Universität Stuttgart
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Item Open Access Synthese, Charakterisierung und Degradation ionisch vernetzter Blendmembranen für den Brennstoffzellen-Einsatz(2013) Chromik, Andreas; Roduner, Emil (Prof. Dr.)Hier sollen die relevanten Ergebnisse dieser Arbeit zusammengefasst werden. Diese stellen wichtige Resultate für die Polymersynthese mittels Polykondensation, das Verhalten und die Kriterien für die Eignung von Polymeren für Blends, deren Degradationsverhalten, ihre BZ-Leistung und die Eignung von verschiedenen Analysetechniken zur Polymer- und Membrancharakterisierung bereit. Speziell bei der Polykondensation von Arylpolymeren können sehr viel höhere Molekularmassen erzielt werden als bisher. Es zeigte sich, dass die unterschiedlichen Molekularmassen einen starken Einfluss auf die Topografie, Morphologie und darüber auf die BZ-Leistung der Blends ausüben. Es wurde ebenfalls gezeigt, dass die GPC eine gute Methode darstellt, um Degradationsvorgänge in der Membran zu detektieren und zu charakterisieren. Zudem kann die GPC als ein wichtiges Instrument zur Qualitätssicherung betrachtet werden. Die hier gewonnenen Daten zeigen, dass man die theoretische Betrachtung von Degradationsmechanismen und deren Auswirkung auf die MWD auch in die Praxis übertragen kann. Des Weiteren konnte nachgewiesen werden, wie die Degradation der Membranen mit der Molekularmasse zusammenhängt und die Molekularmasse sich auf die Topographie, Morphologie und darüber auf die Brennstoffzellenleistung und Stabilität auswirkt. Zudem konnten als wichtige Analysemethoden die GPC- und AFM-Technik implementiert werden, um zum einen das Degradationsverhalten zu untersuchen und zum anderen ein tieferes Verständnis für die mikroskopischen Zusammenhänge von Morphologie und Brennstoffzellenleistung zu gewinnen.Item Open Access Chirale lyotrop-lamellare Flüssigkristalle : Phasenverhalten neuer chiraler Amphiphile und Nachweis des elektroklinen Effektes(2019) Harjung, Marc D.; Gießelmann, Frank (Prof. Dr.)Ziel dieser Arbeit ist es, neue Erkenntnisse zur Bildung lyotroper SmC*-Phasen zu gewinnen und einen elektroklinen Effekt in chiral lamellaren Phasen nachzuweisen.Item Open Access Die protonenleitende Membran - Schlüsselkomponente einer Brennstoffzelle(2001) Roduner, Emil; Kerres, JochenDie Brennstoffzellentechnologie wird wohl eine der Schlüsseltechnologien dieses Jahrhunderts werden, da die Energieausnutzung von Brennstoffzellenaggregaten deutlich höher ist als der Wirkungsgrad von Verbrennungsmotoren. Mögliche Anwendungsfelder von Brennstoffzellen sind sowohl stationäre Einsätze in Blockkraftwerken als auch mobile Anwendungen als Energiequelle in Fahrzeugen wie Personen- und Lastkraftwagen, Bussen oder Schienenfahrzeugen und nicht zuletzt in mobilen Elektronikgeräten wie Funktelefonen oder Laptops.Item Open Access Charakterisierung der Adsorptionskomplexe von Benzol und Sauerstoff auf Cu/Y-Zeolithen und deren Bedeutung für die katalytische Umsetzung von Benzol zu Phenol(2010) Archipov, Tanja; Roduner, Emil (Prof. Dr.)Diese Arbeit beschäftigt sich mit der Charakterisierung von Benzol- und Sauerstoff-Adsorptionskomplexen auf den Cu-haltigen und Cu-freien HY- und NaY-Zeolithen. Als Untersuchungsmethoden wurden FTIR, ESR, XANES, EXAFS, AAS, RFA, GC und NMR eingesetzt. Die Interpretationen der Spektren wurden mit den Resultaten der DFT-basierenden quantenchemischen Rechnungen verglichen, die in der Kooperation mit den Arbeitsgruppen von Prof. G. Rauhut und Prof. H. Stoll entstanden sind. Diese Arbeit besteht im Wesentlichen aus vier Teilen. Der erste Teil widmet sich den Resultaten und der Interpretation von Untersuchungen der aktiven Zentren der Cu(I)HY-, Cu(II)HY- und Cu(II)NaY-Zeolithe. Die nächsten Teile beschäftigen sich mit der Charakterisierung der Benzol- und Sauerstoff-Adsorptionskomplexe auf diesen aktiven Zentren. Schliesslich, im letzten Teil wird die Koadsorption von Benzol und Sauerstoff auf Cu-haltigen Y-Zeolithen analysiert.Item Open Access Celebrating ten years of covalent organic frameworks for solar energy conversion : past, present and future(2024) Rodríguez‐Camargo, Andrés; Endo, Kenichi; Lotsch, Bettina V.Accelerated anthropogenic emission of greenhouse gases due to increasing energy demands has created a negative impact on our planet. Therefore, the replacement of fossil by