Permeation experiments on Surface Modified Graphite (SMG) membranes showed an improved mixed gas separation factor alpha (H-2/CO2) of 9.2 (+80%) and alpha (H-2/H2O) of 10.2 (+70%) compared with the pristine graphite membranes with alpha (H-2/CO2) of 5.1 and alpha (H-2/H2O) of 6.0. The surface modification was obtained by the treatment with different silanes at 140 degrees C, for 2 h under reflux and continuous stirring in an argon atmosphere. The dried powders were pressed to discs and evaluated for the ternary feed mixture (41 vol.-% H-2/41 vol.-% CO2/18 vol.-% H2O). XPS analysis of the membrane surface revealed sp(2) and sp(3) hybridisation states, carbonyl, carboxyl groups and CFx bonds for the pristine and SMG membranes. Spatially resolved micro focused Raman spectroscopy showed that the improved separation performance correlates with a specific modification of the G and D1 modes of the pristine graphite surface for each functionalisation carried out. Therefore, the improved performances result from the specific chemical and physical interactions of H-2, CO2 and H2O with the applied defect modifications of the SMG membranes.

Solid-state nuclear magnetic resonance spectroscopy is an efficient technique to characterize dynamics and structure of materials. It has been widely used to elucidate ion dynamics in lithium ion conductors. Fast moving lithium ions are needed in energy storage devices, whereas slow ion motion is exploited in some materials used, for example, as blankets in fusion reactors. beta-lithium gallium oxide (LiGaO2) is a slow Li+ ionic conductor similar to gamma-lithium aluminum oxide (LiAlO2). In an ion conductor, in addition to the main diffusion process, localized motions (to-and-fro jumps) may be present. In the present work, with the help of solid-state NMR experiments, we report on the localized movements of Li+ ionic species in beta-LiGaO2 in the temperature range between 300 K and 450 K. In this work, we have mainly extracted the peculiarities of ion dynamics from Li-7 spin-alignment echo NMR measurements and the observation of the motional narrowing of the central transition signal of Li-7.

Abstract Ti-based materials exhibit suitable properties for usage in secondary Li- and Na-ion batteries and were in the focus of several electrochemical and ion conductivity studies. A material of such interest is layer-structured, monoclinic Na2Ti3O7. Additionally, the sodium in Na2Ti3O7 can be replaced completely with lithium to achieve monoclinic Li2Ti3O7, whose electrochemical properties were already investigated as well. Both materials exhibit interesting properties such as zero-strain behavior upon intercalation and high cycling stability. However, there is still a lack of fundamental understanding of the ion diffusivity of both Na and Li in the corresponding host structure. Solid-state nuclear magnetic resonance (NMR) spectroscopy is used here for the first time to reveal the cation dynamics in layered Na2Ti3O7 and Li2Ti3O7. This includes activation energies for the ionic motion and jump rates on the microscopic scale from NMR spin-lattice relaxation (SLR), spin-alignment echo (SAE), and 2D NMR exchange techniques. Moreover, the dimensionality of the ionic motion is investigated by frequency-dependent NMR SLR. Structural details are studied using magic-angle spinning (MAS) NMR spectroscopy. Results for the electric field gradient at the Na and Li site, respectively, are compared with those from theoretical calculations performed within this study. The dynamics are similar for both cations, and the frequency-dependence of the 7Li NMR SLR rate indicates Li motion confined to two dimensions. Thus, these two materials may be regarded a model system for low-dimensional diffusion of two different cations.

Abstract We have studied the Li diffusion in the complex hydride Li3(NH2)2I which appears to exhibit fast Li ion conduction. To get a detailed insight into the Li motion, we have applied 7Li nuclear magnetic resonance spectroscopy methods, such as spin-lattice relaxation in the laboratory and rotating frames of reference, as well as spin-alignment echo. This combined approach allows us to probe Li jump rates over the wide dynamic range (~102–109 s−1). The spin-lattice relaxation data in the range 210–410 K can be interpreted in terms of a thermally-activated Li jump process with a certain distribution of activation energies. However, the low-temperature spin-alignment echo decays at T≤200 K suggest the presence of another Li jump process with the very low effective activation energy.

