Die langfristige Speicherung von überschüssiger Energie aus regenerativen Quellen ist ein ungelöstes Problem der Energiewende. Ein Ansatz zur Lösung dieses Problems bietet das Power-to-Gas-Konzept, bei dem Methan aus Wasserstoff und Kohlenstoffdioxid als Ersatz für Erdgas erzeugt wird. Da diese Reaktion kinetisch limitiert ist, ist die Verwendung eines geeigneten heterogenen Katalysators erforderlich. Aufgrund ihrer hohen Aktivität und ihres niedrigen Preises werden häufig nickelhaltige Katalysatoren eingesetzt. In der vorliegenden Arbeit wurde die Eignung von Fe-Ni-Hydrotalcit-basierten Katalysatoren für dieses Konzept untersucht, wobei der Schwerpunkt auf der Bestimmung der Abhängigkeit der katalytischen Aktivität vom Eisengehalt lag. Dabei zeigte sich, dass geringe Mengen an Eisen die Methanausbeute verbessern. Eine Erklärung für diese Aktivitätssteigerung liefern die Ergebnisse von in-situ-DRIFTS-Untersuchungen an eisenhaltigen und eisenfreien Ni-Hydrotalcit-Katalysatoren.

In this thesis, a series of computational studies based on density functional theory (DFT) and density functional tight-binding (DFTB) is presented to deeply understand experimental results on the synthesis of endohedral fullerenes and graphene/iron hybrids at atomic level. In the first part, a simple and efficient model is proposed to evaluate the strain experienced by clusters encapsulated in endohedral metallofullerenes (EMFs). Calculations for the sole cluster, either in the neutral or the charged state, cannot be used for this goal. However, when the effect of the carbon cage is mimicked by small organic π-systems (such as pentalene and sumanene), the cluster has sufficient freedom to adopt the optimal configuration, and therefore the energetic characteristics of the EMF-induced distortion of the cluster can be evaluated. Both nitride and sulfide clusters were found to be rather flexible. Hence, they can be encapsulated in carbon cages of different size and shape. For carbide M2C2 cluster the situation is more complex. The optimized cluster can adopt either butterfly or linear shapes, and these configurations have substantially different metal-metal distance. Whereas for Sc2C2 both structures are isoenergetic, linear form of the Y2C2 cluster is substantially less stable than the butterfly-shaped configuration. These results show that phenomenon of the “nanoscale fullerene compression” once proposed by Zhang et al. (J. AM. CHEM. SOC. (2012),134(20)) should be “nanoscale fullerene stretching”. Finally, the results also reveal that both Ti2S and Ti2C2 cluster are strained in corresponding EMF molecules, but the origin of the strain is opposite: C78-D3h(5) cage imposes too long Ti···Ti distance for the sulfide cluster and too short distance for the carbide cluster. In the second part of the thesis, possible fullerene geometries and electronic structures have been explored theoretically for the species detected in mass spectra of the Sc-EMF extract synthesized using CH4 as a reactive gas. Two most promising candidates, namely Sc4C@C80-Ih(7) and Sc4C3@C80-Ih(7), have been identified and further studied at the DFT level. For Sc4C@C80, the tetrahedral Sc4 cluster with the central μ4-C atom was found to be 10 kJ/mol more stable than the square cluster. For Sc4C3@C80, the calculation showed that the most stable is the Sc4C3 cluster in which the triangular C3 moiety is η3- and η2-coordinated to Sc atoms. Whereas Sc4C@C80 has rather small HOMO-LUMO gap and low ionization potential, the HOMO-LUMO gap of Sc4C3@C80 is substantially higher and exceeds that of Sc4C2@C80. In the third part, computational studies of structures and reactivity are described for a new type of EMFs with a heptagon that has been produced in the arc-discharge synthesis. DFT computations predict that LaSc2N@Cs(hept)-C80 is more stable than LaSc2N@D5h-C80, so the former should be synthesized in much higher yield than observed. This disagreement may be ascribed to the kinetic factors rather than thermodynamic stability. Because of prospective applications of this EMFs by introducing functional groups, the influence of the heptagon on the chemical properties have been further evaluated. Thermodynamically and kinetically preferred reaction sites are studied computationally for Prato and Bingel-Hirsch cycloaddition reactions. In both types of reactions the heptagon is not affected, and chemical reactivity is determined by the adjacent pentalene