Electrodeposition of PVA-protected PtCo electrocatalysts for the oxygen reduction reaction in H2SO4

In this paper we report the electrosynthesis of PVA-protected PtCo films (PVA = poly(vinylalcohol)) and their activities towards the oxygen reduction reaction (ORR). PtCo electrodeposits were potentiostatically obtained in the presence and absence of PVA at distinct potentials. The film morphology and composition were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), which revealed that the use of PVA in the electrodeposition of PtCo films was decisive to achieve better film composition control. Cyclic voltammetry for PVA-protected PtCo films showed that the electrochemical surface area is dependent on the electrodeposition potentials and suggested different adsorption strengths of oxygen-containing species. Films produced in the presence of PVA presented the following activity order towards ORR as a function of the electrodeposition potential (vs. Ag/AgCl): -0.9 V> -0.8 V> -1.0 V> -0.7 V. In contrast, PtCo films electrodeposited in the absence of PVA displayed very similar activities regardless of the electrodeposition potential. The simplicity of the electrodeposition method combined with its effectiveness enabled the production of "model electrodes" for investigating the fundamental aspects of the reactions taking place in the fuel cell cathodes. (C) 2011 Elsevier B.V. All rights reserved.

The Bologna Stone: history's first persistent luminescent material

In 1603, the Italian shoemaker Vincenzo Cascariolo found that a stone (baryte) from the outskirts of Bologna emitted light in the dark without any external excitation source. However, the calcination of the baryte was needed prior to this observation. The stone later named as the Bologna Stone was among the first luminescent materials and the first documented material to show persistent luminescence. The mechanism behind the persistent emission in this material has remained a mystery ever since. In this work, the Bologna Stone (BaS) was prepared from the natural baryte (Bologna, Italy) used by Cascariolo. Its properties, e. g. impurities (dopants) and their valences, luminescence, persistent luminescence and trap structure, were compared to those of the pure BaS materials doped with different (transition) metals (Cu, Ag, Pb) known to yield strong luminescence. The work was carried out by using different methods (XANES, TL, VUV-UV-vis luminescence, TGA-DTA, XPD). A plausible mechanism for the persistent luminescence from the Bologna Stone with Cu+ as the emitting species was constructed based on the results obtained. The puzzle of the Bologna Stone can thus be considered as resolved after some 400 years of studies.

Origin of Anomalous Activities for Electrocatalysts in Alkaline Electrolytes

Pt extended surfaces and nanoparticle electrodes are used to understand the origin of anomalous activities for electrocatalytic reactions in alkaline electrolytes as a function of cycling/time. Scanning tunneling microscopy (STM) of the surfaces before and after cycling in alkaline electrolytes was used to understand the morphology of the impurities and their impact on the catalytic sites. The nature of the contaminant species is identified as 3d-transition metal cations, and the formation of hydr(oxy)oxides of these elements is established as the main reason for the observed behavior. We find that, while for the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) the blocking of the sites by the undesired 3d-transition metal hydr(oxy)oxide species leads to deactivation of the reaction activities, the CO oxidation reaction and the hydrogen evolution reaction (HER) can have beneficial effects from the same impurities, the latter being dependent on the exact nature of the adsorbing species. These results show the significance of impurities present in real electrolytes and their impact on electrocatalysis.

Development and characterization of non-oxide ceramic composites for mechanical and tribological applications

