Mineral dust aerosols emitted from the Earth crust during various natural and anthropogenic processes continuously alter the chemical balance of the atmosphere via heterogeneous processes and thus, impact on the global climate. Understanding of heterogeneous chemistry and photochemistry on mineral dust has become vital to accurately predict the effect of mineral dust loading on the Earth's atmosphere. Here, laboratory measurements are coupled with model studies to understand heterogeneous chemistry and photochemistry in the atmosphere with the specific focus on reactions on mineral oxide surfaces. Heterogeneous uptake of gas phase HNO3 on well characterized metal oxides, oxyhydroxides and carbonates emphasized binding of nitric acid to these surfaces in different modes including monodentate, bidentate and bridging under dry conditions. It is becoming increasingly clear that the heterogeneous chemistry, including uptake of HNO3, is a function of relative humidity (RH) as water on the surface of these particles can enhance or inhibit its reactivity depending on the reaction. All the studied model systems showed a significant uptake of water with the highest uptake by CaCO3. Quantitative analysis of water uptake indicated formation of multilayers of water over these reactive surfaces. Under humid conditions, two water solvated nitrate coordination modes were observed that is inner-sphere and outer-sphere, which differ by nitrate proximity to the surface. Photochemical conversion of nitric acid to gas phase nitrous oxide, nitric oxide and nitrogen dioxide through an adsorbed nitrate intermediate under different atmospherically relevant conditions is shown using transmittance FTIR and XPS analysis. The relative ratio and product yields of these gas phase products change with relative humidity. Photochemistry of adsorbed nitrate on mineral aerosol dust may be influenced by the presence of other distinct gases in the atmosphere making it complicated to understand. This thesis converses formation of active nitrogen, NOx and N2O, and chlorine, ClOx, species in the presence of co-adsorbed trace gases, that could potentially regulate the peak concentration and geographical distribution of atmospheric ozone. Here we report formation of atmospheric N2O, from the photodecomposition of adsorbed nitrate in the presence of co-adsorbed NH3 via an abiotic mechanism that is favorable in the presence of light, relative humidity and a surface. Estimated annual production of N2O over the continental United States is 9.3+0.7/-5.3 Gg N2O, ~5% of total U.S. anthropogenic N2O emissions. Not only NH2 but also gaseous HCl react with adsorbed nitrate to activate "inert" N and Cl reservoir species, yielding NOCl, NOx, Cl and Cl2, through adsorbed nitrate under different atmospherically relevant conditions. Mineral dust aerosol is a major source of bioavailable iron to the ocean with an annual deposition of ~ 450 Tg of dust into the open ocean waters. In this study, we report enhanced Fe dissolution from nano scale Fe-containing minerals, i.e.alpha-FeOOH, beyond the surface area effects that can be attributed to the presence of more reactive sites on specific crystal planes exposed. We further report with clear evidence that aggregation impacts on dissolution. Proton-promoted dissolution of nanorods is nearly or completely quenched in the aggregated state. Acid type, presence of oxyanions and light are several other key factors responsible for regulating for iron dissolution. The work reported in this thesis provides insight into the heterogeneous chemistry and photochemistry of mineral dust aerosol under different atmospherically relevant conditions.