Abstract Fundamental research on lithium ion dynamics in solids is important to develop functional materials for, e.g. sensors or energy storage systems. In many cases a comprehensive understanding is only possible if experimental data are compared with predictions from diffusion models. Nuclear magnetic resonance (NMR), besides other techniques such as mass tracer or conductivity measurements, is known as a versatile tool to investigate ion dynamics. Among the various time-domain NMR techniques, NMR relaxometry, in particular, serves not only to measure diffusion parameters, such as jump rates and activation energies, it is also useful to collect information on the dimensionality of the underlying diffusion process. The latter is possible if both the temperature and, even more important, the frequency dependence of the diffusion-induced relaxation rates of actually polycrystalline materials is analyzed. Here we present some recent systematic relaxometry case studies using model systems that exhibit spatially restricted Li ion diffusion. Whenever possible we compare our results with data from other techniques as well as current relaxation models developed for 2D and 1D diffusion. As an example, 2D ionic motion has been verified for the hexagonal form of LiBH4; in the high-temperature limit the diffusion-induced 7Li NMR spin-lattice relaxation rates follow a logarithmic frequency dependence as is expected from models introduced for 2D diffusion. A similar behavior has been found for LixNbS2. In Li12Si7 a quasi-1D diffusion process seems to be present that is characterized by a square root frequency dependence and a temperature behavior of the 7Li NMR spin-lattice relaxation rates as predicted. Most likely, parts of the Li ions diffuse along the Si5 rings that form chains in the Zintl phase.

Gas transport through metal-organic framework membranes (MOFs) was switched in situ by applying an external electric field (E-field). The switching of gas permeation upon E-field polarization could be explained by the structural transformation of the zeolitic imidazolate framework ZIF-8 into polymorphs with more rigid lattices. Permeation measurements under a direct-current E-field poling of 500 volts per millimeter showed reversibly controlled switching of the ZIF-8 into polar polymorphs, which was confirmed by x-ray diffraction and ab initio calculations. The stiffening of the lattice causes a reduction in gas transport through the membrane and sharpens the molecular sieving capability. Dielectric spectroscopy, polarization, and deuterium nuclear magnetic resonance studies revealed low-frequency resonances of ZIF-8 that we attribute to lattice flexibility and linker movement. Upon E-field polarization, we observed a defibrillation of the different lattice motions.

In many applications it has been found that the standard generalized gradient approximation (GGA) does not accurately describe weak chemical bond and electronic properties of solids containing transition metals. In this work, we have considered the intercalation material 1T-LixTiS2 (0 ≤ x ≤ 1) as a model system for the evaluation of the accuracy of GGA and corrected GGA with reference to the availabile experimental data. The influence of two different dispersion corrections (D3 and D-TS) and an on-site Coulomb repulsion term (GGA + U) on the calculated structural and electronic properties is tested. All calculations are based on the Perdew-Burke-Ernzerhof (PBE) functional. An effective U value of 3.5 eV is used for titanium. The deviation of the calculated lattice parameter c for TiS2 from experiment is reduced from 14 % with standard PBE to −2 % with PBE + U and Grimme’s D3 dispersion correction. 1T-TiS2 has a metallic ground state at PBE level whereas PBE + U predicts an indirect gap of 0.19 eV in agreement with experiment. The 7Li chemical shift and quadrupole coupling constants are in reasonable agreement with the experimental data only for PBE + U-D3. An activation energy of 0.4 eV is calculated with PBE + U-D3 for lithium migration via a tetrahedral interstitial site. This result is closer to experimental values than the migration barriers previously obtained at LDA level. The proposed method PBE + U-D3 gives a reasonable description of structural and electronic properties of 1T-LixTiS2 in the whole range 0 ≤ x ≤ 1. *Corresponding author: Vanessa Werth, Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover, Callinstr. 3-3a, 30167 Hannover, Germany, e-mail: vanessa.werth@pci.uni-hannover.de Kai Volgmann and Paul Heitjans: Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover, Callinstr. 3-3a, 30167 Hannover, Germany; and Zentrum für Festkörperchemie und Neue Materialien (ZFM), Leibniz Universität Hannover, Callinstr. 3-3a, 30167 Hannover, Germany Mazharul M. Islam and Thomas Bredow: Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Universität Bonn, Beringstr. 4, 53115 Bonn, Germany Bereitgestellt von | Technische Informationsbibliothek Hannover Angemeldet Heruntergeladen am | 10.11.17 15:21 1264 V. Werth et al.