The main reasons for the attention focused on ceramics as possible structural materials are their wear resistance and the ability to operate with limited oxidation and ablation at temperatures above 2000°C. Hence, this work is devoted to the study of two classes of materials which can satisfy these requirements: silicon carbide -based ceramics (SiC) for wear applications and borides and carbides of transition metals for ultra-high temperatures applications (UHTCs). SiC-based materials: Silicon carbide is a hard ceramic, which finds applications in many industrial sectors, from heat production, to automotive engineering and metals processing. In view of new fields of uses, SiC-based ceramics were produced with addition of 10-30 vol% of MoSi2, in order to obtain electro conductive ceramics. MoSi2, indeed, is an intermetallic compound which possesses high temperature oxidation resistance, high electrical conductivity (21·10-6 Ω·cm), relatively low density (6.31 g/cm3), high melting point (2030°C) and high stiffness (440 GPa). The SiC-based ceramics were hot pressed at 1900°C with addition of Al2O3-Y2O3 or Y2O3-AlN as sintering additives. The microstructure of the composites and of the reference materials, SiC and MoSi2, were studied by means of conventional analytical techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (SEM-EDS). The composites showed a homogeneous microstructure, with good dispersion of the secondary phases and low residual porosity. The following thermo-mechanical properties of the SiC-based materials were measured: Vickers hardness (HV), Young’s modulus (E), fracture toughness (KIc) and room to high temperature flexural strength (σ). The mechanical properties of the composites were compared to those of two monolithic SiC and MoSi2 materials and resulted in a higher stiffness, fracture toughness and slightly higher flexural resistance. Tribological tests were also performed in two configurations disco-on-pin and slideron cylinder, aiming at studying the wear behaviour of SiC-MoSi2 composites with Al2O3 as counterfacing materials. The tests pointed out that the addition of MoSi2 was detrimental owing to a lower hardness in comparison with the pure SiC matrix. On the contrary, electrical measurements revealed that the addition of 30 vol% of MoSi2, rendered the composite electroconductive, lowering the electrical resistance of three orders of magnitude. Ultra High Temperature Ceramics: Carbides, borides and nitrides of transition metals (Ti, Zr, Hf, Ta, Nb, Mo) possess very high melting points and interesting engineering properties, such as high hardness (20-25 GPa), high stiffness (400-500 GPa), flexural strengths which remain unaltered from room temperature to 1500°C and excellent corrosion resistance in aggressive environment. All these properties place the UHTCs as potential candidates for the development of manoeuvrable hypersonic flight vehicles with sharp leading edges. To this scope Zr- and Hf- carbide and boride materials were produced with addition of 5-20 vol% of MoSi2. This secondary phase enabled the achievement of full dense composites at temperature lower than 2000°C and without the application of pressure. Besides the conventional microstructure analyses XRD and SEM-EDS, transmission electron microscopy (TEM) was employed to explore the microstructure on a small length scale to disclose the effective densification mechanisms. A thorough literature analysis revealed that neither detailed TEM work nor reports on densification mechanisms are available for this class of materials, which however are essential to optimize the sintering aids utilized and the processing parameters applied. Microstructural analyses, along with thermodynamics and crystallographic considerations, led to disclose of the effective role of MoSi2 during sintering of Zrand Hf- carbides and borides. Among the investigated mechanical properties (HV, E, KIc, σ from room temperature to 1500°C), the high temperature flexural strength was improved due to the protective and sealing effect of a silica-based glassy phase, especially for the borides. Nanoindentation tests were also performed on HfC-MoSi2 composites in order to extract hardness and elastic modulus of the single phases. Finally, arc jet tests on HfC- and HfB2-based composites confirmed the excellent oxidation behaviour of these materials under temperature exceeding 2000°C; no cracking or spallation occurred and the modified layer was only 80-90 μm thick.

Vibrational spectroscopy on thermally and optically switchable spin crossover compounds