renewable energy resources has become of paramount interest, both societally and scientifically. It is within this setting that organic photocatalysts have emerged as a new generation of earth‐abundant catalysts for the conversion of solar radiation into chemical energy. In 2014, the first example of a covalent organic framework (COF) photocatalyst for the hydrogen evolution reaction was reported by our group, which has not only marked the beginning of COF photocatalysis for solar fuel production but also helped to accelerate research into “soft photocatalysis” based on porous polymers in general. In the last decade, significant progress has been made toward developing COFs as robust, molecularly precise platforms emulating artificial photosynthesis. This mini‐review commemorates the 10th anniversary of COF photocatalysis and gives a brief historical overview of the milestones in the field since its inception in 2014. We review milestones in the development of COFs for solar fuel production and related photocatalytic transformations, including hydrogen evolution, oxygen evolution, overall water splitting, CO2 reduction, N2 fixation, oxygen reduction, and alcohol oxidation. We discuss lessons learned for the design of structure‐property‐function relationships in COF photocatalysts, and future perspectives and challenges for the field of “soft photocatalysis” are given.Item Open Access Correlation between the microstructure of porous materials and the adsorption properties of H2 and D2(2011) Krkljus, Ivana; Roduner, Emil (Prof. Dr.)One of the most challenging tasks toward the full implementation of the hydrogen based economy is the reversible storage of hydrogen for portable applications. Three main approaches have been investigated to store the hydrogen, storage as a compressed gas or a liquid, or through a direct chemical bond between the hydrogen atom and the material. The alternative approach, the most recently investigated, is the storage of hydrogen at cryogenic conditions. Storage by physisorption within porous adsorbents has particular advantages of complete reversibility, the fast refueling time, the low heat evolution, and above all increased safety. The nature of interaction of hydrogen, deuterium, and gas mixtures with porous adsorbents was exploited by performing thermal desorption spectroscopy (TDS) measurements. This sensitive experimental technique gives qualitative information about the different adsorption sites, which show different desorption temperatures depending on the interaction energy. After an appropriate calibration the amount of gas desorbed may be quantified. To gain a more fundamental insight into the available adsorption sites multiple TDS spectra were recorded, corresponding to different surface coverages (in the pressure range of 1 to 700 mbar), and different heating regimes. Different kind of porous adsorbents, conventional carbon–based materials and novel Metal Organic Framework Materials (MOFs), were used to investigate the hydrogen/deuterium physisorption mechanism. For carbon materials an increase in the hydrogen interaction potential was observed for adsorbents with narrow pore size. The confined geometry, where hydrogen simultaneously interacts with all the surrounding adsorbent walls, strengthens the interaction potential with the adsorbate molecule, thus, maximizing the total van der Waals force on the adsorbate. Crystalline MOFs are a new class of porous materials assembled from discrete metal centers, which act as framework nodes, and organic ligands, employed as linkers. The material properties can be optimized by changing these two main components. Owing to their high porosity, high storage capacity at low temperature, and excellent reversibility kinetics, MOFs have attracted a considerable attention as potential solid–state hydrogen storage materials. This novel class of porous adsorbents has been extensively investigated within this thesis. The greatest challenge for porous adsorbents is to increase the strength of the H2 binding interaction, and bring adsorption closer to RT conditions. Several strategies, aimed at improving hydrogen adsorption potential in MOFs are closely investigated. These strategies comprise the inclusion of open metal sites and the optimization of the pore size and, thus, the adsorption energy by ligand modification. The influence