5 Naturwissenschaften und Mathematik
Design of Robust Heterogeneous Catalysts for Sustainable Chemistry
- Robust heterogeneous catalysts based on polymer derived non-oxide ceramics (SiC/SiCN) with integrated late transition metal (Ni, Pd) nanoparticles (NPs) were designed, characterized and tested for their catalytic potential. For the decoration of SiCN support with metal NPs, a molecular approach was applied which makes the use of amido metal complexes for the chemical modification of the preceramic polymer (commercially known as HTT 1800, Clariant Advanced Materials GmbH, Sulzbach, Germany) by the transfer of metal to its nitrogen functions. The modified polymer was transformed to an amorphous SiCN material containing metal NPs by controlled pyrolysis under an inert atmosphere. By following aforementioned approach, Cu@SiCN materials (non-porous) have already been developed in our research group.
In present thesis, attempts were made to fabricate porous SiCN/SiC materials with good accessibility of metal (Ni and Pd) NPs. Firstly, the synthesis of palladium containg SiCN materials by the pyrolysis of palladium modified polysilazane was attempted. It was found that during polymer to ceramic transformation, palladium reacts with silicon to form intermetallic palladium silicide particles. The materials were characterized (TGA, FT-IR, NMR, PXRD and TEM) and applied in catalytic hydrogenation reactions. The activity of the catalysts was found low because of the silicidation of metal and non-porous SiCN support.
The issues of the low porosity of SiCN ceramics and the formation of metal silicides were addressed in the synthesis of Ni@SiCN materials. Controlled pyroylsis of Ni modified polysilazane at 600°C provided microporous Ni@SiCN materials, whereas at 1100°C the formation of nickel silicides in non-porous SiCN matrix was observed. Ni@SiCN materials were characterized by solid state NMR, FT-IR, PXRD, TGA, N2-physisorption and TEM. Hyperpolarized 129Xe NMR was used to study the effect of nickel loading and annealing time on pore structure. The microporous materials showed good thermal stability and activity in selective hydrogenation of aryl acetylenes but as polymer to ceramic conversion was not complete at 600°C, the materials possessed low hydrothermal stability at higher temperatures due to the hydrolysis of Si-N bonds.
In order to replace Si-N bonds with more stable Si-C bonds, a commercial polycarbosilane was used to fabricate SiC materials with integrated Ni NPs by the pyrolysis of nanostructured polycarbosilane-block-polyethylene (PCS-b-PE) polymer. PCS-b-PE was synthesized by the reaction of PCS with PE-OH catalyzed by an amido nickel complex followed by its nanostructuring via microphase separation technique. The length of PE block affected the type of pores generated in SiC material and the synthesis of highly porous materials with micro-, meso- and hierarchical porosity was achieved. Ni@SiC materials were characterized by TGA, PXRD, N2-physisorption and TEM. Hierarchically porous Ni@SiC selectively cleaves aromatic C-O bond of aryl ethers in water and was found a reusable catalyst.
Lastly, controlled pyrolysis of palladium modified polysilzane provided nanoporous SiCN materials with very small and highly accessible Pd NPs. This shows the role of SiCN support whose nitrogen atoms have a stabilizing effect on small metal nanoparticles. Moreover, SiCN support ineracts nicely with the NPs leading to the prevention of their leaching. In general, NPs have agglomeration tendencies with increasing metal loading but in the case of Pd@SiCN, size of metal NPs remained small even with a palladium loading of 14 wt%. Pulse titration with hydrogen showed high accessibility of Pd. Pd@SiCN materials showed better activity than commercial Pd/C catalyst in oxidation of alcohols in the absence of air or oxygen with the evolution of hydrogen.
M@SiCN/SiC materials may find applications in sustainable production of chemicals and fuels from renewable sources, for instance, from lignocellulosic biomass.
Coarse-grained Modeling of Protein Dynamics using Elastic Network Models
- Dynamics is crucial for the functioning of biological macromolecules. Because of severe limitations in studying protein dynamics experimentally or with all-atom simulations, coarse-grained methods, especially elastic network models (ENMs), are frequently employed. In the last years, studies on various proteins showed that ENMs reliably reproduce experimental data, despite the simplified protein representation and the purely harmonic potential function. This work on two proteins with very different dynamical properties highlights the remarkable success of ENMs and shows which kind of questions can be answered using coarse-grained methods.
The allosteric enzyme aminoglycoside phosphotransferase(3')-IIIa (APH), which confers resistance against a broad range of aminoglycoside antibiotics to pathogenic bacteria, drastically changes its flexibility upon binding of substrates, but without changing its average conformation. In contrast, the homotrimeric vesicular stomatitis virus glycoprotein G (VSV-G), which triggers the pH-dependent fusion of viral and host membrane, undergoes a large structural rearrangement. A striking difference between the two proteins is their shape. VSV-G contains weakly constrained protein segments, the fusion loops, which can undergo large-scale motions at low energetic cost, whereas APH is not obviously arranged into different protein segments. Nevertheless, ENM calculations show that also APH consists of independently moving segments with correlated internal motion, so-called dynamic domains. The concept of dynamic domains can explain the differential effects of ligand binding on the dynamics of APH.