Oktaedrisch koordinierte Übergangsmetalle mit der Elektronenkonfiguration [Ar]3d4 - 3d7 können in zwei unterschiedlichen elektronischen Zuständen existieren: im High-Spin (HS) oder im Low-Spin (LS) Zustand. Zum Beispiel kann Fe(II) in 1A1g (LS) oder 5T2g (HS) Konfiguration auftreten.Besonderes Interesse besteht in der Aufklärung des Mechanismus der kooperativen Wechselwirkung, die den Spinübergang im Festkörper bestimmt. Hierzu müssen zunächst die internen Freiheitsgrade der molekularen Einheiten bekannt sein. Besonders der Beitrag der molekularen Schwingungen zur Entropiedifferenz, die die Triebkraft des Spinübergangs darstellt, ist von entscheidender Bedeutung. Bisher existieren nur wenige detaillierte Untersuchungen zu den Schwingungseigenschaften der Spincrossovermoleküle.In Rahmen der vorliegenden Arbeit wurden die Schwingungseigenschaften einiger Komplexverbindungen, die Spincrossover zeigen, im Detail untersucht. Dazu wurden temperaturabhängige Raman-, Fern- und Mittel-Infrarot-Spektroskopie, Isotopensubstitution und Normalkoordinatenanalysen (NKA) in Verbindung mit Dichtefunktional-Rechnungen (DFT) verwendet.Die gewonnenen Werte der zugeordneten Schwingungsfrequenzen und die bestimmten Kraftkonstantenänderungen können nun zur Verfeinerung von theoretischen Modellen zur Beschreibung des Spinübergangs verwendet werden.

Mg for hydrogen storage: synthesis, nanostructure and thermodynamics properties

Nowadays alternative energies are an extremely important topic and the possibility of using hydrogen as an energy carrier must be explored. Many problems infer the technological application of this abundant and powerful resource, one of them the possibility of storage. In the framework of suitable materials for hydrogen storage, magnesium has been the center of this study because it is cheap and the amount of stored hydrogen that it achieves (7.6 wt%) is extremely appealing. Nanostructure helps to overcome the slow hydrogen diffusion and the functionalization of surfaces with transition metals or oxides favors the hydrogen molecule dissociation/recombination. The aim of this research is the investigation of the metal-hydride transformation in magnesium nanoparticles synthesized by inert-gas condensation, exploiting the fact that they are a simple model system. The so produced nanostructured powder has been analyzed in response to nanoparticles surface functionalization by transition metal clusters, specifically palladium, nickel and titanium, chosen on the basis of their completely different Mg-related phase diagrams. The role of the intermetallic phases formed upon heating and hydrogenation treatments will be presented to provide a comprehensive picture of hydrogen sorption in this class of nanostructured storage materials.

Partial melting within the lower crust – Constraints from bulk rock geochemical data and trace element mineral analyses of granulites and migmatites from Central Finland