units. Thermodynamically controlled Prato addition is predicted to proceed regioselectively across the pentagon/pentagon edges, whereas the most reactive sites in kinetically-controlled Bingel-Hirsch reaction are the carbon atoms next to the pentagon/pentagon edge. Fourth, although various EMFs have been successfully synthesized and characterized, the formation mechanism is still not known in details, and hence control of the synthesis products is rather poor. Therefore, EMF self-assembly process in Sc/carbon vapor in the presence and absence of cooling gas (helium) and reactive gas (NH3 and CH4) is systematically investigated using quantum chemical molecular dynamics (QM/MD) simulations based on the DFTB potentials. The cooling gas effect is that the presence of He atoms accelerates formation of pentagons and hexagons and reduces the size of formed carbon cages in comparison to the analogous He-free simulations. As a result, the Sc/C/He system yields a large number of successful trajectories (i.e. leading to the Sc-EMFs) with more realistic cage-size distribution than the Sc/C system. Encapsulation of Sc atoms within the carbon cage was found to proceed via two parallel mechanisms. The main mechanism involves nucleation of the several hexagons and pentagons with Sc atoms already at the early stages of the carbon vapor condensation. In such proto-cages, both Sc–C σ-bonds and coordination bonds between Sc atoms and the π-system of the carbon network are present. Sc atoms are thus rather labile and can move along the carbon network, but the overall bonding is sufficiently strong to prevent dissociation even at high temperatures. Further growth of the carbon cage results in encapsulation of one or two Sc atoms within the forming fullerene. Another encapsulation mechanism is observed in rare cases. In this process, the closed cage is formed with Sc being a part of the carbon network, i.e. being bonded by three or four Sc–C σ-bonds. However, such intermediates are found to be unstable, and transform into the endohedral fullerenes within few picoseconds of annealing. In perfect agreement with experimental studies, extension of the simulation to Fe and Ti showed that Fe-EMFs are not formed at all, whereas Ti is prone to form Ti-EMFs with small cage sizes, including Ti@C28-Td and Ti@C30-C2v(3). The role of “reactive gas” in the EMF synthesis is revealed in dedicated simulations of the fullerene formation in the presence of several molecules of CH4 or NH3. When concentration of reactive gas is high, carbon vapor tends to form graphene flakes or other carbon species terminated by hydrogen atoms, whereas the yield of empty fullerenes is very low. Conversely, with additional metal atoms (Sc) and the same number of NH3 molecules, the yield of fullerenes constantly increase from 5 to 65% which is ascribed to the catalytic activity of metal atoms in the nucleation of carbon cages already at early stage. Moreover, due to the presence of hydrogen atoms from the reactive gas, the carbon cage formation requires much longer time, which provides sufficient reaction time to encapsulate 3 or 4 Sc atoms within one cage. It explains preferential formation of clusterfullerenes in experiments with reactive gas. At the same time, monometallofullerenes and dimetallofullerenes are the main products in absence of reactive gas. We also provide possible growth mechanisms of carbide and cyano-clusterfullerenes in details to elucidate how the intracluster goes into the cage. A possible growth mechanism of nitride clusterfullerenes has been proposed based on DFT results. In the last part, a free-standing crystalline single-atom thick layer of Fe has been studied theoretically. By investigating the energy difference, ΔE, between a suspended Fe monolayer and a nanoparticle using the equivalent number of Fe atoms, one can estimate that the largest stable membrane should be ca. 12 atoms wide or 3 × 3 nm2 which is in excellent agreement with the experimental observation. Otherwise, the possibility of C, O, N atoms embedded into the Fe membrane can been fully excluded by DFTB and DFT simulations, which agrees with electron energy loss spectroscopy (EELS) measurement. A significantly enhanced magnetic moment for single atom thick Fe membranes (3.08 μB) is predicted by DFT as compared to the bulk BCC Fe (2.1 μB), which originates from the 2D nature of the Fe membrane since the dz2 orbital is out-of-plane while the dxy orbital is in-plane.

