The Combustion Institute.
    view abstractdoi: 10.1016/j.proci.2020.06.204
  • 2020 • 149 Flexible energy conversion and storage via high-temperature gas-phase reactions: The piston engine as a polygeneration reactor
    Atakan, B. and Kaiser, S.A. and Herzler, J. and Porras, S. and Banke, K. and Deutschmann, O. and Kasper, T. and Fikri, M. and Schießl, R. and Schröder, D. and Rudolph, C. and Kaczmarek, D. and Gossler, H. and Drost, S. and Bykov...
    Renewable and Sustainable Energy Reviews 133 (2020)
    Piston engines are typically considered devices converting chemical energy into mechanical power via internal combustion. But more generally, their ability to provide high-pressure and high-temperature conditions for a limited time means they can be used as chemical reactors where reactions are initiated by compression heating and subsequently quenched by gas expansion. Thus, piston engines could be “polygeneration” reactors that can flexibly change from power generation to chemical synthesis, and even to chemical-energy storage. This may help mitigating one of the main challenges of future energy systems – accommodating fluctuations in electricity supply and demand. Investments in devices for grid stabilization could be more economical if they have a second use. This paper presents a systematic approach to polygeneration in piston engines, combining thermodynamics, kinetics, numerical optimization, engineering, and thermo-economics. A focus is on the fuel-rich conversion of methane as a fuel that is considered important for the foreseeable future. Starting from thermodynamic theory and kinetic modeling, promising systems are selected. Mathematical optimization and an array of experimental kinetic investigations are used for model improvement and development. To evaluate technical feasibility, experiments are then performed in both a single-stroke rapid compression machine and a reciprocating engine. In both cases, chemical conversion is initiated by homogeneous-charge compression-ignition. A thermodynamic and thermo-economic assessment of the results is positive. Examples that illustrate how the piston engine can be used in polygeneration processes to convert methane to higher-value chemicals or to take up carbon dioxide are presented. Open issues for future research are addressed. © 2020 The Authors
    view abstractdoi: 10.1016/j.rser.2020.110264
  • 2020 • 148 Generalized stability criterion for exploiting optimized mechanical properties by a general correlation between phase transformations and plastic deformations
    Huang, L. and Lin, W. and Zhang, Y. and Feng, D. and Li, Y. and Chen, X. and Niu, K. and Liu, F.
    Acta Materialia 201 167-181 (2020)
    Designing structured materials with optimized mechanical properties generally focuses on engineering microstructures, which are closely determined by the processing routes, such as phase transformations. However, the direct connection between phase transformations and mechanical properties remains largely unexplored. Here, we propose a new concept of generalized stability (GS) to correlate phase transformations with plastic deformations in terms of the trade-off relationship that exists between thermodynamics and kinetics. We then suggest that, to achieve structured materials with excellent strength–plasticity combinations, phase transformations and/or plastic deformations with high GS, thermodynamic driving force (ΔG), and kinetic activation energy (Q), are highly expected. We verify the GS concept against a phase transformation-modulated nanostructured Fe alloy, for which an ultrahigh yield strength of 2.61 GPa and an ultimate compressive strength of 3.32 GPa while having a total strain to failure of 35% are achieved via multiple strengthening and hardening mechanisms. A theoretical analysis, in combination with microstructural characterization, indicates that the desired thermo-kinetic parameter triplets (i.e., high GS-high ΔG-high Q) could be inherited from the phase transformation to the plastic deformation, which ultimately yields good mechanical performance. The proposed concept can be regarded as the first theoretical criterion or a general rule that correlates phase transformation with plastic deformation, and can assist in the rapid selection of phase transformations to facilitate superior mechanical properties. © 2020
    view abstractdoi: 10.1016/j.actamat.2020.10.005
  • 2020 • 147 Growth kinetics of σ-phase precipitates and underlying diffusion processes in CrMnFeCoNi high-entropy alloys
    Laplanche, G.
    Acta Materialia 199 193-208 (2020)
    Key mechanisms and elementary diffusion processes that control the growth kinetics of σ precipitates in high-entropy alloys were investigated in the present study. For this purpose, an off-equiatomic Cr26Mn20Fe20Co20Ni14 alloy with an initially single-phase FCC structure was subjected to isothermal heat treatments, which are known to promote the formation of σ phase, i.e., aging between 600 °C and 1000 °C for times ranging from 0.1 h to 1000 h. The growth kinetics of σ precipitates at grain boundaries of the FCC matrix and those located within the interior of the grains were analyzed separately. The latter precipitates are found to grow through direct substitutional diffusion of Cr-solutes towards and Mn, Fe, Co, and Ni away from them and the growth rate of the allotriomorphs can be rationalized by the collector plate mechanism of interfacial diffusion-aided growth. From the growth-kinetics data obtained in the present study, lattice interdiffusion coefficients as well as diffusivities along crystalline defects were obtained. Above 800 °C, the growth kinetics are dominated by lattice interdiffusion of Cr in the FCC matrix described by DL = 9.8 × 10-4 exp[(-300 kJ/mol)/(RT)] m2/s. At lower temperatures, the growth kinetics are enhanced by fast interdiffusion along dislocation pipes, which temperature dependence is given by DD = 5.0 × 10-3 exp[(-205 kJ/mol)/(RT)] m2/s. The Cr-diffusivity along σ/FCC interphase boundaries deduced from the thickening kinetics of grain boundary precipitates can be represented by the Arrhenius relationship DI = 0.5 × 10-4 exp[(-145 kJ/mol)/(RT)] m2/s, which is similar to that found for grain boundary interdiffusion in metals and alloys. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.08.023
  • 2020 • 146 Influence of the bias-voltage, the argon pressure and the heating power on the structure and the tribological properties of HiPIMS sputtered MoSx films
    Tillmann, W. and Wittig, A. and Stangier, D. and Moldenhauer, H. and Thomann, C.-A. and Debus, J. and Aurich, D. and Bruemmer, A.
    Surface and Coatings Technology 385 (2020)
    Understanding the growth process and its correlation to the structure of MoSx thin films is essential to control the friction behavior. Nevertheless, structural changes related to kinetic and thermal processes occurring during the deposition are not yet fully understood within the context of MoSx sputtered thin films. Therefore, MoSx films were synthesized by HiPIMS (High Power Impulse Magnetron Sputtering) technique using the one factor at a time method. By systematically changing the bias-voltage (0 to −200 V), the argon pressure (200 mPa to 600 mPa) or the heating power (0 to 3000 W) the interaction between the deposition parameters and their impact on the structure and the tribological properties was analyzed.The results show significant differences regarding the influence of kinetic and thermal effects. The investigation of the crystallographic orientation by XRD measurements reveals that a high kinetic energy induced by a high bias-voltage favors the growth of the (100) edge plane. A deposition process with a low deposition temperature and thus a low deposition rate leads to a more pronounced (002) basal plane due to the lower surface energy of the (002) surface. A high kinetic energy is also related to a densification of the morphology and a decrease in the sulfur content, which results in a thicker tribofilm and thus a lower wear and friction. Films deposited with a high heating power on the other show a low friction, but at the same time a columnar microstructure and high wear. Thus, the structure affects the amount of generated wear particles during the sliding, but more important is the ability of keeping them in the contact area during the tribo-tests. © 2020
    view abstractdoi: 10.1016/j.surfcoat.2020.125358
  • 2020 • 145 Ionization probability of sputtered indium atoms under impact of slow highly charged ions
    Herder, M. and Ernst, P. and Skopinski, L. and Weidtmann, B. and Schleberger, M. and Wucher, A.
    Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics 38 (2020)
    In order to investigate the different role of kinetic and potential projectile energy for secondary ion formation, the authors have measured the ionization probability of indium atoms sputtered from a clean indium surface under irradiation with rare gas (Xeq+) ions of different charge states q at the same kinetic impact energy of 20 keV. In this energy range, the kinetic energy of the projectile is predominantly deposited via nuclear stopping, leading to a collision-dominated sputtering process. The authors find that the ionization probability increases significantly if a highly charged ion is used as a projectile, where the ionization energy becomes comparable to or even exceeds the kinetic energy, indicating that a higher level of electronic substrate excitation induced by the potential energy stored in the projectile can boost the secondary ion formation process. This experimental result is discussed in terms of microscopic model calculations describing the secondary ion formation process. At the same time, the authors observe a significant change of the emission velocity distribution of the sputtered particles, leading to a pronounced low-energy contribution at higher projectile charge states. It is shown that this "potential sputtering"contribution strongly depends on surface chemistry even under conditions where the surface is dynamically cleaned by interleaved 5 keV Ar+ ion bombardment. © 2020 Author(s).
    view abstractdoi: 10.1116/6.0000171
  • 2020 • 144 Kinetic approach to nuclear-spin polaron formation
    Fischer, A. and Kleinjohann, I. and Anders, F.B. and Glazov, M.M.
    Physical Review B 102 (2020)
    Under optical cooling of nuclei, a strongly correlated nuclear-spin polaron state can form in semiconductor nanostructures with localized charge carriers due to the strong hyperfine interaction of the localized electron spin with the surrounding nuclear spins. Here we develop a kinetic-equation formalism describing the nuclear-spin polaron formation. We present a derivation of the kinetic equations for an electron-nuclear spin system coupled to reservoirs of different electron and nuclear spin temperatures which generate the exact thermodynamic steady state for equal temperatures independent of the system size. We illustrate our approach using the analytical solution of the central spin model in the limit of an Ising form of the hyperfine coupling. For homogeneous hyperfine coupling constants, i.e., the box model, the model is reduced to an analytically solvable form. Based on the analysis of the nuclear-spin distribution function and the electron-nuclear spin correlators, we derive a relation between the electron and nuclear spin temperatures, where the correlated nuclear-spin polaron state is formed. In the limit of large nuclear baths, this temperature line coincides with the critical temperature of the mean-field theory for polaron formation. The criteria of the polaron formation in a finite-size system are discussed. We demonstrate that the system's behavior at the transition temperature does not depend on details of the hyperfine-coupling distribution function but only on the effective number of coupled bath spins. In addition, the kinetic equations enable the analysis of the temporal formation of the nuclear-polaron state, where we find the build-up process predominated by the nuclear spin-flip dynamics. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.165309
  • 2020 • 143 Monitoring formaldehyde in a shock tube with a fast dual-comb spectrometer operating in the spectral range of 1740–1790 cm–1
    Fjodorow, P. and Allmendinger, P. and Horvath, R. and Herzler, J. and Eigenmann, F. and Geiser, M. and Fikri, M. and Schulz, C.
    Applied Physics B: Lasers and Optics 126 (2020)
    A dual-frequency-comb spectrometer based on two quantum-cascade lasers is applied to kinetics studies of formaldehyde (HCHO) in a shock tube. Multispectral absorption measurements are carried out in a broad spectral range of 1740–1790 cm–1 at temperatures of 800–1500 K and pressures of 2–3 bar. The formation of HCHO from thermal decomposition of 1,3,5-trioxane (C3H6O3, 0.9% diluted in argon) and the subsequent oxidation of formaldehyde is monitored with a time resolution of 4 µs. The rate coefficient of the decomposition of C3H6O3 (i.e., HCHO formation) is found to be k1 = 6.0 × 1015 exp(− 205.58 kJ mol−1/RT) s–1. For the oxidation studies, mixtures of 0.36% C3H6O3 and 1% O2 in argon are used. The information of all laser lines, along with the consideration of individual signal variance of each line, is utilized for kinetic and spectral analysis. The experimental kinetic profiles of HCHO are compared with simulations based on the mechanisms of Zhou et al. (Combust Flame, 197:423–438, 2018) and Cai and Pitsch (Combust Flame, 162:1623–1637, 2015). © 2020, The Author(s).
    view abstractdoi: 10.1007/s00340-020-07545-x
  • 2020 • 142 Numerical simulations of cyclic voltammetry for lithium-ion intercalation in nanosized systems: finiteness of diffusion versus electrode kinetics
    Gavilán-Arriazu, E.M. and Mercer, M.P. and Pinto, O.A. and Oviedo, O.A. and Barraco, D.E. and Hoster, H.E. and Leiva, E.P.M.
    Journal of Solid State Electrochemistry 24 3279-3287 (2020)
    The voltammetric behavior of Li+ intercalation/deintercalation in/from LiMn2O4 thin films and single particles is simulated, supporting very recent experimental results. Experiments and calculations both show that particle size and geometry are crucial for the electrochemical response. A remarkable outcome of this research is that higher potential sweep rates, of the order of several millivolts per second, may be used to characterize nanoparticles by voltammetry sweeps, as compared with macroscopic systems. This is in line with previous conclusions drawn for related single particle systems using kinetic Monte Carlo simulations. The impact of electrode kinetics and finite space diffusion on the reversibility of the process and the finiteness of the diffusion in ion Li / LiMn2O4 (de)intercalation is also discussed in terms of preexisting modeling. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s10008-020-04717-9
  • 2020 • 141 Particle emission from two-dimensional MoS2 induced by highly charged ion impact
    Skopinski, L. and Ernst, P. and Herder, M. and Schleberger, M.
    Journal of Physics: Conference Series 1412 (2020)
    For many attractive applications of single layer MoS2 such as in optoelectronics e.g., the sample is supported by a substrate. Its importance for the modification through ion irradiation is here experimentally investigated by the analysis of sputtered particle of MoS2 on SiO2 and Au substrates under highly charged ion irradiation. The velocity distribution of the sputtered atoms is less affected by the substrate using highly charged projectiles than using slightly charged ones. Furthermore, we can show that potential sputtering causes additional emission of particles with lower kinetic energy. © 2019 Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/1412/20/202007
  • 2020 • 140 Targeting Her2-insYVMA with Covalent Inhibitors - A Focused Compound Screening and Structure-Based Design Approach
    Lategahn, J. and Hardick, J. and Grabe, T. and Niggenaber, J. and Jeyakumar, K. and Keul, M. and Tumbrink, H.L. and Becker, C. and Hodson, L. and Kirschner, T. and Klövekorn, P. and Ketzer, J. and Baumann, M. and Terheyden, S. an...
    Journal of Medicinal Chemistry 63 11725-11755 (2020)
    Mutated or amplified Her2 serves as a driver of non-small cell lung cancer or mediates resistance toward the inhibition of its family member epidermal growth factor receptor with small-molecule inhibitors. To date, small-molecule inhibitors targeting Her2 which can be used in clinical routine are lacking, and therefore, the development of novel inhibitors was undertaken. In this study, the well-established pyrrolopyrimidine scaffold was modified with structural motifs identified from a screening campaign with more than 1600 compounds, which were applied against wild-type Her2 and its mutant variant Her2-A775_G776insYVMA. The resulting inhibitors were designed to covalently target a reactive cysteine in the binding site of Her2 and were further optimized by means of structure-based drug design utilizing a set of obtained complex crystal structures. In addition, the analysis of binding kinetics and absorption, distribution, metabolism, and excretion parameters as well as mass spectrometry experiments and western blot analysis substantiated our approach. ©
    view abstractdoi: 10.1021/acs.jmedchem.0c00870
  • 2020 • 139 Thermodynamics and kinetics of glycolytic reactions. Part i: Kinetic modeling based on irreversible thermodynamics and validation by calorimetry
    Vogel, K. and Greinert, T. and Reichard, M. and Held, C. and Harms, H. and Maskow, T.
    International Journal of Molecular Sciences 21 1-20 (2020)
    In systems biology, material balances, kinetic models, and thermodynamic boundary conditions are increasingly used for metabolic network analysis. It is remarkable that the reversibility of enzyme‐catalyzed reactions and the influence of cytosolic conditions are often neglected in kinetic models. In fact, enzyme‐catalyzed reactions in numerous metabolic pathways such as in glycolysis are often reversible, i.e., they only proceed until an equilibrium state is reached and not until the substrate is completely consumed. Here, we propose the use of irreversible thermodynamics to describe the kinetic approximation to the equilibrium state in a consistent way with very few adjustable parameters. Using a flux‐force approach allowed describing the influence of cytosolic conditions on the kinetics by only one single parameter. The approach was applied to reaction steps 2 and 9 of glycolysis (i.e., the phosphoglucose isomerase reaction from glucose 6‐ phosphate to fructose 6‐phosphate and the enolase‐catalyzed reaction from 2‐phosphoglycerate to phosphoenolpyruvate and water). The temperature dependence of the kinetic parameter fulfills the Arrhenius relation and the derived activation energies are plausible. All the data obtained in this work were measured efficiently and accurately by means of isothermal titration calorimetry (ITC). The combination of calorimetric monitoring with simple flux‐force relations has the potential for adequate consideration of cytosolic conditions in a simple manner. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ijms21218341
  • 2020 • 138 Vanishing influence of the band gap on the charge exchange of slow highly charged ions in freestanding single-layer MoS2
    Creutzburg, S. and Schwestka, J. and Niggas, A. and Inani, H. and Tripathi, M. and George, A. and Heller, R. and Kozubek, R. and Madauß, L. and McEvoy, N. and Facsko, S. and Kotakoski, J. and Schleberger, M. and Turchanin, A. and...
    Physical Review B 102 (2020)
    Charge exchange and kinetic energy loss of slow highly charged xenon ions transmitted through freestanding monolayer MoS2 are studied. Two distinct exit charge state distributions, characterized by high and low charge states, are observed. They are accompanied by smaller and larger kinetic energy losses, as well as scattering angles, respectively. High charge exchange is attributed to two-center neutralization processes, which take place in close impact collisions with the target atoms. Experimental findings are compared to graphene as a target material and simulations based on a time-dependent scattering potential model. Independent of the target material, experimentally observed charge exchange can be modeled by the same electron capture and de-excitation rates for MoS2 and graphene. A common dependence of the kinetic energy loss on the charge exchange for MoS2 as well as graphene is also observed. Considering the similarities of the zero band-gap material graphene and the 1.9 eV band-gap material MoS2, we suggest that electron transport on the femtosecond timescale is dominated by the strong influence of the ion's Coulomb potential in contrast to the dispersion defined by the material's band structure. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.045408
  • 2019 • 137 Assessment of coal pyrolysis kinetics for Barracuda or Ansys Fluent
    Krusch, S. and Scherer, V. and Solimene, R. and Senneca, O.
    Energy Procedia 158 1999-2004 (2019)
    In the present work different methodologies have been used to analyze pyrolysis of a German bituminous coal "Auguste Victoria" with the aim to obtain pyrolysis kinetic submodels suitable for combustion models developed under Barracuda or other CFD environment such as Ansys Fluent. Intrinsic kinetics have been obtained by conventional TGA analysis at low heating rate (2-20 K/min) and with small particle size (in the order of 0.1 mm) and described by a two-step pyrolysis model. Additionally, the rates of pyrolysis were measured under fluidized bed conditions for larger particle size fractions using a method based on the time-resolved measurement of pressure in the freeboard induced by volatiles release and by a flow restriction at the exhaust. Results of thermogravimetric experiments have been worked out to obtain submodels of coal pyrolysis, comparing and discussing alternative reaction networks of different complexity. However, in fluidized bed experiments, using larger particle sizes, transport limitations become important. As expected, for this regime the reactions rates become lower the larger the particle size, reducing the relevance of intrinsic kinetic submodels. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 - The 10th International Conference on Applied Energy.
    view abstractdoi: 10.1016/j.egypro.2019.01.459
  • 2019 • 136 Beyond the Rate-Determining Step in the Oxygen Evolution Reaction over a Single-Crystalline IrO2(110) Model Electrode: Kinetic Scaling Relations
    Exner, K.S. and Over, H.
    ACS Catalysis 9 6755-6765 (2019)
    Electrochemical water splitting is a key technology for moving toward a promising energy scenario based on renewable (regenerative) energy resources in that wind and solar energy can be stored and buffered in chemical bonds, such as in H2. The efficiency of water electrolysis is, however, limited by the sluggish oxygen evolution reaction (OER) at the anode, for which IrO2-based electrodes are considered to be the best compromise of a stable and reasonably active OER electrocatalyst in acidic medium. To improve existing OER electrocatalysts and to advance a rational search of promising alternative electrode materials, it is imperative to identify the rate-determining step (rds). We apply here the concept of the free energy diagram along the reaction coordinate to identify the rate-determining step (rds) in the oxygen evolution reaction (OER) over an IrO2(110) model anode in both acidic and basic media. The free energy diagram as a function of the applied electrode potential is constructed from experimental Tafel plots and ab initio Pourbaix diagrams. Quite in contrast to common perception, the rds for the OER over IrO2(110) at high overpotentials is identified with the decomposition of the OOH adsorbate via a decoupled electron-proton transfer to form gaseous O2. Combining linear scaling relationships with the free energy diagram approach leads to the introduction of kinetic scaling relations, which allow us to predict the rate-determining step (rds) of the OER over general transition metal oxide electrocatalysts in the high-overpotential regime by a single descriptor, namely, the free formation energy of oxygen with respect to the OH adsorbate (Î"G2) on the anode surface. On the basis of kinetic scaling relations we suggest that further improvement of the catalytic OER performance may require a decoupling of the electron-proton transfer in the rds. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.9b01564
  • 2019 • 135 Combined co-solvent and pressure effect on kinetics of a peptide hydrolysis: An activity-based approach
    Knierbein, M. and Wangler, A. and Luong, T.Q. and Winter, R. and Held, C. and Sadowski, G.
    Physical Chemistry Chemical Physics 21 22224-22229 (2019)
    The application of co-solvents and high pressure has been reported to be an efficient means to tune the kinetics of enzyme-catalyzed reactions. Co-solvents and pressure can lead to increased reaction rates without sacrificing enzyme stability, while temperature and pH operation windows are generally very narrow. Quantitative prediction of co-solvent and pressure effects on enzymatic reactions has not been successfully addressed in the literature. Herein, we are introducing a thermodynamic approach that is based on molecular interactions in the form of activity coefficients of substrate and of enzyme in the multi-component solution. This allowed us to quantitatively predict the combined effect of co-solvent and pressure on the kinetic constants, i.e.The Michaelis constant KM and the catalytic constant kcat, of an α-CT-catalyzed peptide hydrolysis reaction. The reaction was studied in the presence of different types of co-solvents and at pressures up to 2 kbar, and quantitative predictions could be obtained for KM, kcat, and finally even primary Michaelis-Menten plots using activity coefficients provided by the thermodynamic model PC-SAFT. This journal is © the Owner Societies.
    view abstractdoi: 10.1039/c9cp03868j
  • 2019 • 134 Comparative study of flame-based SiO 2 nanoparticle synthesis from TMS and HMDSO: SiO-LIF concentration measurement and detailed simulation
    Chrystie, R.S.M. and Janbazi, H. and Dreier, T. and Wiggers, H. and Wlokas, I. and Schulz, C.
    Proceedings of the Combustion Institute 37 1221-1229 (2019)
    Depending on the chemical nature of precursor species, the flame-based synthesis of silicon dioxide nanoparticles in lean hydrogen/oxygen flames proceeds via different chemical routes, which affects the generated particle characteristics. Modeling the flame chemistry and particle formation therefore can provide valuable understanding of the underlying gas-phase and particle-formation pathways. In the present study we compare experimentally obtained temperature and semi-quantified SiO-concentration profiles in low-pressure (3 kPa), lean (? < 0.6), inert-gas diluted H 2 /O 2 /Ar burner-stabilized flat flames doped with 200-4000 ppm hexamethyldisiloxane (HMDSO) or tetramethylsilane (TMS) with results from kinetics modeling. Temperature fields in the flames were determined via multi-line laser-induced fluorescence (LIF) imaging using both added NO and native SiO as target species. Gas-phase silicon monoxide (SiO) was detected via LIF by exciting the rovibrational Q(42) transition in the A 1 Π-X 1 Σ + (1,0) vibronic band system at 230.998 nm that provides a weak temperature dependence when analyzing relative SiO mole fractions. Semi-quantitative SiO mole-fraction profiles as a function of height-above-burner (HAB) were obtained for all flames from the measured SiO-LIF intensities corrected for variations of the temperature-dependent ground-state population and the collisional quenching using measured temperatures and effective fluorescence lifetimes, respectively. The experimental data were compared with results of appropriate chemical kinetics mechanisms from the literature with suitable modifications to best reproduce measured SiO mole-fraction profiles. Modeling initial cluster formation is important in this study to unravel the observed 'double-peak'-structure of the SiO concentration profiles assumed to originate from resublimed SiO from early-formed SiO 2 nanoparticles in the rising temperature gradient during initial particle nucleation, and which may be altered by the availability of oxygen in the precursor species. © 2018 The Combustion Institute.
    view abstractdoi: 10.1016/j.proci.2018.07.024
  • 2019 • 133 Comparative study of flame-based SiO2 nanoparticle synthesis from TMS and HMDSO: SiO-LIF concentration measurement and detailed simulation
    Chrystie, R.S.M. and Janbazi, H. and Dreier, T. and Wiggers, H. and Wlokas, I. and Schulz, C.