This PhD thesis concerns geochemical constraints on recycling and partial melting of Archean continental crust. A natural example of such processes was found in the Iisalmi area of Central Finland. The rocks from this area are Middle to Late Archean in age and experienced metamorphism and partial melting between 2.7-2.63 Ga. The work is based on extensive field work. It is furthermore founded on bulk rock geochemical data as well as in-situ analyses of minerals. All geochemical data were obtained at the Institute of Geosciences, University of Mainz using X-ray fluorescence, solution ICP-MS and laser ablation-ICP-MS for bulk rock geochemical analyses. Mineral analyses were accomplished by electron microprobe and laser ablation ICP-MS. Fluid inclusions were studied by microscope on a heating-freezing-stage at the Geoscience Center, University Göttingen. Part I focuses on the development of a new analytical method for bulk rock trace element determination by laser ablation-ICP-MS using homogeneous glasses fused from rock powder on an Iridium strip heater. This method is applicable for mafic rock samples whose melts have low viscosities and homogenize quickly at temperatures of ~1200°C. Highly viscous melts of felsic samples prevent melting and homogenization at comparable temperatures. Fusion of felsic samples can be enabled by addition of MgO to the rock powder and adjustment of melting temperature and melting duration to the rock composition. Advantages of the fusion method are low detection limits compared to XRF analyses and avoidance of wet-chemical processing and use of strong acids as in solution ICP-MS as well as smaller sample volumes compared to the other methods. Part II of the thesis uses bulk rock geochemical data and results from fluid inclusion studies for discrimination of melting processes observed in different rock types. Fluid inclusion studies demonstrate a major change in fluid composition from CO2-dominated fluids in granulites to aqueous fluids in TTG gneisses and amphibolites. Partial melts were generated in the dry, CO2-rich environment by dehydration melting reactions of amphibole which in addition to tonalitic melts produced the anhydrous mineral assemblages of granulites (grt + cpx + pl ± amph or opx + cpx + pl + amph). Trace element modeling showed that mafic granulites are residues of 10-30 % melt extraction from amphibolitic precursor rocks. The maximum degree of melting in intermediate granulites was ~10 % as inferred from modal abundances of amphibole, clinopyroxene and orthopyroxene. Carbonic inclusions are absent in upper-amphibolite facies migmatites whereas aqueous inclusion with up to 20 wt% NaCl are abundant. This suggests that melting within TTG gneisses and amphibolites took place in the presence of an aqueous fluid phase that enabled melting at the wet solidus at temperatures of 700-750°C. The strong disruption of pre-metamorphic structures in some outcrops suggests that the maximum amount of melt in TTG gneisses was ~25 vol%. The presence of leucosomes in all rock types is taken as the principle evidence for melt formation. However, mineralogical appearance as well as major and trace element composition of many leucosomes imply that leucosomes seldom represent frozen in-situ melts. They are better considered as remnants of the melt channel network, e.g. ways on which melts escaped from the system. Part III of the thesis describes how analyses of minerals from a specific rock type (granulite) can be used to determine partition coefficients between different minerals and between minerals and melt suitable for lower crustal conditions. The trace element analyses by laser ablation-ICP-MS show coherent distribution among the principal mineral phases independent of rock composition. REE contents in amphibole are about 3 times higher than REE contents in clinopyroxene from the same sample. This consistency has to be taken into consideration in models of lower crustal melting where amphibole is replaced by clinopyroxene in the course of melting. A lack of equilibrium is observed between matrix clinopyroxene / amphibole and garnet porphyroblasts which suggests a late stage growth of garnet and slow diffusion and equilibration of the REE during metamorphism. The data provide a first set of distribution coefficients of the transition metals (Sc, V, Cr, Ni) in the lower crust. In addition, analyses of ilmenite and apatite demonstrate the strong influence of accessory phases on trace element distribution. Apatite contains high amounts of REE and Sr while ilmenite incorporates about 20-30 times higher amounts of Nb and Ta than amphibole. Furthermore, trace element mineral analyses provide evidence for magmatic processes such as melt depletion, melt segregation, accumulation and fractionation as well as metasomatism having operated in this high-grade anatectic area.

Sviluppo dei materiali innovativi e studio della loro interazione con sistemi biologici e biomimetici

Nell’ambito della Chimica Sostenibile e dell’applicazione dei suoi principi per la salvaguardia dell’ambiente, il progetto di dottorato ha riguardato lo sviluppo di materiali innovativi e lo studio della loro interazione con sistemi biologici e biomimetici. In particolare l’attività si è focalizzata sulla sintesi di liquidi ionici ed indagini delle interazioni con membrane cellulari e sull’utilizzo ed isolamento di molecole da fonti rinnovabili. I liquidi ionici sono sali organici liquidi a temperature inferiori ai 100 °C; sono considerati promettenti solventi a ridotta tossicità, ma vanno chiarite a pieno le modalità di interazione con i sistemi biologici ed i meccanismi di tossicità. A questo scopo è stata impiegata una batteria di test bio-chimici, con saggi di fluorescenza e colorimetrici, che hanno permesso di discriminare le diverse tipologie di interazioni con varie strutture di membrana. Le informazioni raccolte sono servite per progettare sostanze meno dannose per le strutture cellulari, al fine di scegliere le funzionalità molecolari che consentano ai liquidi ionici di mantenere la loro attività ma di essere meno dannosi per l’ambiente. Per quanto riguarda l’utilizzo ed isolamento di molecole da fonte rinnovabili, si è utilizzata la tecnica della pirolisi per l’ottenimento di starting materials ed il loro impiego nella sintesi di chemicals in alternativa a composti derivanti da fonti fossili. La pirolisi tradizionale della cellulosa fornisce una molecola interessante, per semplicità denominata LAC, in quantità insufficienti ad un uso applicativo. Nell’ambito delle ricerche svolte è stato scoperto che la pirolisi condotta in presenza di catalizzatori meso-strutturati (MCM-41) drogati con metalli di transizione, fornisce buone quantità di LAC. LAC si è dimostrato promettente sia per la produzione di nuove molecole con possibili applicazioni nella chimica fine e farmaceutica, che come monomero per nuovi polimeri (copolimero ed omopolimero).

