03 Fakultät Chemie
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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 Röntgenabsorptionsspektroskopie an nach dem Sol-Gel-Prozeß hergestellten Übergangsmetalloxiden(2003) Bauer, MatthiasIm Rahmen dieser Diplomarbeit wurden Aero- und Xerogele von Zirkonium- und Wolframoxid sowie deren Calcinierungsprodukte untersucht. Ziel war es, die atomare Nahordnung vor allem im Hinblick auf gebildete ZrO2- und WO3-Strukturen aufzuklären. Die Identifizierung dieser Strukturen erfolgte mit Hilfe der EXAFS-Spektroskopie und durch zusätzlichen Vergleich mit den entsprechenden kristallinen Referenzsub-stanzen. Auf diese Weise wurden die Kristallinität bzw. der Grad der Unordnung der untersuchten Systeme bestimmt. Bei den Wolfram-Verbindungen konnte die Existenz von Keggin-Strukturen neben kristallinen und amorphen Strukturen nachgewiesen werden. Die Vielzahl der verschiedenen Spezies begrenzte die Aussagekraft der EXAFS-Analyse. Diese Schwierigkeit konnte in vielen Fällen durch die Hinzunahme der Raman-Spektroskopie kompensiert werden. Eine Quantifizierung der verschiedenen vorkommenden Strukturtypen in den jeweiligen Proben konnte auch damit nicht erreicht werden. Da die ermittelten Strukturen nur Mittelwerte aller vorhandenen Spezies sind, stellt dies die Grenzen der EXAFS-Spektroskopie klar. Der chemischen Natur entsprechend weisen Wolfram-Verbindungen eine größere Bandbreite an ähnlichen Formen einer bestimmten Stöchiometrie auf als entsprechende Zirkoniumkomponenten.Item Open Access Insights into the first multi-transition-metal containing Ruddlesden-Popper-type cathode for all-solid-state fluoride ion batteries(2024) Vanita, Vanita; Waidha, Aamir Iqbal; Vasala, Sami; Puphal, Pascal; Schoch, Roland; Glatzel, Pieter; Bauer, Matthias; Clemens, OliverPromising cathode materials for fluoride-ion batteries (FIBs) are 3d transition metal containing oxides with Ruddlesden-Popper-type structure. So far, the multi-elemental compositions have not been investigated, but it could alternate the electrochemical performance similar to what has been found for cathode materials for lithium-ion batteries. In this study, we investigate RP type La2Ni0.75Co0.25O4.08 as an intercalation-based active cathode material for all-solid-state FIBs. We determine the structural changes of La2Ni0.75Co0.25O4.08 during fluoride intercalation/de-intercalation by ex situ X-ray diffraction, which showed that F- insertion leads to transformation of the parent phase to three different phases. Changes in the Ni and Co oxidation states and coordination environment were examined by X-ray absorption spectroscopy and magnetic measurements in order to understand the complex reaction behaviour of the phases in detail, showing that the two transition metals behave differently in the charging and discharging process. Under optimized operating conditions, a cycle life of 120 cycles at a critical cut-off capacity of 40 mA h g-1 against Pb/PbF2 was obtained, which is one of the highest observed for intercalation electrode materials in FIBs so far. The average coulombic efficiencies ranged from 85% to 90%. Thus, La2Ni0.75Co0.25O4.08 could be a promising candidate for cycling-stable high-energy cathode materials for all-solid-state FIBs.Item Open Access Hydrogen spillover through hydride transfer : the reaction of ZnO and ZrO2 with strong hydride donors(2024) Benz, Michael; Bunjaku, Osman; Nowakowski, Michal; Allgaier, Alexander; Biswas, Indro; Slageren, Joris van; Bauer, Matthias; Estes, Deven P.Hydrogen spillover, transfer of H2 from a metal surface to a support (often metal oxides), is pivotal for many heterogeneous catalytic processes, including Cu/ZnO and Cu/ZrO2 catalyzed methanol synthesis. Little is known about hydrogen spillover on ZnO or ZrO2, due to the high complexity of the metal-metal oxide interface. Here, we model hydrogen spillover on ZnO and ZrO2 by reacting them with molecular metal hydrides to see how the properties of the hydrides affect hydrogen spillover. While the good H· donors HV(CO)4dppe (1) and CpCr(CO)3H (2) do not react with the metal oxide surfaces, the strong hydride donors iBu2AlH (3), Cp2ZrHCl (4), and [HCu(PPh3)]6 (5) do reduce ZnO and ZrO2 to give defect sites with the same EPR signatures as obtained via hydrogen spillover. We also observe new M-O bonds to the surface using X-ray absorption spectroscopy (XAS). We propose that these metal oxides undergo hydrogen spillover via initial hydride transfer followed by tautomerization of the surface hydride, giving reduced sites and OH bonds. This mechanism is in contrast to the traditional spillover mechanism involving discrete proton- and electron transfer steps. We also observe that ZnO is easier to reduce than ZrO2, explaining the difficulty observing spillover on Cu/ZrO2.