The first chapter of this thesis describes how experimental evidence for the importance of dynamics successively replaced the former idea of static proteins, and explains the dynamic basis of ligand binding, allostery and conformational changes. In the second chapter, theoretical methods for the analysis of protein dynamics are introduced, with emphasis on the ENM-based methods used in my studies. The studies are summarized in the third chapter. In the study on APH, I employ the Gaussian network model to analyze the ligand-dependent dynamics, the broad substrate specificity and the perturbation-sensitivity of the ligand binding sites. In a second study, ENM-based as well as all-atom molecular dynamics simulations are used to analyze the conformational change of VSV-G. Both approaches detect the fusion loops of VSV-G as most flexible parts of the protein, and thus as most likely starting point for the structural rearrangement, but only the all-atom model can generate deviations from the average structure at low pH. The last study describes the implementation and application of a dynamic domain assignment method, called CovarDom, which is based on covariances of residue fluctuations. Calculation of dynamic domains for a large protein set demonstrates the general applicability of CovarDom.
Monte Carlo Simulation Methods for Studying the Thermodynamics of Ligand Binding & Transfer Processes in Biomolecules
R. Thomas Ullmann
- The binding and transfer of ligands is of central
importance for the function of many biomolecular
systems. The main topic of this thesis is the
development and application of Monte Carlo (MC)
simulation methods for studying complex ligand
binding equilibria which can also involve
conformational changes. The simulated systems
were described by microstates within a continuum
electrostatics/molecular mechanics (CE/MM) model
of the receptor-ligand system. The CE/MM modeling
methodology was improved. The improvements led to
more detailed molecular models that enable a more
realistic reproduction of system properties and
environmental conditions. The developed simulation
methods were applied to biomolecular systems whose
function involves aspects that are important for
the understanding of bioenergetic energy
transduction. The results of this thesis are
presented in five articles that are published in
peer reviewed scientific journals.
Manuscript A presents the Monte Carlo simulation
software GMCT which was largely developed in this
thesis. The software offers a variety of different
simulation methods that allow the user to harness
the full potential of CE/MM models in the simulation
of complex receptor systems.
Manuscript B presents a novel theoretical framework
for free energy calculations with the free energy
perturbation method. The novel framework is more
broadly applicable and can lead to more efficient
simulations than previous formulations. The
derivation of the formalism also led to interesting
insights into general statistical mechanics. The
formalism was implemented in GMCT and could already
be used fruitfully for the free energy calculations
presented in Manuscripts C and D.
Manuscript C demonstrates the application of free
energy measures of cooperativity to study the
coupling of protonation, reduction and conformational
change in azurin from Pseudomonas aeruginosa (PaAz).
Such a coupling is prototypic for bioenergetic systems
because it forms the thermodynamic basis of their
energy transducing function. PaAz is an experimentally
well characterized, small electron transport protein.
For this reason, PaAz was used here as model system
to demonstrate the usefulness of cooperativity free
energies in detecting and quantifying thermodynamic
coupling between events in complex biomolecular
systems. The results of this study led to new insight
that could help to determine the still enigmatic
physiological role of PaAz.
In Manuscript D, free energy calculations were
applied to study the thermodynamics of transport
through the ammonium transporter Amt-1 from
Archaeoglobus fulgidus (AfAmt-1). Ammonium is the most
directly utilizable nitrogen source for plants and
microorganisms. AfAmt-1 and its homologues facilitate
the transport of ammonia/ammonium across biological
membranes in living beings from all domains of life.
It is intensely debated how these proteins perform
their function and whether ammonia or its protonated
form ammonium is actually transported. The study
extended upon previous theoretical studies by
including the effects of substrate concentration,
electrochemical transmembrane gradients,
proton-coupled binding equilibria and competitive
binding of different ligand species. It was found
that the transported species is most likely the
ammonium ion. An ammonia/proton symport mechanism
that involves a pair of coplanar histidine residues
at the center of the transmembrane pore as transient
proton acceptor is made plausible by the high
genetic conservation of these residues.
Manuscript E presents a first application of the
microstate description within a CE/MM model to the
simulation of the non-equilibrium dynamics of a
molecular system. We simulated the re-reduction
kinetics of the primary electron donor in the
photocycle of the bacterial photosynthetic reaction
center from Blastochloris viridis. The simulation
results are in very good agreement with
experimentally measured data.