Using sulfur isotope fractionation to understand the atmospheric oxidation of SO 2

Sulfate aerosol plays an important but uncertain role in cloud formation and radiative forcing of the climate, and is also important for acid deposition and human health. The oxidation of SO2 to sulfate is a key reaction in determining the impact of sulfate in the environment through its effect on aerosol size distribution and composition. This thesis presents a laboratory investigation of sulfur isotope fractionation during SO2 oxidation by the most important gas-phase and heterogeneous pathways occurring in the atmosphere. The fractionation factors are then used to examine the role of sulfate formation in cloud processing of aerosol particles during the HCCT campaign in Thuringia, central Germany. The fractionation factor for the oxidation of SO2 by ·OH radicals was measured by reacting SO2 gas, with a known initial isotopic composition, with ·OH radicals generated from the photolysis of water at -25, 0, 19 and 40°C (Chapter 2). The product sulfate and the residual SO2 were collected as BaSO4 and the sulfur isotopic compositions measured with the Cameca NanoSIMS 50. The measured fractionation factor for 34S/32S during gas phase oxidation is αOH = (1.0089 ± 0.0007) − ((4 ± 5) × 10−5 )T (°C). Fractionation during oxidation by major aqueous pathways was measured by bubbling the SO2 gas through a solution of H2 O2

Glycerol oxidehydration to acrylic acid on complex mixed-metal oxides

The project of this Ph.D. thesis is based on a co-supervised collaboration between Università di Bologna, ALMA MATER STUDIORUM (Italy) and Instituto de Tecnología Química, Universitat Politècnica de València ITQ-UPV (Spain). This Ph.D. thesis is about the synthesis, characterization and catalytic testing of complex mixed-oxide catalysts mainly related to the family of Hexagonal Tungsten Bronzes (HTBs). These materials have been little explored as catalysts, although they have a great potential as multifunctional materials. Their peculiar acid properties can be coupled to other functionalities (e.g. redox sites) by isomorphous substitution of tungsten atoms with other transition metals such as vanadium, niobium and molybdenum. In this PhD thesis, it was demonstrated how it is possible to prepare substituted-HTBs by hydrothermal synthesis; these mixed-oxide were fully characterize by a number of physicochemical techniques such as XPS, HR-TEM, XAS etc. They were also used as catalysts for the one-pot glycerol oxidehydration to acrylic acid; this reaction might represent a viable chemical route to solve the important issue related to the co-production of glycerin along the biodiesel production chain. Acrylic acid yields as high as 51% were obtained and important structure-reactivity correlations were proved to govern the catalytic performance; only fine tuning of acid and redox properties as well as the in-framework presence of vanadium are fundamental to achieve noteworthy yields into the acid monomer. The overall results reported herein might represent an important contribution for future applications of HTBs in catalysis as well as a general guideline for a multifaceted approach for their physicochemical characterization.