    Proceedings of the Combustion Institute 37 1221-1229 (2019)
    Depending on the chemical nature of precursor species, the flame-based synthesis of silicon dioxide nanoparticles in lean hydrogen/oxygen flames proceeds via different chemical routes, which affects the generated particle characteristics. Modeling the flame chemistry and particle formation therefore can provide valuable understanding of the underlying gas-phase and particle-formation pathways. In the present study we compare experimentally obtained temperature and semi-quantified SiO-concentration profiles in low-pressure (3 kPa), lean (? < 0.6), inert-gas diluted H2/O2/Ar burner-stabilized flat flames doped with 200-4000 ppm hexamethyldisiloxane (HMDSO) or tetramethylsilane (TMS) with results from kinetics modeling. Temperature fields in the flames were determined via multi-line laser-induced fluorescence (LIF) imaging using both added NO and native SiO as target species. Gas-phase silicon monoxide (SiO) was detected via LIF by exciting the rovibrational Q(42) transition in the A1Π-X1Σ+ (1,0) vibronic band system at 230.998 nm that provides a weak temperature dependence when analyzing relative SiO mole fractions. Semi-quantitative SiO mole-fraction profiles as a function of height-above-burner (HAB) were obtained for all flames from the measured SiO-LIF intensities corrected for variations of the temperature-dependent ground-state population and the collisional quenching using measured temperatures and effective fluorescence lifetimes, respectively. The experimental data were compared with results of appropriate chemical kinetics mechanisms from the literature with suitable modifications to best reproduce measured SiO mole-fraction profiles. Modeling initial cluster formation is important in this study to unravel the observed 'double-peak'-structure of the SiO concentration profiles assumed to originate from resublimed SiO from early-formed SiO2 nanoparticles in the rising temperature gradient during initial particle nucleation, and which may be altered by the availability of oxygen in the precursor species. © 2018 The Combustion Institute.
    view abstractdoi: 10.1016/j.proci.2018.07.024
  • 2019 • 132 Design criteria for oxygen evolution electrocatalysts from first principles: Introduction of a unifying material-screening approach
    Exner, K.S.
    ACS Applied Energy Materials 2 7991-8001 (2019)
    The oxygen evolution reaction (OER) is the bottleneck in proton-exchange membrane (PEM) electrolyzers as substantial overpotentials are required for the formation of gaseous oxygen at anode side to reach a satisfying current density. In the past years, substantial research investigations were dedicated to search for electrode materials with an ameliorated OER activity. Therein, different frameworks are proposed in the literature. The conventional method based on the computational hydrogen electrode approach relies on an assessment of simple binding energies by deriving linear scaling relationships that translate to a volcano plot at zero overpotential. Recently, the traditional volcano concept was extended, in that the applied overpotential and kinetics were accounted for by deducing overpotential-dependent volcano curves or kinetic scaling relations, respectively. An alternative framework corresponds to the electrochemical-step symmetry index (ESSI), which was suggested as an improved measure within the search of potential OER electrocatalysts. Hitherto, there is no connection between these diverse methods, and it remains elusive which of these approaches is most suitable for material-screening purposes. On the example of the OER over transition-metal oxides, porphyrins, perovskites, metal oxides, and functionalized graphitic materials, a powerful combination of linear scaling relationships, kinetic scaling relations, overpotential-dependent Volcano plots, and ESSI is presented. While the computational costs for this unifying approach are the same as for the traditional volcano analysis, the inclusion of a single experimental input parameter in the underlying framework enables gaining unprecedented insights into catalyst design, thereby considering various aspects, such as binding energies, applied overpotential, decisive reaction intermediate, rate-determining reaction step, and catalytic symmetry. It is suggested to employ the concept of activity maps, as introduced in this contribution, for material-screening studies of multielectron processes in energy and environmental science. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b01480
  • 2019 • 131 Experimental and numerical investigation on the flow in a return channel of multistage centrifugal compressors
    Dolle, B. and Brillert, D. and Dohmen, H.J. and Benra, F.-K.
    Proceedings of the ASME Turbo Expo 2B-2019 (2019)
    Multistage radial compressors are major components in plenty of industrial applications. Today, compressor downsizing for CAPEX reduction is of utmost importance. Accordingly, the pressure ratio must be increased, accommodated in a most compact design, accepting nearly no penalties in the performance level achieved so far. In order to investigate the complex flow in stator parts of multistage centrifugal compressors and to increase the confidence level of numerical methods a new test rig is developed and taken into operation. This test rig allows to separate stationary flow effects from time variant effects leading to an in depth insight into the physical flow features. The aim is to investigate the flow in different stator designs in detail for varying flow coefficients. Additive manufacturing techniques are applied to achieve low costs simultaneously with short production time for the tested parts. In this publucation, the measured flow field in the stator parts is discussed. The experimental results comprise velocity and pressure data from five-hole-probes and unsteady velocity data from single-film hot-film-probes. Using unsteady velocity data, turbulent statistics such as turbulence intensity and the turbulent kinetic energy will be determined. Subsequently, the experimental results are compared to numerical results. Therefore, (U)RANS simulations are performed using a commercial CFD-code. The simulation results are evaluated at planes appropriate to the measuring planes in the test rig to guarantee a directly comparable data base. Copyright © 2019 ASME.
    view abstractdoi: 10.1115/GT2019-90455
  • 2019 • 130 Fully-resolved simulations of coal particle combustion using a detailed multi-step approach for heterogeneous kinetics
    Tufano, G.L. and Stein, O.T. and Kronenburg, A. and Gentile, G. and Stagni, A. and Frassoldati, A. and Faravelli, T. and Kempf, A.M. and Vascellari, M. and Hasse, C.
    Fuel 75-83 (2019)
    Fully-resolved simulations of the heating, ignition, volatile flame combustion and char conversion of single coal particles in convective gas environments are conducted and compared to experimental data (Molina and Shaddix, 2007). This work extends a previous computational study (Tufano et al., 2016) by adding a significant level of model fidelity and generality, in particular with regard to the particle interior description and heterogeneous kinetics. The model considers the elemental analysis of the given coal and interpolates its properties by linear superposition of a set of reference coals. The improved model description alleviates previously made assumptions of single-step pyrolysis, fixed volatile composition and simplified particle interior properties, and it allows for the consideration of char conversion. The results show that the burning behavior is affected by the oxygen concentration, i.e. for enhanced oxygen levels ignition occurs in a single step, whereas decreasing the oxygen content leads to a two-stage ignition process. Char conversion becomes dominant once the volatiles have been depleted, but also causes noticeable deviations of temperature, released mass, and overall particle conversion during devolatilization already, indicating an overlap of the two stages of coal conversion which are usually considered to be consecutive. The complex pyrolysis model leads to non-monotonous profiles of the combustion quantities which introduce a minor dependency of the ignition delay time τign on its definition. Regardless of the chosen extraction method, the simulations capture the measured values of τign very well. © 2018 The Authors
    view abstractdoi: 10.1016/j.fuel.2018.11.139
  • 2019 • 129 Is Thermodynamics a Good Descriptor for the Activity? Re-Investigation of Sabatier's Principle by the Free Energy Diagram in Electrocatalysis
    Exner, K.S.
    ACS Catalysis 5320-5329 (2019)
    The computational hydrogen electrode (CHE) approach has spurred ab initio investigations in the field of electrocatalysis, since the underlying concept enables to quantify free energy changes, ?G (thermodynamics), for the formation of reaction intermediates on an electrocatalyst surface. The connection between thermodynamics and kinetics (activity) is achieved by Sabatier's principle: the optimum situation to realize an active electrocatalyst is ascribed to reaction intermediates that are thermoneutrally bound (?G = 0 eV) at zero overpotential. In order to validate the linkage between thermodynamics and kinetics at zero overpotential for two-electron processes, free energy diagrams as a function of the applied electrode potential are compiled. Herein, the chlorine evolution reaction (CER) over RuO2(110), one of the best understood model systems in electrocatalysis, is used as a starting point for this investigation. It turns out that the connection between thermodynamics and kinetics at zero overpotential does not reproduce activity trends correctly if the Tafel slope is overpotential dependent. Therefore, it appears expedient to include the applied overpotential into the thermodynamic framework: for electrocatalysts with a change in the Tafel slope, it is suggested to employ the absolute free energy change for the formation of a reaction intermediate at respective overpotential ?, |?G(?)|, as thermodynamic descriptor for the kinetics of two-electron processes, which may aid the construction of overpotential-dependent Volcano plots for improved material screening. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.9b00732
  • 2019 • 128 Martensite to austenite reversion in a high-Mn steel: Partitioning-dependent two-stage kinetics revealed by atom probe tomography, in-situ magnetic measurements and simulation
    Souza Filho, I.R. and Kwiatkowski da Silva, A. and Sandim, M.J.R. and Ponge, D. and Gault, B. and Sandim, H.R.Z. and Raabe, D.
    Acta Materialia 166 178-191 (2019)
    Austenite (γ) reversion in a cold-rolled 17.6 wt.% Mn steel was tracked by means of dilatometry and in-situ magnetic measurements during slow continuous annealing. A splitting of the γ-reversion into two stages was observed to be a result of strong elemental partitioning between γ and α′-martensite during the low temperature stage between 390 and 575 °C. Atom probe tomography (APT) results enable the characterization of the Mn-enriched reversed-γ and the Mn-depleted remaining α′-martensite. Because of its lower Mn content, the reversion of the remaining α′-martensite into austenite takes place at a higher temperature range between 600 and 685 °C. APT results agree with partitioning predictions made by thermo-kinetic simulations of the continuous annealing process. The critical composition for γ-nucleation was predicted by thermodynamic calculations (Thermo-Calc) and a good agreement was found with the APT data. Additional thermo-kinetic simulations were conducted to evaluate partitioning-governed γ-growth during isothermal annealing at 500 °C and 600 °C. Si partitioning to γ was predicted by DICTRA and confirmed by APT. Si accumulates near the moving interface during γ-growth and homogenizes over time. We used the chemical composition of the remaining α′-martensite from APT data to calculate its Curie temperature (TCurie) and found good agreement with magnetic measurements. These results indicate that elemental partitioning strongly influences not only γ-reversion but also the TCurie of this steel. The results are important to better understand the thermodynamics and kinetics of austenite reversion for a wide range of Mn containing steels and its effect on magnetic properties. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.046
  • 2019 • 127 Neural-Network-Based Path Collective Variables for Enhanced Sampling of Phase Transformations
    Rogal, J. and Schneider, E. and Tuckerman, M.E.
    Physical Review Letters 123 (2019)
    The investigation of the microscopic processes underlying structural phase transformations in solids is extremely challenging for both simulation and experiment. Atomistic simulations of solid-solid phase transitions require extensive sampling of the corresponding high-dimensional and often rugged energy landscape. Here, we propose a rigorous construction of a 1D path collective variable that is used in combination with enhanced sampling techniques for efficient exploration of the transformation mechanisms. The path collective variable is defined in a space spanned by global classifiers that are derived from local structural units. A reliable identification of the local structural environments is achieved by employing a neural-network-based classification scheme. The proposed path collective variable is generally applicable and enables the investigation of both transformation mechanisms and kinetics. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.123.245701
  • 2019 • 126 Optical Measurement Method of Particle Suspension in Stirred Vessels
    Wolinski, S. and Ulbricht, M. and Schultz, H.J.
    Chemie-Ingenieur-Technik 91 1326-1332 (2019)
    Suspending particles in liquids is an important and versatile case for industrial stirring processes. By using advanced optical, non-invasive measurement techniques like particle image velocimetry (PIV), it is possible to gain deep insights into the involved fluid dynamics without affecting the flow. However, for suspensions, the application of PIV is not trivial since both, suspended and tracer particles are present and need to be discerned during experiments. The here presented method development solves this problem and thus leads to a better insight into turbulent kinetic energy distribution, which can be utilized for process optimization through improved stirred vessel design. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cite.201800099
  • 2019 • 125 Relationship between bubble characteristics and hydrodynamic parameters for single bubbles in presence of surface active agents
    Lewandowski, B. and Fertig, M. and Ulbricht, M. and Krekel, G.
    Chemical Engineering Science 199 179-198 (2019)
    Rising single air bubbles were investigated in aqueous solutions of hexadecylamine (HDA) and methyl isobutyl carbinol (MIBC) as surface active agents at varying concentrations at a constant gas flow rate. Shadowgraphy was applied to determine main bubble characteristics, such as equivalent diameter, morphology, and rising velocity. From these characteristics, critical parameters like the concentration at the minimum bubble velocity were derived. Simultaneous application of Particle Image Velocimetry (PIV) provided information about hydrodynamic parameters, e.g. the induced liquid velocities and vortex shedding. From surface tension measurements, the concentration of adsorbed species on the interface and packing densities of HDA and MIBC on the bubble surface could be calculated. HDA exhibited a better adsorption and a higher packing density on the bubble surface compared to MIBC due to the ionic character and the straight hydrocarbon chain. The bubble characteristics were therefore more strongly affected by HDA than by MIBC. Combining the Shadowgraphy and PIV results it was found that the mean liquid velocity as well as the amount of induced turbulent kinetic energy increased with increasing concentration of surfactants in the solutions, while the investigated bubble characteristics such as equivalent diameter and rising velocity decreased. The increase in mean liquid velocity and induced turbulent kinetic energy could be correlated with the oscillating frequency of the bubble trajectory, which also increased with increasing surfactant concentration. The vortex shedding process could be visualised using Proper Orthogonal Decomposition (POD) revealing the micro-process of energy cascading. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.ces.2019.01.026
  • 2019 • 124 Scale-up of the rounding process in pelletization by extrusion-spheronization
    Evers, M. and Weis, D. and Antonyuk, S. and Thommes, M.
    Pharmaceutical Development and Technology 24 1014-1020 (2019)
    Previously described scaling models for the spheronization process of wet extrudates are incomplete, often concluding with an adjustment of the plate speed according to the spheronizer diameter, but neglecting to give guidelines on the adjustment of the load or the process duration. In this work, existing scaling models were extended to include the load and the process time. By analyzing the final particle size and shape distributions as well as the rounding kinetics for various loads and plate speeds in spheronizers with plate diameters of 0.12 m, 0.25 m and 0.38 m, the found scaling model was validated. The peripheral speed was found to be the main influence on the rounding kinetic, while the load and the plate diameter only showed minor influence. Higher peripheral speeds, higher loads and a larger spheronizer diameter led to an increase in rounding kinetic, allowing for shorter residence times and increased throughput. However, lower peripheral speed, lower loads and lower plate diameters led to particles of increased sphericity. © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/10837450.2019.1621900
  • 2019 • 123 Single entity electrochemistry for the elucidation of lithiation kinetics of TiO 2 particles in non-aqueous batteries
    Löffler, T. and Clausmeyer, J. and Wilde, P. and Tschulik, K. and Schuhmann, W. and Ventosa, E.
    Nano Energy 57 827-834 (2019)
    In battery research, the development of analytical techniques is of key importance for determining intrinsic properties of active materials ultimately dictating the battery performance. We report the application of nano-impact electrochemistry to gain insight into the intrinsic properties of commercial battery materials i.e. TiO 2 particles in non-aqueous media. Potentiostatic lithiation measurements do not only provide qualitative information about the rate-limiting step in the lithiation process, but also demonstrate that nano-impact electrochemistry is a suitable technique in non-aqueous media in complete absence of oxygen and water. Our results reveal that the intrinsic lithiation rate of individual TiO 2 particles is not – as generally assumed – determined by interfacial ion transfer kinetics, mobility of ion and/or electrons in the bulk of the particle, but by the solid-solid electron transfer. These findings have important implications for future studies of fundamental properties of battery materials considering that charge transfer in battery electrodes does not always obey Butler-Volmer kinetics. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.nanoen.2018.12.064
  • 2019 • 122 Single entity electrochemistry for the elucidation of lithiation kinetics of TiO2 particles in non-aqueous batteries
    Löffler, T. and Clausmeyer, J. and Wilde, P. and Tschulik, K. and Schuhmann, W. and Ventosa, E.
    Nano Energy 57 827-834 (2019)
    In battery research, the development of analytical techniques is of key importance for determining intrinsic properties of active materials ultimately dictating the battery performance. We report the application of nano-impact electrochemistry to gain insight into the intrinsic properties of commercial battery materials i.e. TiO2 particles in non-aqueous media. Potentiostatic lithiation measurements do not only provide qualitative information about the rate-limiting step in the lithiation process, but also demonstrate that nano-impact electrochemistry is a suitable technique in non-aqueous media in complete absence of oxygen and water. Our results reveal that the intrinsic lithiation rate of individual TiO2 particles is not – as generally assumed – determined by interfacial ion transfer kinetics, mobility of ion and/or electrons in the bulk of the particle, but by the solid-solid electron transfer. These findings have important implications for future studies of fundamental properties of battery materials considering that charge transfer in battery electrodes does not always obey Butler-Volmer kinetics. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.nanoen.2018.12.064
  • 2019 • 121 Supercritical CO 2 - assisted production of PLA and PLGA foams for controlled thymol release
    Milovanovic, S. and Markovic, D. and Mrakovic, A. and Kuska, R. and Zizovic, I. and Frerich, S. and Ivanovic, J.
    Materials Science and Engineering C 99 394-404 (2019)
    Amorphous, medical grade poly(D,L-lactic acid) (PLA) and poly(D,L-lactic-co-glycolic acid) (PLGA) were used to develop systems for controlled release of a natural bioactive substance - thymol. Supercritical carbon dioxide (scCO 2 ) was successfully used both as an impregnation medium for thymol incorporation into the polymer matrix and a foaming agent in a single-step batch process. Impregnation of samples using low to moderate scCO 2 densities (273 kg/m 3 and 630 kg/m 3 ) and short processing times (2 h and 4 h) enabled thymol loading of 0.92%–6.62% and formation of microcellular foams upon system depressurization. Thymol effect on structural and thermal properties on foamed samples was proven by FTIR and DSC. The effect of CO 2 under elevated pressure on the neat polymers was analysed by high pressure DSC. Foaming of polymers with lower molecular weight by CO 2 of higher density yielded foams with smaller pores. All tested foams released thymol in a controlled manner in phosphate buffered saline (PBS) at 37 °C within 3 to 6 weeks. Higher loading and lower cell density favoured thymol release rate, while its concentration in PBS for the tested period depended on foam interaction with the medium. Representative PLGA foam sample with the highest thymol loading (6.62%) showed controlled thymol release within 72 h in mediums having pH values from 1.1 to 7.4. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2019.01.106
  • 2019 • 120 Tail state limited photocurrent collection of thick photoactive layers in organic solar cells
    Wu, J. and Luke, J. and Lee, H.K.H. and Shakya Tuladhar, P. and Cha, H. and Jang, S.-Y. and Tsoi, W.C. and Heeney, M. and Kang, H. and Lee, K. and Kirchartz, T. and Kim, J.-S. and Durrant, J.R.
    Nature Communications 10 (2019)
    We analyse organic solar cells with four different photoactive blends exhibiting differing dependencies of short-circuit current upon photoactive layer thickness. These blends and devices are analysed by transient optoelectronic techniques of carrier kinetics and densities, air photoemission spectroscopy of material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We conclude that, for the device series studied, the photocurrent loss with thick active layers is primarily associated with the accumulation of photo-generated charge carriers in intra-bandgap tail states. This charge accumulation screens the device internal electrical field, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail state distribution and density and increase the maximal photoactive thickness for efficient operation. Our work suggests that selecting organic photoactive layers with a narrow distribution of tail states is a key requirement for the fabrication of efficient, high photocurrent, thick organic solar cells. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-12951-7
  • 2019 • 119 The influence of selected aromatic fluorescence tracers on the combustion kinetics of iso-octane
    Sommerer, J. and Fikri, M. and Herzler, J. and Maas, U. and Schießl, R. and Schulz, C.
    Fuel 244 559-568 (2019)
    The influence of four single-ring aromatic compounds on the ignition process of iso-octane is investigated by shock-tube experiments and numerical simulations. Methylbenzene (toluene), dimethylbenzene (xylene), trimethylbenzene (without distinction of isomers), and methoxybenzene (anisole) are often used as tracers in laser-induced fluorescence (LIF) combustion diagnostics where the presence of the tracer should have a minimal influence on the engine performance. Ignition delay times for tracer-blended iso-octane/air mixtures were measured at 750–1500 K at a pressure of 40 bar, and for equivalence ratios ϕ of 0.5 and 1.0. Measured ignition delay times of the blends containing 5–10 vol.% of the respective tracer are very close to the ignition delay times of pure iso-octane/air mixtures at the same pressure, temperature and equivalence ratio. Simulations of the auto-ignition involving detailed chemical kinetics of the alkane and the aromatics predict ignition delay times that are in good agreement with the experimental results. The simulations are used for a more detailed analysis of the interaction between the alkane and the aromatics during ignition. It is shown that the tracers and the fuel are consumed at similar rates in the blended mixtures, although mixtures of pure tracer with air are significantly less reactive than the corresponding mixtures of fuel and air. Sensitivity analyses are used to investigate the coupling between the sub-mechanisms of the tracers and the fuel. The chain branching reactions of the fuel leading to the formation of OH radicals are found to be the most sensitive reactions in the pre-ignition phase. The interaction between tracer and fuel at low temperatures can be characterized as follows: The fuel initiates radical formation, and the aromatic compound accompanies the commencing reaction by using the radicals originating from the fuel for their own decomposition. In the early steps of reaction, chain branching is caused by the tracer at a slower pace compared to the fuel. Therefore, the tracer-based reaction channels merely follow the reaction progress by the pace defined by the fuel. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2019.01.190
  • 2019 • 118 The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol over Cu/ZrO 2 in the aqueous phase
    Gabrysch, T. and Muhler, M. and Peng, B.
    Applied Catalysis A: General 47-53 (2019)
    The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO 2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H 2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO 2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol -1 for Cu/ZrO 2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2019.03.001
  • 2019 • 117 The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol over Cu/ZrO2 in the aqueous phase
    Gabrysch, T. and Muhler, M. and Peng, B.
    Applied Catalysis A: General 576 47-53 (2019)
    The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol-1 for Cu/ZrO2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2019.03.001
  • 2019 • 116 Unraveling the Metastability of Cn 2+ (n = 2-4) Clusters
    Peng, Z. and Zanuttini, D. and Gervais, B. and Jacquet, E. and Blum, I. and Choi, P.-P. and Raabe, D. and Vurpillot, F. and Gault, B.
    Journal of Physical Chemistry Letters 10 581-588 (2019)
    Pure carbon clusters have received considerable attention for a long time. However, fundamental questions, such as what the smallest stable carbon cluster dication is, remain unclear. We investigated the stability and fragmentation behavior of Cn 2+ (n = 2-4) dications using state-of-the-art atom probe tomography. These small doubly charged carbon cluster ions were produced by laser-pulsed field evaporation from a tungsten carbide field emitter. Correlation analysis of the fragments detected in coincidence reveals that they only decay to Cn-1 + + C+. During C2 2+ ? C+ + C+, significant kinetic energy release (5.75-7.8 eV) is evidenced. Through advanced experimental data processing combined with ab initio calculations and simulations, we show that the field-evaporated diatomic 12C2 2+ dications are either in weakly bound 3?u and 3Sg - states, quickly dissociating under the intense electric field, or in a deeply bound electronic 5Su - state with lifetimes >180 ps. © Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.8b03449
  • 2018 • 115 A comparison of the torrefaction behavior of wood, miscanthus and palm kernel shells: Measurements on single particles with geometries of technical relevance
    Becker, A. and Scherer, V.
    Fuel 224 507-520 (2018)
    A torrefaction test rig was designed to investigate large single biomass particles up to characteristic sizes of 25 mm, typical for industrial reactors. Time-resolved mass loss for such particles is measured with a magnetic suspension balance at well-defined torrefaction conditions (temperature, residence time, gas atmosphere). This paper comprises the results of woody and non-woody biomass: pine, a coniferous, and beech, a deciduous, wood, palm kernel shells and miscanthus. Influence of process temperature (240 to 320 °C), residence time (up to 1 h) and type of solid biomass on time-resolved mass loss is presented. Additional tests with oxygen in the process gas (0–15 vol%), typical for industrial torrefaction systems, are carried out for selected samples of beech wood. The differences in torrefaction behaviour of bark, sap- and heartwood of pine are evaluated. Finally, it is shown that the torrefaction reactor developed allows to derive kinetic parameters for mass loss. At temperatures up to 300 °C the mass loss for palm kern shells is highest followed by miscanthus, and pine. By examining pine, as an example, it is shown that heartwood is significantly more reactive than sapwood and bark. Finally, it is demonstrated, that for the particle sizes considered here heat and mass transfer limitations can be neglected for the determination of torrefaction kinetics. Kinetic data agree well with data from literature. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.fuel.2018.01.095
  • 2018 • 114 Adsorption Thermodynamics and Kinetics of Light Hydrocarbons on Microporous Activated Carbon at Low Temperatures
    Birkmann, F. and Pasel, C. and Luckas, M. and Bathen, D.