of the coordinatively unsaturated metal centers, liberated by the removal of metal–bound volatile species, has been particularly investigated. As for carbon materials, the H2–MOF interaction potential is especially enhanced in materials with the pore size comparable to the kinetic diameter of the hydrogen molecule. Such effects may result from the overlap of the potential field due to the proximity of the pore wall, which strengthen the interaction potential with the adsorbate molecule. However, smaller pores prevent hydrogen penetration and induce diffusion limitations. Furthermore, the molecular transport in confined pores at low temperatures may be significantly affected by quantum effects.Item Open Access Polymer electrolyte membrane degradation and oxygen reduction in fuel cells : an EPR and DFT investigation(2004) Panchenko, Alexander; Roduner, Emil (Prof. Dr.)The present work deals with the investigation and characterisation of different processes which take place in a working PEMFC. The SQUID technique was used to investigate magnetic properties of the electrode material. The EPR was the main approach to investigate the radical formation in a working fuel cell. The second part of the thesis comprises the results and discussion of the quantum chemical calculations of the O2 and its reduction intermediates adsorption on low index Pt surfaces. The work aims at an understanding of the pathways of oxidative degradation of membranes, and it wants to provide guidance in the choice of favorable fuel cell operating conditions and in the preparation of alternative membranes with improved durability. For this a miniature fuel cell which can operate in a resonator of an X-band EPR spectrometer was constructed. The concentration of free radicals produced in a fuel cell is extremely low and their lifetime is relatively short, so that it is not possible with conventional methods to observe them directly. We therefore employed the spin trapping technique, using the spin trap molecules POBN, DMPO, DBNBS, and DEPMPO. Radical formation was studied separately at the anode and cathode side of the in situ EPR fuel cell. At the anode side of the cell formal addition of hydrogen atoms to the spin trap molecules was observed. No traces of membrane degradation were detected at the anode side of the fuel cell for any membrane used. At the cathode side we were able to demonstrate the ·OH radical formation during the oxygen reduction by introducing the DMPO spin trap water solution into the cell equipped with the Nafion-115 membrane. The formed ·OH radicals manifested their destructive nature when F-free membranes were used instead of a very stable Nafion membrane. They attacked the membrane and formed different organic radicals on the membrane surface. The formation of radicals was confirmed by the addition of a spin trap water solution at the cathode side. The spin trap molecules react with the radicals under formation of the stable spin trap adducts. In the theoretical part of the manuscript we investigated the energetics of the oxygen reduction reaction intermediates on Pt surfaces in the conditions relevant for the fuel cells. The presence of applied electric fields and coadsorption of water were considered. The adsorption properties of the oxygen molecule and intermediates of the ORR (Oxygen Reduction Reaction) on the (111), (100), and (110) platinum surfaces were calculated in the DFT-GGA framework (PW91-GGA/PAW) using periodic boundary conditions and a slab model of the Pt surface. The electric field dependence of the adsorbate properties was studied using a cluster model of the adsorption system. The B3LYP functional and a 6-311G** basis set for the O and H atoms and a LANL2DZ basis set for the Pt atoms were employed in this case. For all adsorbed species the applied electric field is predicted to have a strong impact and to cause considerable changes in the bond lengths, charge transfer characteristics and vibrational frequencies. The presence of coadsorbed water on the catalyst surfaces was modeled by the coadsorption of two water molecules together with the O2 and ·OH species. The presence of water leads to the formation of hydrogen bonds and strengthens the adsorption significantly.Item Open Access Macrocyclization of dienes under confinement with cationic tungsten imido/oxo alkylidene N‐heterocyclic