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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 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 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.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 Röntgenabsorptionsspektroskopische Untersuchungen an katalytisch aktiven Metallkomplexen und nanostrukturierten anorganisch-organischen Hybridmaterialien(2003) Feth, Martin Philipp; Bertagnolli, Helmut (Prof. Dr.)In der vorliegenden Arbeit wurden Fragestellungen aus dem Bereich der organischen Katalyse-Chemie und materialwissenschaftliche Aspekte der Polymer- und Sol-Gel-Chemie mit Hilfe verschiedener spektroskopischer und analytischer Verfahren bearbeitet. Der Schwerpunkt der durchgeführten Untersuchungen lag hierbei auf dem Gebiet der Röntgenabsorptionsspektroskopie (XAS) mittels Synchrotronstrahlung. Die XAS-Messungen wurden an Elektronen- bzw. Positronen-Speicherringen im In- (Hamburger Synchrotronstrahlungslabor in Hamburg, Berliner Elektronenspeicherring - Gesellschaft für Synchrotronstrahlung m.b.H. in Berlin) und Ausland (Stanford Synchrotron Radiation Laboratory in Stanford, USA) durchgeführt. Die Messungen am neueingerichteten Strahlrohr KMC-2 des Berliner Synchrotrons BESSY II stellen hierbei die ersten XAS-Daten einer Nutzergruppe an diesem Meßplatz dar. Testmessungen zeigten, daß sich dieses Strahlrohr nicht nur durch seine besonders gute Energieauflösung, welche durch den Einsatz von Silizium-Germanium-Gradientenkristallen (SiGe(220)) im Monochromator erreicht wird, auszeichnet, sondern daß auch Fluoreszenz- und µ-XANES-Untersuchungen problemlos möglich sind. Für die hiesige Arbeitsgruppe bedeutet der Meßplatz KMC-2 eine neue Möglichkeit für XAS-Messungen im Röntgenbereich zwischen 4.0 bis 15.0 keV. Im Themenbereich der organischen Katalyse wurden metallorganische Komplexe der Elemente Mangan, Nickel und Kupfer mittels FT-IR-, FT-Raman-, UV/Vis-, EXAFS- und XANES-Spektroskopie untersucht, welche sowohl in der Olefin- bzw. Olefin/CO-Co-polymerisation als auch der asymmetrischen Synthese (Jacobsen-Katsuki-Epoxidierung von Olefinen, enantioselektive Diels-Alder-Reaktionen) ihre Anwendung finden. Mit Hilfe von in situ-XANES- und EXAFS-Messungen wurden die in den Reaktionslösungen auftretenden katalytisch aktiven Spezies identifiziert und die Nahordnung wie auch der Oxidationszustand der Metallatome in diesen Verbindungen aufgeklärt. Ergebnisse aus der Infrarot- und Raman-Spektroskopie untermauerten die Resultate der Röntgenabsorptionsspektroskopie. Zeitaufgelöst durchgeführte XANES-Untersuchungen lieferten kinetische Daten zum Ligandenaustauschverhalten der Metallkomplexe und ergänzen damit die Werte aus den ebenfalls durchgeführten UV/Vis-spektroskopischen Messungen. In Verbindung mit den anderen spektroskopischen Methoden konnten aus den XAS-Ergebnissen Reaktionsmechanismen abgeleitet werden, deren Kenntnis für den erfolgreichen Einsatz dieser Komplexe als Katalysatoren hilfreich sein könnte. Ein weiteres Thema dieser Arbeit waren die Untersuchungen an nanostrukturierten anorganisch-organischen Hybridmaterialien und deren Ausgangsstoffen aus dem Sol-Gel-Prozeß. Anorganisch-organischen Hybridmaterialien sind Reaktionsprodukte der Polymerisation (freie radikalische Polymerisation) von Polyoxometallaten mit organischen Monomeren (Methacrylsäure, Methylmethacrylat, Styrol). Diese Hybridpolymere verknüpfen die Materialeigenschaften von anorganischen Verbindung (z.B. thermische und mechanische Stabilität) mit denen organischer Kunststoffe (leichte Verformbarkeit etc.). In der vorliegenden Arbeit wurde der Reaktionsmechanismus zur Sol-Gel-Herstellung von Polyoxometallaten näher untersucht. Exemplarisch geschah dies für zwei Zirkonium(IV)-Oxo-Cluster. Ein schon existierendes Reaktionsschema wurde mittels Raman-, EXAFS- und XANES-Spektroskopie sowie Hochdruckdruckflüssigchromatographie (HPLC) auf seine Richtigkeit hin überprüft. Wichtige Erkenntnisse über die Geschwindigkeit einzelner Reaktionsschritte konnten aus diesen Untersuchungen gezogen werden. In einem weiteren Schritt wurde die strukturelle Stabilität der Cluster in unterschiedlichen Lösungsmitteln mittels XAS-Spektroskopie überprüft. Anschließend an die Studien zur Herstellung der Polyoxometallate wurden die eigentlichen Hybridpolymere, welche röntgenamorph sind, strukturell untersucht. Es wurden EXAFS-Messungen an Hybridpolymeren durchgeführt, die Polyoxometallate der Elemente Titan, Zirkonium bzw. Hafnium enthalten. Ziel dieser Messung war es festzustellen, ob es möglich ist Oxo-Cluster in