    Industrial and Engineering Chemistry Research 57 8023-8035 (2018)
    The removal of light hydrocarbons from exhaust air and process gas is important for a variety of applications, e.g., in natural-gas treatment. However, particularly at lower concentrations, the removal of C1 and C2 hydrocarbons is either very expensive or unfeasible with standard technology. Adsorption processes at temperatures below 0 °C may provide a technical solution, but until today, no systematic study on the dynamics of trace adsorption at low temperatures is available. Therefore, in this work, we present breakthrough curve experiments of ethane, propane, and n-butane over a temperature range from +20 to -80 °C and a concentration range from 5 to 1000 Pa on microporous activated carbon. Equilibrium loadings are calculated and modeled with the temperature-dependent Toth equation. From dynamic simulations of the experimental breakthrough curves, kinetic parameters are determined. The lowering of temperature results in the slowdown of kinetics, which, however, is overcompensated by a simultaneous capacity gain. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.8b00678
  • 2018 • 113 Co-solvent effects on reaction rate and reaction equilibrium of an enzymatic peptide hydrolysis
    Wangler, A. and Canales, R. and Held, C. and Luong, T.Q. and Winter, R. and Zaitsau, D.H. and Verevkin, S.P. and Sadowski, G.
    Physical Chemistry Chemical Physics 20 11317-11326 (2018)
    This work presents an approach that expresses the Michaelis constant KaM and the equilibrium constant Kth of an enzymatic peptide hydrolysis based on thermodynamic activities instead of concentrations. This provides KaM and Kth values that are independent of any co-solvent. To this end, the hydrolysis reaction of N-succinyl-l-phenylalanine-p-nitroanilide catalysed by the enzyme α-chymotrypsin was studied in pure buffer and in the presence of the co-solvents dimethyl sulfoxide, trimethylamine-N-oxide, urea, and two salts. A strong influence of the co-solvents on the measured Michaelis constant (KM) and equilibrium constant (Kx) was observed, which was found to be caused by molecular interactions expressed as activity coefficients. Substrate and product activity coefficients were used to calculate the activity-based values KaM and Kth for the co-solvent free reaction. Based on these constants, the co-solvent effect on KM and Kx was predicted in almost quantitative agreement with the experimental data. The approach presented here does not only reveal the importance of understanding the thermodynamic non-ideality of reactions taking place in biological solutions and in many technological applications, it also provides a framework for interpreting and quantifying the multifaceted co-solvent effects on enzyme-catalysed reactions that are known and have been observed experimentally for a long time. © the Owner Societies.
    view abstractdoi: 10.1039/c7cp07346a
  • 2018 • 112 Competition between formation of carbides and reversed austenite during tempering of a medium-manganese steel studied by thermodynamic-kinetic simulations and atom probe tomography
    Kwiatkowski da Silva, A. and Inden, G. and Kumar, A. and Ponge, D. and Gault, B. and Raabe, D.
    Acta Materialia 147 165-175 (2018)
    We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-0.1C-0.5Si medium manganese steel during low temperature tempering. The material was annealed up to 24 h at 450 °C in order to follow the kinetics of precipitation. Using thermodynamics and kinetics simulations, we predicted the growth of M23C6 carbides according to the local-equilibrium negligible partition (LENP) mode where carbide growth is controlled by the diffusion of carbon, while maintaining local chemical equilibrium at the interface. Atom-probe tomography (APT) measurements performed on samples annealed for 1, 6 and 24 h at 450 °C confirmed that LENP is indeed the mode of carbide growth and that Mn segregation is necessary for the nucleation. Additionally, we observed the heterogeneous nucleation of transition carbides with a carbon content between 6 and 8 at% at segregated dislocations and grain boundaries. We describe these carbides as a complex face-centered cubic transition carbide type (CFCC-TmC phase) obtained by the supersaturation of the FCC structure by carbon that will act as precursor to the more stable γ-M23C6 carbide that forms at the dislocations and grain boundaries. The results suggest that the addition of carbon does not directly favor the formation of austenite, since Mn is consumed by the formation of the carbides and the nucleation of austenite is thus retarded to later stages of tempering as every FCC nucleus in the initial stages of tempering is readily converted into a carbide nucleus. We propose the following sequence of transformation: (i) carbon and Mn co-segregation to dislocations and grain boundaries; (ii) formation of FCC transition carbides; (iii) growth controlled according to the LENP mode and (iv) austenite nucleation and growth. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.01.022
  • 2018 • 111 Direct monitoring of the conformational equilibria of the activation loop in the mitogen-activated protein kinase p38α
    Roser, P. and Weisner, J. and Simard, J.R. and Rauh, D. and Drescher, M.
    Chemical Communications 54 12057-12060 (2018)
    Conformational transitions in protein kinases are crucial for the biological function of these enzymes. Here, we characterize and assess conformational equilibria of the activation loop and the effect of small molecule inhibitors in the MAP kinase p38α. Our work experimentally revealed the existence of a two-state equilibrium for p38α while the addition of inhibitors shifts the equilibrium between these two states. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8cc06128a
  • 2018 • 110 Entropically driven Polymeric Enzyme Inhibitors by End-Group directed Conjugation
    Hijazi, M. and Krumm, C. and Cinar, S. and Arns, L. and Alachraf, W. and Hiller, W. and Schrader, W. and Winter, R. and Tiller, J.C.
    Chemistry - A European Journal 24 4523-4527 (2018)
    A new generic concept for polymeric enzyme inhibitors is presented using the example of poly(2-methyl-2-oxazoline) (PMOx) terminated with an iminodiacetate (IDA) function. These polymers are shown to be non-competitive inhibitors for horseradish peroxidase (HRP). Mechanistic investigations revealed that the polymer is directed to the protein by its end group and collapses at the surface in an entropy-driven process as shown by isothermal titration calorimetry. The dissociation constant of the complex was determined as the inhibition constant Ki using HRP kinetic activity measurements. Additional experiments suggest that the polymer does not form a diffusion layer around the protein, but might inhibit by inducing minor conformational changes in the protein. This kind of inhibitor offers new avenues towards designing bioactive compounds. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201800168
  • 2018 • 109 Filament Growth and Resistive Switching in Hafnium Oxide Memristive Devices
    Dirkmann, S. and Kaiser, J. and Wenger, C. and Mussenbrock, T.
    ACS Applied Materials and Interfaces 10 14857-14868 (2018)
    We report on the resistive switching in TiN/Ti/HfO2/TiN memristive devices. A resistive switching model for the device is proposed, taking into account important experimental and theoretical findings. The proposed switching model is validated using 2D and 3D kinetic Monte Carlo simulation models. The models are consistently coupled to the electric field and different current transport mechanisms such as direct tunneling, trap-assisted tunneling, ohmic transport, and transport through a quantum point contact have been considered. We find that the numerical results are in excellent agreement with experimentally obtained data. Important device parameters, which are difficult or impossible to measure in experiments, are calculated. This includes the shape of the conductive filament, width of filament constriction, current density, and temperature distribution. To obtain insights in the operation of the device, consecutive cycles have been simulated. Furthermore, the switching kinetics for the forming and set process for different applied voltages is investigated. Finally, the influence of an annealing process on the filament growth, especially on the filament growth direction, is discussed. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b19836
  • 2018 • 108 Grain boundary-constrained reverse austenite transformation in nanostructured Fe alloy: Model and application
    Huang, L. and Lin, W. and Wang, K. and Song, S. and Guo, C. and Chen, Y. and Li, Y. and Liu, F.
    Acta Materialia 154 56-70 (2018)
    Reverse austenite transformation (RAT) is critical for designing advanced high-strength steels (AHSS), which, however, has not been sufficiently studied in nanostructured (NS) steels or Fe alloys, and hence not fully understood yet. Herein, the RAT (e.g. ferrite to austenite) kinetics in the NS Fe alloy upon continuous heating was experimentally and theoretically investigated, where, the ultrafine austenite characterized by a sluggish growth velocity and a high thermal stability, and additionally, an appreciable solute partitioning detected using atom probe microscopy, indicate the diffusion-controlled mechanism of RAT. The double-edged role of grain boundaries (GBs) in the NS alloy is elucidated, i.e. enhancing the diffusivity due to the type-A kinetics, and simultaneously, facilitating the formation of constrained diffusion field mainly due to the segmented effect of GB nucleation. On this basis, a modified diffusion model incorporating the effect of GBs is implemented to understand the GB-constrained austenite growth and the associated partitioning behavior, and further complemented with Cahn model, an austenite growth model is applied to predict the overall kinetics of RAT in the NS Fe alloy. It then follows that a strategy by combination of diffusion-controlled growth model and microstructure model could serve as a framework to predict the kinetics of RAT in the NS alloys. Regarding the RAT kinetics in the NS alloys, the present work uncovers the ‘GB-constrained’ mechanism, which is expected to offer the potential application for nanostructure manipulation in the development of AHSS. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.05.021
  • 2018 • 107 High resolution AFM studies of irradiated mica - Following the traces of swift heavy ions under grazing incidence
    Gruber, E. and Bergen, L. and Salou, P. and Lattouf, E. and Grygiel, C. and Wang, Y. and Benyagoub, A. and Levavasseur, D. and Rangama, J. and Lebius, H. and Ban-D'Etat, B. and Schleberger, M. and Aumayr, F.
    Journal of Physics Condensed Matter 30 (2018)
    High resolution AFM imaging of swift heavy ion irradiated muscovite mica under grazing incidence provides detailed insight into the created nanostructure features. Swift heavy ions under grazing incidence form a complex track structure along the surface, which consists of a double track of nanohillocks at the impact site accompanied by a single, several 100 nm long protrusion. Detailed track studies by varying the irradiation parameters, i.e. the angle of incidence (0.2°-2°) and the kinetic energy of the impinging ions (23, 55, 75, 95 MeV) are presented. Moreover, the track formation in dependence of the sample temperature (between room temperature and 600 °C) and of the chemical composition (muscovite mica and fluorphlogopite mica) is studied. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aac7f7
  • 2018 • 106 How evolving multiaxial stress states affect the kinetics of rafting during creep of single crystal Ni-base superalloys
    Cao, L. and Wollgramm, P. and Bürger, D. and Kostka, A. and Cailletaud, G. and Eggeler, G.
    Acta Materialia 158 381-392 (2018)
    Miniature tensile creep specimens are used to investigate the effect of mild circular notches on microstructural evolution during [001] tensile creep of a Ni-base single crystal superalloy. Creep deformed material states from a uniaxial (950 °C, uniaxial stress: 300 MPa) and a circular notched creep specimen (950 °C, net section stress in notch root: 300 MPa) are compared. For both types of tests, creep experiments were interrupted after 81, 169 and 306 h. Quantitative scanning electron microscopy (SEM) is used to assess the evolution of the γ/γ′-microstructure from rafting to topological inversion. Scanning transmission electron microscopy (STEM) was applied to study the evolution of dislocation densities during creep. As a striking new result it is shown that in circular notched specimen, the microstructural evolution is well coupled to the kinetics of the stress redistribution during creep. Rafting, the directional coarsening of the γ′-phase, and the increase of γ-channel dislocation density, start in the notch root before the center of the specimen is affected. When stresses in the circular notched specimens are fully redistributed, the microstructural differences between the notch root and the center of the circular notched specimen disappear. The comparison of the mechanical data and the microstructural findings in uniaxial and circular notched specimens contribute to a better understanding of the role of mild notches, of stress multiaxiality and of strain accumulation in the microstructure evolution of single crystal Ni-base superalloys during creep. The results obtained in the present work are discussed in the light of previous work published in the literature. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.07.061
  • 2018 • 105 Investigating phase separation in amorphous solid dispersions via Raman mapping
    Luebbert, C. and Klanke, C. and Sadowski, G.
    International Journal of Pharmaceutics 535 245-252 (2018)
    The bioavailability of poorly-water-soluble active pharmaceutical ingredients (APIs) can be significantly improved by so-called amorphous solid dispersions (ASDs). However, the long-term stability of ASDs might be impaired by API recrystallization and/or amorphous phase separation (APS). So far, no methods have been reported to quantify APS in ASDs. In this work, phase-separation kinetics as well as the compositions of the two amorphous phases evolving due to APS were quantitatively determined for the first time using confocal Raman spectroscopy. Raman spectra were evaluated via non-linear multivariate Indirect Hard Modeling and verified by differential scanning calorimetry and hot-stage microscopy. APS in water-free ASDs of ibuprofen and poly (DL-lactic-co-glycolic acid) was investigated considering the influence of temperature and polymer architecture (linear vs. star-shaped). Water absorbed at 40 °C and 75% relative humidity (RH) promotes APS which was quantified for formulations of felodipine/poly(vinyl pyrrolidone) and ibuprofen/poly(vinyl pyrrolidone). © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.ijpharm.2017.11.014
  • 2018 • 104 Kinetic prediction of fast curing polyurethane resins by model-free isoconversional methods
    Stanko, M. and Stommel, M.
    Polymers 10 (2018)
    In this work, the characterisation of reaction kinetics of a methylene diphenyl diisocyanate (MDI)-based fast curing polyurethane resin (PUR) and the mathematical description of its curing process are presented. For the modelling of the reaction process isoconversional methods, which are also called model-free approaches, are used instead of model-based approaches. One of the main challenges is the characterisation of a reactive system with a short pot life, which already starts to crosslink below room temperature. The main focus is the evaluation of the applicability of isoconversional methods for predicting the reaction kinetics of fast curing polyurethane resins. In order to realise this, a repeatable methodology for the determination of time- and temperature-dependent reaction curves using differential scanning calorimetry (DSC) is defined. The cure models defined by this method serve as the basis for process simulations of PUR processing technologies such as resin transfer moulding (RTM) or reactive injection moulding (RIM) and reactive extrusion (REX). The characterisation of the reaction kinetics using DSC measurements is carried out under isothermal and non-isothermal conditions. Within this work isoconversional methods have been applied successfully to experimentally determined DSC data sets. It is shown that the reaction kinetics of fast curing polyurethane resins can be predicted using this methods. Furthermore, it is demonstrated that the time-dependent change of conversion of the considered polyurethane under isothermal curing conditions can also be predicted using isoconversional methods based on non-isothermal DSC measurements. This results in a significant reduction in the experimental effort required to characterise and model the curing process of polyurethanes. © 2018 by the authors.
    view abstractdoi: 10.3390/polym10070698
  • 2018 • 103 Mechanical twinning induced alteration in the kinetics of martensitic phase transformation in TRIP-maraging steels
    Gupta, S. and Ma, A. and Hartmaier, A.
    International Journal of Solids and Structures 155 213-224 (2018)
    A computational study is conducted to examine the influence of mechanical twins on the kinetics of martensitic transformation in TRIP-maraging steel. Simulations are performed using a non-local crystal plasticity (CP) phase transformation model describing the deformation behavior of TRIP-maraging steel. This work is inspired by the experimental observations of Wang et al. (2014), where authors reported a twinning induced stability against the phase transformation in austenite islands embedded in the martensitic matrix. Finite element (FE) analysis is used to simulate the stability of austenite against the phase transformation in the presence and absence of mechanical twins, respectively. Depending on the parent crystal orientation, macroscopic outcome of FE calculations manifested a slower/faster transformation kinetics for austenitic islands with mechanical twins as compared to islands without twins which is found to be in a good qualitative agreement to experimental observations. Models with two austenite islands of different sizes confirmed the twinning induced alteration in the transformation kinetics. Microscopic observations from the simulations demonstrated that the twin orientations generated during mechanical twinning are less or more favorable for the strain induced or stress assisted transformations. This directly indicates that the basis of the alteration in the transformation kinetics could be the strong orientation dependence of the martensitic transformation. Thus, a model based analysis was used to explain the experimental findings related to phase transformation kinetics of TRIP-maraging steels. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijsolstr.2018.07.021
  • 2018 • 102 Monitoring Potential-Induced DNA Dehybridization Kinetics for Single Nucleotide Polymorphism Detection by using In Situ Surface Enhanced Raman Scattering
    Kayran, Y.U. and Cinar, N. and Jambrec, D. and Schuhmann, W.
    ChemElectroChem (2018)
    Changes in temperature, ionic strength or electrical field are generally employed to dehybridize double-stranded DNA (dsDNA). In contrast, we propose potential-induced dsDNA dehybridization to distinguish fully matched target DNA (tDNA) from tDNA with a single nucleotide polymorphism (SNP) by following their dehybridization kinetics through insitu surface enhanced Raman scattering (SERS). The determination of the potential that evokes notable dehybridization of dsDNA without causing any desorption of the immobilized probe DNA (pDNA) from the electrode surface was performed by investigating the desorption kinetics of labelled single-stranded DNA (ssDNA) and dehybridization kinetics of dsDNA with labelled tDNA. This comparatively simple approach allows for SNP detection within minutes. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201701220
  • 2018 • 101 Phase stability and kinetics of σ-phase precipitation in CrMnFeCoNi high-entropy alloys
    Laplanche, G. and Berglund, S. and Reinhart, C. and Kostka, A. and Fox, F. and George, E.P.
    Acta Materialia 161 338-351 (2018)
    Although the phase stability of high-entropy alloys in the Cr-Mn-Fe-Co-Ni system has received considerable attention recently, knowledge of their thermodynamic equilibrium states and precipitation kinetics during high-temperature exposure is limited. In the present study, an off-equiatomic Cr26Mn20Fe20Co20Ni14 high-entropy alloy was solutionized and isothermally aged at temperatures between 600 °C and 1000 °C for times to 1000 h. In the original single-phase fcc matrix, an intermetallic σ phase was found to form at all investigated temperatures. Its morphology and composition were determined and the precipitation kinetics analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a time-temperature-transformation diagram (TTT diagram) is constructed for this off-equiatomic alloy. We combine our findings with theories of precipitation kinetics developed for traditional polycrystalline fcc alloys to calculate a TTT diagram for the equiatomic CrMnFeCoNi HEA. The results of our investigation may serve as a guide to predict precipitation kinetics in other complex alloys in the Cr-Mn-Fe-Co-Ni system. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.09.040
  • 2018 • 100 Soot formation in shock-wave-induced pyrolysis of acetylene and benzene with H 2 , O 2 , and CH 4 addition
    Drakon, A. and Eremin, A. and Mikheyeva, E. and Shu, B. and Fikri, M. and Schulz, C.
    Combustion and Flame 198 158-168 (2018)
    Experiments on the pyrolysis of C 2 H 2 /Ar and C 6 H 6 /Ar mixtures with addition of H 2 , O 2 , and CH 4 have been carried out behind reflected shock waves at temperatures ranging from 1400 to 2600 K. Soot formation was measured by laser extinction at 633 nm. Time-resolved temperature measurements were performed via two-color CO absorption on the P(8) and R(21) lines at 2111.54 and 2191.50 cm -1 using quantum-cascade lasers. For this purpose, 0.5–0.8% CO was added to the gas mixtures. The measured temperature dependence of soot formation in experiments with added O 2 , and CH 4 was corrected for the temperature effect caused by the thermochemistry of either endothermic pyrolysis or exothermic oxidation or reactions that cause time-dependent deviation from the initial frozen-shock temperatures. In all mixtures, the addition of H 2 resulted in a noticeable decrease of the soot yield. A considerable increase in the soot yield was found with addition of methane to acetylene mixtures. In contrast, in benzene mixtures, the addition of methane caused a decrease of the soot yield. The qualitative analysis of the kinetics of the gas-phase stage of the pyrolysis reactions elucidated the influence of all investigated additives on the change in the key routes of initial stages of PAH and soot formation. We observed that the addition of H 2 to acetylene inhibits the initial stages of the pyrolysis reaction, while the addition of CH 4 and O 2 opens up new ways for the formation of benzene and phenyl and following growth of pyrene. In contrast to that, in benzene all the additives studied lead to the suppression of the kinetics pathways for the formation of pyrene and the subsequent growth of soot. © 2018 The Combustion Institute
    view abstractdoi: 10.1016/j.combustflame.2018.09.014
  • 2018 • 99 Surface structure modification of single crystal graphite after slow, highly charged ion irradiation
    Alzaher, I. and Akcöltekin, S. and Ban-d'Etat, B. and Manil, B. and Dey, K.R. and Been, T. and Boduch, P. and Rothard, H. and Schleberger, M. and Lebius, H.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 420 23-26 (2018)
    Single crystal graphite was irradiated by slow, highly charged ions. The modification of the surface structure was studied by means of Low-Energy Electron Diffraction. The observed damage cross section increases with the potential energy, i.e. the charge state of the incident ion, at a constant kinetic energy. The potential energy is more efficient for the damage production than the kinetic energy by more than a factor of twenty. Comparison with earlier results hints to a strong link between early electron creation and later target atom rearrangement. With increasing ion fluence, the initially large-scale single crystal is first transformed into μm-sized crystals, before complete amorphisation takes place. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.nimb.2018.01.024
  • 2018 • 98 Thermodynamic prediction of the solvent effect on a transesterification reaction
    Lemberg, M. and Schomäcker, R. and Sadowski, G.
    Chemical Engineering Science 176 264-269 (2018)
    This work focuses on the thermodynamic prediction of solvent effects on the transesterification of butyl acetate with ethanol to butanol and ethyl acetate in the solvent heptane at 293.15 K and 303.15 K. Both, the reaction equilibrium and the reaction kinetics have been investigated experimentally by Schmidt et al. (1999). They found that the solvent heptane does not affect the reaction equilibrium but significantly influences the reaction kinetics. They described the solvent effect on the reaction kinetics by empirically correlating the experimentally-observed apparent rate constants with the dielectric constants of the different reaction mixtures. In this work we re-evaluated the experimental data and now present a thermodynamic approach to consistently predict the solvent effect on both, the reaction equilibrium and the reaction kinetics. Accounting for the activity coefficients of the reactants/products obtained from the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) allowed for considering the interactions of the reactants/products among themselves and also with the solvent heptane. Accounting for those, it is shown that the solvent effect on the reaction equilibrium as well as on the reaction rate can even be predicted in very good agreement with the experimental data. © 2017
    view abstractdoi: 10.1016/j.ces.2017.10.033
  • 2017 • 97 A kinetic Monte Carlo approach to diffusion-controlled thermal desorption spectroscopy
    Schablitzki, T. and Rogal, J. and Drautz, R.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 375 (2017)
    Atomistic simulations of thermal desorption spectra for effusion from bulk materials to characterize binding or trapping sites are a challenging task as large system sizes as well as extended time scales are required. Here, we introduce an approach where we combine kinetic Monte Carlo with an analytic approximation of the superbasins within the framework of absorbing Markov chains. We apply our approach to the effusion of hydrogen from BCC iron, where the diffusion within bulk grains is coarse grained using absorbingMarkov chains, which provide an exact solution of the dynamics within a superbasin. Our analytic approximation to the superbasin is transferable with respect to grain size and elliptical shapes and can be applied in simulations with constant temperature as well as constant heating rate. The resulting thermal desorption spectra are in close agreement with direct kinetic Monte Carlo simulations, but the calculations are computationally much more efficient. Our approach is thus applicable to much larger system sizes and provides a first step towards an atomistic understanding of the influence of structural features on the position and shape of peaks in thermal desorption spectra. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
    view abstractdoi: 10.1098/rsta.2016.0404
  • 2017 • 96 A quantum chemical and kinetics modeling study on the autoignition mechanism of diethyl ether
    Sakai, Y. and Herzler, J. and Werler, M. and Schulz, C. and Fikri, M.
    Proceedings of the Combustion Institute 36 195-202 (2017)
    A detailed chemical kinetics model has been developed to elucidate the auto-ignition behavior of diethyl ether (DEE) under conditions relevant for internal combustion engines. The present model is composed of a C0-C4 base module from literature and a DEE module. For the low-temperature oxidation mechanism, the reactions of ROO and QOOH radicals were studied previously with a quantum-chemical and transition state theory approach by Sakai et al. (2015). In the present study, the potential energy surfaces for the unimolecular reactions of OOQOOH isomers and 1- and 2-ethoxyethyl radicals were determined with a CBSQB3 composite method. In the presence of an OOH group, the reaction barrier of the hydrogen shift from the β site (terminal carbon atom) decreases as it does in alkane oxidation but there is no effect on the hydrogen shift from the α site (next to the ether oxygen atom). Therefore, the reaction barriers of OOQOOH isomers have the same trend as the corresponding ROO radical and rate constants for the reactions of OOQOOH isomers were determined. The constructed model was validated against the recent data of ignition delay times provided in literature by Werler et al. (2015). The agreement is good over the temperature range 500-1300K and pressure range 1-40bar, although, open questions remain regarding the non-consensus at 900-1150K and 40bar. Reaction-path and sensitivity analyses attribute the importance of the reactivity at the α site to the decrease of the C H bond dissociation energy due to the ether oxygen atom. © 2016.
    view abstractdoi: 10.1016/j.proci.2016.06.037
  • 2017 • 95 Clarifying the Physics of Flow Separations in Steam Turbine Inlet Valves at Part Load Operation and Improved Design Considerations
    Domnick, C.B. and Brillert, D. and Musch, C. and Benra, F.-K.