carbene complexes(2023) Ziegler, Felix; Bruckner, Johanna R.; Nowakowski, Michal; Bauer, Matthias; Probst, Patrick; Atwi, Boshra; Buchmeiser, Michael R.Macrocyclization reactions are still challenging due to competing oligomerization, which requires the use of small substrate concentrations. Here, the cationic tungsten imido and tungsten oxo alkylidene N-heterocyclic carbene complexes [[W(N-2,6-Cl2-C6H3)(CHCMe2Ph(OC6F5)(pivalonitrile)(IMes)+ B(ArF)4-] (W1) and [W(O (CHCMe2Ph(OCMe(CF3)2)(IMes)(CH3CN)+ B(ArF)4-] (W2) (IMes=1,3-dimesitylimidazol-2-ylidene; B(ArF)4-=tetrakis(3,5-bis(trifluoromethyl)phenyl borate) have been immobilized inside the pores of ordered mesoporous silica (OMS) with pore diameters of 3.3 and 6.8 nm, respectively, using a pore-selective immobilization protocol. X-ray absorption spectroscopy of W1@OMS showed that even though the catalyst structure is contracted due to confinement by the mesopores, both the oxidation state and structure of the catalyst stayed intact upon immobilization. Catalytic testing with four differently sized α,ω-dienes revealed a dramatically increased macrocyclization (MC) and Z-selectivity of the supported catalysts compared to the homogenous progenitors, allowing high substrate concentrations of 25 mM. With the supported complexes, a maximum increase in MC-selectivity from 27 to 81 % and in Z-selectivity from 17 to 34 % was achieved. In general, smaller mesopores exhibited a stronger confinement effect. A comparison of the two supported tungsten-based catalysts showed that W1@OMS possesses a higher MC-selectivity, while W2@OMS exhibits a higher Z-selectivity which can be rationalized by the structures of the catalysts.Item Open Access Chirality effects in thermotropic and lyotropic nematic liquid crystals under confined geometries(2019) Dietrich, Clarissa; Giesselmann, Frank (Prof. Dr.)Chirality is a phenomenon in nature that appears across all disciplines of natural science, from biology to mathematics. The spontaneous formation of chiral structures in a system of achiral components is known as spontaneous mirror symmetry breaking and is by itself of fundamental interest leading also towards the question of the origin of homochirality in nature in general. In this work, we show that by means of the topology imposed by the confining geometry and by interfacial boundary conditions - in combination with the physical properties of a liquid crystal - spontaneous mirror symmetry broken structures can be obtained. They are analyzed, inter alia, with respect to the types of geometrical confinements used, e.g. how the confinement amplifies, induces, and influences the detection of chirality effects in order to facilitate the measurement of tiny amounts of chiral additives qualitatively and quantitatively.Item Open Access Solid state NMR studies of the guest molecules in urea inclusion compounds(2007) Yang, Xiaorong; Mueller, Klaus (Prof. Dr.)Urea inclusion compounds are suitable to study the dynamics of the guest molecules and the interactions between guest and guest as well as guest and host molecules under very confined spatial conditions. The understanding of the functional group interactions and the molecular dynamics can provide useful information for practical applications. 1,10-Dibromodecane and 1,11-dibromoundecane/urea inclusion compounds The structural features of urea inclusion compounds with the guest molecules 1,10-dibromodecane and 1,11-dibromoundecance were examined by solid-state 13C CP MAS and 1H MAS NMR spectroscopy. The comparison of the 13C and 1H chemical shifts of the guest species in urea and solution shows downfield shifts upon enclathration for both samples with 1,10-dibromodecane and 1,11-dibromoundecane, which supports an almost all-trans conformation of the guests in urea. The dynamic properties of urea inclusion compounds with the guest molecules 1,10-dibromodecane and 1,11-dibromoundecance selectively deuterated at both end groups, were studied by means of dynamic 2H NMR spectroscopy. Variable temperature line shapes, spin-spin relaxation and spin-lattice relaxation data were obtained between 100 K and room temperature. By examining the various motional models, it is shown that 2H NMR data can at best be reproduced by a superposition of a non-degenerate 3-site or 6-site jump model and an overall chain wobbling motion with a maximal wobbling angle of 24° at room temperature. 