Plasticity, Intraspecific Variability and Local Adaptation to Climatic Extreme Events of Ecotypes/Provenances of Key Plant Species
- Climate change, and especially an increase of magnitude and frequency of climatic extreme events such as drought periods or heatwaves, will alter growing conditions for plants in the future. Persistent ecosystems, with long-living organisms, such as forest or permanent grassland will be particularly impacted by this development. The velocity of these changes is likely to occur at a pace, which species may not be able to keep track with by natural dispersal or genetic adaptation. Agriculture, forestry and ecosystem management must develop counteracting practices to secure the persistence and functioning of these ecosystems and thus their provision of goods and services. Therefore it is important to develop a better understanding how species and ecosystems may respond to future climatic stressors. Impact assessments, e.g. via climatic envelope modelling are prone to misinterpretations of the adaptive capacity of species, as they do not incorporate the intraspecific genetic and phenotypic differences that exist within the populations accross the distribution range of a species.
Yet, intraspecific variation may exhibit potential tools for the development of climate change adaptation strategies. Here, I focus on key ecosystems in Central Europe. In particular the selective use of plant provenances or ecotypes may help to make ecosystems climate-resilient without a potentially more problematic introduction of exotic species. Especially provenances from warmer, drought-prone regions, with a current climate similar to the projected one for Central Europe recently came into focus as potential substitutes for local provenances, as they might have developed local adaptations to climate conditions at their location of origin. Insights about the response of these provenances to changing averages and extreme event regimes are crucial for a reasonable use of within-species diversity in climate change adaptation.
First, the concept of assisted colonization or migration of species or ecotypes and the role it can play as an adaptation strategy in agriculture, forestry or nature conservation is introduced (Manuscript1). It is suggested that a focus should be laid on keystone species that ensure ecosystem persistence and functioning as they govern the habitat structure and microclimate of a site. The assisted colonization of pre-adapted ecotypes of keystone species from climates similar to future projections for the target site is proposed.
Furthermore, provenances of selected grassland and forest key-species were exposed to drought and warming in two experiments in Bayreuth and Landau, and their ecological responses were analysed. Results suggest that local adaptations to climatic stressors exist. However, the magnitude and direction of responses strongly depend on species and climatic variables. For grassland species, e.g. differences in drought sensitivity could be demonstrated in some cases (Manuscript 4). Fagus sylvatica exhibited differences between the provenances in response to drought conditions, as well (Manuscript 3). It seems that marginal provenances, from the dry margins of the distribution range, show less increment reduction due to the drought treatment. Yet, under more favourable conditions of water supply these provenances did not yield the same high increment rates than more central provenances, indicating a trade-off between stability under stress and yield under non-stress conditions. A pine species that is generally considered to be rather drought-resistant, Pinus nigra, which is a potential substitute for climate-threatened conifers on dry sites in Central Europe, did not show any differences in response to drought and warming (Manuscript 2), maybe due to a weak selective pressure as a result of high drought-resistance across the whole distribution range. The impacts of drought on increment became not visible before the second year after the treatment, stressing the need for more long-time experiments in climate impact research.
Even in a generally warmer environment, cold extremes in winter or spring are expected still to prevail in the future. Therefore, the provenances of the selected species were tested for their cold-hardiness and late frost resistance (Manuscripts 5-7). Growth of the grassland species and F. sylvatica were negatively impacted by a late frost event and differences in late-frost sensitivity between provenances or ecotypes were identified. The (sub-) mediterranean species P. nigra showed differences between provenances in their winter cold hardiness. Correlations between performance under cold stress and winter conditions or late frost proneness of the places of origin could be established for almost all species. However, preceding climate experience, such as the warming or drought treatment of the plants altered their reaction to cold extremes compared to the control treatment, indicating the complexity of the interactive impacts of climate factors on ecosystem and plant performance.
The uncertainty of climate projections and the multitude of changing climatic stressors, though, make the prospect of an easy and rapid success in the search for single “best-adapted” provenances very questionable. In economics the portfolio effect shows that a diversification of investments decreases the risk of a total loss of profits. Hence, in a modelling procedure based on the increment data from the above mentioned experiment it was tested if a “portfolio investment” in several provenances in one stand decreases the risk of yield losses (Manuscript 8). Results indicate that the higher the number of provenances the higher the chance for a “best-performer” to be included in the set. So the likelihood of higher yields, under different climatic conditions increases, yet the risk of low yields stays stable.
Generally, it seems that the selective use of plant species and ecotypes in climate change adaptation can be a feasible tool to maintain ecosystem functionality and productivity. However, the uncertain projections, the multitude of climatic stressors and their interplay with other environmental factors and the potential impacts of assisted colonization of ecotypes on the genetic diversity within species and populations require further research.