    Journal of Fluids Engineering, Transactions of the ASME 139 (2017)
    In steam turbine inlet valves used to adjust the power output of large steam turbines, the through-flow is reduced by lowering the valve plug and hence reducing the cross-sectional area between the plug and the seat. At throttled operation, a supersonic jet is formed between the plug and the seat. This jet bearing tremendous kinetic energy flows into the valve diffuser where it is dissipated. Depending on the dissipation process, a certain portion of the kinetic energy is converted to sound and subsequently to structural vibration, which can be harmful to the valve plug. The flow topology in the valve diffuser has a strong influence on the conversion of kinetic energy to sound and hence vibrations. Several studies show that an annular flow attached to the wall of the valve diffuser causes significantly less noise and vibrations than a detached flow in the core of the diffuser. The relation between the flow topology and the vibrations is already known, but the physics causing the transition from the undesired core flow to the desired annular flow and the dependency on the design are not fully understood. The paper presented here reveals the relation between the flow topology in the steam valve and the separation of underexpanded Coand? wall jets. The physics of the jet separations are clarified and a method to predict the flow separations with a low numerical effort is shown. Based on this, safe operational ranges free of separations can be predicted and improved design considerations can be made. © 2017 by Siemens AG.
    view abstractdoi: 10.1115/1.4036263
  • 2017 • 94 Conversion of an instantaneous activating K+ channel into a slow activating inward rectifier
    Baumeister, D. and Hertel, B. and Schroeder, I. and Gazzarrini, S. and Kast, S.M. and Van Etten, J.L. and Moroni, A. and Thiel, G.
    FEBS Letters 591 295-303 (2017)
    The miniature channel, Kcv, is a structural equivalent of the pore of all K+ channels. Here, we follow up on a previous observation that a largely voltage-insensitive channel can be converted into a slow activating inward rectifier after extending the outer transmembrane domain by one Ala. This gain of rectification can be rationalized by dynamic salt bridges at the cytosolic entrance to the channel; opening is favored by voltage-sensitive formation of salt bridges and counteracted by their disruption. Such latent voltage sensitivity in the pore could be relevant for the understanding of voltage gating in complex Kv channels. © 2016 Federation of European Biochemical Societies
    view abstractdoi: 10.1002/1873-3468.12536
  • 2017 • 93 Design, Synthesis, and Biological Evaluation of Novel Type I1/2 p38α MAP Kinase Inhibitors with Excellent Selectivity, High Potency, and Prolonged Target Residence Time by Interfering with the R-Spine
    Walter, N.M. and Wentsch, H.K. and Bührmann, M. and Bauer, S.M. and Döring, E. and Mayer-Wrangowski, S. and Sievers-Engler, A. and Willemsen-Seegers, N. and Zaman, G. and Buijsman, R. and Lämmerhofer, M. and Rauh, D. and Laufer, S.A.
    Journal of Medicinal Chemistry 60 8027-8054 (2017)
    We recently reported 1a (skepinone-L) as a type I p38α MAP kinase inhibitor with high potency and excellent selectivity in vitro and in vivo. However, as a type I inhibitor, it is entirely ATP-competitive and shows just a moderate residence time. Thus, the scope was to develop a new class of advanced compounds maintaining the structural binding features of skepinone-L scaffold like inducing a glycine flip at the hinge region and occupying both hydrophobic regions I and II. Extending this scaffold with suitable residues resulted in an interference with the kinase's R-Spine. By synthesizing 69 compounds, we could significantly prolong the target residence time with one example to 3663 s, along with an excellent selectivity score of 0.006 and an outstanding potency of 1.0 nM. This new binding mode was validated by cocrystallization, showing all binding interactions typifying type I1/2 binding. Moreover, microsomal studies showed convenient metabolic stability of the most potent, herein reported representatives. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jmedchem.7b00745
  • 2017 • 92 Diffusion processes during cementite precipitation and their impact on electrical and thermal conductivity of a heat-treatable steel
    Klein, S. and Mujica Roncery, L. and Walter, M. and Weber, S. and Theisen, W.
    Journal of Materials Science 52 375-390 (2017)
    The thermal conductivity of heat-treatable steels is highly dependent on their thermo-mechanical history and the alloying degree. Besides phase transformations like the martensitic γ → α ′ or the degree of deformation, the precipitation of carbides exerts a strong influence on the thermal conductivity of these steels. In the current work, thermal and electrical conductivity of a 0.45 mass% C steel is investigated during an isothermal heat treatment at 700 ∘C and correlated with the precipitation kinetics of cementite. To include processes in the short-term as well as in the long-term range, annealing times from 1 s to 200 h are applied. This investigation includes microstructural characterization, diffusion simulations, and electrical and thermal conductivity measurements. The precipitation of carbides is connected with various microstructural processes which separately influence the thermophysical properties of the steel from the solution state to the short-term and long-term annealing states. In the early stages of cementite growth, an interstitial-dominated diffusion reaction takes place (carbon diffusion in the metastable condition of local equilibrium non-partitioning). Afterwards, substitutional-dominated diffusion controls the kinetics of the reaction. The electrical and thermal conductivity increase differently during the two stages of the carbide precipitation. The increment is associated to the binding of alloying elements into the carbides and to the reduction of the distortion of the martensitic matrix. Both factors increase the electron density and reduce the electron and phonon scattering. The correlation of the precipitation kinetics and the thermophysical properties are of general interest for the design of heat-treatable steels. © 2016, The Author(s).
    view abstractdoi: 10.1007/s10853-016-0338-1
  • 2017 • 91 Effects of incident N atom kinetic energy on TiN/TiN(001) film growth dynamics: A molecular dynamics investigation
    Edström, D. and Sangiovanni, D.G. and Hultman, L. and Petrov, I. and Greene, J.E. and Chirita, V.
    Journal of Applied Physics 121 (2017)
    Large-scale classical molecular dynamics simulations of epitaxial TiN/TiN(001) thin film growth at 1200 K, a temperature within the optimal range for epitaxial TiN growth, with an incident N-to-Ti flux ratio of four, are carried out using incident N energies EN = 2 and 10 eV and incident Ti energy ETi = 2 eV. To further highlight the effect of EN, we grow a bilayer film with EN = 2 eV initially and then switch to EN = 10 eV. As-deposited layers are analyzed as a function of composition, island-size distribution, island-edge orientation, and vacancy formation. Results show that growth with EN = 2 eV results in films that are globally overstoichiometric with islands bounded by N-terminated polar 110 edges, whereas films grown with EN = 10 eV are flatter and closer to stoichiometric. However, EN = 10 eV layers exhibit local N deficiency leading to the formation of isolated 111-oriented islands. Films grown by changing the incident energy from 2 to 10 eV during growth are more compact than those grown entirely with EN = 2 eV and exhibit greatly reduced concentrations of upper-layer adatoms, admolecules, and small clusters. Islands with 110 edges formed during growth with EN = 2 eV transform to islands with 100 edges as EN is switched to 10 eV. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4972963
  • 2017 • 90 Energy Transfer Kinetics in Photosynthesis as an Inspiration for Improving Organic Solar Cells
    Nganou, C. and Lackner, G. and Teschome, B. and Deen, M.J. and Adir, N. and Pouhe, D. and Lupascu, D.C. and Mkandawire, M.
    ACS Applied Materials and Interfaces 9 19030-19039 (2017)
    Clues to designing highly efficient organic solar cells may lie in understanding the architecture of light-harvesting systems and exciton energy transfer (EET) processes in very efficient photosynthetic organisms. Here, we compare the kinetics of excitation energy tunnelling from the intact phycobilisome (PBS) light-harvesting antenna system to the reaction center in photosystem II in intact cells of the cyanobacterium Acaryochloris marina with the charge transfer after conversion of photons into photocurrent in vertically aligned carbon nanotube (va-CNT) organic solar cells with poly(3-hexyl)thiophene (P3HT) as the pigment. We find that the kinetics in electron hole creation following excitation at 600 nm in both PBS and va-CNT solar cells to be 450 and 500 fs, respectively. The EET process has a 3 and 14 ps pathway in the PBS, while in va-CNT solar cell devices, the charge trapping in the CNT takes 11 and 258 ps. We show that the main hindrance to efficiency of va-CNT organic solar cells is the slow migration of the charges after exciton formation. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b04028
  • 2017 • 89 Influence of carbon microstructure on the Li–O2 battery first-discharge kinetics
    Wijaya, O. and Hoster, H.E. and Rinaldi, A.
    International Journal of Energy Research 41 889-898 (2017)
    Defects in the carbon microstructure have been reported to enhance the discharge performance of Li–O2 battery. However, systematic studies correlating the presence of defects with the discharge kinetics have not addressed the variation of carbon electrode surface areas. In this work, carbon blacks and carbon nanofibers with different defect densities were investigated for their discharge properties. The electrolyte-accessible areas of the carbon electrodes were obtained from Cyclic voltammetry measurements. The microstructure and surface areas of the carbons were characterized by Raman spectroscopy, electron microscopy, and N2 isotherm. Linear sweep voltammetry and galvanostatic discharge experiments consistently demonstrated that graphitic carbons have more negative onset potentials and more negative discharge potentials at the same current density than defective carbons. The linear sweep voltammetry data were normalized to the carbon masses, Brunauer–Emmet–Teller surface areas, and double layer capacitance-derived areas for comparison. Plot of inverse charge transfer resistance and double layer capacitance from electrochemical impedance spectroscopy measurements were used to extract current density values without knowledge of electrode areas. The current densities from impedance measurements exhibited good agreement with the data from linear sweep experiments. The electrochemical experiments conclusively showed that defects on the graphitic microstructure increase the discharge kinetics of the Li–O2 battery. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/er.3690
  • 2017 • 88 Investigation of Numerical Effects on the Flow and Combustion in les of ICE
    Nguyen, T. and Kempf, A.M.
    Oil and Gas Science and Technology 72 (2017)
    This work investigates the influence of numerical dissipation on the modeled combustion in Large Eddy Simulations (LES). It is well known that capturing the dynamics of the in-cylinder flow is crucial for engine simulations, as it strongly affects flame propagation. The flame propagation during the power-stroke highly depends on the turbulence level that is developed throughout compression. This turbulence level will be strongly influenced by the accuracy of the numerical schemes employed. Even a small extent of upwinding, filtering, low-order implicit time-stepping, cell-stretching or mapping between grids may affect the flow field, the turbulence level, and hence the turbulent flame speed and the pressure curve. To provide a reference, the LES in-house code PsiPhi is used, which ensures a minimum of dissipation due to high order explicit time-stepping, homogeneous and isotropic filters and cells. Good stability of the code permits the use of a second-order Central Differencing Scheme (CDS) for the transport of momentum, avoiding numerical dissipation. To analyse the effect of numerical dissipation, simulations of a fired engine are performed using different numerical schemes for the convection of momentum. Physical quantities including the total kinetic energy, the velocity gradient, the turbulent viscosity, the in-cylinder pressure, the flame propagation or the burning rate of different test cases are evaluated and compared to each other to show the numerical effects on combustion. Furthermore, the suitability of common LES quality criteria including an energy criterion and viscosity ratio is discussed based on the comparison of simulations with less and more accurate numerics. It is shown that these LES quality indicators can be highly misleading. © T. Nguyen and A.M. Kempf, published by IFP Energies nouvelles, 2017.
    view abstractdoi: 10.2516/ogst/2017023
  • 2017 • 87 In–situ TEM study of diffusion kinetics and electron irradiation effects on the Cr phase separation of a nanocrystalline Cu–4 at.% Cr thin film alloy
    Harzer, T.P. and Duarte, M.J. and Dehm, G.
    Journal of Alloys and Compounds 695 1583-1590 (2017)
    The Cr phase separation process and kinetics of a metastable Cu96Cr4 alloy film were investigated by isothermal annealing at different temperatures of up to 500 °C using transmission electron microscopy. It is shown that the Cr phase separation proceeds predominantly via enrichment of Cr at grain boundaries and grain boundary diffusion. Temperature dependent diffusion coefficients and the activation energy for grain boundary diffusion of Cr in face–centered cubic Cu are determined from analytical in–situ transmission electron microscopy experiments. In addition, the influence of electron beam irradiation on the diffusion kinetics is considered. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2016.10.302
  • 2017 • 86 Kinetic arrest of the ferrimagnetic state in indium-doped Mn1.82Co0.18Sb
    Tekgül, A. and Acet, M. and Farle, M. and Ünal, N.
    Journal of Alloys and Compounds 695 418-425 (2017)
    Substituting Mn with small amounts of Co in Mn2Sb leads to kinetic arrest and magnetic frustration effects. By adding small amounts of In in place of Sb, the tetragonality and thereby the kinetic arrest properties can be modified and controlled. We investigate the kinetic arrest effect in Mn1.82Co0.18Sb1−xInx with x = 0 and 0.5 at the first-order antiferromagnetic-ferrimagnetic magnetostructural transition. Kinetic arrest is observed in both systems, whereby the arrest is partial under 5 T field for Mn1.82Co0.18Sb and full for Mn1.82Co0.18Sb0.95In0.05. The FI state remains arrested as long as the cooling-field is maintained. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2016.11.093
  • 2017 • 85 Relaxation dynamics and transformation kinetics of deeply supercooled water: Temperature, pressure, doping, and proton/deuteron isotope effects
    Lemke, S. and Handle, P.H. and Plaga, L.J. and Stern, J.N. and Seidl, M. and Fuentes-Landete, V. and Amann-Winkel, K. and Köster, K.W. and Gainaru, C. and Loerting, T. and Böhmer, R.
    Journal of Chemical Physics 147 (2017)
    Above its glass transition, the equilibrated high-density amorphous ice (HDA) transforms to the low-density pendant (LDA). The temperature dependence of the transformation is monitored at ambient pressure using dielectric spectroscopy and at elevated pressures using dilatometry. It is found that near the glass transition temperature of deuterated samples, the transformation kinetics is 300 times slower than the structural relaxation, while for protonated samples, the time scale separation is at least 30 000 and insensitive to doping. The kinetics of the HDA to LDA transformation lacks a proton/deuteron isotope effect, revealing that this process is dominated by the restructuring of the oxygen network. The x-ray diffraction experiments performed on samples at intermediate transition stages reflect a linear combination of the LDA and HDA patterns implying a macroscopic phase separation, instead of a local intermixing of the two amorphous states. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4993790
  • 2017 • 84 Sunlight-Dependent Hydrogen Production by Photosensitizer/Hydrogenase Systems
    Adam, D. and Bösche, L. and Castañeda-Losada, L. and Winkler, M. and Apfel, U.-P. and Happe, T.
    ChemSusChem 10 894-902 (2017)
    We report a sustainable in vitro system for enzyme-based photohydrogen production. The [FeFe]-hydrogenase HydA1 from Chlamydomonas reinhardtii was tested for photohydrogen production as a proton-reducing catalyst in combination with eight different photosensitizers. Using the organic dye 5-carboxyeosin as a photosensitizer and plant-type ferredoxin PetF as an electron mediator, HydA1 achieves the highest light-driven turnover number (TONHydA1) yet reported for an enzyme-based in vitro system (2.9×106 mol(H2) mol(cat)−1) and a maximum turnover frequency (TOFHydA1) of 550 mol(H2) mol(HydA1)−1 s−1. The system is fueled very effectively by ambient daylight and can be further simplified by using 5-carboxyeosin and HydA1 as a two-component photosensitizer/biocatalyst system without an additional redox mediator. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201601523
  • 2017 • 83 Synergistic Effect of Cobalt and Iron in Layered Double Hydroxide Catalysts for the Oxygen Evolution Reaction
    Yang, F. and Sliozberg, K. and Sinev, I. and Antoni, H. and Bähr, A. and Ollegott, K. and Xia, W. and Masa, J. and Grünert, W. and Cuenya, B.R. and Schuhmann, W. and Muhler, M.
    ChemSusChem 10 156-165 (2017)
    Co-based layered double hydroxide (LDH) catalysts with Fe and Al contents in the range of 15 to 45 at % were synthesized by an efficient coprecipitation method. In these catalysts, Fe3+ or Al3+ ions play an essential role as trivalent species to stabilize the LDH structure. The obtained catalysts were characterized by a comprehensive combination of surface- and bulk-sensitive techniques and were evaluated for the oxygen evolution reaction (OER) on rotating disk electrodes. The OER activity decreased upon increasing the Al content for the Co- and Al-based LDH catalysts, whereas a synergistic effect in Co- and Fe-based LDHs was observed, which resulted in an optimal Fe content of 35 at %. This catalyst was spray-coated on Ni foam electrodes and showed very good stability in a flow-through cell with a potential of approximately 1.53 V at 10 mA cm−2 in 1 m KOH for at least 48 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201601272
  • 2017 • 82 The influence of iron oxide on the oxidation kinetics of synthetic char derived from thermogravimetric analysis and fixed-bed experiments under isothermal and temperature-programmed conditions
    Düdder, H. and Lotz, K. and Wütscher, A. and Muhler, M.
    Fuel 201 99-104 (2017)
    The catalytic effect of iron oxide on the oxidation kinetics of synthetic char was investigated in a fixed-bed reactor and in a conventional thermobalance for comparison. Synthetic char doped with iron oxide was obtained by pyrolyzing hydrochar at 800. °C, which had been synthesized by hydrothermal carbonization of cellulose in the presence of iron oxide. Isothermal char oxidation in the fixed-bed reactor resulted in the most reliable kinetic results. According to model-free kinetic analysis of these experiments at 15% conversion, iron oxide decreased the activation energy of char oxidation from 149. kJ/mol to 133. kJ/mol. Modeling of the conversion-time curves was first performed by using the uniform reaction model and then improved by using a . n-th order power law. In the temperature range of 440-490. °C a very good agreement with the experimental data was achieved using . n = 0.6. Activation energies amounting to 149. kJ/mol and 134. kJ/mol were derived for the undoped and iron oxide-doped char, respectively, well in line with the model-free analysis. © 2016.
    view abstractdoi: 10.1016/j.fuel.2016.09.076
  • 2017 • 81 Time-resolved impact electrochemistry for quantitative measurement of single-nanoparticle reaction kinetics
    Saw, E.N. and Kratz, M. and Tschulik, K.
    Nano Research 1-10 (2017)
    Single-nanoparticle electrochemistry has been established as a tool to characterize various nanomaterials based on the charge passed during their random impact at an electrode. Here it is demonstrated that the duration and shape of the resulting current peak can be used to quantify the reaction kinetics on a single-particle basis. Both the chemical rate constant and reaction mechanism for oxidation of single nanoparticles in different electrolytes can be determined directly from the duration of the current signal recorded in high-speed, highsensitivity current measurements. Using 29-nm-sized Ag particles in four different electrolytes as a proof of concept for this general approach, hitherto inaccessible insights into single-particle reactivity are provided. While comparable rate constants were measured for the four electrolytes at low overpotentials, transport-limited impacts at high overpotentials were found to depend strongly on the type and quantity of anions present in solution. [Figure not available: see fulltext.] © 2017 Tsinghua University Press and Springer-Verlag GmbH Germany
    view abstractdoi: 10.1007/s12274-017-1578-3
  • 2016 • 80 A new setup for the investigation of swift heavy ion induced particle emission and surface modifications
    Meinerzhagen, F. and Breuer, L. and Bukowska, H. and Bender, M. and Severin, D. and Herder, M. and Lebius, H. and Schleberger, M. and Wucher, A.
    Review of Scientific Instruments 87 (2016)
    The irradiation with fast ions with kinetic energies of >10 MeV leads to the deposition of a high amount of energy along their trajectory (up to several ten keV/nm). The energy is mainly transferred to the electronic subsystem and induces different secondary processes of excitations, which result in significant material modifications. A new setup to study these ion induced effects on surfaces will be described in this paper. The setup combines a variable irradiation chamber with different techniques of surface characterizations like scanning probe microscopy, time-of-flight secondary ion, and neutral mass spectrometry, as well as low energy electron diffraction under ultra high vacuum conditions, and is mounted at a beamline of the universal linear accelerator (UNILAC) of the GSI facility in Darmstadt, Germany. Here, samples can be irradiated with high-energy ions with a total kinetic energy up to several GeVs under different angles of incidence. Our setup enables the preparation and in situ analysis of different types of sample systems ranging from metals to insulators. Time-of-flight secondary ion mass spectrometry enables us to study the chemical composition of the surface, while scanning probe microscopy allows a detailed view into the local electrical and morphological conditions of the sample surface down to atomic scales. With the new setup, particle emission during irradiation as well as persistent modifications of the surface after irradiation can thus be studied. We present first data obtained with the new setup, including a novel measuring protocol for time-of-flight mass spectrometry with the GSI UNILAC accelerator. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4939899
  • 2016 • 79 Barrierless growth of precursor-free, ultrafast laser-fragmented noble metal nanoparticles by colloidal atom clusters - A kinetic in situ study
    Jendrzej, S. and Gökce, B. and Amendola, V. and Barcikowski, S.
    Journal of Colloid and Interface Science 463 299-307 (2016)
    Unintended post-synthesis growth of noble metal colloids caused by excess amounts of reactants or highly reactive atom clusters represents a fundamental problem in colloidal chemistry, affecting product stability or purity. Hence, quantified kinetics could allow defining nanoparticle size determination in dependence of the time. Here, we investigate in situ the growth kinetics of ps pulsed laser-fragmented platinum nanoparticles in presence of naked atom clusters in water without any influence of reducing agents or surfactants. The nanoparticle growth is investigated for platinum covering a time scale of minutes to 50 days after nanoparticle generation, it is also supplemented by results obtained from gold and palladium. Since a minimum atom cluster concentration is exceeded, a significant growth is determined by time resolved UV/Vis spectroscopy, analytical disc centrifugation, zeta potential measurement and transmission electron microscopy. We suggest a decrease of atom cluster concentration over time, since nanoparticles grow at the expense of atom clusters. The growth mechanism during early phase (<1. day) of laser-synthesized colloid is kinetically modeled by rapid barrierless coalescence. The prolonged slow nanoparticle growth is kinetically modeled by a combination of coalescence and Lifshitz-Slyozov-Wagner kinetic for Ostwald ripening, validated experimentally by the temperature dependence of Pt nanoparticle size and growth quenching by Iodide anions. © 2015.
    view abstractdoi: 10.1016/j.jcis.2015.10.032
  • 2016 • 78 Drug Release Kinetics and Mechanism from PLGA Formulations
    Ji, Y. and Lesniak, A.K. and Prudic, A. and Paus, R. and Sadowski, G.
    AIChE Journal 62 4055-4065 (2016)
    The release kinetics of indomethacin (IND) and hydrochlorothiazide (HCT) from drug/PLGA formulations with different copolymer composition and molecular weight of PLGA were measured in vitro by using a rotating disk system (USP II). The release mechanism of IND and HCT from their PLGA formulations was analyzed using a chemical-potential-gradient model combined with the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT). Furthermore, the release kinetics of IND and HCT from the PLGA formulations with different copolymer composition and molecular weight of PLGA were correlated and predicted in good accordance with the experimental data. It was found that the chemical-potential-gradient model combined with the PC-SAFT helped to understand the drug release mechanism from the drug/PLGA formulations. It also well correlated and predicted the drug release kinetics as function of copolymer composition and molecular weight of PLGA as well as of drug type. It helps to save time and costs for determination of the long-term drug release kinetics, especially for sustained drug release as obtained from the drug/PLGA formulations in this work. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4055–4065, 2016. © 2016 American Institute of Chemical Engineers
    view abstractdoi: 10.1002/aic.15282
  • 2016 • 77 Experimentation for char combustion kinetics measurements: Bias from char preparation
    Vorobiev, N. and Geier, M. and Schiemann, M. and Scherer, V.
    Fuel Processing Technology 151 155-165 (2016)
    As an alternative to direct use in char burnout kinetics experiments, coal and other solid fuels may be devolatilized and converted to char in a separate step in order to eliminate the influence of volatile release and combustion on measured char conversion properties. In this study, the effects of char preparation conditions on char burnout characteristics during pulverized coal combustion are investigated. Untreated particles of a Colombian high-volatile bituminous coal as well as three chars from that coal, which were externally produced in three different reactors, were tested in a combustion-driven entrained flow reactor. The reactivities of the chars were quantified as pre-exponential factors for commonly employed single-film burnout models, which were estimated from optically measured temperatures and sizes of individual burning particles. The results indicate that char production with devolatilization under heating rates greater than 2 × 104 K/s yields suitable materials for experimental research on char consumption kinetics. The characteristics of high-heating-rate production methods appear to affect char reactivities only in the early stages of the consumption process (less than 20-25% conversion of the initial char mass). © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.fuproc.2016.05.005
  • 2016 • 76 Exploring the mineral-water interface: Reduction and reaction kinetics of single hematite (α-Fe2O3) nanoparticles
    Shimizu, K. and Tschulik, K. and Compton, R.G.
    Chemical Science 7 1408-1414 (2016)
    In spite of their natural and technological importance, the intrinsic electrochemical properties of hematite (α-Fe2O3) nanoparticles are not well understood. In particular, particle agglomeration, the presence of surface impurities, and/or inadequate proton concentrations are major obstacles to uncover the fundamental redox activities of minerals in solution. These are particularly problematic when samples are characterized in common electrochemical analyses such as cyclic voltammetry in which nanoparticles are immobilized on a stationary electrode. In this work, the intrinsic reaction kinetics and thermodynamics of individual hematite nanoparticles are investigated by particle impact chronoamperometry. The particle radius derived from the integrated area of spikes recorded in a chronoamperogram is in excellent agreement with electron microscopy results, indicating that the method provides a quantitative analysis of the reduction of the nanoparticles to the ferrous ion. A key finding is that the suspended individual nanoparticles undergo electrochemical reduction at potentials much more positive than those immobilized on a stationary electrode. The critical importance of the solid/water interface on nanoparticle activity is further illustrated by a kinetic model. It is found that the first electron transfer process is the rate determining step of the reductive dissolution of hematite nanoparticles, while the overall process is strongly affected by the interfacial proton concentration. This article highlights the effects of the interfacial proton and ferrous ion concentrations on the reductive dissolution of hematite nanoparticles and provides a highly effective method that can be readily applied to study a wide range of other mineral nanoparticles. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5sc03678j
  • 2016 • 75 Full Kinetics from First Principles of the Chlorine Evolution Reaction over a RuO2(110) Model Electrode
    Exner, K.S. and Anton, J. and Jacob, T. and Over, H.