<p>Thorium complexes primarily exist in the thermodynamically stable (IV) oxidation state with only a few low-valent thorium(III) and thorium(II) complexes having been isolated. As a result, redox chemistry with thorium at the metal center is synthetically challenging without carefully selected ligand systems. This redox-restricted nature of thorium(IV) makes redox-active ligands (RALs) an attractive option to facilitate multi-electron redox chemistry with thorium. In this work, first, a series of thorium(IV) complexes featuring the redox-active iminoquinone ligand and its derivatives, including the iminosemiquinone and amidophenolate species, were synthesized and characterized. Rare thorium oxygen multiple bonds were then accessed by exploiting the RALs on the thorium center and using dioxygen in dry air. Other oxidation chemistry was attempted with the thorium amidophenolate complexes as well. Second, armed with the knowledge of synthesizing multiple bonds with thorium(IV) complexes, similar chemistry was explored with cerium as it is in the same group as thorium. A series of cerium(III) and cerium(IV) complexes featuring the redox-active iminoquinone ligand and its derivatives were synthesized. Oxidation chemistry was explored with the cerium amidophenolate complexes and a rare cerium oxo was isolated. Finally, with interest in expanding and addressing a gap in the literature related to the synthesis, characterization, and utility of thorium alkyls, several tetrabenzylthorium complexes were synthesized, characterized, and some reactivity was explored. A highlight of this work involved the isolation of the first crystal structure of ligand and solvent free tetrabenzylthorium since its first synthesis in 1974. Full spectroscopic and structural characterization of the complexes was performed via ¹H NMR spectroscopy, X-ray crystallography, EPR spectroscopy, electronic absorption spectroscopy, and SQUID magnetometry, which all confirmed the identity and electronic structure of these complexes. </p>

Manufactured CeO2 nanomaterials (CeO2-MNMs) are used for a wide variety of applications including diesel fuel additives and chemical/mechanical planarization media. To test the effects of CeO2-MNM surface coating charge on to model organism Caenorhabditis elegans, we synthesized 4 nm CeO2 with cationic (DEAE-), anionic (CM-), and neutral (DEX) coatings. In L3 nematodes exposed for 24 hours, DEAE-CeO2 induced lethality at lower concentrations than CM- or DEX-CeO2. Feeding slightly decreased CeO2 toxicity, regardless of coating. In L2 nematodes exposed for 48 hours with feeding, DEAE-CeO2 caused lethality at the lower concentrations as compared to CM- and DEX-CeO2. Sublethal effects were measured by observing reproduction and oxidative/nitrosative protein damage. Low concentrations of DEAE-CeO2 induced similar reductions as CM- and DEX-CeO2 that were two orders of magnitude higher. Using immunochemical slot blots to explore oxidative/nitrosative stress, no treatments produced significant changes in protein carbonyl or 3-nitrotyrosine formation; however, the statistical power of our assay was low. All treatments caused large but not statistically significant increases in protein carbonyl levels. DEAE-CeO2 exposure caused a significant reduction in 4-hydroxy-2-nonenol levels. This research suggests that cationic coatings render CeO2 significantly more toxic to C. elegans than neutral or anionic coatings.

<p>Cu-zeolites have received renewed attention as catalytic materials that facilitate partial methane oxidation (PMO) to methanol, with a variety of mononuclear, binuclear, and multinuclear Cu active site motifs that have been proposed in prior literature. Our approach to more precisely identify and probe the Cu structures that activate O₂ and reduce in CH₄relies on the synthesis of model supports with varying composition and well-defined Cu speciation, which also facilitates connections between experimental data and theoretical models. Chabazite (CHA) zeolites are high-symmetry frameworks that contain a single lattice tetrahedral site (T-site), in which Cu²⁺ ions exchange at paired Al sites in a six-membered ring (6-MR) while CuOH⁺ species exchange at isolated 6-MR Al sites, the latter of which can react to form binuclear O/O₂-bridged Cu structures. In this work, Cu-CHA zeolites were synthesized to contain predominantly Cu²⁺ (Z₂Cu) or CuOH⁺ (ZCuOH) species of varying density, or a mixture of Z₂Cu and ZCuOH sites. Z₂Cu and ZCuOH sites were quantified by titration of residual Brønsted acid sites with NH₃, which respectively exchange with 2:1 or 1:1 H⁺:Cu²⁺ stoichiometry. Stoichiometric PMO reaction cycles on Cu-zeolites involved high-temperature (723 K) activation in O₂, and then moderate-temperature (473 K) reduction in CH₄ and