Detektion von Nickel- und Cobaltspezies in einzelnen lebenden Zellen mit hoher lateraler Auflösung

Übergangsmetallen wie Nickel und Cobalt kommt meist eine große Bedeutung als Cofaktor in Enzymen oder Metallkomplexen im Metabolismus von Lebewesen zu. Da eine sehr geringe Konzentration dieser Übergangsmetalle in einer Zelle für deren Funktionalität ausreicht, ist eine konstante Konzentration der Spurenelemente in einer Zelle angestrebt. Durch meist anthropogene Einflüsse sind Pflanzen und Menschen zunehmend hohen Konzentrationen von Übergangsmetallen ausgesetzt, die in Abhängigkeit von ihrer Spezies, der Konzentration und der Lokalisation unterschiedliche Toxizitäten aufweisen können. Die Speziation von Metallen wurde bisher mittels gängiger Analyseverfahren, wie der ICP-MS und ähnlicher Verfahren, anhand von bulk-Material durchgeführt. Durch die Entwicklung von optischen Sensoren für Metallionen war es möglich, diese Metalle auch in lebenden Zellen mittels Fluoreszenzmikroskopie zu lokalisieren. Ke und Kollegen (2006, 2007) nutzten einen solchen optischen Sensor - Newport Green DCF, um die Aufnahme von Nickel in humane A543 Lungenbronchialepithelzellen nach Inkubation mit dem wasserlöslichen NiCl2 (0,5 mM und 1 mM) sowie den wasserunlöslichen Verbindungen Ni3S2 (0,5 µg/cm2 und 1 µg/cm2) und NiS (2,5 µg/cm2) nachzuweisen und zu lokalisieren und konnten damit eine Akkumulation von Nickel im Zytoplasma und im Zellkern aufzeigen. Dabei war bei wasserlöslichen und wasserunlöslichen Nickelverbindungen Nickel nach 24 h im Zytoplasma und erst nach 48 h im Zellkern zu beobachten.rnrnDa Nickel und Cobalt keine detektierbare Eigenfluoreszenz unter den gegebenen Bedingungen zeigten, wurde für den optischen Nachweis von Nickel und Cobalt mit dem konfokalen Laser-Raster Mikroskop (CLSM) nach der Zugabe der verschiedenen wasserlöslichen und wasserunlöslichen Metallverbindungen NiCl2, NiSO4, Ni3S2 und CoCl2 in einzelnen lebenden humanen Gingiva-Fibroblasten, sowie in Pflanzenzellen in dieser Arbeit ebenfalls der optische Sensor Newport Green DCF genutzt. Korrespondierend zu den Ergebnissen früherer Arbeiten von Ke et al. (2006, 2007), in denen die Nickelaufnahme bei Konzentrationen von >0,5 mM NiCl2 bzw. >0,5 µg/cm2 Ni3S2 gezeigt wurde, wurde Nickel in Fibroblasten in Abhängigkeit von der Spezies mit steigender Metallkonzentration von 100 µM bis 500 µM nach 16 h im Zytoplasma und zunehmend nach 24 h bis 48 h im Zellkern detektiert. Bei der wasserunlöslichen Verbindung Ni3S2 war der Nachweis von Nickel im Zellkern bereits nach 16 h bis 24 h erfolgreich. Zusätzlich wurden weitere Strukturen wie das Endoplasmatische Retikulum, die Mitochondrien und die Nukleoli durch eine starke Fluoreszenz des optischen Sensors bei Colokalisationsexperimenten mit Organell-spezifischen Fluoreszenzfarbstoffen als target für die Nickelbindung vermutet. Die Lokalisation von Cobalt in den Fibroblasten entsprach weitgehend der Lokalisation von Nickel. Im Zellkern war die Cobaltlokalisation jedoch auf die Nukleoli beschränkt. Weiterführende Versuche an humanen Gingiva-Fibroblasten zeigten, dass die Aufnahme der Metalle in die Fibroblasten pH-Wert abhängig war. Niedrige pH-Werte im sauren pH-Bereich verringerten die Aufnahme der Metalle in die Zellen, wobei ein pH-Wert im basischen Bereich keinen bedeutenden Unterschied zum neutralen pH-Bereich aufwies. Im Vergleich zu den Fibroblasten war in Pflanzenzellen zu jedem Zeitpunkt, auch bei geringen Konzentrationen der Metallverbindungen sowie des optischen Sensors, Nickel und Cobalt in den Zellkernen detektierbar. Durch die Eigenschaft der Pflanzenzellen eine Vakuole zu besitzen, war Nickel und Cobalt hauptsächlich in den Vakuolen lokalisiert. Weitere Strukturen wie das Endoplasmatische Retikulum, die Mitochondrien oder auch die Zellwand kamen bei Pflanzenzellen als target in Frage.rnrnDie Fluoreszenz und Lokalisation der Metalle in den Fibroblasten waren unabhängig von der Spezies sehr ähnlich, sodass in den Zellen die Spezies anhand der fluoreszenzmikroskopischen Aufnahmen kaum unterschieden werden konnten. Lambda-Scans in verschiedenen regions of interest (ROI) wurden durchgeführt, um durch die Fluoreszenzspektren Hinweise auf eine charakteristische Beeinflussung der Bindungspartner von Nickel und Cobalt oder dieser Metalle selbst in den Zellen auf den optischen Sensor zu bekommen und diese dadurch identifizieren zu können. Das Ziel der parallelen Detektion bzw. Lokalisation und gleichzeitigen Speziation bestimmter Nickel- und Cobaltpezies in einzelnen lebenden Zellen konnte in dieser Arbeit durch den optischen Sensor Newport Green DCF nicht erreicht werden.