Zahlreiche strikt anaerob lebende Mikroorganismen, darunter acetogene Bakterien, Sulfatreduzierer und methanogene Archaeen, nutzen den reduktiven Acetyl-CoA-Weg zur autotrophen Kohlenstoff-Fixierung oder Energiegewinnung. Die letzten Schritte der Acetyl-CoA-Bildung beruhen hierbei auf dem Zusammenspiel dreier Proteine, dem Corrinoid-Eisen/Schwefel-Protein (CoFeSP), der Methyltetrahydrofolat:CoFeSP-Methyltransferase (MeTr) und dem Acetyl-CoA-Synthase/CO-Dehydrogenase-Komplex (ACS/CODH). In der vorliegenden Arbeit wurde die Substratbindung an MeTr durch thermodynamische und kinetische Messungen untersucht. MeTHF bindet stärker an das Enzym als das demethylierte Produkt Tetrahydrofolat (THF) und scheint dabei einem einstufigen Bindungsmodell zu folgen. Das Substrat wird bei der Bindung an MeTr protoniert, wobei Asn200 eine protonierte H-N5(+)-CH3-Position des MeTHF durch eine alternative Konformation stabilisieren könnte. Asp44 und Asp76 bilden eine funktionelle Dyade bei der Substratbindung, kommen als Protondonoren zur Substrataktivierung jedoch nicht in Frage. Die Kristallstruktur von CoFeSP wurde erstmals vollständig mit der flexiblen N-terminalen [4Fe4S]-Cluster-Bindedomäne bestimmt. Die für die Cobalamin-Bindedomäne erwarteten Konformationsänderungen wurden anhand der Interaktion mit dem reduktiven Aktivator von CoFeSP (RACo) analysiert. Durch Förster-Resonanzenergietransfer wurde eine Annäherung der ortsspezifisch markierten CoFeSP-Positionen beobachtet und anhand des Fluoreszenzsignals die Kinetik der Komplexbildung mit RACo bestimmt. Durch gepulste Elektronendoppelresonanz konnte ebenfalls eine Abstandsänderung nachgewiesen werden. ACS wurde als apo-Enzym gereinigt und durch NiCl2-Rekonstitution in die aktive Form überführt. Durch die Kristallisation der C-terminalen ACS-Domäne wurden hochaufgelöste Strukturen erzeugt, welche eine Diskussion der strukturellen Details des aktiven Zentrums ermöglichen. / Several anaerobic microorganisms, including acetogenic bacteria, sulfate-reducing bacteria and methanogenic archaea operate the reductive acetyl-CoA pathway for autotrophic carbon fixation or to gain energy. The last steps of acetyl-CoA formation rely on three enzymes, the corrinoid-iron/sulfur-protein (CoFeSP), the methyltetrahydrofolate:CoFeSP methyltransferase (MeTr) and the acetyl-CoA synthase/CO dehydrogenase complex (ACS/CODH). Substrate binding to MeTr was investigated by thermodynamic and kinetic meassurements. MeTHF binds slightly stronger than the demethylated product tetrahydrofolate (THF), likely following a simple one-step-binding mechanism. Substrate binding to MeTr is coupled to proton uptake. A H-N5(+)-CH3-transition state of MeTHF could be stabilized by an alternative conformation of Asn200. Asp44 and Asp76 form a functional dyade in substrate binding but can be excluded as proton donors for substrate activation. The crystal structure of CoFeSP was solved completely, including the previously disordered N-terminal [4Fe4S]-cluster binding domain. The expected conformational change of the corrinoid binding domain was characterized by analyzing the interaction between CoFeSP and its reductive activator (RACo). An approach of the labeled CoFeSP positions in the CoFeSP:RACo complex was observed by Förster resonance energy transfer. Based on the corresponding fluorescence signal, the kinetics of complex formation were meassured in solution. Pulsed electron double resonance also showed that the labeled positions approach upon complex formation. Full-length ACS was purified in the apo state. A reconstitution of the A-cluster with NiCl2 resulted in active enzyme. Different crystal structures of the isolated C-terminal domain of ACS were solved at high resolution. Therefore, structural details of the active site could be discussed.