treatment in H₂O (473 K) to extract CH₃OH. <i>I</i><i>n-situ</i> UV-Visible spectroscopy under oxidizing (O₂, 723 K) and reducing (CO, 523 K; CH₄, 473 K; He, 723 K) conditions detected the presence of mononuclear and binuclear Cu site types, while <i>in-situ</i> Cu K-edge X-ray absorption spectroscopy after such treatments was used to quantify Cu(I) and Cu(II) contents and <i>in situ</i> Raman spectroscopy was used to identify the Cu structures formed. ZCuOH, but not Z₂Cu sites, are precursors to binuclear O/O₂-bridged Cu sites that form upon O₂ activation and subsequently produce methanol after stoichiometric PMO cycles, at yields (per total Cu) that increased systematically with ZCuOH site density. The fraction of Cu(II) sites that undergo auto-reduction in inert at high temperatures (He, 723 K) is identical, within experimental error, to the fraction that reduces in CH₄ at temperatures relevant for PMO (473 K), providing a quantitative link between the binuclear Cu site motifs involved in both reaction pathways and motivating refinement of currently postulated PMO reaction mechanisms. These Cu-CHA zeolites were also studied for other redox chemistries including the selective catalytic reduction (SCR) of NO_x with NH₃. <i>In situ </i>UV-Visible and X-ray absorption spectroscopies were used to monitor and quantify the transient partial reduction of Cu(II) to Cu(I) during exposure to NH₃ (473 K), in concert with titration methods that use NO and NH₃ co-reductants to fully reduce all Cu(II) ions that remain after treatment in NH₃ alone to the Cu(I) state, providing quantitative evidence that both Z₂Cu and ZCuOH sites are able to reduce in NH₃ alone to similar extents as a function of time. These findings provide new insight into the reaction pathways and mechanisms in which NH₃ behaves as a reductant of mononuclear Cu(II) sites in zeolites, which are undesired side-reactions that occur during steady-state NO_x SCR and that often unintendedly result in Cu(II) reduction prior to spectroscopic or titrimetric characterization. Overall, the strategy in this dissertation employs synthetic methods to control framework Al density and arrangement in zeolite supports to emphasize extra-framework Cu site motifs of different structure and at different spatial densities, and to interrogate these model materials using a combination of <i>in situ</i> spectroscopic techniques together with theory, in order to elucidate active site structure and proximity requirements in redox catalysis. This work demonstrates how quantitative reactivity and site titration data, brought together with an arsenal of tools available in contemporary catalysis research, can provide detailed mechanistic insights into transition metal-catalyzed redox cycles on heterogeneous catalysts.</p>

Neptunium, with its rich redox chemistry, has a special position in the chemistry of actinides. With a decades-long history of development of aqueous separation methods for used nuclear fuel (UNF), management of neptunium remains an unresolved issue because of its not clearly defined redox speciation. Neptunium is present in two, pentavalent (V) and hexavalent (VI) oxidation states, both in their dioxocation O=Np=O neptunyl form, which differ greatly in their solvent extraction behavior. While the neptunium(VI) dioxocation is being very well extracted, the dioxocation of pentavalent neptunium is practically non-extractable by an organic solvent. As a result, neptunium is not well separated and remains distributed in both organic and aqueous extraction phases. The aim of this study was to develop or enhance the understanding of several key topics governing the redox behavior of neptunium in nitric acid medium, which are of vital importance for the engineering design of industrial-scale liquid-liquid separation systems. In this work, reactions of neptunium(V) and (VI) with vanadium(V) and acetohydroxamic acid - two redox agents envisioned for adjusting the neptunium oxidation state in aqueous separations ��� were studied in order to determine their kinetic characteristics, rate laws and rate constants, as a function of temperature and nitric acid concentration. Further were analyzed the interactions of neptunium(V) and (VI) with nitrous acid, which is formed as a product of radiolytic degradation of nitric acid caused by high levels of radioactivity present in such systems. Once HNO��� is distributed between both the aqueous solutions and organic solvent, nitrous acid is also formed in both phases and has a key influence on redox speciation of neptunium; therefore, the effects of gamma-radiation on the redox speciation of neptunium were investigated. The work also includes the results of examination of scavenging of nitrous acid by hydrogen peroxide, which is generated along with nitrous acid during radiolysis of aqueous solutions of nitric acid, and also by chemical reactions with added scavenging agents (methylurea, acetohydroxamic acid). / Graduation date: 2013