Effect of Dopants on the Local Atomic Structure and Sintering Behavior of Bismuth Sodium Titanate
- The most commonly used piezoceramic is lead zirconate titanate Pb(ZrxTi(1-x))O (PZT). It possesses outstanding piezoelectric properties which can be modified for numerous applications by the addition of dopants. However, because of environmental and health concerns regarding lead, lead-free alternatives are demanded by politics.
One of the two most promising lead-free replacement materials is the ferroelectric bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT). Like PZT, it crystallizes in the perovskite structure.
Since the dielectric and piezoelectric properties of pure BNT ceramics are insufficient for application, BNT is often modified by the addition of dopants. These influence a great variety of material properties to different degrees, e.g. the sintering behavior, the dielectric and piezoelectric properties and their respective temperature stabilities. Doping of BNT aims to decrease the sintering temperature in order to avoid Bi vaporization, to increase the depolarization temperature and to enhance the piezoelectric coefficient.
The effects of numerous dopants on the resulting performance of BNT were studied extensively in the literature. However, so far little attention has been payed to the way in which dopants interact with the piezomaterial. Nevertheless, it is the understanding of these relationships that would make targeted modifications and improvements of BNT possible.
The primary goal of this study was to investigate and explain the effects of a model dopant -cobalt- on the phase formation, sintering behavior and microstructure of BNT as well as on the resulting dielectric and piezoelectric properties. In this regard, a core issue was to determine the preferred lattice site of Co in BNT.
BNT was synthesized from oxide powders using the classic solid-state route and sintered at temperatures ranging from 1000 °C to 1150 °C. Cobalt was added in concentrations between 0.1 mol% and 2.6 mol% Co prior to the calcination as Co3O4.
About one third of the total cobalt amount was incorporated into the BNT lattice on the perovskite B-site, that is, it substituted for Ti. The cobalt in BNT appeared to be in equilibrium with the secondary phase Co2TiO4, which invariably formed at cobalt concentrations greater than 0.1 mol% Co. For charge balancing reasons, oxygen vacancies were created in the lattice of cobalt-doped BNT. These markedly enhanced the diffusivity. As a result, the sintering temperature of doped BNT decreased with increasing cobalt concentrations, and high final densities were achieved. However, in highly doped BNT sample swelling occurred at elevated temperatures of the sintering cycle. This phenomenon was attributed to evaporating oxygen caused by the valence transition of Co3+ to Co2+.
Up to 950 °C, BNT was found to densify via solid state sintering mechanisms. Above this temperature, a small amount of liquid phase was present, which probably formed from decomposing BNT because of a slight Ti-deficiency due to doping. Increased Bi vaporization from the melt above 1000 °C appeared to have stabilized sodium cobalt titanate, an additional secondary phase.
The rotation of the iso-lines in the kinetic field diagram of doped BNT was interpreted such that the activation energy for grain growth was higher than the activation energy for densification. Possible reasons are the solute-drag effect and the pinning of domain walls by secondary phase particles.
Both the depolarization temperature and the piezoelectric coefficient d33 decreased with increasing cobalt concentrations. The dielectric properties deteriorated as well. This was attributed to the high electrical conductivity of the doped samples, which prevented full poling.
Crystalline-core micelles based on triblock terpolymers with polyethylene middle blocks
- This thesis is focused on the crystallization-induced structure formation of polyethylene containing triblock terpolymers in organic solvents to surface-compartmentalized worm-like crystalline-core micelles (wCCMs). Obtaining profound knowledge of the parameters controlling the self-assembly process allowed the production of a variety of complex one-dimensional micellar architectures with many potential applications, such as adaptive surfactants.
At first, the basic parameters that control the crystallization-induced self-assembly were explored using symmetric polystyrene-block-polyethylene-block-poly(methyl methacrylate) (PS-b-PE-b-PMMA) triblock terpolymers and a PS-b-PE-b-PS triblock copolymer. In good solvents for the PE block, e.g. THF and toluene, the selective formation of wCCMs was observed over a wide range of concentration, applied crystallization temperature and polymer composition. Whereas wCCMs produced by PS-b-PE-b-PS showed a homogeneous PS corona, a patch-like compartmentalization of the corona was observed if the micelles were formed by PS-b-PE-b-PMMA. As THF shows equal solvent quality for both corona blocks, wCCMs with almost alternating PS and PMMA compartments of about 15 nm were observed in this solvent. However, if structure formation was conducted in bad solvents for PE, such as dioxane or dimethylacetamide, spherical micelles with amorphous PE cores were formed already before crystallization. Hence, the subsequent crystallization of PE resulted in spherical CCMs with a patchy or a homogeneous corona depending on the used triblock. These findings allow the highly selective production of stable spherical or worm-like CCMs from the same polymer.