Im biogeochemischen Wasserstoffkreislauf erfolgt der überwiegende Teil der H2-Aufnahme aus der Atmosphäre durch die Böden. Erst seit kurzem ist bekannt, dass die Oxidation von Wasserstoff in Böden mutmaßlich durch eine Reihe von Bodenbakterien vermittelt wird, die zur Aufnahme von Wasserstoff in atmosphärischen Konzentrationen befähigt sind. Diese Bakterien codieren [NiFe]-Hydrogenasen einer neuen Gruppe, die als Gruppe 5 der [NiFe]-Hydrogenasen klassifiziert wurde. Auch das beta Proteobakterium Ralstonia eutropha besitzt die Gene einer derartigen Hydrogenase, die aufgrund ihrer Ähnlichkeit zu den sonst überwiegend in Actinobakterien gefundenen Vertretern der Gruppe 5 als „Actinobakterielle Hydrogenase“ (AH) benannt wurde. In der vorliegenden Arbeit wurde die AH aus R. eutropha als erste Gruppe 5-[NiFe]-Hydrogenase in reiner Form isoliert und eingehend durch unterschiedliche biochemische, spektroskopische und röntgenkristallographische Verfahren untersucht. Die hierbei erhaltenen Ergebnisse unterstützen die für Gruppe-5-[NiFe]-Hydrogenasen postulierte Funktion im Erhaltungsstoffwechsel der Organismen unter besonderen Bedingungen, schließen jedoch eine Beteiligung der AH an der hochaffinen Oxidation von Wasserstoff in Böden aus. Jedoch zeigt das Enzym die neuartige Eigenschaft der sauerstoffinsensitiven Wasserstoff-Oxidation, was auf die Anwesenheit eines ungewöhnlichen, durch 1 Aspartat und 3 Cysteine koordinierten [4Fe4S]-Clusters und der vermuteten Kopplung der Elektronentransportketten in der mutmaßlich physiologischen doppeldimeren Form des Enzyms zurückzuführen sein dürfte. Die Arbeit erweitert somit die Kenntnisse auf dem Gebiet der Sauerstofftoleranz von Hydrogenasen sowie der Eigenschaften der Gruppe 5-[NiFe]-Hydrogenasen und ihrer physiologischen Rolle in den betreffenden Organismen. / In the biogeochemical hydrogen cycle, the dominating process for hydrogen uptake from the atmosphere is performed in soils. Only recently it was shown that hydrogen oxidation in soils is presumably mediated by a number of soil-dwelling actinobacteria, which are enabled in high-affinity hydrogen uptake. These bacteria encode [NiFe] hydrogenases of a novel group classified as group 5 of [NiFe] hydrogenases. A hydrogenase of this group is also found in the beta proteobacterium Ralstonia eutropha and was named „Actinobacterial Hydrogenase“ (AH) for its similarity to the group 5 [NiFe] hydrogenases found in actinobacteria. In this work, the AH from R. eutropha was, as the first group 5 [NiFe] hydrogenase, purified to homogeinity and thoroughly characterized by various biochemical, spectroscopic and X-ray crystallographic methods. The results obtained hereby support the function in maintaining a basal metabolism under challenging conditions, that was postulated for group 5 [NiFe] hydrogenases. Yet, the results also exclude the possibility of the AH contributing to high-affinity hydrogen uptake in soils. However, the enzyme shows the novel property of being able of oxygen-insensitive hydrogen oxidation. This property is obviously connected to an unusual [4Fe4S] cluster coordinated by 1 aspartate and 3 cysteines, as well as to a supposed coupling of the electron transport chains in the double dimeric native form of the enzyme. Hence, this work broadens the knowledge in the field of oxygen tolerant hydrogen oxidation and provides new insights in the function of group 5 [NiFe] hydrogenases and their physiological role in the organisms.

In dieser Dissertation wurden das Auftreten und die ökologische Bedeutung Vivianits in Süßwassersedimenten erforscht. Vivianit ist das am weitensten verbreitete reduzierte Eisenphosphatmineral, das sich in Gewässersedimenten bildet. Über die Mechanismen der Vivianitbildung in Sedimenten und die quantitative Rolle des Minerals für die Speicherung von Phosphor ist bisher wenig bekannt. Die neuen Erkenntnisse dieser Arbeit basieren auf der Entwicklung einer neuartigen Methode, die eine direkte Identifikation Vivianits mittels Röntgendiffraktometrie in Sedimenten erlaubte. Es gelang erstmalig, Vivianit in Oberflächensedimenten zu quantifizieren. Die vorliegende Arbeit zeigt, dass Vivianit signifikant, mit 10-40 %, zur Phosphorretention in Süßwassersedimenten beitragen kann. Die Untersuchung der Bildungsbedingungen Vivianits in unterschiedlichen Gewässersedimenten Norddeutschlands zeigte, dass das molare Schwefel zu Eisen Verhältnis des Sediments als ein wichtiger Indikator für die Bedingungen identifiziert, welche die Triebkräfte für die An- und Abwesenheit Vivianits darstellen. Eine Eutrophierung von Gewässern und der damit verbundene Anstieg der Sulfidproduktion kann dabei die Bildung Vivianits beeinträchtigen, und eine Abnahme des Phosphorbindungsvermögens des Sediments zur Folge haben. Die vorliegende Arbeit macht deutlich, dass eine artifizielle Erhöhung des Eisengehaltes des Sediments im Rahmen einer Seenrestaurierung eine Vivianitbildung induzieren kann und so langfristig zu einem erhöhten Phosphorrückhalt führt. Sättigungsberechnungen