Organic/inorganic polyoxometalate-based hybrids for oxidation catalysis and energy conversion

This thesis work has been carried out during the Erasmus exchange period at the “Université Paris 6 – Pierre et Marie Curie”, in the “Edifices PolyMétalliques – EPOM” team, leaded by Prof. Anna Proust, belonging to the “Institut Parisien de Chimie Moléculaire”, under the supervision of Dr. Guillaume Izzet and Dr. Geoffroy Guillemot. The redox properties of functionalized Keggin and Dawson POMs have been exploited in photochemical, catalytic and reactivity tests. For the photochemical purposes, the selected POMs have been functionalized with different photoactive FGs, and the resulting products have been characterized by CV analyses, luminescence tests and UV-Vis analyses. In future, these materials will be tested for hydrogen photoproduction and polymerization of photoactive films. For the catalytic purposes, POMs have been firstly functionalized with silanol moieties, to obtain original coordination sites, and then post-functionalized with TMs such as V, Ti and Zr in their highest oxidation states. In this way, the catalytic properties of TMs were coupled to the redox properties of POM frameworks. The redox behavior of some of these hybrids has been studied by spectro-electrochemical and EPR methods. Catalytic epoxidation tests have been carried out on allylic alcohols and n-olefins, employing different catalysts and variable amounts of them. The performances of POM-V hybrids have been compared to those of VO(iPrO)3. Finally, reactivity of POM-VIII hybrids has been studied, using styrene oxide and ethyl-2-diazoacetate as substrates. All the obtained products have been analyzed via NMR techniques. Cyclovoltammetric analyses have been carried out in order to determine the redox behavior of selected hybrids.