As the corona structure of the patchy micelles self-assembled from triblock terpolymers was mainly deduced from transmission electron microscopy (TEM) performed on dried samples, a small-angle neutron scattering (SANS) study was performed in order to elucidate the morphology in solution. Therefore a partly deuterated triblock terpolymer was synthesized and measured at different contrasts to allow the selective detection of the different corona compartments. The resulting SANS curves could be interpreted using a form factor model for core-shell cylinders with alternating PS and PMMA hemishells including interparticle interactions, thus validating the TEM observations. Notably, Janus-type and patchy cylinders can be clearly distinguished using the applied form factor model.
Moreover, the controlled formation of wCCMs with tunable corona composition and structure was achieved using the cocrystallization of different triblock copolymers. Via random cocrystallization of PS-b-PE-b-PMMA and PS-b-PE-b-PS the corona morphology could be tuned continuously from a mixed corona at low PMMA content over spherical PMMA patches of increasing number and size to alternating PS and PMMA patches. This approach allows to manufacture wCCMs with predefined corona structure omitting the need to synthesize a new tailor-made triblock terpolymer for every desired morphology.
By establishing the controlled crystallization-driven self-assembly of triblock terpolymers with PE middle blocks, it was further possible to prepare wCCMs with predefined average lengths up to 500 nm and length polydispersities as low as Lw/Ln = 1.1. Here, self-assembled spherical CCMs of PS-b-PE-b-PS were used as seeds for the controlled growth of PS-b-PE-b-PS unimers. Upon further addition of PS-b-PE-b-PMMA unimers these grew epitaxially onto the preexisting wCCMs, resulting in triblock co-micelles that consisted of middle blocks with a homogeneous PS corona and outer blocks with alternating PS/PMMA compartments. These structures represent not only the first block co-micelles including blocks with a patchy corona, but also the first ones produced from purely organic block copolymers.
In view of application, the ability of patchy wCCMs formed by PS-b-PE-b-PMMA to stabilize interfaces was investigated using pendant-drop tensiometry. The observed reduction of the interfacial tension at the toluene/water interface was significantly higher than that of comparable triblock terpolymer single chains and that of wCCMs with a homogeneous PS corona. Interestingly, the obtained equilibrium interfacial tension equaled that of Janus cylinders with similar dimensions. To explain this unexpected finding the corona chains were proposed to adapt to the interface via selective collapse and shielding of the incompatible part of the corona chains. Studying wCCMs formed by several triblock terpolymers with different compositions, the interfacial activity was found to increase with increasing overall length of the corona chains, and to a certain extent with the molar fraction of PS units in the corona.
Iron spin crossovers at high pressures and temperatures and their effects on materials relevant tot he Earth’s lower mantle and core
- Iron is the most abundant element by mass in the Earth. The iron content and its spin or oxidation state have a major influence on the physical properties of the main phases in the Earth’s interior. Therefore it is of vast importance to understand the behavior of iron in mineral phases at the temperature and pressure conditions of the Earth’s interior. This cumulative thesis investigates Fe spin crossovers in iron-containing magnesium aluminum silicates, iron-bearing silicate glasses, the iron carbide Fe3C and the effect of Fe spin crossovers on the Fe/Mg partitioning between perovskite and ferropericlase in pyrolitic model system of the Earth’s lower mantle. The goal is first to understand the nature of the Fe spin crossover in respect to its oxidation state and second to estimate the consequences of their occurrence to processes and the structure in the Earth. Central tools in these studies are laser heated diamond anvil cells, to reach the pressure and temperature conditions of the Earth’s interior, Mössbauer spectroscopy, which is a sensitive probe for detecting structural and spin changes in Fe-bearing materials, and analytical transmission electron microscopy, as a probe of chemistry and oxidation state on the nm-scale. In this cumulative thesis I present the results of five research articles. For the analysis of conventional and recently developed synchrotron energy domain Mössbauer spectra the computer program MossA is introduced, which builds the basis for the analysis and interpretation of the results for the other studies. Based on synchrotron Mössbauer spectroscopy and electrical conductivity measurements of Fe-bearing silicate aluminum perovskite it is shown that Fe3+ occupies the dodecahedral A-site of the perovskite structure and remains in the high-spin state throughout the pressure and temperature conditions of the Earth’s lower mantle. Furthermore, a study on the electronic behavior of Fe in a Fe2+-rich aluminous silicate glass and a Fe3+-rich sodium silicate glass infers that no sharp high spin to low spin crossover occurs in silicate melts in the Earth’s lower mantle. This result excludes the possibility of negatively buoyant melts in the lower mantle in an early magma ocean solely due to strong preferential partitioning of iron into the melt phase, which would be induced by a Fe low-spin bearing melt. New insights into to decoupled partitioning behavior of Fe2+ and Fe3+ between the two dominant phases of the Earth’s lower mantle, perovskite and ferropericlase, are presented. The intermediate spin to low spin crossover of Fe2+ in perovskite at about 110 GPa seems to have a strong effect on partitioning and oxidation state of Fe. It leads to a change of the partitioning behavior of Fe between perovskite and ferropericlase and induces a reduction of Fe3+ to Fe2+ in perovskite. Finally, a Mössbauer spectroscopic and single-crystal x-ray diffraction study of Fe3C reveals a two-stage loss of magnetism in Fe3C at high pressures at room temperature: a ferro- to paramagnetic transition around 8-10 GPa and a para- to nonmagnetic transition at about 22 GPa.