Polymere einzubauen, ohne daß die Struktur der Cluster verloren geht. Weiterhin wurde der Einfluß studiert, den die Art des Monomers und das eingesetzte Verhältnis der Ausgangsverbindungen auf die strukturelle Stabilität des Oxo-Clusters im Polymer ausübt. Es zeigte sich, daß die Struktur der Oxo-Cluster, unabhängig von der Art des eingesetzten Monomers, bei Cluster-Monomer-Verhältnissen von 1:50 im Polymer erhalten bleibt. Gemischtmetallische Titan-Zirkonium-Oxo-Cluster sind bis zu Verhältnissen von 1:100 strukturell stabil, wohingegen bei reinen Zirkonium- und Hafnium-Oxo-Clustern ab einem molaren Verhältnis von 1:100 ein teilweiser Abbau der Struktur des Clusters auftritt.Item Open Access Spectroscopic study of the adsorption complexes of benzene and oxygen on Cu/HZSM5 zeolites(2010) Ene, Augusta Bianca; Roduner, Emil (Prof. Dr.)The focus of this study lies in the understanding on a molecular level of the oxidation of benzene to phenol with molecular oxygen over copper-containing HZSM5 zeolites. This direct gas phase oxidation reaction would be a promising alternative to the Cumene process if the catalyst could be optimized to give higher yields. Thus we started our work by characterizing the initial state of the zeolite and the ion exchanged copper ions and then followed the changes they undergo during the adsorption of the reactants and the oxidation reaction. Hereby not only a qualitative analysis of the oxidation processes was performed, but a quantitative evaluation of the adsorbed molecules and the involved catalytic centers. By combining different experimental techniques (FTIR, EPR, XAFS, UV-Vis, GC/MS) with theoretical calculations and results obtained on the catalytically inactive CuHY zeolite, we were able to suggest a reaction path and determine the cause for the small phenol yields. Since the copper ion exchange method has a tremendous influence on the amount of exchanged ions and their oxidation state, we observed in our study samples prepared by either liquid or solid state ion exchange, the latter performed at different temperatures, between 773 and 1273 K. The cation of the parent zeolite, also plays an important role, leading us to study both copper-free and copper-containing H- and Na-ZSM5 zeolites. After the ion exchange, no further oxidative or reductive treatment was applied to the samples. Thus, in their initial form, the zeolites contain a mixture of copper oxidation states, Cu(I) and Cu(II). In literature besides single copper units, a wide range of possible copper oxygen complexes has been discussed, but no concurrent conclusions could be drawn. Single oxygen bridged EPR silent Cu(II)-O2--Cu(II) complexeshave been reported as well as dioxygen bridged species. The study of the low temperature magnetic behavior of our samples resulted in the first experimental evidence of the antiferromagnetic behavior of the copper dimers in Cu/HZSM5 zeolites. The diffusion route through the zeolite pore structure and the adsorption order of benzene and oxygen on both copper and the hydroxyl groups of the ZSM5 zeolite has been determined quantitatively, so that the location of the molecules is known for any loading. Since we observed a strong interaction of both reactants with the copper ions as well as with the hydroxyl groups, we suggest a reaction path which relies on the bifunctionality of the catalyst. The reaction mechanism consists of three intertwined cycles. The high Si/Al ratio of Cu/HZSM5 has the consequence that the second charge of the observed Cu(II) has to be compensated by the presence of a second Al ion or extraframework anions, as in [Cu(I)OH_]+. The homolytic dissociation of this ion pair delivers hydroxyl radicals which drive the other two catalytic cycles. In both these cycles phenol is formed with not only copper but also the Brønsted sites acting as catalytic centers. Hydrogen peroxide is also formed and subsequently dissociated by copper through a Fenton’s reaction. The interplay of both the copper ions and the Brønsted sites has the advantage that phenol is formed on both catalytic centers. The knowledge of the reaction mechanism on a molecular level and of the factors hindering a successful catalytic cycle offers now the possibility of an intentional tailoring of a new catalyst.Item Open Access Preparation and characterization of mesoscale confined materials for asymmetric catalysis(2024) Abitaev, Karina; Sottmann, Thomas (Apl. Prof. Dr.)Item Open Access Structures, properties, and applications of new ferroelectric nematic liquid crystals(2024) Nacke, Pierre; Gießelmann, Frank (Prof. Dr.)