Investigation of acceptor dopants in ZrNiSn half-Heusler materials

Thermoelectric generators (TEG) are solid state devices and are able to convert thermal energy directly into electricity and thus could play an important role in waste heat recovery in the near future. Half-Heusler (HH) compounds with the general formula MNiSn (M = Ti, Zr, Hf) built a promising class of materials for these applications because of their high Seebeck coefficients, their environmentally friendliness and their cost advantage over conventional thermoelectric materials.rnrnMuch of the existing literature on HH deals with thermoelectric characterization of n-type MNiSn and p-type MCoSb compounds. Studies on p-type MNiSn-based HHs are far fewer in number. To fabricate high efficient thermoelectric modules based on HH compounds, high performance p-type MNiSn systems need to be developed that are compatible with the existing n-type HH compounds. This thesis explores synthesis strategies for p-type MNiSn based compounds. In particular, the efficacy of transition metals (Sc, La) and main group elements (Al, Ga, In) as acceptor dopants on the Sn-site in ZrNiSn, was investigated by evaluating their thermoelectric performance. The most promising p-type materials could be achieved with transition metal dopants, where the introduction of Sc on the Zr side, yielded the highest Seebeck coefficient in a ternary NiSn-based HH compound up to this date. Hall effect and band gap measurements of this system showed, that the high mobility of minority carrier electrons dominate the transport properties at temperatures above 500 K. It could be shown that this is the reason, why n-type HH are successful TE materials for high temperature applications, and that p-types are subjected to bipolar effects which will lead to diminished thermoelectric efficiencies at high temperatures.rnrnTo complement the experimental investigations on different metal dopants and their influence on the TE properties of HH compounds, numerical solutions to the Boltzmann transport equation were used to predict the optimum carrier concentration where the maximum TE efficiency occurs for p-type HH compounds. The results for p-type samples showed that can not be treated within a simple parabolic band model approach, due to bipolar and multi-band effects.rnrnThe parabolic band model is commonly used for bulk TE materials. It is most accurate when the transport properties are dominated by one single carrier type. Since the transport properties of n-type HH are dominated by only one carrier type (high mobility electrons), it could be shown, that the use of a simple parabolic band model lead to a successful prediction of the optimized carrier concentration and thermoelectric efficiency in n-type HH compounds. rn

Synthesis and metal complexes of C2 symmetric ligands obtained from (R)-(+)-Betti and dialdehydes for asymmetric induction reactions

The aim of this master’s research thesis was the employment of an enantiopure 1,3-aminoalcohol, the 1-(α-aminobenzyl)-2-naphthol, known as Betti base, for the synthesis of some novel compounds which show a C2 symmetry. Some of these compounds, after derivatization, were used as ligands in association with transition metals to prepare some catalysts for enantioselective catalytic reactions. Some aminoalcohol (Salan-type) derivatives of these compounds were obtained upon reduction and in some cases it was possible to obtain complexes with transition metals such as Mn, Ni, Co and Cu. Furthermore a novel 6-membered analogue bisoxazoline ligand, 2,6-bis((R)-1-Phenyl-1H-naphtho[1,2-e][1,3]oxazin-3-yl)pyridine, was obtained and from it two Cu-complexes were prepared. The metal complexes were employed in some reactions to test the asymmetric induction, which was in some cases up to discrete values.

1,2-Bis(2-benzimidazolyl)-1,2-ethanediol, a chiral, tridentate, facially coordinating ligand

The ligand 1,2-bis(1H-benzimidazol-2-yl)-1,2-ethanediol, 1, and its methylated derivative 2 are readily synthesized from tartaric acid, and act as chiral, facially coordinating tridentate ligands, forming complexes of composition ML2 with octahedral transition metals. The copper(II) complexes show distorted 4 + 2 coordination with benzimidazoles occupying the equatorial sites and alcohol functions weakly binding in the axial sites. Nickel(II) complexes in three different states of protonation show regular octahedral geometry with the alcohols mutually cis. Deprotonation of the coordinated alcohol produces little structural change but the monodeprotonated complex forms a hydrogen bonded dimer. Magnetic measurements show the hydrogen bonded bridge to offer a pathway for weak antiferromagnetic coupling. UV-Visible spectroscopy shows the ligand to have a field intermediate between water and pyridine. The diastereoselectivity of complexation depends on the geometry: nickel(II) shows a weak preference for the homochiral complex, whereas copper(II) forms almost exclusively homochiral complexes.