Energy-domain synchrotron radiation Mössbauer source for physics under extreme conditions
- Iron is one of the most abundant elements on Earth, and it is an important component in minerals. Electronic and magnetic properties of iron-bearing materials significantly affect processes occurring in the deep interior of the Earth. In the materials that make up the Earth’s lower mantle iron may exist in different valence, spin states and crystallographic environments. Most of the existing experimental techniques either do not allow to separately follow evolution of different iron sites or are not suitable for measurements under high-pressure/high-temperature conditions. This makes studies of iron electronic structure under such conditions extremely challenging.
The current Ph.D. thesis is divided into two major parts. The first part is dedicated to the development of a Synchrotron Mössbauer Source (SMS). This device allows energy domain Mossbauer spectroscopy to be performed on a sample under pressures above 100 GPa in laser heated diamond anvil cells. The second part is dedicated to studying the behavior of iron in iron/alumina-bearing silicate perovskite under conditions of the Earth’s lower mantle.
1. Synchrotron Mössbauer Source
There are several techniques that allow magnetic and electronic properties of materials under extreme conditions to be probed: X-ray Emission Spectroscopy (XES), X-ray absorption near edge structure (XANES), Nuclear Resonance Spectroscopes, etc. For elements in which observation of Mössbauer effect is possible the most mature, sensitive, and suitable technique for studies of magnetic and electronic properties is energy-domain Mössbauer spectroscopy.
However, due to low brilliance of utilized radioactive sources and low natural abundance of iron in lower mantle minerals measurements using conventional energy- resolved Mössbauer spectroscopy require very long time and usually are limited to moderate pressures. The problem can be solved by combining the outstanding properties of synchrotron radiation (high brilliance, possibility for extreme focusing) with the energy-resolved approach. In brief, what is needed is a synchrotron source of Mössbauer radiation. Construction of such source was the primary task of my PhD work. The possibility to develop such a source was demonstrated at the Nuclear Resonance beamline ID18 at the European Synchrotron Radiation Facility (ESRF) by Smirnov et al. (1997). The source is based on pure nuclear reflections existing in antiferromagnetic 57FeBO3 crystals.
￼The major technical goals of my PhD work were to (a) construct a device that would be permanently ready for operation, and (b) optimize it to have the best possible resolution and highest possible intensity. In order to achieve these goals theoretical calculations were first conducted to understand how to best improve the performance. Second, several experiments were performed to confirm the theoretically predicted results. Third, several optical schemes of the SMS were tested in order to determine the optimal setup.
As a result of the research and development program a powerful Synchrotron Mössbauer Source (SMS) for high-pressure applications was constructed at the Nuclear Resonance beamline (ID18) of the ESRF. Using results obtained in the combined theoretical/experimental study of angular dependence of energy and temporal distributions of the pure nuclear reflections of iron borate crystal, the SMS was optimized for the highest possible intensity and best possible resolution. The bandwidth of radiation provided by the SMS is between 10-15 neV (2-3 Γ0, where Γ0 is a natural linewidth of Mössbauer resonance for Iron), the intensity is ~2.5×104 photons/s and the typical scanning velocity range is about ±12 mm/s (±0.6 μeV). In contrast to conventional radioactive sources, the SMS gives the possibility to focus the beam to tens of microns. SMS is the in-line monochromator, permanently located in the optics hutch and operational immediately after moving it into the incident beam position. The source can be used with all existing sample environments in the experimental hutches downstream of the beamline.
The implementation of this device opens the possibility for studying systems with complex hyperfine structure utilizing energy-resolved approach under various extreme conditions, for example at high-pressure. Furthermore, the SMS allows for very short collection times of only a few minutes, which enables data to be collected during laser heating. Several high-pressure and high-pressure/high-temperature studies that have already been performed are described in the second part of this Ph.D. thesis. The almost 100% recoilless resonant radiation delivered by the source and its high brightness allow a broad field of SMS applications. The SMS can be utilized in any mode of synchrotron storage ring operation.