    Angewandte Chemie - International Edition 55 7501-7504 (2016)
    Current progress in modern electrocatalysis research is spurred by theory, frequently based on ab initio thermodynamics, where the stable reaction intermediates at the electrode surface are identified, while the actual energy barriers are ignored. This approach is popular in that a simple tool is available for searching for promising electrode materials. However, thermodynamics alone may be misleading to assess the catalytic activity of an electrochemical reaction as we exemplify with the chlorine evolution reaction (CER) over a RuO2(110) model electrode. The full procedure is introduced, starting from the stable reaction intermediates, computing the energy barriers, and finally performing microkinetic simulations, all performed under the influence of the solvent and the electrode potential. Full kinetics from first-principles allows the rate-determining step in the CER to be identified and the experimentally observed change in the Tafel slope to be explained. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201511804
  • 2016 • 74 Heating of the Mn spin system by photoexcited holes in type-II (Zn, Mn)Se/(Be, Mn)Te quantum wells
    Debus, J. and Maksimov, A.A. and Dunker, D. and Yakovlev, D.R. and Filatov, E.V. and Tartakovskii, I.I. and Ivanov, V.Y. and Waag, A. and Bayer, M.
    Physica Status Solidi (B) Basic Research 251 1694-1699 (2016)
    The efficiency of the Mn-spin system heating under pulsed laser excitation is studied in diluted magnetic semiconductor heterostructures Zn0.99 Mn0.01 Se/Be0.93 Mn0.07 Te with type-II band alignment by means of time-resolved photoluminescence and pump-probe reflectivity. An essential role in the heating is played by multiple spin-flip scatterings of a hole with localized spins of Mn2+ ions. The efficiency of the spin and energy transfer from photoexcited holes to Mn ions of the Zn0.99 Mn0.01 Se layer considerably depends on the hole lifetime in this layer. This lifetime can be limited by the hole relaxation into the Be0.93 Mn0.07 Te layers and is strongly sensitive to the excitation power and Zn0.99 Mn0.01 Se layer thickness. These dependences allow us to determine a characteristic time of about 20ps for the spin and energy transfer from photoexcited holes to the Mn spin system. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201350320
  • 2016 • 73 Kinetic interpretation of resonance phenomena in low pressure capacitively coupled radio frequency plasmas
    Wilczek, S. and Trieschmann, J. and Eremin, D. and Brinkmann, R.P. and Schulze, J. and Schuengel, E. and Derzsi, A. and Korolov, I. and Hartmann, P. and Donkó, Z. and Mussenbrock, T.
    Physics of Plasmas 23 (2016)
    Low pressure capacitive radio frequency (RF) plasmas are often described by equivalent circuit models based on fluid approaches that predict the self-excitation of resonances, e.g., high frequency oscillations of the total current in asymmetric discharges, but do not provide a kinetic interpretation of these effects. In fact, they leave important questions open: How is current continuity ensured in the presence of energetic electron beams generated by the expanding sheaths that lead to a local enhancement of the conduction current propagating through the bulk? How do the beam electrons interact with cold bulk electrons? What is the kinetic origin of resonance phenomena? Based on kinetic simulations, we find that the energetic beam electrons interact with cold bulk electrons (modulated on a timescale of the inverse local electron plasma frequency) via a time dependent electric field outside the sheaths. This electric field is caused by the electron beam itself, which leaves behind a positive space charge, that attracts cold bulk electrons towards the expanding sheath. The resulting displacement current ensures current continuity by locally compensating the enhancement of the conduction current. The backflow of cold electrons and their interaction with the nonlinear plasma sheath cause the generation of multiple electron beams during one phase of sheath expansion and contribute to a strongly non-sinusoidal RF current. These kinetic mechanisms are the basis for a fundamental understanding of the electron power absorption dynamics and resonance phenomena in such plasmas, which are found to occur in discharges of different symmetries including perfectly symmetric plasmas. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4953432
  • 2016 • 72 Kinetics of chemotaxis, cytokine, and chemokine release of NR8383 macrophages after exposure to inflammatory and inert granular insoluble particles
    Schremmer, I. and Brik, A. and Weber, D.G. and Rosenkranz, N. and Rostek, A. and Loza, K. and Brüning, T. and Johnen, G. and Epple, M. and Bünger, J. and Westphal, G.A.
    Toxicology Letters 263 68-75 (2016)
    Accumulation of macrophages and neutrophil granulocytes in the lung are key events in the inflammatory response to inhaled particles. The present study aims at the time course of chemotaxis in vitro in response to the challenge of various biopersistent particles and its functional relation to the transcription of inflammatory mediators. NR8383 rat alveolar macrophages were challenged with particles of coarse quartz, barium sulfate, and nanosized silica for one, four, and 16 h and with coarse and nanosized titanium dioxide particles (rutile and anatase) for 16 h only. The cell supernatants were used to investigate the chemotaxis of unexposed NR8383 macrophages. The transcription of inflammatory mediators in cells exposed to quartz, silica, and barium sulfate was analyzed by quantitative real-time PCR. Challenge with quartz, silica, and rutile particles induced significant chemotaxis of unexposed NR8383 macrophages. Chemotaxis caused by quartz and silica was accompanied by an elevated transcription of CCL3, CCL4, CXCL1, CXCL3, and TNFα. Quartz exposure showed an earlier onset of both effects compared to the nanosized silica. The strength of this response roughly paralleled the cytotoxic effects. Barium sulfate and anatase did not induce chemotaxis and barium sulfate as well caused no elevated transcription. In conclusion, NR8383 macrophages respond to the challenge with inflammatory particles with the release of chemotactic compounds that act on unexposed macrophages. The kinetics of the response differs between the various particles. © 2016 Elsevier Ireland Ltd
    view abstractdoi: 10.1016/j.toxlet.2016.08.014
  • 2016 • 71 Metal Nanoparticle-Catalyzed Reduction Using Borohydride in Aqueous Media: A Kinetic Analysis of the Surface Reaction by Microfluidic SERS
    Xie, W. and Grzeschik, R. and Schlücker, S.
    Angewandte Chemie - International Edition 55 13729-13733 (2016)
    Hydrides are widely used in reduction reactions. In protic solvents, their hydrolysis generates molecular hydrogen as a second reducing agent. The competition between these two parallel reduction pathways has been overlooked so far since both typically yield the same product. We investigated the platinum-catalyzed reduction of 4-nitrothiophenol to 4-aminothiophenol in aqueous sodium borohydride solution as a prominent model reaction, by using label-free SERS monitoring in a microfluidic reactor. Kinetic analysis revealed a strong pH dependence. Surprisingly, only at pH>13 the reduction is driven exclusively by sodium borohydride. This study demonstrates the potential of microfluidics-based kinetic SERS monitoring of heterogeneous catalysis in colloidal suspension. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201605776
  • 2016 • 70 Photoelectrochemistry: Zinc Ferrite Photoanode Nanomorphologies with Favorable Kinetics for Water-Splitting (Adv. Funct. Mater. 25/2016)
    Hufnagel, A.G. and Peters, K. and Müller, A. and Scheu, C. and Fattakhova-Rohlfing, D. and Bein, T.
    Advanced Functional Materials 26 4425 (2016)
    doi: 10.1002/adfm.201670155
  • 2016 • 69 Single-shot mega-electronvolt ultrafast electron diffraction for structure dynamic studies of warm dense matter
    Mo, M.Z. and Shen, X. and Chen, Z. and Li, R.K. and Dunning, M. and Sokolowski-Tinten, K. and Zheng, Q. and Weathersby, S.P. and Reid, A.H. and Coffee, R. and Makasyuk, I. and Edstrom, S. and McCormick, D. and Jobe, K. and Hast, C...
    Review of Scientific Instruments 87 (2016)
    We have developed a single-shot mega-electronvolt ultrafast-electron-diffraction system to measure the structural dynamics of warm dense matter. The electron probe in this system is featured by a kinetic energy of 3.2 MeV and a total charge of 20 fC, with the FWHM pulse duration and spot size at sample of 350 fs and 120 μm respectively. We demonstrate its unique capability by visualizing the atomic structural changes of warm dense gold formed from a laser-excited 35-nm freestanding single-crystal gold foil. The temporal evolution of the Bragg peak intensity and of the liquid signal during solid-liquid phase transition are quantitatively determined. This experimental capability opens up an exciting opportunity to unravel the atomic dynamics of structural phase transitions in warm dense matter regime. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4960070
  • 2016 • 68 Spectra library: An assumption-free in situ method to access the kinetics of catechols binding to colloidal ZnO quantum dots
    Lin, W. and Haderlein, M. and Walter, J. and Peukert, W. and Segets, D.
    Angewandte Chemie - International Edition 55 932-935 (2016)
    Assumption-free and in situ resolving of the kinetics of ligand binding to colloidal nanoparticles (NPs) with high time resolution is still a challenge in NP research. A unique concept of using spectra library and stopped-flow together with a "search best-match" Matlab algorithm to access the kinetics of ligand binding in colloidal systems is reported. Instead of deconvoluting superimposed spectra using assumptions, species absorbance contributions (ligand@ZnO NPs and ligand in solution) are obtained by offline experiments. Therefrom, a library of well-defined targets with known ligand distribution between particle surface and solution is created. Finally, the evolution of bound ligand is derived by comparing in situ spectra recorded by stopped-flow and the library spectra with the algorithm. Our concept is a widely applicable strategy for kinetic studies of ligand adsorption to colloidal NPs and a big step towards deep understanding of surface functionalization kinetics. Well-established libraries of target spectra that are derived by means of careful offline analysis and identification of equilibrium data within larger kinetic datasets can be used for any particle-ligand system. Kinetics of ligand binding to nanoparticles can be derived free of assumption, in situ, and with high time resolution. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201508252
  • 2016 • 67 Swift heavy ion irradiation of CaF2 - From grooves to hillocks in a single ion track
    Gruber, E. and Salou, P. and Bergen, L. and El Kharrazi, M. and Lattouf, E. and Grygiel, C. and Wang, Y. and Benyagoub, A. and Levavasseur, D. and Rangama, J. and Lebius, H. and Ban-D'Etat, B. and Schleberger, M. and Aumayr, F.
    Journal of Physics Condensed Matter 28 (2016)
    A novel form of ion-tracks, namely nanogrooves and hillocks, are observed on CaF2 after irradiation with xenon and lead ions of about 100 MeV kinetic energy. The irradiation is performed under grazing incidence (0.3°-3°) which forces the track to a region in close vicinity to the surface. Atomic force microscopy imaging of the impact sites with high spatial resolution reveals that the surface track consists in fact of three distinct parts: each swift heavy ion impacting on the CaF2 surface first opens a several 100 nm long groove bordered by a series of nanohillocks on both sides. The end of the groove is marked by a huge single hillock and the further penetration of the swift projectile into deeper layers of the target is accompanied by a single protrusion of several 100 nm in length slowly fading until the track vanishes. By comparing experimental data for various impact angles with results of a simulation, based on a three-dimensional version of the two-temperature-model (TTM), we are able to link the crater and hillock formation to sublimation and melting processes of CaF2 due to the local energy deposition by swift heavy ions. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/28/40/405001
  • 2016 • 66 Thermodynamics of Bioreactions
    Held, C. and Sadowski, G.
    Annual Review of Chemical and Biomolecular Engineering 7 395-414 (2016)
    Thermodynamic principles have been applied to enzyme-catalyzed reactions since the beginning of the 1930s in an attempt to understand metabolic pathways. Currently, thermodynamics is also applied to the design and analysis of biotechnological processes. The key thermodynamic quantity is the Gibbs energy of reaction, which must be negative for a reaction to occur spontaneously. However, the application of thermodynamic feasibility studies sometimes yields positive Gibbs energies of reaction even for reactions that are known to occur spontaneously, such as glycolysis. This article reviews the application of thermodynamics in enzyme-catalyzed reactions. It summarizes the basic thermodynamic relationships used for describing the Gibbs energy of reaction and also refers to the nonuniform application of these relationships in the literature. The review summarizes state-of-the-art approaches that describe the influence of temperature, pH, electrolytes, solvents, and concentrations of reacting agents on the Gibbs energy of reaction and, therefore, on the feasibility and yield of biological reactions. Copyright © 2016 by Annual Reviews. All rights reserved.
    view abstractdoi: 10.1146/annurev-chembioeng-080615-034704
  • 2016 • 65 Time-Resolved InSitu X-ray Diffraction Reveals Metal-Dependent Metal-Organic Framework Formation
    Wu, Y. and Henke, S. and Kieslich, G. and Schwedler, I. and Yang, M. S. and Fraser, D. A. X. and O'Hare, D.
    Angewandte Chemie-international Edition 55 14081--14084 (2016)
    Versatility in metal substitution is one of the key aspects of metal-organic framework (MOF) chemistry, allowing properties to be tuned in a rational way. As a result, it important to understand why MOF syntheses involving different metals arrive at or fail to produce the same topological outcome. Frequently, conditions are tuned by trial-and-error to make MOFs with different metal species. We ask: is it possible to adjust synthetic conditions in a systematic way in order to design routes to desired phases? We have used insitu X-ray powder diffraction to study the solvothermal formation of isostructural M-2(bdc)(2)dabco (M=Zn, Co, Ni) pillared-paddlewheel MOFs in real time. The metal ion strongly influences both kinetics and intermediates observed, leading in some cases to multiphase reaction profiles of unprecedented complexity. The standard models used for MOF crystallization break down in these cases; we show that a simple kinetic model describes the data and provides important chemical insights on phase selection.
    view abstractdoi: 10.1002/anie.201608463
  • 2016 • 64 Zinc Ferrite Photoanode Nanomorphologies with Favorable Kinetics for Water-Splitting
    Hufnagel, A.G. and Peters, K. and Müller, A. and Scheu, C. and Fattakhova-Rohlfing, D. and Bein, T.
    Advanced Functional Materials 26 4435-4443 (2016)
    The n-type semiconducting spinel zinc ferrite (ZnFe2O4) is used as a photoabsorber material for light-driven water-splitting. It is prepared for the first time by atomic layer deposition. Using the resulting well-defined thin films as a model system, the performance of ZnFe2O4 in photoelectrochemical water oxidation is characterized. Compared to benchmark α-Fe2O3 (hematite) films, ZnFe2O4 thin films achieve a lower photocurrent at the reversible potential. However, the oxidation onset potential of ZnFe2O4 is 200 mV more cathodic, allowing the water-splitting reaction to proceed at a lower external bias and resulting in a maximum applied-bias power efficiency (ABPE) similar to that of Fe2O3. The kinetics of the water oxidation reaction are examined by intensity-modulated photocurrent spectroscopy. The data indicate a considerably higher charge transfer efficiency of ZnFe2O4 at potentials between 0.8 and 1.3 V versus the reversible hydrogen electrode, attributable to significantly slower surface charge recombination. Finally, nanostructured ZnFe2O4 photoanodes employing a macroporous antimony-doped tin oxide current collector reach a five times higher photocurrent than the flat films. The maximum ABPE of these host–guest photoanodes is similarly increased. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201600461
  • 2015 • 63 A new hybrid scheme for simulations of highly collisional RF-driven plasmas
    Eremin, D. and Hemke, T. and Mussenbrock, T.
    Plasma Sources Science and Technology 25 (2015)
    This work describes a new 1D hybrid approach for modeling atmospheric pressure discharges featuring complex chemistry. In this approach electrons are described fully kinetically using particle-in-cell/Monte-Carlo (PIC/MCC) scheme, whereas the heavy species are modeled within a fluid description. Validity of the popular drift-diffusion approximation is verified against a 'full' fluid model accounting for the ion inertia and a fully kinetic PIC/MCC code for ions as well as electrons. The fluid models require knowledge of the momentum exchange frequency and dependence of the ion mobilities on the electric field when the ions are in equilibrium with the latter. To this end an auxiliary Monte-Carlo scheme is constructed. It is demonstrated that the drift-diffusion approximation can overestimate ion transport in simulations of RF-driven discharges with heavy ion species operated in the γ mode at the atmospheric pressure or in all discharge simulations for lower pressures. This can lead to exaggerated plasma densities and incorrect profiles provided by the drift-diffusion models. Therefore, the hybrid code version featuring the full ion fluid model should be favored against the more popular drift-diffusion model, noting that the suggested numerical scheme for the former model implies only a small additional computational cost. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/25/1/015009
  • 2015 • 62 Combined experiment and theory approach in surface chemistry: Stairway to heaven?
    Exner, K.S. and Heß, F. and Over, H. and Seitsonen, A.P.
    Surface Science 640 165-180 (2015)
    In this perspective we discuss how an intimate interaction of experiments with theory is able to deepen our insight into the catalytic reaction system on the molecular level. This strategy is illustrated by discussing various examples from our own research of surface chemistry and model catalysis. The particular examples were carefully chosen to balance the specific strength of both approaches - theory and experiment - and emphasize the benefit of this combined approach. We start with the determination of complex surface structures, where diffraction techniques in combination with theory are clear-cut. The promoter action of alkali metals in heterogeneous catalysis is rationalized with theory and experiment for the case of CO coadsorption. Predictive power of theory is limited as demonstrated with the apparent activity of chlorinated TiO2(110) in the oxidation of HCl: Even if we know all elementary reaction steps of a catalytic reaction mechanism, the overall kinetics may remain elusive and require the application kinetic Monte Carlo simulations. Catalysts are not always stable under reaction conditions and may chemically transform as discussed for the CO oxidation reaction over ruthenium. Under oxidizing reaction conditions ruthenium transforms into RuO2, a process which is hardly understood on the molecular level. Lastly we focus on electrochemical reactions. Here theory is clearly ahead since spectroscopic methods are not available to resolve the processes at the electrode surface. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2015.01.006
  • 2015 • 61 Continuous delivery of rhBMP2 and rhVEGF165 at a certain ratio enhances bone formation in mandibular defects over the delivery of rhBMP2 alone - An experimental study in rats
    Lohse, N. and Moser, N. and Backhaus, S. and Annen, T. and Epple, M. and Schliephake, H.
    Journal of Controlled Release 220 201-209 (2015)
    The aim of the present study was to test the hypothesis that different amounts of vascular endothelial growth factor and bone morphogenic protein differentially affect bone formation when applied for repair of non-healing defects in the rat mandible. Porous composite PDLLA/CaCO3 carriers were fabricated as slow release carriers and loaded with rhBMP2 and rhVEGF165 in 10 different dosage combinations using gas foaming with supercritical carbon dioxide. They were implanted in non-healing defects of the mandibles of 132 adult Wistar rats with additional lateral augmentation. Bone formation was assessed both radiographically (bone volume) and by histomorphometry (bone density). The use of carriers with a ratio of delivery of VEGF/BMP between 0.7 and 1.2 was significantly related to the occurrence of significant increases in radiographic bone volume and/or histologic bone density compared to the use of carriers with a ratio of delivery of ≤ 0.5 when all intervals and all outcome parameters were considered. Moreover, simultaneous delivery at this ratio helped to "save" rhBMP2 as both bone volume and bone density after 13 weeks were reached/surpassed using half the dosage required for rhBMP2 alone. It is concluded, that the combined delivery of rhVEGF165 and rhBMP2 for repair of critical size mandibular defects can significantly enhance volume and density of bone formation over delivery of rhBMP2 alone. It appears from the present results that continuous simultaneous delivery of rhVEGF165 and rhBMP2 at a ratio of approximately 1 is favourable for the enhancement of bone formation. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jconrel.2015.10.032
  • 2015 • 60 Effect of the specific surface area on thermodynamic and kinetic properties of nanoparticle anatase TiO2 in lithium-ion batteries
    Madej, E. and Klink, S. and Schuhmann, W. and Ventosa, E. and La Mantia, F.
    Journal of Power Sources 297 140-148 (2015)
    Anatase TiO<inf>2</inf> nanoparticles with a specific surface area of 100 m2 g-1 and 300 m2 g-1 have been investigated as negative insertion electrode material for lithium-ion batteries. Galvanostatic intermittent titration (GITT) and electrochemical impedance spectroscopy (EIS) were used to investigate the effect of the specific surface area on the performance of the material. GITT was performed at C/10 rate, followed by an EIS measurement after each relaxation step. Separation of kinetic and thermodynamic contributions to the overpotential of the phase transformation on Li+ (de-)insertion allowed revealing a dependency of both terms on the specific surface area. The material with higher surface area undergoes intrinsic transformation during the initial cycles affecting the thermodynamics of (de-)insertion while the sample with lower surface area shows large and asymmetric kinetic hindrances. For the material with 15 nm particles, Li+ de-insertion appears to have a higher resistance than lithium insertion. © 2015, Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jpowsour.2015.07.079
  • 2015 • 59 Fluctuating multicomponent lattice Boltzmann model
    Belardinelli, D. and Sbragaglia, M. and Biferale, L. and Gross, M. and Varnik, F.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 91 (2015)
    Current implementations of fluctuating lattice Boltzmann equations (FLBEs) describe single component fluids. In this paper, a model based on the continuum kinetic Boltzmann equation for describing multicomponent fluids is extended to incorporate the effects of thermal fluctuations. The thus obtained fluctuating Boltzmann equation is first linearized to apply the theory of linear fluctuations, and expressions for the noise covariances are determined by invoking the fluctuation-dissipation theorem directly at the kinetic level. Crucial for our analysis is the projection of the Boltzmann equation onto the orthonormal Hermite basis. By integrating in space and time the fluctuating Boltzmann equation with a discrete number of velocities, the FLBE is obtained for both ideal and nonideal multicomponent fluids. Numerical simulations are specialized to the case where mean-field interactions are introduced on the lattice, indicating a proper thermalization of the system. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.91.023313
  • 2015 • 58 Hydrogenation of CO2 to methanol and CO on Cu/ZnO/Al2O3: Is there a common intermediate or not? This work is dedicated to the memory and achievements of Dr. Haldor Topsøe.
    Kunkes, E.L. and Studt, F. and Abild-Pedersen, F. and Schlögl, R. and Behrens, M.
    Journal of Catalysis 328 43-48 (2015)
    H/D exchange experiments on a Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> catalyst have shown that methanol synthesis and RWGS display a strong thermodynamic isotope effect, which is attributed to differences in the zero-point energy of hydrogenated vs. deuterated species. The effect is larger for methanol synthesis and substantially increases the equilibrium yield in deuterated syngas. In the kinetic regime of CO<inf>2</inf> hydrogenation, an inverse kinetic isotope effect of H/D substitution was observed, which is stronger for methanol synthesis than for CO formation suggesting that the two reactions do not share a common intermediate. Similar observations were also made on other catalysts such as Cu/MgO, Cu/SiO<inf>2</inf>, and Pd/SiO<inf>2</inf>. In contrast to CO<inf>2</inf> hydrogenation, the CO hydrogenation on Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> did not show such a strong kinetic isotope effect indicating that methanol formation from CO<inf>2</inf> does not proceed via consecutive reverse water gas shift and CO hydrogenation steps. The inverse KIE is consistent with formate hydrogenation being the rate-determining step of methanol synthesis from CO<inf>2</inf>. Differences in the extent of product inhibition by water, observed for methanol synthesis and reverse water gas shift indicate that the two reactions proceed on different surface sites in a parallel manner. The consequences for catalyst design for effective methanol synthesis from CO<inf>2</inf> are discussed. © 2014 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2014.12.016
  • 2015 • 57 Kinetic arrest in magnetically inhomogeneous C-deficient Mn3GaC
    Cakir, O. and Acet, M. and Farle, M. and Dias, E. and Priolkar, K.
    Journal of Magnetism and Magnetic Materials 390 96--99 (2015)
    Thermal broadening of the first order magnetostructural transition and enhancement in ferromagnetic exchange occurs in carbon deficient Mn3GaC0.9. We show from temperature and field-dependent magnetization measurements that this leads to inhomogeneous magnetism and causes frustration and kinetic arrest effects. The arrested state is deactivated and the system returns to its ground state when the cooling-field is removed. This causes open hysteresis loops with which we study the kinetic arrest effect in this system. (C) 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2015.04.084
  • 2015 • 56 Large Eddy Simulation of coal combustion in a large-scale laboratory furnace
    Rabaçal, M. and Franchetti, B.M. and Marincola, F.C. and Proch, F. and Costa, M. and Hasse, C. and Kempf, A.M.