ergaben, dass ein hinsichtlich Vivianits übersättigtes Porenwasser kein sicheres Indiz für die Anwesenheit des Minerals ist. Die Berechnungen sind nicht in der Lage die kleinskaligen chemischen Bedingungen im Porenraum des Sediments abzubilden. Die Untersuchungen zeigen, dass die Bildung von Vivianit einen wichtigen Prozess der Phosphorbindung in Gewässersedimenten darstellt, der bislang jedoch weitestgehend vernachlässigt wurde. / In this thesis, the occurrence and environmental relevance of vivianite in freshwater sediments were explored. Vivianite is the most common reduced iron phosphate mineral which forms in sedimentary environments. Not much is known about the mechanisms which lead to vivianite formation in surface sediments, and about the quantitative role of vivianite in phosphorus sequestration. The development of a novel sediment preparation technique allowed the direct identification of vivianite by powder X-ray diffraction. Notably, for the first time, vivianite was quantified in surface freshwater sediments. The study examplifies that vivianite can significantly contribute to the phosphorus retention in surface freshwater sediments, accounting for 10-40 % of total sedimentary phosphorus. The exploration of vivianite in different surface freshwater sediments located in northern Germany revealed that the sedimentary sulphur to iron ratio is a valuable indicator for the conditions that are important drivers behind the formation or absence of vivianite. It has been demonstrated that eutrophication and the accompanied increase in sulphide production hampers vivianite formation, leading to a decreased phosphorus binding capacity of sediments through increased sediment sulphidization. The present study also revealed, that an iron addition as a measure of lake restoration can trigger vivianite formation, and significantly increases the long-term phosphorus retention of sediments. Pore water equilibrium calculations demonstrated that supersaturated pore water is not sufficient to predict the occurrence of the mineral in situ. Those calculations often fail to predict the occurrence of vivianite because they do not adequately represent chemical conditions within sediment microenvironments. In summary, the formation of vivianite in aquatic sediments constitutes an important process in phosphorus sequestration which has so far largely been ignored.

This work has been devoted to the study of phase transformations involving chemical ordering and magnetic properties evolution in bulk Fe-Pt alloys composed of nanometer-sized grains. A comprehensive study of phase transformations and ordering in Fe-Pt alloys is performed by a combination of in-situ neutron powder diffraction and thermal analysis. The dependence of ordering processes on the alloy composition and initial microstructure (homogeneous A1 phase or multilayer-type) is established. Through the use of mechanical alloying and subsequent heat treatment it has been possible to achieve the formation of chemically highly ordered L10 FePt and, in the case of the Fe-rich and Pt-rich compositions, L12 Fe3Pt and FePt3 phases, respectively. Whereas in Pt-rich alloys the decoupling effect of the FePt3 phase leads to coercivity improvement, in Fe-rich nanocomposites a peculiar nanometer scale multilayer structure gives rise to remanence enhancement due to large effects of exchange interactions between the crystallites of the phases. The structure, magnetic properties and magnetisation reversal processes of these alloys are investigated. Experimentally observed phenomena are understood on the basis of a simple two-particle interaction model. Neutron diffraction has also been used for the investigation of the magnetic structure of ordered and partially ordered nanocrystalline Fe-Pt alloys. It has been shown that the magnetic moment of Fe atoms in L10-type Fe Pt alloys is sensitive to the compositional order. The results are compared to density functional calculations.

The concomitant occurrence and reported discrepant behavior of uranium and arsenic in water bodies is a major health and environmental concern. This study combined batch and column experiments, hydrogeochemical simulations and XAFS spectroscopy to uncover the exchange mechanisms governing uranium fate between water and scrap metallic iron, minerals and Shewanella putrefaciens surfaces in the presence of arsenic. The main results suggest that both water chemistry and the solid phase composition influence uranium fate in the presence of arsenic. The importance of uranyl-arsenate species as a major control of uranium behavior in the presence of arsenic is shown. The toxicity of arsenic and the presence of nitrate are interpreted as limiting factors of the enzymatic reduction of both toxins. Besides, XANES fingerprinting and EXAFS modeling have confirmed precipitation/co-precipitation of uranyl-arsenates as a major mechanism controlling uranium behavior in the presence of arsenic.

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