2. Study of the spin state of Fe3+ ions in perovskite
Silicate perovskite (Mg,Fe)(Si,Al)O3 is the most abundant phase in the Earth’s lower mantle. Knowledge of its properties is indispensable for understanding lower mantle behavior. Dynamic, thermodynamic, and transport properties of silicate perovskite can be significantly affected by the valence and spin state of iron. Silicate perovskite with 5-10 mol% of Fe (where Fe3+/ΣFe ~50–75% (McCammon et al., ￼1997)) and Al, is dominant phase in Earth lower mantle (~75 vol%) (Zhang et al., 2006; Stackhouse et al., 2007). The behavior of Fe electronic properties under conditions close to those of the Earths lower mantle remains strongly controversial.
The second part of my Ph.D. work is dedicated to an investigation of the spin state of iron in Fe3+- rich silicate perovskite at high pressure. Four different silicate perovskite samples with different stoichiometry were studied using the Synchrotron Mössbauer Source. SMS spectra were collected at room temperature and pressures up to 122 GPa using diamond anvil cells, with or without laser annealing of the samples.
The hyperfine parameters, i.e., centre shift and quadrupole splitting, for the same phases, which were extracted from measured spectra for all perovskite samples studied in this work, are the same at each pressure within experimental error. Moreover, there is no change in Fe3+/ΣFe for individual samples over the entire pressure range of the experiment. The hyperfine parameters of the Fe3+ doublet are consistent with the high-spin state (Gütlich et al., 2011), and their smooth variation with pressure indicates that Fe3+ does not undergo spin crossover within the entire pressure range. All observed changes in the spectra are associated with abrupt changes in the electronic state of Fe2+. The hyperfine parameters of the low QS Fe2+ doublet correspond to the high-spin state (McCammon et al., 2008), while the doublet with high quadrupole splitting, whose intensity grows with pressure at the expense of the Fe2+ high-spin state, corresponds either to intermediate-spin (IS) Fe2+ (McCammon et al., 2008) or a distortion of the site occupied by high-spin Fe2+ (Hsu et al., 2010). Based on results presented in a work of Narygina (2010), we indentify changes in Fe2+ electronic structure as high-spin to intermediate spin transition. Irrespective of the interpretation of the Fe2+ spin state, conclusions regarding the absence of spin crossover in Fe3+ remain valid.
These results show that the previously reported spin crossover of Fe3+ ions does not occur when Fe3+ occupies the A-site. In both alumina-containing and alumina-free silicate perovskites Fe3+ ions remain in the high-spin state up to at least 122 GPa, i.e., almost up to the pressure corresponding to the lower mantle - outer core boundary. The results also indicate that Fe3+ ions do not diffuse from the A-site to the B-site in perovskite after high-temperature annealing at high pressure, Mössbauer spectra of before and after annealing are identical. There is also no evidence for high-spin to low-spin crossover of Fe3+ ions due to site change. In contrast, the results confirm that Fe2+ ions undergo a transition from a high-spin to an intermediate spin state, without reaching a low-spin state within the studied pressure range at room temperature. These results suggest that the seismic velocity anomalies in the lower mantle cannot be attributed to spin crossover in Fe3+.
Synthesis and investigation of boron phases at high pressures and temperatures
- Boron, discovered as an element in 1808 and produced in pure form in 1909, still remains one of the most complicated light elements full of surprises. Even the number of pure boron polymorphs is a subject of intensive discussions. It is proven the existence of α-, β- and γ-boron phases. Structural details of the most common boron phase (β-B) are still not fully revealed. For decades boron remained the last stable element in the periodic table, whose ground state was not determined. It has been a subject of a longstanding controversy, whether α-B or β-B is the thermodynamically stable phase at ambient pressure and temperature.
The existence of the α-tetragonal boron phase T-50 has been an open question since its first discovery. It was not clear if T-50 could be synthesized as a pure boron phase or its structure must be stabilized by the presence of carbon or nitrogen. Theorists claimed that T-50 could not exist at all because of its unstable electronic configuration.
We have developed a method of synthesis of single crystals of α-boron. They were crystallized from a boron-platinum melt at high pressures (6-11 GPa) and high temperatures (1450-1875 K). An average size of the as-grown isometric crystals was 60 μm to 80 μm in maximum dimension. The crystal structure is in good agreement with the literature data. Detailed investigation of single crystals of α-boron using Raman spectroscopy was performed under elevated pressures and temperatures. The behaviour of the Raman modes under pressure was studied both theoretically and experimentally. Single crystals of β-boron were grown at temperatures above 1550 K and pressures up to 11 GPa using the similar methodology like that worked out for synthesis of α-boron.
In a series of experiments above 8 GPa we synthesized single crystals of tetragonal δ-boron (also known in literature as α-tetragonal boron or T-50) and refined the crystal structure of this phase based on synchrotron X-ray diffraction data. The purity of δ-boron was confirmed by means of the microprobe analysis and the electron energy loss spectroscopy (EELS).