    Proceedings of the Combustion Institute 35 3609-3617 (2015)
    A detailed Large Eddy Simulation (LES) of pulverised coal combustion in a large-scale laboratory furnace is presented. To achieve a detailed representation of the flow, mixing and particle dispersion, a massively parallel LES was performed. Different phenomenological network models were applied and compared to each other in order to obtain the most adequate devolatilization kinetic data for the LES. An iterative procedure allowed to optimise the devolatilization kinetic data for the studied coal and operating conditions. The particle combustion history is studied by analysing particle instantaneous properties giving a perspective on coal combustion that currently is not available by other means than LES. Predicted major species and temperature were compared with measurements and a good agreement was obtained. The finely resolved near burner region revealed that the flame is stabilised very close to the burner. Furthermore, two distinct zones of CO2 production were found - one in the internal recirculation zone (IRZ) due to gaseous combustion, and one downstream of the vortex breakdown, due to intense char combustion. It was found that particle properties are inhomogeneous within the IRZ, whereas in the external recirculation zone (ERZ) and downstream of the vortex breakdown they were found to be homogeneous. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2014.06.023
  • 2015 • 55 Ligand-free Gold Nanoparticles as a Reference Material for Kinetic Modelling of Catalytic Reduction of 4-Nitrophenol
    Gu, S. and Kaiser, J. and Marzun, G. and Ott, A. and Lu, Y. and Ballauff, M. and Zaccone, A. and Barcikowski, S. and Wagener, P.
    Catalysis Letters 145 1105-1112 (2015)
    The reduction of 4-nitrophenol by sodium borohydride is a common model reaction to test the catalytic activity of metal nanoparticles. As all reaction steps proceed solely on the surface of the metal nanoparticles (Langmuir-Hinshelwood model), ligand-coverage of metal nanoparticles impedes the merging of theory and experiment. Therefore we analyzed the catalytic activity of bare gold nanoparticles prepared by laser ablation in liquid without any stabilizers or ligands. The catalytic reaction is characterized by a full kinetic analysis including 4-hydroxylaminophenol as an intermediate species. Excellent agreement between theory and experiment is found. Moreover, the suspension of the nanoparticles remains stable. Hence, ligand-free nanoparticles can be used as a reference material for mechanistic studies of catalytic reactions. In addition, the analysis shows that gold nanoparticles synthesized by laser ablation are among the most active catalysts for this reaction. (Graph Presented). © 2015 Springer Science+Business Media.
    view abstractdoi: 10.1007/s10562-015-1514-7
  • 2015 • 54 Modelling and evaluation of hydrogen desorption kinetics controlled by surface reaction and bulk diffusion for magnesium hydride
    Drozdov, I.V. and Vaßen, R. and Stöver, D.
    RSC Advances 5 5363-5371 (2015)
    The 'shrinking core' model has been applied for the evaluation of hydrogen desorption kinetics during decomposition of magnesium hydride. According to our estimation, the full desorption time is expected to have a quadratic dependence on the size of powder particles, if the bulk diffusion of hydrogen atoms in magnesium is a rate controlling step. However, for the actual diffusion rate for hydrogen in magnesium bulk the diffusion cannot significantly influence the overall desorption kinetics for microand nano-powders. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4ra08089k
  • 2015 • 53 Modelling and proper evaluation of volumetric kinetics of hydrogen desorption by metal hydrides
    Drozdov, I.V. and Kochubey, V. and Meng, L. and Mauer, G. and Vaßen, R. and Stöver, D.
    International Journal of Hydrogen Energy 40 10111-10122 (2015)
    A simple model of the hydrogen desorption kinetics of metal hydrides is formulated and solved analytically. The particle surface reaction is assumed to be a rate-controlling-step. Then a volumetric measurement of hydrogen desorption process is evaluated on an example of wet ball milled magnesium hydride, and can be applied generally for any metal hydride. The solution reproduces the shape of experimental curves for desorption process. In the case of surface-controlled kinetics, a volumetric measurement requires a special evaluation of results, predicted by the solution of the model. An improved evaluation of the volumetric measurement of hydrogen desorption from magnesium hydride powders using the model has been demonstrated. © 2015 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2015.05.119
  • 2015 • 52 Physics of the Advanced Plasma Source: A review of recent experimental and modeling approaches
    Brinkmann, R.P. and Harhausen, J. and Schröder, B. and Lapke, M. and Storch, R. and Styrnoll, T. and Awakowicz, P. and Foest, R. and Hannemann, M. and Loffhagen, D. and Ohl, A.
    Plasma Physics and Controlled Fusion 58 (2015)
    The Advanced Plasma Source (APS), a gridless hot cathode glow discharge capable of generating an ion beam with an energy of up to 150 eV and a flux of 1019s-1, is a standard industrial tool for the process of plasma ion-assisted deposition (PIAD). This manuscript details the results of recent experimental and modeling work aimed at a physical understanding of the APS. A three-zone model is proposed which consists of (i) the ionization zone (the source itself) where the plasma is very dense, hot, and has a high ionization rate, (ii) the acceleration zone (of ~20 cm extension) where a strong outward-directed electric field accelerates the primary ions to a high kinetic energy, and (iii) a drift zone (the rest of the process chamber) where the emerging plasma beam is further modified by resonant charge exchange collisions that neutralize some of the energetic ions and generate, at the same time, a flux of slow ions. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0741-3335/58/1/014033
  • 2015 • 51 Ripening kinetics of laser-generated plasmonic nanoparticles in different solvents
    Gökce, B. and Van't Zand, D.D. and Menéndez-Manjón, A. and Barcikowski, S.
    Chemical Physics Letters 626 96-101 (2015)
    Abstract Pulsed laser ablation in liquid is considered to be a fast nanoparticle-synthesis method taking place on ps to μs timescale. Here, we report the comparably slow ripening kinetics of laser-generated plasmonic nanoparticles (copper, silver, and gold) immediately after ablation. The growth dynamics is studied in situ by following the surface plasmon resonance and correlated to known models. We thereby identify a two-step diffusion-controlled coalescence and growth mechanism, quantify their kinetic constants and show the effect of different solvents (water, acetone, ethanol, and ethyl acetate). © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.cplett.2015.03.010
  • 2015 • 50 Targeting Drug Resistance in EGFR with Covalent Inhibitors: A Structure-Based Design Approach
    Engel, J. and Richters, A. and Getlik, M. and Tomassi, S. and Keul, M. and Termathe, M. and Lategahn, J. and Becker, C. and Mayer-Wrangowski, S. and Grütter, C. and Uhlenbrock, N. and Krüll, J. and Schaumann, N. and Eppmann, S. ...
    Journal of Medicinal Chemistry 58 6844-6863 (2015)
    Receptor tyrosine kinases represent one of the prime targets in cancer therapy, as the dysregulation of these elementary transducers of extracellular signals, like the epidermal growth factor receptor (EGFR), contributes to the onset of cancer, such as non-small cell lung cancer (NSCLC). Strong efforts were directed to the development of irreversible inhibitors and led to compound CO-1686, which takes advantage of increased residence time at EGFR by alkylating Cys797 and thereby preventing toxic effects. Here, we present a structure-based approach, rationalized by subsequent computational analysis of conformational ligand ensembles in solution, to design novel and irreversible EGFR inhibitors based on a screening hit that was identified in a phenotype screen of 80 NSCLC cell lines against approximately 1500 compounds. Using protein X-ray crystallography, we deciphered the binding mode in engineered cSrc (T338M/S345C), a validated model system for EGFR-T790M, which constituted the basis for further rational design approaches. Chemical synthesis led to further compound collections that revealed increased biochemical potency and, in part, selectivity toward mutated (L858R and L858R/T790M) vs nonmutated EGFR. Further cell-based and kinetic studies were performed to substantiate our initial findings. Utilizing proteolytic digestion and nano-LC-MS/MS analysis, we confirmed the alkylation of Cys797. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jmedchem.5b01082
  • 2015 • 49 Theoretical rovibrational spectroscopy of NO2+
    Botschwina, P. and Bargholz, A. and Sebald, P. and Stein, C. and Schröder, B. and Oswald, R.
    Journal of Molecular Spectroscopy 311 12-18 (2015)
    Accurate near-equilibrium potential energy functions (PEFs) have been constructed for the nitronium ion (NO2+) by composite methods using either CCSD(T)-F12b or explicitly correlated multi-reference methods (MRCI-F12+Q or MRACPF-F12) as dominant contributions. Up to pentuple substitutions are required in the coupled-cluster based approach to reach convergence in the wavenumbers of the fundamentals to ca. 1 cm-1. These are predicted to be ν1=1386.0cm-1,ν1=621.1 cm-1 and ν3=2342.8 cm-1. All values differ significantly from the results of previous studies by zero-kinetic energy (ZEKE) spectroscopy and reanalysis or remeasurement is suggested. Compared to neon-matrix IR spectroscopic work of Jacox and coworkers the present calculations yield smaller wavenumbers of Δν3=-5.4 cm-1 and Δ(ν1+ν3)=-7.9 cm-1 so that blueshifting is predicted for those absorptions. The calculated isotopic shifts for both bands are in excellent agreement with the corresponding experimental values. Accurate values for rotational and centrifugal distortion constants of NO2+ in different vibrational states are predicted which should be of help in the search for forthcoming high-resolution spectra of that cation. © 2014 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jms.2014.11.005
  • 2015 • 48 Toxicity Inhibitors Protect Lipid Membranes from Disruption by Aβ42
    Malishev, R. and Nandi, S. and Kolusheva, S. and Levi-Kalisman, Y. and Klärner, F.-G. and Schrader, T. and Bitan, G. and Jelinek, R.
    ACS Chemical Neuroscience 6 1860-1869 (2015)
    Although the precise molecular factors linking amyloid β-protein (Aβ) to Alzheimer's disease (AD) have not been deciphered, interaction of Aβ with cellular membranes has an important role in the disease. However, most therapeutic strategies targeting Aβ have focused on interfering with Aβ self-assembly rather than with its membrane interactions. Here, we studied the impact of three toxicity inhibitors on membrane interactions of Aβ42, the longer form of Aβ, which is associated most strongly with AD. The inhibitors included the four-residue C-terminal fragment Aβ(39-42), the polyphenol (-)-epigallocatechin-3-gallate (EGCG), and the lysine-specific molecular tweezer, CLR01, all of which previously were shown to disrupt different steps in Aβ42 self-assembly. Biophysical experiments revealed that incubation of Aβ42 with each of the three modulators affected membrane interactions in a distinct manner. Interestingly, EGCG and CLR01 were found to have significant interaction with membranes themselves. However, membrane bilayer disruption was reduced when the compounds were preincubated with Aβ42, suggesting that binding of the assembly modulators to the peptide attenuated their membrane interactions. Importantly, our study reveals that even though the three tested compounds affect Aβ42 assembly differently, membrane interactions were significantly inhibited upon incubation of each compound with Aβ42, suggesting that preventing the interaction of Aβ42 with the membrane contributes substantially to inhibition of its toxicity by each compound. The data suggest that interference with membrane interactions is an important factor for Aβ42 toxicity inhibitors and should be taken into account in potential therapeutic strategies, in addition to disruption or remodeling of amyloid assembly. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acschemneuro.5b00200
  • 2014 • 47 A thermodynamic investigation of the glucose-6-phosphate isomerization
    Hoffmann, P. and Held, C. and Maskow, T. and Sadowski, G.
    Biophysical Chemistry 195 22-31 (2014)
    In this work, ΔRg+ values for the enzymatic G6P isomerization were determined as a function of the G6P equilibrium molality between 25 °C and 37 °C. The reaction mixtures were buffered at pH = 8.5. In contrast to standard literature work, ΔRg+ values were determined from activity-based equilibrium constants instead of molality-based apparent values. This yielded a ΔRg+ value of 2.55 ± 0.05 kJ mol- 1 at 37 °C, independent of the solution pH between 7.5 and 8.5. Furthermore, ΔRh + was measured at pH = 8.5 and 25 °C yielding 12.05 ± 0.2 kJ mol- 1. Accounting for activity coefficients turned out to influence ΔRg+ up to 30% upon increasing the G6P molality. This result was confirmed by predictions using the thermodynamic model ePC-SAFT. Finally, the influence of the buffer and of potassium glutamate as an additive on the reaction equilibrium was measured and predicted with ePC-SAFT in good agreement. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.bpc.2014.08.002
  • 2014 • 46 Constraints on the nature and evolution of the magma plumbing system of Mt. Etna volcano (1991-2008) from a combined thermodynamic and kinetic modelling of the compositional record of minerals
    Kahl, M. and Chakraborty, S. and Pompilio, M. and Costa, F.
    Journal of Petrology 56 2025-2068 (2014)
    Deciphering the evolution of the internal dynamics of magmatic plumbing systems and identifying the key parameters that drive such dynamics are major goals of modern volcanology. Here we present a novel petrological approach that combines kinetic modelling of the diffusive relaxation of chemical zoning patterns in olivine crystals with thermodynamic modelling (MELTS) to constrain the nature and evolution of the plumbing system of Mt. Etna and the processes governing its internal dynamics. We investigated the compositional and temporal record preserved in 180 olivine crystals that were erupted between 1991 and 2008. Detailed systemization of the information stored in the sequential zoning record of the olivines reveals the existence of at least five compositionally different magmatic environments (MEs), characterized by different olivine compositions: M0 (Fo79-83), M1 (Fo75-78), M2 (Fo70-72), M3 (Fo65-69) and mm1 (Fo73-75). Several routes of magma transfer connect these environments. We identified three prominent magma passageways between the environments M0:M1, M2:mm1 and M1:M2 that were active during the entire period of observation between 1991 and 2008. Modelling the diffusive relaxation of the olivine zoning patterns reveals that the transfer of magma along such routes can occur over fairly heterogeneous timescales ranging from days to 2 years. Although some of the passageways have been sporadically active in the months and sometimes years before an eruption, the magma migration activity increases clearly in the weeks and days prior to an eruptive event. In this context, major transfer routes such as M2:mm1 might represent temporary passageways that are activated only shortly before eruptive events. A forward modelling approach was developed using thermodynamic calculations with the MELTS software to identify the key intensive variables associated with the different magmatic environments. In this approach the observed populations of mineral compositions (e.g. Fo79-83), rather than single compositions, are associated with thermodynamic parameters [pressure, temperature, water content, oxygen fugacity (fO2) and bulk composition of the melt] to identify the most plausible set corresponding to each ME. We found that temperature, water content and possibly oxidation state are the main distinguishing features of the different magmatic environments. The most primitive olivine population M0 (Fo79-83) and some of its associated clinopyroxenes formed at high melt water contents (3·5-5·2 wt %), at fO2 conditions buffered at quartz-fayalite-magnetite (QFM) or Ni-NiO (NNO), at temperatures ≥ 1110°C and at pressures ranging between 1·5 and 3·0 kbar (or higher). The intermediate population M1 (Fo75-78) can be produced over a broad spectrum of conditions, but all require similar temperatures and lower water contents (0·1-1·4 wt %). The most evolved, more Fe-rich olivines of M2 and M3 are products of melts with much lower water contants (0·2-1·1 wt % H2O for M2, < 0·5 wt % H2O for M3), and at probably somewhat more reducing (QFM) conditions at somewhat lower temperatures (~1080°C). The M3 environment characterized by very low water contents and reducing conditions could be related to an enhanced flux of CO2. Combination of the characteristics of the various magmatic environments with temporal information (residence time in different environments and timing of magma transfer between these environments) allows a dynamic model of the plumbing system beneath Mt. Etna to be constructed. © The Author 2015.
    view abstractdoi: 10.1093/petrology/egv063
  • 2014 • 45 Cu-based catalyst resulting from a Cu,Zn,Al hydrotalcite-like compound: A microstructural, thermoanalytical, and in situ XAS study
    Kühl, S. and Tarasov, A. and Zander, S. and Kasatkin, I. and Behrens, M.
    Chemistry - A European Journal 20 3782-3792 (2014)
    A Cu-based methanol synthesis catalyst was obtained from a phase pure Cu,Zn,Al hydrotalcite-like precursor, which was prepared by co-precipitation. This sample was intrinsically more active than a conventionally prepared Cu/ZnO/Al2O3 catalyst. Upon thermal decomposition in air, the [(Cu0.5Zn0.17Al0.33)(OH) 2(CO3)0.17]×mH2O precursor is transferred into a carbonate-modified, amorphous mixed oxide. The calcined catalyst can be described as well-dispersed "CuO" within ZnAl 2O4 still containing stabilizing carbonate with a strong interaction of Cu2+ ions with the Zn-Al matrix. The reduction of this material was carefully analyzed by complementary temperature-programmed reduction (TPR) and near-edge X-ray absorption fine structure (NEXAFS) measurements. The results fully describe the reduction mechanism with a kinetic model that can be used to predict the oxidation state of Cu at given reduction conditions. The reaction proceeds in two steps through a kinetically stabilized CuI intermediate. With reduction, a nanostructured catalyst evolves with metallic Cu particles dispersed in a ZnAl2O4 spinel-like matrix. Due to the strong interaction of Cu and the oxide matrix, the small Cu particles (7 nm) of this catalyst are partially embedded leading to lower absolute activity in comparison with a catalyst comprised of less-embedded particles. Interestingly, the exposed Cu surface area exhibits a superior intrinsic activity, which is related to a positive effect of the interface contact of Cu and its surroundings. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201302599
  • 2014 • 44 FLiK: A direct-binding assay for the identification and kinetic characterization of stabilizers of inactive kinase conformations
    Simard, J.R. and Rauh, D.
    Methods in Enzymology 548 147-171 (2014)
    Despite the hundreds of kinase inhibitors currently in discovery and preclinical phases, the number of FDA-approved kinase inhibitors remains very low by comparison, a discrepancy which reflects the challenges which accompanies kinase inhibitor development. Targeting protein kinases with ATP-competitive inhibitors has been the classical approach to inhibit kinase activity, but the highly conserved nature of the ATP-binding site often contributes to the poor inhibitor selectivity. To address this problem, we developed a high-throughput screening technology that can discriminate for inhibitors, which stabilize inactive kinase conformations by binding within allosteric pockets in the kinase domain. Here, we describe how to use the Fluorescence Labels in Kinases approach to measure the Kd of ligands as well as how to kinetically characterize the binding and dissociation of ligands to the kinase. We also describe how this technology can be used to rapidly screen small molecule libraries in high throughput. © 2014 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/B978-0-12-397918-6.00006-9
  • 2014 • 43 History effects in lithium-oxygen batteries: How initial seeding influences the discharge capacity
    Rinaldi, A. and Wijaya, O. and Hoster, H.E. and Yu, D.Y.W.
    ChemSusChem 7 1283-1288 (2014)
    In laboratory experiments, Li-O2 systems show "sudden death" at capacities far below the theoretical value. Identifying how discharge products limit the total capacity is crucial in Li-O2 system. We investigated the effect of Li2O2 seed layer deposited on carbon cathode under potentiostatic conditions at increasing overpotentials to the subsequent slow discharge at galvanostatic condition. The discharge capacity attainable in the second step is found to vary by more than a factor of 3 depending on the history, i.e., the seed layer. These results provide evidence that the battery history is decisive for the total discharge capacities. History lesson: The discharge product will at some point form the surface of the ongoing electrochemical reaction in Li-O2 battery. The nature of Li2O2 deposits are crucial for a battery's capacity performance. The discharge profiles of carbon cathodes that are precovered by Li2O2 seed layers are compared. The layers are Coulometrically equal but are deposited at varying deposition rates, and demonstrate how faster initial seeding leads to lower total discharge capacities. © 2014 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201300986
  • 2014 • 42 Immobilization of proteins in their physiological active state at functionalized thiol monolayers on ATR-germanium crystals
    Schartner, J. and Gavriljuk, K. and Nabers, A. and Weide, P. and Muhler, M. and Gerwert, K. and Kötting, C.
    ChemBioChem 15 2529-2534 (2014)
    Protein immobilization on solid surfaces has become a powerful tool for the investigation of protein function. Physiologically relevant molecular reaction mechanisms and interactions of proteins can be revealed with excellent signal-to-noise ratio by vibrational spectroscopy (ATR-FTIR) on germanium crystals. Protein immobilization by thiol chemistry is well-established on gold surfaces, for example, for surface plasmon resonance. Here, we combine features of both approaches: a germanium surface functionalized with different thiols to allow specific immobilization of various histidine-tagged proteins with over 99% specific binding. In addition to FTIR, the surfaces were characterized by XPS and fluorescence microscopy. Secondary-structure analysis and stimulus-induced difference spectroscopy confirmed protein activity at the atomic level, for example, physiological cation channel formation of Channelrhodopsin 2. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cbic.201402478
  • 2014 • 41 Influence of storage condition on properties of MCC II-based pellets with theophylline-monohydrate
    Krueger, C. and Thommes, M. and Kleinebudde, P.
    European Journal of Pharmaceutics and Biopharmaceutics 88 483-491 (2014)
    Microcrystalline cellulose II (MCC II1) is a polymorph of commonly used MCC I; in 2010 it was introduced as new pelletization aid in wet-extrusion/spheronization leading to fast disintegrating pellets. Previous investigations suggested that the storage of the resulting pellets affect the disintegration behavior, the non-hygroscopic substance chloramphenicol that showed no polymorphism or hydrate formation due to relative humidity was used for the investigations. Therefore, theophylline-monohydrate that can dehydrate during storage, but also during manufacturing and drying was used for this study to confirm the results of the previous study and give a more detailed overview of the influence of recrystallization of theophylline monohydrate on disintegration. Storage recommendations should be derived. MCC II-based pellets were prepared of binary mixtures containing 10%, 20% or 50% MCC II as pelletization aid and theophylline-monohydrate as API. These pellets were stored at different relative humidity (0-97%rH; 20 °C); the influence on their disintegration and drug release was investigated. The storage conditions had an impact on pellet disintegration. Low relative humidities (≤40%rH) led to a conversion of the monohydrate to the anhydrous form. Newly grown crystals formed a kind of network around the pellet and inhibited the disintegration. High relative humidity (>80%rh) affected the disintegration caused by changes in the MCC II as already seen in the previous study. Due to the changed disintegration behavior also the drug release and release kinetic changed. Therefore, for theophylline containing pellets a storage humidity of 55%rH to 80%rH (20 °C) is recommended. All in all, these investigations substantiate the knowledge of MCC II-based pellets providing a better basis for adequate storage conditions of MCC II based pellets. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.ejpb.2014.06.006
  • 2014 • 40 Molecular basis for preventing α-synuclein aggregation by a molecular tweezer
    Acharya, S. and Safaie, B.M. and Wongkongkathep, P. and Ivanova, M.I. and Attar, A. and Klärner, F.-G. and Schrader, T. and Loo, J.A. and Bitan, G. and Lapidus, L.J.
    Journal of Biological Chemistry 289 10727-10737 (2014)
    Recent work on α-synuclein has shown that aggregation is controlled kinetically by the rate of reconfiguration of the unstructured chain, such that the faster the reconfiguration, the slower the aggregation. In this work we investigate this relationship by examining α-synuclein in the presence of a small molecular tweezer, CLR01, which binds selectively to Lys side chains. We find strong binding to multiple Lys within the chain as measured by fluorescence and mass-spectrometry and a linear increase in the reconfiguration rate with concentration of the inhibitor. Top-down mass-spectrometric analysis shows that the main binding of CLR01 to α-synuclein occurs at the N-terminal Lys-10/Lys- 12. Photo-induced cross-linking of unmodified proteins (PICUP) analysis shows that under the conditions used for the fluorescence analysis, α-synuclein ispredominantlymonomeric.Theresultscan be successfully modeled using a kineticschemein which two aggregation- pronemonomerscanformanencountercomplexthat leads to further oligomerization but can also dissociate back to monomers if the reconfiguration rate is sufficiently high.Takentogether, the data provide important insights into the preferred binding site of CLR01 on α-synuclein and the mechanism by which the molecular tweezer prevents self-assembly into neurotoxic aggregates by α-synuclein and presumably other amyloidogenic proteins. © 2014 by The American Society for Biochemistry and Molecular Biology, Inc.
    view abstractdoi: 10.1074/jbc.M113.524520
  • 2014 • 39 Ostwald-Freundlich diffusion-limited dissolution kinetics of nanoparticles
    Ely, D.R. and Edwin García, R. and Thommes, M.
    Powder Technology 257 120-123 (2014)
    For many years, nanoparticles have garnered increasing interest in pharmaceutical investigations. It is well known that the solubility of nanoparticles increases with decreasing size due to the Gibbs-Thomson effect. However, there are currently no analytical models to describe the kinetics of nanoparticle dissolution. The purpose of this article is to provide a Thermodynamics-based description of the kinetics of nanoparticle dissolution. In particular, the Ostwald-Freundlich relation is used to correct dissolution times for small particles, which have higher solubilities than larger particles. The developed model is an extension of the Hixson-Crowell cube root law in which the total normalized dissolution time is corrected by a "solubility size factor" that approaches unity for increasing initial particle size. This model enables rapid estimation of the total dissolution time of spherical nanoparticles in a gently agitated, zero solute concentration reservoir. The total dissolution time predicted differs from Hixson-Crowell by nearly 10% for initial particle sizes fifty times larger than the characteristic particle size, and increases to more than a factor of six at the characteristic particle size. This work provides a physics-based description of the nanoparticle dissolution kinetics and details the reaches and limitations of the developed model. The theoretical framework provided herein is valid for a wide range of dissolution processes and size scales affording it a high level of practicality. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2014.01.095
  • 2014 • 38 Rheological changes of polyamide 12 under oscillatory shear
    Mielicki, C. and Gronhoff, B. and Wortberg, J.