Ti-based materials exhibit suitable properties for usage in secondary Liand Na-ion batteries and were in the focus of several electrochemical and ion conductivity studies. A material of such interest is layer-structured, monoclinic Na2Ti3O7. Additionally, the sodium in Na2Ti3O7 can be replaced completely with lithium to achieve monoclinic Li2Ti3O7, whose electrochemical properties were already investigated as well. Both materials exhibit interesting properties such as zero-strain behavior upon intercalation and high cycling stability. However, there is still a lack of fundamental understanding of the ion diffusivity of both Na and Li in the corresponding host structure. Solid-state nuclear magnetic resonance (NMR) spectroscopy is used here for the first time to reveal the cation dynamics in layered Na2Ti3O7 and Li2Ti3O7. This includes activation energies for the ionic motion and jump rates on the microscopic scale from NMR spin-lattice relaxation (SLR), spin-alignment echo (SAE), and 2D NMR exchange techniques. Moreover, the dimensionality of the ionic motion is investigated by frequency-dependent NMR SLR. Structural details are studied using magic-angle spinning (MAS) NMR spectroscopy. Results for the electric field gradient at the Na and Li site, respectively, are compared with those from theoretical calculations performed within *Corresponding authors: Kai Volgmann and Paul Heitjans, Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover, Callinstr. 3 – 3a, 30167 Hannover, Germany; and Zentrum für Festkörperchemie und Neue Materialien (ZFM), Leibniz Universität Hannover, Callinstr. 3 – 3a, 30167 Hannover, Germany, e-mail: volgmann@pci.uni-hannover.de (K. Volgmann); heitjans@pci.uni-hannover.de (P. Heitjans) Vanessa Werth: Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover, Callinstr. 3 – 3a, 30167 Hannover, Germany; and Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Universität Bonn, Beringstr. 4, 53115 Bonn, Germany Suliman Nakhal and Martin Lerch: Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany Thomas Bredow: Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Universität Bonn, Beringstr. 4, 53115 Bonn, Germany Bereitgestellt von | Technische Informationsbibliothek Hannover Angemeldet Heruntergeladen am | 10.11.17 15:23 1244 K. Volgmann et al. this study. The dynamics are similar for both cations, and the frequency-dependence of the 7Li NMR SLR rate indicates Li motion confined to two dimensions. Thus, these two materials may be regarded a model system for low-dimensional diffusion of two different cations.

Abstract In many applications it has been found that the standard generalized gradient approximation (GGA) does not accurately describe weak chemical bond and electronic properties of solids containing transition metals. In this work, we have considered the intercalation material 1T-Li x TiS2 (0≤x≤1) as a model system for the evaluation of the accuracy of GGA and corrected GGA with reference to the availabile experimental data. The influence of two different dispersion corrections (D3 and D-TS) and an on-site Coulomb repulsion term (GGA+U) on the calculated structural and electronic properties is tested. All calculations are based on the Perdew-Burke-Ernzerhof (PBE) functional. An effective U value of 3.5 eV is used for titanium. The deviation of the calculated lattice parameter c for TiS2 from experiment is reduced from 14 % with standard PBE to −2 % with PBE+U and Grimme’s D3 dispersion correction. 1T-TiS2 has a metallic ground state at PBE level whereas PBE+U predicts an indirect gap of 0.19 eV in agreement with experiment. The 7Li chemical shift and quadrupole coupling constants are in reasonable agreement with the experimental data only for PBE+U-D3. An activation energy of 0.4 eV is calculated with PBE+U-D3 for lithium migration via a tetrahedral interstitial site. This result is closer to experimental values than the migration barriers previously obtained at LDA level. The proposed method PBE+U-D3 gives a reasonable description of structural and electronic properties of 1T-Li x TiS2 in the whole range 0≤x≤1.

Hydrogen production by catalytic steam reforming of renewable hydrocarbons like bio-methane or bio-ethanol has become an attractive goal of sustainable chemistry. Side reactions as in ethanol steam reforming decrease the hydrogen selectivity. A low-temperature catalytic membrane reactor with a hydrogen-selective membrane is expected to solve this problem. Three different carbon membranes are investigated with respect to their performance to extract hydrogen selectively from the binary and ternary reaction mixtures (H2/CO2), (H2/CO2/H2O), and (H2/ethanol) as model systems for bio-ethanol steam reforming. The three carbon membranes under study are (i) an amorphous carbon layer prepared by physical vapour deposition (PVD) of carbon on an porous alumina support using a carbon fibre yarn, (ii) a turbostratic carbon layer obtained by pyrolysis of a supported organic polymer blend as precursor, and (iii) a crystalline carbon prepared by pressing of graphite flakes into a self-supporting disc. For the equimolar binary feed mixture (H2/CO2) all carbon membranes were found to be hydrogen selective. For the ternary feed mixture (41vol.-% H2/41vol.-% CO2/18vol.-% H2O), in the case of the amorphous and crystalline carbon membrane, the hydrogen selectivity remains also in the presence of steam. The turbostratic carbon membrane separates preferentially steam (H2O) from the ternary feed mixture (H2/CO2/H2O).