A new, so far unknown boron phase, ε-boron, was synthesized at pressures of 8-10 GPa and temperatures between 2000-2250 K. The microprobe analysis and EELS revealed that the samples were not contaminated. The crystal structure of the new phase was determined by means of single crystal X-ray diffraction. ε-boron crystallizes in a R-3m space group with the unit cell parameters a = 5.5940(7) Å and c = 12.0756(16) Å (in hexagonal setting). The unit cell contains 15 boron atoms. The structure can be presented by the network of B12 icosahedra with a group of three boron atoms in the inter-icosahedra space. This phase is isostructural to boron carbide B13C2 (if carbon atoms are substituted by boron ones). Measured hardness is ~60 GPa which places ε-boron in the family of superhard materials.
We have demonstrated that δ-boron and ε-boron are metastable polymorphs because (a) they were found only together with other stable boron phases (α-, β-, or γ-B), and (b) upon heating at high pressure, both δ-B and ε-B transform to β- or γ-B, if the PT conditions correspond to the fields of stability of the latter.
Summarising, in the course of the present work the high-pressure high-temperature synthesis of the five boron polymorphs was established as a reproducible, verifiable and well-documented process. Following the synthesis prescription one can grow single crystals of α-B, β-B, γ-B, δ-B, and ε-B phases. Based on results of numerous HPHT experiments, the phase boundaries between the stable boron phases (α-B, β-B, γ-B) were found. Thus, our serial exploration of the pressure-temperature field using the large volume press synthesis technique resulted in establishing the phase diagram of boron (showing also the PT fields of the appearance of its two metastable phases, δ-B and ε-B) in the pressure interval of 3 GPa to 18 GPa at temperatures between 1073 K and 2423 K. Based on our experimental data and linear extrapolation of the α/β phase boundary down to ambient pressure we could resolve a longstanding controversy on the ground state of boron in favour of the α-B phase.
"The New Chemistry" - Sustainable Catalysis with Alcohols
- Subject of the thesis are new iridium complexes stabilized by anionic P,N- or P,N,P-ligands. These complexes were used in homogeneous catalysis. Furthermore, mechanistic studies were performed to provide an insight into the catalytic cycles. Synthesis protocols for a multitude of different product classes have been developed.
The iridium complex 1, stabilized by a neutral P,N-ligand, reacts under basic conditions with 2-aminopyridines. By elimination of dipyridylamine the new catalyst species 2a was formed, which is more stable than catalyst 1.
Based on this finding eight new anionic P,N-ligands and the resulting iridium complexes were synthesized.
After optimization of the reaction conditions (solvent, base, temperature and catalyst loading) these catalysts were used in BH (borrowing hydrogen)/HA (hydrogen autotransfer) reactions. The selective monoalkylation of anilines with primary alcohols was investigated. In comparative experiments the superiority of the new class of catalysts versus the original catalyst 1 was clearly shown. Under mild reaction conditions (70 °C) the selectivity profile with respect to the monoalkylation has been preserved.
The catalytic protocol was subsequently extended to the alkylation of aromatic diamines. Therefore various diaminobenzenes were used as substrates. Also Dapsone®, an important drug in treatment of leprosy could be used as starting material. We succeeded in both symmetric and unsymmetric monoalkylations of diamines. Due to the selectivity profile of the catalyst regarding aromatic amines, also unprotected amino alcohols could be used as alkylating reagents.
By the use of tridentate P,N,P-ligands, a novel class of more stable catalysts compared to complexes 2a-9a, could be developed. Due to sealing the synthesis reactor with a semipermeable membrane, the retransfer of the “borrowed” hydrogen could be prevented and H2 is released. Dehydrogenation and condensation steps are now possible instead of BH/HA. By reacting secondary alcohols with β-amino alcohols, pyrroles were accessible.
After adapting the synthesis protocol to this new class of products the tolerance of functional groups was tested. Diversely functionalized alcohols were used. Under mild reaction conditions (90 °C) and very low catalyst loadings (down to 0.03 mol% iridium), a large number of novel pyrroles was accessible. Using this protocol 21 differently substituted α,α-pyrroles, 12 bicyclic pyrroles, symmetrically as well as non-symmetrically substituted oligopyrroles and three β-aminopyrroles were synthesized. The catalyst resting state was identified by NMR experiments and X-ray structure analysis to be an iridium trihydride. This trihydride is formed under catalytic conditions, by treatment of the pre-catalyst with alcohols or in hydrogen atmosphere.
In the final part of the work, a catalytic pyridine synthesis was developed. In this so far unknown heterocycle synthesis up to four different substituents could be introduced within a single reaction step. 2,6-, 2,5-, 2,4- and 2,3-substituted pyridines were synthesized selectively by using variously substituted primary or secondary alcohols and γ-amino alcohols. Furthermore, both the synthesis of bicyclic pyridines as well as the synthesis of pyridines that bear chiral substituents is possible.