    AIP Conference Proceedings 1593 231-235 (2014)
    Changes in material properties as well as process deviation prevent Laser Sintering (LS) technology from manufacturing of quality assured parts in a series production. In this context, the viscosity of Polyamide 12 (PA12) is assumed to possess the most significant influence, as it determines the sintering velocity, the resistance towards melt formation and the bonding strength of sintered layers. Moreover, the viscosity is directly related to the structure of the molten polymer. In particular, it has been recently reported that LS process conditions lead to structural changes of PA12 affecting viscosity and coalescence of adjacent polymer particles, i.e. melt formation significantly. Structural change of PA12 was understood as a post condensation. Its influence on viscosity was described by a time and temperature depending rheological model whereas time dependence was considered by a novel structural change shift factor which was derived from melt volume rate data. In combination with process data that was recorded using online thermal imaging, the model is suitable to control the viscosity (processability of the material) as result of material and process properties. However, as soon as laser energy is exposed to the powder bed PA12 undergoes a phase transition from solid to molten state. Above the melting point, structural change is expected to occur faster due to a higher kinetic energy and free volume of the molten polymer. Oscillatory shear results were used to study the influence of aging time and for validation of the novel structural change shift factor and its model parameters which were calibrated based on LS processing condition. © 2014 American Institute of Physics.
    view abstractdoi: 10.1063/1.4873770
  • 2014 • 37 The influence of kinetics, mass transfer and catalyst deactivation on the growth rate of multiwalled carbon nanotubes from ethene on a cobalt-based catalyst
    Voelskow, K. and Becker, M.J. and Xia, W. and Muhler, M. and Turek, T.
    Chemical Engineering Journal 244 68-74 (2014)
    CNT growth experiments on a cobalt-based catalyst were conducted in a tubular fixed bed reactor at different temperatures and ethene concentrations. The measured kinetic data were analyzed with an isothermal, dynamic reactor model taking into account pore and film diffusion as well as the size of CNT agglomerates as a function of time. Based on previously published results it was found that the CNT agglomerates are enlarged by an average factor of 6.5 compared to the original diameter of the catalyst particle. Under these conditions, the development of the agglomerate diameter with time can be described with a single parameter which is independent of the reaction conditions. The rate of the CNT growth was determined to be first order in the ethene concentration with an activation energy of 107. kJ/mol. The catalyst deactivation by cumulative encapsulation of active sites was found to be second order with respect to the consumed amount of ethene with a rate constant independent of the temperature. Nevertheless, deactivation takes place faster at higher temperatures and/or ethene concentrations, since the deactivation process is directly coupled to the rate of CNT synthesis. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.cej.2014.01.024
  • 2014 • 36 Thermogravimetric analysis of activated carbons, ordered mesoporous carbide-derived carbons, and their deactivation kinetics of catalytic methane decomposition
    Shilapuram, V. and Ozalp, N. and Oschatz, M. and Borchardt, L. and Kaskel, S. and Lachance, R.
    Industrial and Engineering Chemistry Research 53 1741-1753 (2014)
    This study presents the deactivation kinetics of methane decomposition for the activated carbons Fluka-05105 and Fluka-05120, ordered mesoporous carbon (CMK-3), and ordered mesoporous carbide-derived carbon (DUT-19). The experimental and thermodynamically predicted carbon deposition, the average and total hydrogen production, and the effect of flow rate on carbon formation rate of these catalysts were investigated. Results indicate that the experimental conditions chosen were within the reaction control regime. Catalytic activity was calculated via two different definitions present in literature: one in terms of carbon deposition rate and the other in terms of carbon mass deposited. Deactivation kinetics were obtained by fitting the experimental data by nonlinear regression analysis. Differences between the two methods in determining activity resulted in significant changes in the estimation of deactivation kinetics. The activity calculated based on the rate method results in the best fit of experimentally collected data. A deactivation order and methane concentration dependency of approximately 1.0 and 0.5 were determined for all the catalysts tested (Fluka-05105, Fluka-05120, CMK-3, and DUT-19). The activation energy of deactivation (Ed) was determined to be 192, 154, 166, and 181 kJ/mol for Fluka-05120, Fluka-05105, CMK-3, and DUT-19, respectively. DUT-19 was the best performing catalyst in terms of carbon formation rate, total carbon production, hydrogen production rate, average hydrogen production, and total hydrogen production. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ie402195q
  • 2014 • 35 Tribolayer formation in a metal-on-metal (MoM) hip joint: An electrochemical investigation
    Mathew, M.T. and Nagelli, C. and Pourzal, R. and Fischer, A. and Laurent, M.P. and Jacobs, J.J. and Wimmer, M.A.
    Journal of the Mechanical Behavior of Biomedical Materials 29 199-212 (2014)
    The demand for total hip replacement (THR) surgery is increasing in the younger population due to faster rehabilitation and more complete restoration of function. Up to 2009, metal-on-metal (MoM) hip joint bearings were a popular choice due to their design flexibility, post-operative stability and relatively low wear rates. The main wear mechanisms that occur along the bearing surface of MoM joints are tribochemical reactions that deposit a mixture of wear debris, metal ions and organic matrix of decomposed proteins known as a tribolayer. No in-depth electrochemical studies have been reported on the structure and characteristics of this tribolayer or about the parameters involved in its formation.In this study, we conducted an electrochemical investigation of different surfaces (bulk-like: control, nano-crystalline: new implant and tribolayer surface: retrieved implant) made out of two commonly used hip CoCrMo alloys (high-carbon and low-carbon). As per ASTM standard, cyclic polarization tests and electrochemical impedance spectroscopy tests were conducted. The results obtained from electrochemical parameters for different surfaces clearly indicated a reduction in corrosion for the tribolayer surface (Icorr: 0.76μA/cm2). Further, polarization resistance (Rp:2.39±0.60MΩ/cm2) and capacitance (Cdl:15.20±0.75μF/cm2) indicated variation in corrosion kinetics for the tribolayer surface, that attributed to its structure and stability in a simulated body environment. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2013.08.018
  • 2013 • 34 A kinetic study of oxygen reduction reaction and characterization on electrodeposited gold nanoparticles of diameter between 17 nm and 40 nm in 0.5 M sulfuric acid
    Wang, Y. and Laborda, E. and Ward, K.R. and Tschulik, K. and Compton, R.G.
    Nanoscale 5 9699-9708 (2013)
    Kinetic and mechanistic studies of the oxygen reduction reaction (ORR) in oxygen saturated 0.5 M sulfuric acid at 298 K at a gold macroelectrode and at an electrodeposited gold nanoparticle-modified glassy carbon electrode are reported. The conditions of electrodeposition are optimized to obtain small nanoparticles of diameter from 17 nm to 40 nm. The mechanism and kinetics of ORR on the gold macroelectrode are investigated and compared with those obtained for nanoparticle-modified electrodes. The mechanism for this system includes two electron and two proton transfers and hydrogen peroxide as the final product. The first electron transfer step corresponding to the reduction of O2 to O2 - is defined as the rate determining step. No significant changes are found for the nanoparticles here employed: electron transfer rate constant (k0) is k0,bulk = 0.30 cm s -1 on the bulk material and k0,nano = 0.21 cm s -1 on nanoparticles; transfer coefficient (α) changes from αbulk = 0.45 on macro-scale to αnano = 0.37 at the nano-scale. © The Royal Society of Chemistry 2013.
    view abstractdoi: 10.1039/c3nr02340k
  • 2013 • 33 Aging-associated enzyme human clock-1: Substrate-mediated reduction of the diiron center for 5-demethoxyubiquinone hydroxylation
    Lu, T.-T. and Lee, S.J. and Apfel, U.-P. and Lippard, S.J.
    Biochemistry 52 2236-2244 (2013)
    The mitochondrial membrane-bound enzyme Clock-1 (CLK-1) extends the average longevity of mice and Caenorhabditis elegans, as demonstrated for Δclk-1 constructs for both organisms. Such an apparent impact on aging and the presence of a carboxylate-bridged diiron center in the enzyme inspired this work. We expressed a soluble human CLK-1 (hCLK-1) fusion protein with an N-terminal immunoglobulin binding domain of protein G (GB1). Inclusion of the solubility tag allowed for thorough characterization of the carboxylate-bridged diiron active site of the resulting GB1-hCLK-1 by spectroscopic and kinetic methods. Both UV-visible and Mössbauer experiments provide unambiguous evidence that GB1-hCLK-1 functions as a 5-demethoxyubiquinone-hydroxylase, utilizing its carboxylate-bridged diiron center. The binding of DMQn (n = 0 or 2) to GB1-hCLK-1 mediates reduction of the diiron center by nicotinamide adenine dinucleotide (NADH) and initiates O2 activation for subsequent DMQ hydroxylation. Deployment of DMQ to mediate reduction of the diiron center in GB1-hCLK-1 improves substrate specificity and diminishes consumption of NADH that is uncoupled from substrate oxidation. Both Vmax and k cat/KM for DMQ hydroxylation increase when DMQ0 is replaced by DMQ2 as the substrate, which demonstrates that an isoprenoid side chain enhances enzymatic hydroxylation and improves catalytic efficiency. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/bi301674p
  • 2013 • 32 Continuum and kinetic simulations of the neutral gas flow in an industrial physical vapor deposition reactor
    Bobzin, K. and Brinkmann, R.P. and Mussenbrock, T. and Bagcivan, N. and Brugnara, R.H. and Schäfer, M. and Trieschmann, J.
    Surface and Coatings Technology 237 176-181 (2013)
    Magnetron sputtering used for physical vapor deposition processes often requires gas pressures well below 1. Pa. Under these conditions the gas flow in the reactor is usually determined by a Knudsen number of about one, i.e., a transition regime between the hydrodynamic and the rarefied gas regime. In the first, the gas flow is well described by the Navier-Stokes equations, while in the second a kinetic approach via the Boltzmann equation is necessary. In this paper the neutral gas flow of argon and molecular nitrogen gas inside an industrial scale plasma reactor was simulated using both a fluid model and a fully kinetic Direct Simulation Monte Carlo model.By comparing both model results the validity of the fluid model was checked. Although in both models a Maxwell-Boltzmann energy distribution of the neutral particles is the natural outcome, the results of the gas flow differ significantly. The fluid model description breaks down, due to the inappropriate assumption of a fluid continuum. This is due to exclusion of non-local effects in the multi dimensional velocity space, as well as invalid gas/wall interactions. Only the kinetic model is able to provide an accurate physical description of the gas flow in the transition regime. Our analysis is completed with a brief investigation of different definitions of the local Knudsen number. We conclude that the most decisive parameter - the spatial length scale L - has to be very careful chosen in order to obtain a reasonable estimate of the gas flow regime. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2013.08.018
  • 2013 • 31 Controlled growth of protein resistant PHEMA brushes via S-RAFT polymerization
    Zamfir, M. and Rodriguez-Emmenegger, C. and Bauer, S. and Barner, L. and Rosenhahn, A. and Barner-Kowollik, C.
    Journal of Materials Chemistry B 1 6027-6034 (2013)
    The reversible addition-fragmentation chain transfer polymerization of 2-hydroxyethyl methacrylate (HEMA) from surfaces (S-RAFT) using an R-group-attached chain transfer agent (CTA) is presented. The approach was exploited for the efficient preparation of well-defined PHEMA brushes of up to 50 nm thickness in a controlled fashion without using any cytotoxic catalyst. The chemical composition, morphology and wettability of the samples were assessed by X-ray photoelectron spectroscopy, atomic force microscopy and water contact angle measurements, while the growth kinetics were studied by monitoring the dry thickness via spectroscopic ellipsometry. The mechanism and kinetics of the RAFT polymerization on the surface-in the presence of a sacrificial CTA and of solvent mixtures with different polarities-were investigated. A marked effect of the concentration of the sacrificial CTA on the kinetics was observed. Importantly-and for the first time-the living PHEMA brushes were exploited as macroRAFT agents for chain extension, and thicknesses up to 70 nm were achieved. The prepared PHEMA brushes were challenged with protein solutions demonstrating their resistance to fouling. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3tb20880j
  • 2013 • 30 Direct gas-phase synthesis of single-phase β-FeSi2 nanoparticles
    Bywalez, R. and Orthner, H. and Mehmedovic, E. and Imlau, R. and Kovacs, A. and Luysberg, M. and Wiggers, H.
    Journal of Nanoparticle Research 15 (2013)
    For the first time, phase-pure β-FeSi2 nanoparticles were successfully produced by gas-phase synthesis. We present a method to fabricate larger quantities of semiconducting β-FeSi2 nanoparticles, with crystallite sizes between 10 and 30 nm, for solar and thermoelectric applications utilizing a hot-wall reactor. A general outline for the production of those particles by thermal decomposition of silane and iron pentacarbonyl is provided based on kinetic data. The synthesized particles are investigated by X-ray diffraction and transmission electron microscopy, providing evidence that the as-prepared materials are indeed β-FeSi2, while revealing morphological characteristics inherent to the nanoparticles created. © 2013 Springer Science+Business Media.
    view abstractdoi: 10.1007/s11051-013-1878-9
  • 2013 • 29 Influence of MCC II fraction and storage conditions on pellet properties
    Krueger, C. and Thommes, M. and Kleinebudde, P.
    European Journal of Pharmaceutics and Biopharmaceutics 85 1039-1045 (2013)
    Microcrystalline cellulose II (MCC II) - a polymorph of commonly used MCC I - was introduced as new pelletization aid in wet-extrusion/spheronization leading to fast disintegrating pellets. Previous investigations suggested that pellet properties were influenced by the fraction of MCC II. Furthermore, it is unknown whether the storage conditions can affect the disintegration behavior. Therefore, the effects of MCC II fraction and the storage conditions on several pellet properties were investigated. MCC II-based pellets were prepared of pure MCC II or binary mixtures containing 10-50% (steps of 10%) MCC II as pelletization aid and theophylline, chloramphenicol or lactose. The pellets were characterized by their aspect ratio, equivalent diameter, water content, tensile strength, porosity as well as shrinking, and disintegration behavior and drug release according to their MCC II fraction. Furthermore, the pellets were stored at different relative humidities (0-97%rh), and the influence on their disintegration and drug release was investigated. With increasing MCC II fraction, the pellets became lager in size, decreased their porosity, and required higher water contents for spheronization. Moreover, the disintegration time increased and the disintegration itself was incomplete. Furthermore, the storage conditions had an impact on the disintegration properties of MCC II-based pellets. The disintegrating was affected irreversibly after storage at high humidity (80-97%rh) resulting in a slow drug release. Therefore, MCC II-based pellets need to be stored below 80%rh to secure a fast disintegration. A better knowledge of the properties of MCC II-based pellets was obtained providing a basis for a successful manufacturing and adequate storage of MCC II-based pellets prepared by extrusion/spheronization. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.ejpb.2013.07.001
  • 2013 • 28 Kinetic simulation of the sheath dynamics in the intermediate radio frequency regime
    Shihab, M. and Elgendy, A.T. and Korolov, I. and Derzsi, A. and Schulze, J. and Eremin, D. and Mussenbrock, T. and Donkó, Z. and Brinkmann, R.P.
    Plasma Science and Technology 22 (2013)
    The dynamics of temporally modulated plasma boundary sheaths is studied in the intermediate radio frequency regime where the applied radio frequency and the ion plasma frequency (or the reciprocal of the ion transit time) are comparable. Two fully kinetic simulation algorithms are employed and their results are compared. The first is a realization of the well-known particle-in-cell technique with Monte Carlo collisions and simulates the entire discharge, a planar radio frequency capacitively coupled plasma with an additional ionization source. The second code is based on the recently published scheme Ensemble-in-Spacetime (EST); it resolves only the sheath and requires the time-resolved voltage across and the ion flux into the sheath as input. Ion inertia causes a temporal asymmetry (hysteresis) of the charge-voltage relation; other ion transit time effects are also found. The two algorithms are in good agreement, both with respect to the spatial and temporal dynamics of the sheath and with respect to the ion energy distributions at the electrodes. It is concluded that the EST scheme may serve as an efficient post-processor for fluid or global simulations and for measurements: it can rapidly and accurately calculate ion distribution functions even when no genuine kinetic information is available. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/22/5/055013
  • 2013 • 27 Nanocarbon Paste Electrodes
    Lowinsohn, D. and Gan, P. and Tschulik, K. and Foord, J.S. and Compton, R.G.
    Electroanalysis 25 2435-2444 (2013)
    The electrochemical behaviour of carbon paste electrodes prepared using nanocarbon and mineral oil was investigated and the results contrasted with different carbon and carbon pastes electrodes. The composition of carbon paste was studied by performing cyclic voltammetry performed in 0.1M KCl solution in the presence of 4.0mM Ru(NH3)6Cl3, a well-characterized redox system commonly used to test the electrode behaviour. After optimisation of the paste composition, the sensors chosen were tested for the analysis and characterization of three different systems: Ru(NH3)6 3+/2+, FcCH2OH/FcCH2OH+ and acetaminophen. The ability to obtain high quality voltammetry from the nanocarbon electrode was demonstrated and simulation of the voltammetry allowed the extraction of electrode kinetic parameters with high precision. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/elan.201300364
  • 2013 • 26 Separating the initial growth rate from the rate of deactivation in the growth kinetics of multi-walled carbon nanotubes from ethene over a cobalt-based bulk catalyst in a fixed-bed reactor
    Becker, M.J. and Xia, W. and Xie, K. and Dittmer, A. and Voelskow, K. and Turek, T. and Muhler, M.
    Carbon 58 107-115 (2013)
    The initial growth kinetics of multi-walled carbon nanotubes (CNTs) was investigated using a highly active Co-based mixed-oxide catalyst in a tubular fixed-bed reactor under plug-flow conditions with ethene as carbon source. The growth temperature and the ethene concentration were systematically varied in the range from 758 to 923 K and from 5 to 45 vol.%, respectively. The carbon mass accumulation was derived from the ethene conversion and analyzed by a kinetic model, from which the initial CNT growth rate and the mean lifetime of the active sites were derived permitting the prediction of the maximum theoretical CNT yield. With increasing growth temperatures up to 923 K both the initial growth rate and the mean lifetime of active sites increased strongly with a significantly prolonged lifetime above 848 K. The initial growth rate was slow at lower ethene concentrations, but the mean life time was very high. Increasing the ethene concentration up to 45 vol.% led to a much higher initial growth rate, but shortened the mean lifetime strongly. Due to the fast deactivation at high ethene concentrations, the predicted maximum yield decreased considerably approaching the yield obtained after 5 min of time on stream. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2013.02.038
  • 2013 • 25 The role of activity coefficients in bioreaction equilibria: Thermodynamics of methyl ferulate hydrolysis
    Hoffmann, P. and Voges, M. and Held, C. and Sadowski, G.
    Biophysical Chemistry 173-174 21-30 (2013)
    The Gibbs energy of reaction (ΔRg) is the key quantity in the thermodynamic characterization of biological reactions. Its calculation requires precise standard Gibbs energy of reaction (ΔRg +) values. The value of ΔRg+ is usually determined by measuring the apparent (concentration-dependent) equilibrium constants K, e.g., the molality-based Km. However, the thermodynamically consistent determination of ΔRg+ requires the thermodynamic (activity-based) equilibrium constant Ka. These values (Km and Ka) are equal only if the ratio of the activity coefficients of the reactants to the activity coefficients of the products (Kγ) is equal to unity. In this work, the impact of Kγ on the estimation of Ka for biological reactions was investigated using methyl ferulate (MF) hydrolysis as a model reaction. The value of Kγ was experimentally determined from Km values that were measured at different reactant concentrations. Moreover, K γ was independently predicted using the thermodynamic model ePC-SAFT. Both the experimentally determined and the predicted K γ values indicate that this value cannot be assumed to be unity in the considered reaction. In fact, in the reaction conditions considered in this work, Kγ was shown to be in the range of 3 < K γ < 6 for different reactant molalities (2 < mmol MF kg- 1 < 10). The inclusion of Kγ and thus the use of the thermodynamically correct Ka value instead of Km lead to remarkable differences (almost 40%) in the determination of ΔRg+. Moreover, the new value for ΔRg+ increases the concentration window at which the reaction can thermodynamically occur. The influence of additives was also investigated both experimentally and theoretically. Both procedures consistently indicated that the addition of NaCl (0 to 1 mol kg- 1 water) moderately decreased the value of Kγ, which means that the values of Km increase and that a higher amount of products is obtained as a result of the addition of salt. Additionally, Km was found to strongly depend on pH. A ten-fold increase in the Km values was observed in the pH range of 6 to 7; this increase corresponds to a change of more than 100% in the value of ΔRg+. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.bpc.2012.12.006
  • 2012 • 24 Autoignition of surrogate biodiesel fuel (B30) at high pressures: Experimental and modeling kinetic study
    Ramirez Lancheros, H.P. and Fikri, M. and Cancino, L.R. and Moréac, G. and Schulz, C. and Dagaut, P.
    Combustion and Flame 159 996-1008 (2012)
    Ignition delay times of surrogate biodiesel fuels were measured in a high-pressure shock tube over a wide range of experimental conditions (pressures of 20 and 40. bar, equivalence ratios in the range 0.5-1.5, and temperatures ranging from 700 to 1200. K). A detailed chemical kinetic mechanism developed for the oxidation of a biodiesel fuel and a B30 biodiesel surrogate (49% n-decane, 21% 1-methylnaphthalene, and 30% methyloctanoate in mol%) was used to simulate the present experiments. Cross reactions between radicals from the three fuel components and reactions of methylnaphthalene oxidation recently proposed in the literature were introduced into the model in order to improve ignition delay time predictions at low temperatures. The new scheme (7865 reversible reactions and 1975 species) yields improved model predictions of concentration profiles measured earlier in a jet-stirred reactor, and also represents fairly well the present experimental data over the entire range of conditions of this study. Sensitivity analyses and reaction path analyses were used to rationalize the results. © 2011 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2011.10.006
  • 2012 • 23 Design, Synthesis, and biological evaluation of novel disubstituted dibenzosuberones as highly potent and selective inhibitors of p38 mitogen activated protein kinase
    Koeberle, S.C. and Fischer, S. and Schollmeyer, D. and Schattel, V. and Grütter, C. and Rauh, D. and Laufer, S.A.
    Journal of Medicinal Chemistry 55 5868-5877 (2012)
    Synthesis, biological testing, structure-activity relationships (SARs), and selectivity of novel disubstituted dibenzosuberone derivatives as p38 MAP kinase inhibitors are described. Hydrophilic moieties were introduced at the 7-, 8-, and 9-position of the 2-phenylamino-dibenzosuberones, improving physicochemical properties as well as potency. Extremely potent inhibitors were obtained, with half-maximal inhibitory concentration (IC 50) values in the low nM range in a whole blood assay measuring the inhibition of cytokine release. The high potency of the target compounds together with the outstanding selectivity of this novel class of compounds toward p38 mitogen activated protein (MAP) kinase as compared to other kinases indicate them to be most applicable as tools in pharmacological research and eventually they may foster a new generation of anti-inflammatory drugs. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jm300327h
  • 2012 • 22 Detailed kinetic modeling of methanol synthesis over a ternary copper catalyst
    Peter, M. and Fichtl, M.B. and Ruland, H. and Kaluza, S. and Muhler, M. and Hinrichsen, O.
    Chemical Engineering Journal 203 480-491 (2012)
    Three differently detailed kinetic models for methanol synthesis are derived for experimental data measured over a ternary copper catalyst. Two global reactor models for reaction design, including a power law and a Langmuir-Hinshelwood-Hougen-Watson approach, are presented. In addition a microkinetic model is adapted to describe the whole experimental data and is used to discuss dynamical changes occurring during methanol synthesis. The first global model based on power law kinetics is very precisely in predicting the integral rates of methanol production. The power law requires the inclusion of a water inhibition term to be applicable over the whole range of experiments. A semi-empirical Langmuir-Hinshelwood-Hougen-Watson model, taken from the literature, gives essentially the same results, even upon extrapolation. The third model, a microkinetic model, was successfully fitted with only two variables and is in reasonable agreement with the experimental data. For all models a sensitivity analysis shows the influencing parameters on the methanol production rate. The valid microkinetic model, however, can give qualitative estimations of the structure sensitivity and dynamic behavior of methanol synthesis. The dynamic change of active sites and of site distribution of different copper low-index planes along the reactor length is given and the inhibiting role of water, indicated by the power law and microkinetic model, is analyzed. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.cej.2012.06.066
  • 2012 • 21 Fluorescence labels in kinases: A high-throughput kinase binding assay for the identification of DFG-out binding ligands
    Simard, J.R. and Rauh, D.