LixNa2-xTi6O13 has received attention as 3d-metal oxide based anode material for possible battery application. Generally, titanium oxides represent excellent Li hosts due to their zero-strain behavior, cycling stability and high operating voltage. New developments choose Na as charge carrier, but less effort is put in the investigation of mixed cation conductors. Owing to the synthesis route of LixNa2-xTi6O13 (0 <= x <= 2) the coordination of Na and Li in the channels is different which had been investigated by means of X-ray and neutron diffraction. Up to now, no Raman spectroscopy has been applied. This oxide is highly Raman-active, thus the local structure can also be analyzed in terms of vibrational spectroscopy. Micro-Raman spectroscopy carried out at room temperature with different cation contents (x = 0, 0.33, 1, 2) shows the continuous change from Na to Li by a superposition of the modes for Na2Ti6O13 and Li2Ti6O13. The only exceptions are two distinct modes. They appear either for Li (118 cm(-1)) or Na (135 cm(-1)). The results confirm the high-temperature phase stability of Na2Ti6O13 as well as the anisotropic thermal expansion of the unit cell seen by in situ X-ray powder diffrac-tion under two different gas atmospheres. Combining these results, we suppose that the anisotropic thermal expansion of the lattice parameters is affected by the normal vectors of the corresponding bond vibrations in Na2Ti6O13 and Li2Ti6O13. Crystalline-orbital calculations of the Raman shifts of LixNa2-xTi6O13 were carried out for the cation contents x = 0, 1,2 and Raman modes were assigned to specific bond vibrations supported by theory. Besides, this gives additionally information about the Li/Na ratio in a new and simple way.

Collective vibrational modes of crystal lattices, called phonons, determine fundamental material properties, such as their thermal and electrical conductivities. Bulk phonon spectra are influenced by point defects. More recently, the importance of phonons on nanostructures has come into the focus of attention. Here we show a spatially resolved phonon spectra of point defects that reveal distinctly different signatures for a cavity alone and an impurity atom fully integrated into the surface as opposed to one placed into a cavity. The spectra are indicative for delocalized phonons and localized vibrations, respectively, as confirmed by theory.

Within the system Li2O – TiO2, lithium hexatitanate Li2Ti6O13 (i.e. Li2O ∙ 6 TiO2) was already studied before by electrochemical methods with respect to its application as a potential Li-ion battery anode material [1, 2]. This compound is also attractive for fundamental cation diffusion studies, in particular due to its channel structure and the fact that Li and Na are interchangeable. The present work examines the effect of different Li : Na ratios in LixNa2-xTi6O13 on diffusion using impedance spectroscopy which measures the conductivity over a wide frequency range and thus on different length scales. This also enables the comparison with NMR results obtained by us in a separate study. Additionally, the Hebb-Wagner polarization (DC polarization) method is being used here to discriminate between the cation and a partial electronic conductivity [3].

X-ray Photoelectron Spectroscopy (XPS), Metastable Induced Electron Spectroscopy (MIES) and Ultraviolet Photoelectron Spectroscopy (UPS) were applied to study the interaction of oxygen molecules with iron films. Supplementarily, iron oxide was investigated for comparison.With XPS from the Fe 2p(3/2) range contributions of metallic Fe as well as Fe2+ and Fe3+ can be distinguished. During the interaction with oxygen an oxide film is formed on the iron surface. Nevertheless, XPS still shows metallic contributions even for a surface which is saturated with more than 10(4) L The oxide film hinders the dissociation of further impinging oxygen molecules.The interaction of He* atoms with iron oxide surfaces during MIES is dominated by Auger Neutralization. This surprising result follows from the high work function and the fact that intrinsic defects result in a Fermi level pinning to the conduction band. (C) 2010 Elsevier B.V. All rights reserved.