1,6-Dibromohexane/urea inclusion compounds The structural and dynamic properties of 1,6-dibromohexane in urea were explored by solid-state NMR spectroscopy. 13C CP/MAS NMR spectra show unusual large shifts up to about 5 ppm, which is attributed to the conformation change of the guests from the co-existence of the gauche and trans conformers in solution to exclusive gauche conformational states in urea. Variable temperature 2H NMR studies were performed in a temperature range between 220 and 320 K on two samples with non-deuterated urea, and 1,6-dibromohexane deuterated either at carbons C-1/C-6 (positions) or at carbons C-2/C-5 (-positions). The quantitative analysis of the variable temperature 2H NMR line shape experiments and spin-lattice relaxation studies supports the proposed non-degenerate 2-site jump model which describes mutual exchange between the two gauche conformers within the urea matrix. It is concluded that the gauche-gauche exchange process is the dominant source for 2H and 13C spin-lattice relaxation of 1,6-dibromohexane in urea. Analysis of the T1Z and T1Q data provides the correlation times for this motional process. The activation energy of about 20 kJ/mol for the low temperature range is rather large, and reflects considerable spatial constraints imposed by the surrounding urea matrix. Above room temperature, a lower activation energy of about 9 kJ/mol along with a discontinuous change for the mobility is observed which is attributed to the gain in spatial freedom. In general, compared with its long-chain analogues, 1,6-dibromohexane guest molecules in urea experience slower and more hindered motions. 1-bromodecane /urea inclusion compounds The structural and dynamic properties of 1-bromodecane guest molecules in urea were investigated by solid-state NMR spectroscopy. 13C CP/MAS NMR studies of 1-bromodecane/urea inclusion compounds reveal that there is no preference for a particular arrangement of the chain ends, i.e. no preference for head-head, head-tail or tail-tail arrangement. The dynamic properties of the 1-bromodecane guests were examined by 2H NMR spectroscopy including line shape, spin-spin (T2) and spin-lattice relaxation (T1Z and T1Q studies). The quantitative analysis of the experimental data shows that a non-degenerate 3-site or 6-site jump process can describe the motion of the guests in the low temperature phase while the dynamic characteristics of the guests in the high temperature phase are described by a degenerate 3-site or 6-site jump model. An activation energy of 11 kJ/mole is derived for the high temperature phase and 35 kJ/mole for the low temperature phase, which correspond to the two different slopes of the high and low temperature phases separated by T1Z and T1Q minima at the T1Z and T1Q curves, respectively. For the guest motion, a small angle fluctuation, which gives rise to a reduction of the quadrupole coupling constant in 2H NMR spectra, is also considered in the whole temperature range investigated. MAS NMR studies on selected urea inclusion compounds Urea inclusion compounds with different guest species were investigated by 13C CP MAS and 1H MAS NMR experiments. 13C NMR spectra of 1-fluorodecane/urea show that the methyl group gives rise to two peaks reflecting different end-group environments. However, for carboxylic acid, the resonances of the CH3 or COOH end-group of decanoic acid display a single peak, which is attributed to the formation of exclusively head-head dimers in the urea channels via hydrogen bonding. Urea inclusion compounds with hexadecane and pentadecane in urea were investigated by 13C CP MAS and 1H MAS NMR experiments. The comparison of the 13C NMR chemical shifts of the guest species in urea and in solution shows a downfield shift for the guest species in urea, and these chemical shift alterations are again traced back to conformational changes. 13C relaxation studies show that the 13C T1 and 13C T1values exhibit a distinct position dependence. They both reflect internal chain mobility. From the semi-quantitative 13C T1 analysis of urea inclusion compounds with hexadecane and pentadecane, correlation times of ca. 10-6 s are obtained. It is argued that chain fluctuations and lateral motion of n-alkane guests may contribute to the 13C T1relaxation, although a final proof is still missing.