    Methods in Molecular Biology 800 95-117 (2012)
    Despite the hundreds of kinase inhibitors currently in discovery and pre-clinical phases, the number of kinase inhibitors which have been approved and are on the market remains low by comparison. This discrepancy reflects the challenges which accompany the development of kinase inhibitors which are relatively specific and less toxic. Targeting protein kinases with ATP-competitive inhibitors has been the classical approach to inhibiting kinase activity, but the highly conserved nature of the ATP-binding site contributes to poor inhibitor selectivity, issues which have particularly hampered the development of novel kinase inhibitors. We developed a high-throughput screening technology that can discriminate for inhibitors which stabilize the inactive "DFG-out" kinase conformation by binding within an allosteric pocket adjacent to the ATP-binding site. Here, we describe how to use this approach to measure the K d of ligands, as well as how to kinetically characterize the binding and dissociation of ligands to the kinase. We also describe how this technology can be used to rapidly screen small molecule libraries at high throughput. © 2012 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/978-1-61779-349-3_8
  • 2012 • 20 Folding graphene with swift heavy ions
    Akcöltekin, S. and Bukowska, H. and Akcol̈tekin, E. and Lebius, H. and Schleberger, M.
    Materials Research Society Symposium Proceedings 1354 67-72 (2012)
    Swift heavy ion induced modifications on graphene were investigated by means of atomic force microscopy and Raman spectroscopy. For the experiment graphene was exfoliated onto different substrates (SrTiO3 (100), TiO2(100), Al2O3(1102) and 90 nm SiO 2/Si) by the standard technique. After irradiation with heavy ions of 93 MeV kinetic energy and under glancing angles of incidence, characteristic folding structures are observed. The folding patterns on crystalline substrates are generally larger and are created with a higher efficiency than on the amorphous SiO2. This difference is attributed to the relatively large distance between graphene and SiO2 of d ≈ 1 nm. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/opl.2011.1278
  • 2012 • 19 Impact of single basepair mismatches on electron-transfer processes at Fc-PNA·DNA modified gold surfaces
    Hüsken, N. and Gȩbala, M. and Battistel, A. and La Mantia, F. and Schuhmann, W. and Metzler-Nolte, N.
    ChemPhysChem 13 131-139 (2012)
    Gold-surface grafted peptide nucleic acid (PNA) strands, which carry a redox-active ferrocene tag, present unique tools to electrochemically investigate their mechanical bending elasticity based on the kinetics of electron-transfer (ET) processes. A comparative study of the mechanical bending properties and the thermodynamic stability of a series of 12-mer Fc-PNA·DNA duplexes was carried out. A single basepair mismatch was integrated at all possible strand positions to provide nanoscopic insights into the physicochemical changes provoked by the presence of a single basepair mismatch with regard to its position within the strand. The ET processes at single mismatch Fc-PNA·DNA modified surfaces were found to proceed with increasing diffusion limitation and decreasing standard ET rate constants k 0 when the single basepair mismatch was dislocated along the strand towards its free-dangling Fc-modified end. The observed ET characteristics are considered to be due to a punctual increase in the strand elasticity at the mismatch position. The kinetic mismatch discrimination with respect to the fully-complementary duplex presents a basis for an electrochemical DNA sensing strategy based on the Fc-PNA·DNA bending dynamics for loosely packed monolayers. In a general sense, the strand elasticity presents a further physicochemical property which is affected by a single basepair mismatch which may possibly be used as a basis for future DNA sensing concepts for the specific detection of single basepair mismatches. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201100578
  • 2012 • 18 Interplay of wrinkles, strain, and lattice parameter in graphene on iridium
    Hattab, H. and N'Diaye, A.T. and Wall, D. and Klein, C. and Jnawali, G. and Coraux, J. and Busse, C. and Van Gastel, R. and Poelsema, B. and Michely, T. and Meyer zu Heringdorf, F.-J. and Horn-von Hoegen, M.
    Nano Letters 12 678-682 (2012)
    Following graphene growth by thermal decomposition of ethylene on Ir(111) at high temperatures we analyzed the strain state and the wrinkle formation kinetics as function of temperature. Using the moiré spot separation in a low energy electron diffraction pattern as a magnifying mechanism for the difference in the lattice parameters between Ir and graphene, we achieved an unrivaled relative precision of ±0.1 pm for the graphene lattice parameter. Our data reveals a characteristic hysteresis of the graphene lattice parameter that is explained by the interplay of reversible wrinkle formation and film strain. We show that graphene on Ir(111) always exhibits residual compressive strain at room temperature. Our results provide important guidelines for strategies to avoid wrinkling. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nl203530t
  • 2012 • 17 New insights into hard phases of CoCrMo metal-on-metal hip replacements
    Liao, Y. and Pourzal, R. and Stemmer, P. and Wimmer, M.A. and Jacobs, J.J. and Fischer, A. and Marks, L.D.
    Journal of the Mechanical Behavior of Biomedical Materials 12 39-49 (2012)
    The microstructural and mechanical properties of the hard phases in CoCrMo prosthetic alloys in both cast and wrought conditions were examined using transmission electron microscopy and nanoindentation. Besides the known carbides of M23C6-type (M=Cr, Mo, Co) and M6C-type which are formed by either eutectic solidification or precipitation, a new mixed-phase hard constituent has been found in the cast alloys, which is composed of ~100nm fine grains. The nanosized grains were identified to be mostly of M23C6 type using nano-beam precession electron diffraction, and the chemical composition varied from grain to grain being either Cr- or Co-rich. In contrast, the carbides within the wrought alloy having the same M23C6 structure were homogeneous, which can be attributed to the repeated heating and deformation steps. Nanoindentation measurements showed that the hardness of the hard phase mixture in the cast specimen was ~15.7GPa, while the M23C6 carbides in the wrought alloy were twice as hard (~30.7GPa). The origin of the nanostructured hard phase mixture was found to be related to slow cooling during casting. Mixed hard phases were produced at a cooling rate of 0.2°C/s, whereas single phase carbides were formed at a cooling rate of 50°C/s. This is consistent with sluggish kinetics and rationalizes different and partly conflicting microstructural results in the literature, and could be a source of variations in the performance of prosthetic devices in-vivo. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2012.03.013
  • 2012 • 16 Pearlite revisited
    Steinbach, I. and Plapp, M.
    Continuum Mechanics and Thermodynamics 24 665-673 (2012)
    Zener's model of pearlite transformation in steels can be viewed as the prototype of many microstructure evolution models in materials science. It links principles of thermodynamics and kinetics to the scale of the microstructure. In addition it solves a very practical problem: How the hardness of steel is correlated to the conditions of processing. Although the model is well established since the 1950s, quantitative explanation of growth kinetics was missing until very recently. The present paper will shortly review the classical model of pearlite transformation. Zener's conjecture of maximum entropy production will be annotated by modern theoretical and experimental considerations of a band of stable (sometimes oscillating) states around the state of maximum entropy production. Finally, an explanation of the growth kinetics observed in experiments is proposed based on diffusion fluxes driven by stress gradients due to large transformation strain. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00161-011-0204-y
  • 2012 • 15 Precise hierarchical self-assembly of multicompartment micelles
    Gröschel, A.H. and Schacher, F.H. and Schmalz, H. and Borisov, O.V. and Zhulina, E.B. and Walther, A. and Müller, A.H.E.
    Nature Communications 3 (2012)
    Hierarchical self-assembly offers elegant and energy-efficient bottom-up strategies for the structuring of complex materials. For block copolymers, the last decade witnessed great progress in diversifying the structural complexity of solution-based assemblies into multicompartment micelles. However, a general understanding of what governs multicompartment micelle morphologies and polydispersity, and how to manipulate their hierarchical superstructures using straightforward concepts and readily accessible polymers remains unreached. Here we demonstrate how to create homogeneous multicompartment micelles with unprecedented structural control via the intermediate pre-assembly of subunits. This directed self-assembly leads to a step-wise reduction of the degree of conformational freedom and dynamics and avoids undesirable kinetic obstacles during the structure build-up. It yields a general concept for homogeneous populations of well-defined multicompartment micelles with precisely tunable patchiness, while using simple linear ABC triblock terpolymers. We further demonstrate control over the hierarchical step-growth polymerization of multicompartment micelles into micron-scale segmented supracolloidal polymers as an example of programmable mesoscale colloidal hierarchies via well-defined patchy nanoobjects. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms1707
  • 2012 • 14 Preparation and comparative release characteristics of three anthocyanin encapsulation systems
    Oidtmann, J. and Schantz, M. and Mäder, K. and Baum, M. and Berg, S. and Betz, M. and Kulozik, U. and Leick, S. and Rehage, H. and Schwarz, K. and Richling, E.
    Journal of Agricultural and Food Chemistry 60 844-851 (2012)
    Bilberries (Vaccinium myrtillus L.) and their major polyphenolic constituents, anthocyanins, have preventive activities inter alia against colon cancer and inflammatory bowel diseases. However, anthocyanins are sensitive to environmental conditions; thus their bioavailability in the gastrointestinal tract is an important determinant of their in vivo activity. In the study reported here, the potential benefits of encapsulating an anthocyanin rich bilberry extract (BE) on anthocyanin stability were investigated. Nonencapsulated BE and three different BE loaded microcapsule systems were incubated in simulated gastric fluid (SGF) and fed state simulated intestinal fluid (FeSSIF). After exposure to these media, released anthocyanins were identified and quantified by HPLC with UV/Vis detection. Although a rapid release of anthocyanins was observed within the first 20 min, encapsulation of anthocyanins doubled the amount of available anthocyanins after 150 min of incubation. These results illustrate the ability of encapsulation to inhibit early degradation of anthocyanins in the intestinal system. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jf2047515
  • 2012 • 13 Vortex shedding from a bilge keel in a transient turbulent flow
    Piehl, H. and El Moctar, B.O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 5 907-914 (2012)
    The hydrodynamic damping of a bilge keel during the roll motion of a ship is fairly well understood and its basic principle can be summarized as follows: The larger the bilge keel attached to the hull, the stronger is its roll damping effect. The geometric limitations regarding the size of the bilge keel are set by class regulations mainly dependent on the ship's length, breadth and draft. Most bilge keel shapes are simple flat bars attached to the center of the bilge radius. While the added resistance of the bilge keel has to be kept as low as possible, the effective area for the cross flow generated by the roll motion should be maximized. Therefore the bilge keel's cross section has to be kept small and its camber line parallel to the streamlines. In more sophisticated designs L-shaped bilge keels are applied in order to increase the damping effect on the roll motion. The aspects above need to be considered when defining the geometric limits of a bilge keel, however to further optimize the design of bilge keels numerical simulations are needed. Even with today's computing power, the costs of simulating a full ship hull with a sufficiently high mesh resolution to capture viscous vortex shedding effects would be prohibitive. To address and overcome this restriction a numerical test setup was developed that simulates the flow only in the near vicinity of the bilge keel. By further neglecting the influence of the free surface, it was possible to use a standard single-phase, incompressible, turbulent, transient solver. The open source FVM code Open FOAM was used for all three stages of the simulation: mesh generation, solution process and post-processing. With this simplified simulation model a systematic investigation of the turbulence model, the temporal and spacial discretization, as well as the principles of vortex shedding was carried out. The damping efficiency of the bilge keels was evaluated on basis of the mechanical work - by moving the hull through a viscous fluid - and the kinetic energy transported within the vortices. The findings from these flow simulations provide insights into the principles of bilge keel vortex shedding and their interaction with the hull and enable the development of bilge keel design guidelines. Copyright © 2012 by ASME.
    view abstractdoi: 10.1115/OMAE2012-84013
  • 2012 • 12 Well-defined amphiphilic poly(2-oxazoline) ABA-triblock copolymers and their aggregation behavior in aqueous solution
    Krumm, C. and Fik, C.P. and Meuris, M. and Dropalla, G.J. and Geltenpoth, H. and Sickmann, A. and Tiller, J.C.
    Macromolecular Rapid Communications 33 1677-1682 (2012)
    Self-organization of block copolymers in solution is a way to obtain advanced functional superstructures. The synthesis of well-defined polymethyloxazoline-block-polyphenyloxazoline-block-polymethyloxazoline triblock copolymers is described and proven by 1H NMR spectroscopy, SEC, and ESI-MS. The surprisingly water- soluble block copolymers do self-organize in aqueous solutions uniquely forming three coexisting well-defined structures: unimolecular micelles, micellar aggregates, and very form-stable polymersomes. This is the first example of a polymersome forming ABA-triblock copolymer with a glassy middle block. The spherical vesicles are analysed by scanning electron microscopy and transmission electron microscopy. It could be shown that these vesicles are indeed hollow spheres. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/marc.201200192
  • 2011 • 11 A molecular dynamics investigation of kinetic electron emission from silver surfaces under varying angle of projectile impact
    Duvenbeck, A. and Hanke, S. and Weidtmann, B. and Wucher, A.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 269 1661-1664 (2011)
    We present a computer simulation study on the influence of the impact angle of the projectile on kinetic electron emission yields for 5-keV Ag → Ag bombardment. By means of a hybrid computer simulation model incorporating (i) the particle dynamics following the primary particle impact, (ii) the kinetically induced electronic substrate excitations via electronic friction and electron promotion and (iii) the transport of excitation energy away from the spot of generation, a full three-dimensional electron temperature profile within the volume affected by the atomic collision cascade is calculated. This profile is evaluated at the very surface of the target and taken as input for a thermionic model ('hot-spot-model') for kinetic electron emission. Averaging the results for different choices of the polar angle of incidence Θ over a large set of impact points, the obtained kinetic electron emission yields can be compared with experimental data and predictions from simple geometrical calculations. The presented simulation results appear to be reasonable in comparison with experimental data as well as with simple geometrical considerations of kinetic electron emission under oblique incidence. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.nimb.2010.11.082
  • 2011 • 10 Influence of individual ionic components on the agglomeration kinetics of silver nanoparticles
    Gebauer, J.S. and Treuel, L.
    Journal of Colloid and Interface Science 354 546-554 (2011)
    The precise characteristic of the agglomeration behavior of colloidal suspensions is of paramount interest to many current studies in nanoscience. This work seeks to elucidate the influence that differently charged salts have on the agglomeration state of a Lee-Meisel-type silver colloid. Moreover, we investigate the influence of the chemical nature of individual ions on their potential to induce agglomeration. Raman spectroscopy and surface-enhanced Raman spectroscopy are used to give insights into mechanistic aspects of the agglomeration process and to assess the differences in the influence of different salts on the agglomeration behavior. Finally, we demonstrate the potential of the measurement procedure used in this work to determine the elementary charge on colloidal NPs. © 2010 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2010.11.016
  • 2011 • 9 Martensitic transformation in rapidly solidified Heusler Ni 49Mn39Sn12 ribbons
    Zheng, H. and Wu, D. and Xue, S. and Frenzel, J. and Eggeler, G. and Zhai, Q.
    Acta Materialia 59 5692-5699 (2011)
    In the present work, the microstructure evolution and kinetics of the martensitic transformation are investigated in as-spun and annealed ribbons of Heusler Ni49Mn39Sn12 using electron microscopy, X-ray diffraction and differential scanning calorimetry. Both ribbons undergo a reversible martensitic transformation during thermal cycling and the low-temperature martensite is confirmed to be a modulated four-layered orthorhombic (4O) structure through in situ cooling transmission electronic microscopy investigation. The annealing effect on the martensitic transformation behavior is discussed from the viewpoints of electron concentration, Mn-Mn interatomic distance, atomic order degree and grain size. A strong cooling-rate dependence of phase transition kinetics is found and the mechanism is analyzed. The satisfactory reproducibility obtained during thermal cycling test of this alloy ribbons offers great potential for practical applications. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.05.044
  • 2011 • 8 Mechanistic studies of Fc-PNA(·DNA) surface dynamics based on the kinetics of electron-transfer processes
    Hüsken, N. and Gȩbala, M. and La Mantia, F. and Schuhmann, W. and Metzler-Nolte, N.
    Chemistry - A European Journal 17 9678-9690 (2011)
    N-Terminally ferrocenylated and C-terminally gold-surface-grafted peptide nucleic acid (PNA) strands were exploited as unique tools for the electrochemical investigation of the strand dynamics of short PNA(·DNA) duplexes. On the basis of the quantitative analysis of the kinetics and the diffusional characteristics of the electron-transfer process, a nanoscopic view of the Fc-PNA(·DNA) surface dynamics was obtained. Loosely packed, surface-confined Fc-PNA single strands were found to render the charge-transfer process of the tethered Fc moiety diffusion-limited, whereas surfaces modified with Fc-PNA·DNA duplexes exhibited a charge-transfer process with characteristics between the two extremes of diffusion and surface limitation. The interplay between the inherent strand elasticity and effects exerted by the electric field are supposed to dictate the probability of a sufficient approach of the Fc head group to the electrode surface, as reflected in the measured values of the electron-transfer rate constant, k 0. An in-depth understanding of the dynamics of surface-bound PNA and PNA·DNA strands is of utmost importance for the development of DNA biosensors using (Fc-)PNA recognition layers. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201003764
  • 2011 • 7 Metal ion release kinetics from nanoparticle silicone composites
    Hahn, A. and Brandes, G. and Wagener, P. and Barcikowski, S.
    Journal of Controlled Release 154 164-170 (2011)
    Metal ion release kinetics from silver and copper nanoparticle silicone composites generated by laser ablation in liquids are investigated. The metal ion transport mechanism is studied by using different model equations and their fit to experimental data. Results indicate that during the first 30 days of immersion, Fickian diffusion is the dominant transport mechanism. After this time period, the oxidation and dissolution of nanoparticles from the bulk determine the ion release. This second mechanism is very slow since the dissolution of the nanoparticle is found to be anisotropic. Silver ion release profile is best described by pseudo-first order exponential equation. Copper ion release profile is best described by a second order exponential equation. For practical purposes, the in vitro release characteristics of the bioactive metal ions are evaluated as a function of nanoparticle loading density, the chemistry and the texture of the silicone. Based on the proposed two-step release model, a prediction of the release characteristics over a time course of 84 days is possible and a long-term ion release could be demonstrated. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jconrel.2011.05.023
  • 2011 • 6 Preliminary study on calcium aluminosilicate glass as a potential host matrix for radioactive 90Sr-An approach based on natural analogue study
    Sengupta, P. and Fanara, S. and Chakraborty, S.
    Journal of Hazardous Materials 190 229-239 (2011)
    Given the environmental-, safety- and security risks associated with sealed radioactive sources it is important to identify suitable host matrices for 90Sr that is used for various peaceful applications. As SrO promotes phase separation within borosilicate melt, aluminosilicate bulk compositions belonging to anorthite-wollastonite-gehlenite stability field are studied in this work. Tests for their homogeneity, microstructural characteristics and resistance to phase separation narrowed the choice down to the composition CAS11 (CaO=35wt%, Al2O3=20wt%, SiO2=45wt%). We find that up to 30wt% SrO can be loaded in this glass without phase separation (into Ca, Sr-rich and Sr-poor, Si-rich domains). Leaching behaviour of the glasses differs depending on the content and distribution of Sr. In general, the elemental leach rates determined from conventional PCT experimental procedure yield values better than 10-7gcm-2day-1 for both CAS11 base glass as well as SrO doped glass. It was noted that leach rates calculated on the basis of Ca2+ and Sr2+ were of the same order and bit higher compared to those calculated on the basis of Si4+ and Al3+. During accelerated leaching tests, zeolite and zeolite+epidote were found to have developed on CAS11 base glass and SrO doped glasses respectively. The Sr bulk diffusion coefficients is found to vary from ~10-15 to 10-13cm2/s at temperature intervals as high as 725-850°C. Based on the experimental observations, it is suggested that CAS11 glass can be used as host matrix of 90Sr for various applications of radioactive Sr-pencils. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.jhazmat.2011.03.031
  • 2011 • 5 The influence of the potassium promoter on the kinetics and thermodynamics of CO adsorption on a bulk iron catalyst applied in Fischer-Tropsch synthesis: A quantitative adsorption calorimetry, temperature-programmed desorption, and surface hydrogenation study
    Graf, B. and Muhler, M.
    Physical Chemistry Chemical Physics 13 3701-3710 (2011)
    The adsorption of carbon monoxide on an either unpromoted or potassium-promoted bulk iron catalyst was investigated at 303 K and 613 K by means of pulse chemisorption, adsorption calorimetry, temperature-programmed desorption and temperature-programmed surface reaction in hydrogen. CO was found to adsorb mainly molecularly in the absence of H 2 at 303 K, whereas the presence of H 2 induced CO dissociation at higher temperatures leading to the formation of CH 4 and H 2O. The hydrogenation of atomic oxygen chemisorbed on metallic iron was found to occur faster than the hydrogenation of atomically adsorbed carbon. At 613 K CO adsorption occurred only dissociatively followed by recombinative CO 2 formation according to C ads + 2O ads → CO 2(g). The presence of the potassium promoter on the catalyst surface led to an increasing strength of the Fe-C bond both at 303 K and 613 K: the initial differential heat of molecular CO adsorption on the pure iron catalyst at 303 K amounted to 102 kJ mol -1, whereas it increased to 110 kJ mol -1 on the potassium-promoted sample, and the initial differential heat of dissociative CO adsorption on the unpromoted iron catalyst at 613 K amounted to 165 kJ mol -1, which increased to 225 kJ mol -1 in the presence of potassium. The calorimetric CO adsorption experiments also reveal a change of the energetic distribution of the CO adsorption sites present on the catalyst surface induced by the potassium promoter, which was found to block a fraction of the CO adsorption sites. © the Owner Societies 2011.
    view abstractdoi: 10.1039/c0cp01875a
  • 2010 • 4 Accessing ultrashort reaction times in particle formation with SAXS experiments: ZnS precipitation on the microsecond time scale
    Schmidt, W. and Bussian, P. and Lindén, M. and Amenitsch, H. and Agren, P. and Tiemann, M. and Schüth, F.
    Journal of the American Chemical Society 132 6822-6826 (2010)
    Precipitation of zinc sulfide particles is a very rapid process, and monitoring of the particle growth is experimentally very demanding. Applying a liquid jet flow cell, we were able to follow zinc sulfide particle formation on time scales down to 10 -5 s. The flow cell was designed in such a way that data acquisition on the microsecond time scale was possible under steady-state conditions along a liquid jet (tubular reactor concept), allowing SAXS data accumulation over a time scale of minutes. We were able to monitor the growth of zinc sulfide particles and found experimental evidence for very rapid particle aggregation processes within the liquid jet. Under the experimental conditions the particle growth is controlled by mass transfer: i.e., the diffusion of the hydrogen sulfide into the liquid jet. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja101519z
  • 2010 • 3 An efficient nickel catalyst for the reduction of carbon dioxide with a borane
    Chakraborty, S. and Zhang, J. and Krause, J.A. and Guan, H.
    Journal of the American Chemical Society 132 8872-8873 (2010)
    Nickel hydride with a diphosphinite-based ligand catalyzes the highly efficient reduction of CO2 with catecholborane, and the hydrolysis of the resulting methoxyboryl species produces CH3OH in good yield. The mechanism involves a nickel formate, formaldehyde, and a nickel methoxide as different reduced stages for CO2. The reaction may also be catalyzed by an air-stable nickel formate. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja103982t
  • 2010 • 2 Ex-post size control of high-temperature-stable yolk-shell Au,@ZrO 2 catalysts
    Güttel, R. and Paul, M. and Schüth, F.
    Chemical Communications 46 895-897 (2010)
    Yolk-shell catalysts have attracted interest in both academia and industry, since they combine high-temperature stability with a reduced complexity for kinetic and mechanistic investigations. This contribution presents a possibility to adjust the size of an active gold core inside a porous zirconia shell via an ex-post-modification approach. © The Royal Society of Chemistry 2010.
    view abstractdoi: 10.1039/b921792d
  • 2010 • 1 Unique features of the folding landscape of a repeat protein revealed by pressure perturbation
    Rouget, J.-B. and Schroer, M.A. and Jeworrek, C. and Pühse, M. and Saldana, J.-L. and Bessin, Y. and Tolan, M. and Barrick, D. and Winter, R. and Royer, C.A.
    Biophysical Journal 98 2712-2721 (2010)
    The volumetric properties of proteins yield information about the changes in packing and hydration between various states along the folding reaction coordinate and are also intimately linked to the energetics and dynamics of these conformations. These volumetric characteristics can be accessed via pressure perturbation methods. In this work, we report high-pressure unfolding studies of the ankyrin domain of the Notch receptor (Nank1-7) using fluorescence, small-angle x-ray scattering, and Fourier transform infrared spectroscopy. Both equilibrium and pressure-jump kinetic fluorescence experiments were consistent with a simple two-state folding/unfolding transition under pressure, with a rather small volume change for unfolding compared to proteins of similar molecular weight. High-pressure fluorescence, Fourier transform infrared spectroscopy, and small-angle x-ray scattering measurements revealed that increasing urea over a very small range leads to a more expanded pressure unfolded state with a significant decrease in helical content. These observations underscore the conformational diversity of the unfolded-state basin. The temperature dependence of pressure-jump fluorescence relaxation measurements demonstrated that at low temperatures, the folding transition state ensemble (TSE) lies close in volume to the folded state, consistent with significant dehydration at the barrier. In contrast, the thermal expansivity of the TSE was found to be equivalent to that of the unfolded state, indicating that the interactions that constrain the folded-state thermal expansivity have not been established at the folding barrier. This behavior reveals a high degree of plasticity of the TSE of Nank1-7. © 2010 by the Biophysical Society.
    view abstractdoi: 10.1016/j.bpj.2010.02.044