The hydroflux method is a promising new synthesis approach for explorative crystal growth. Various new compounds were synthesized during the preparation of this PhD thesis, doubling the number of substances discovered to date via the hydroflux approach. The product range consists primarily of oxides, hydroxides or a mixture of both, with oxygen-free compounds being obtained for the first time in form of various chalcogenides. The so far barely explored redox chemistry of the hydroflux was elucidated in more detail and novel preparation procedures were developed to intentionally introduce reductive and oxidative conditions. Thus, chalcogenides and highly oxidized cations were obtained. In addition, important reaction parameters of the hydroflux method were derived based on the developed syntheses procedures and properties of the various new compounds. The two largest compound classes within the product range are the hydrogarnets and the oxo(hydroxo)ferrates. Among the various interesting properties of the latter compounds, the potassium ion conductivity stands out, which is closely related to their structure and chemical stability. The structure of the oxohydroxoferrates K2–x(Fe,M)4O7–y(OH)y (M = Fe, Si, Ge, Ti, Mn, Ir) can be described as a parking garage. Honeycomb layers consisting of edge-sharing [FeO6] octahedra form the floors, which are connected by pairs of vertices-sharing [FeO4] tetrahedra representing the pillars. In the pictorial representation of this two-dimensional ion conductor, the potassium ions represent the cars that are mobile within one floor because not all parking lots are occupied, i.e., the structure has a potassium deficit. The substituted elements M influence the potassium content and thus the ion conductivity, which tends to increase with higher potassium deficits. The oxohydroxoferrates hydrolyze slowly in moist air under segregation of potassium hydroxide, which significantly increases the mobility of the potassium ions due to the hygroscopic nature and thus the ion conductivity. The three-dimensional ion conductor K12+6xFe6Te4–xO27 consists of a cubic labyrinth of potassium channels, which are surrounded by an open framework of [FeO5] pyramids and [TeO6] octahedra. Every potassium position is connected with eight large cavities acting as nodes for the potassium channels. However, the potassium positions within the channels are fully occupied, which hinders mobility within the labyrinth to the disadvantage of the ion conductivity. Similar to K2–x(Fe,M)4O7–y(OH)y, K12+6xFe6Te4–xO27 hydrolyzes under ambient conditions decreasing the potassium content within the structure. However, only a slight amount of potassium can be removed before the open framework collapses. Hydrogarnets crystallize in the flexible garnet structure-type and adapt the general formula AE3[M(OH)6]2. The crystal structure consists of a complex three-dimensional framework, in which [MO6] octahedra and empty (O4H4)4– tetrahedra are connected via their vertices and the larger alkaline earth metal cations AE filling the remaining voids. In contrast to garnets (nesosilicates), the hydrogarnets have a lower thermal stability and hardness. For many applications, this instability might be a drawback, but at the same time, it qualifies them for a low temperature and resource efficient application as carbon-free single-source precursors. In case of the rare earth hydrogarnets (AE = Sr, Ba; M = Sc, Y, Ho–Lu), the dehydration at about 550 °C leads to the formation of AEM2O4, which were previously obtainable only at reaction temperatures above 1300 °C.[86–89] Redox chemistry in hydroflux systems had been barely investigated so far, with neither equations nor possible mechanisms discussed to explain redox phenomena. In more than half of the published articles of this thesis, redox reactions were observed, often involving molecular oxygen or nitrate as oxidant. Similar to alkali metal hydroxide melts, molecular oxygen is expected to react with hydroxide ions to form peroxides or even superoxides, while nitrates might be reduced to nitrites. Moreover, higher oxidations states seem to be preferred in the hydroflux medium, as, for example, tellurium(IV), chromium(III) and arsenic(III) were readily oxidized to their maximum oxidation states. Additionally, the partial replacement of KOH by KO2 in the hydroflux medium introduced a high oxygen partial pressure, resulting in the oxidation of iodide(–I) ions to orthoperiodate(VII) ions. This preparation procedure has a great potential to yield compounds with elements in unusual high oxidation states, especially transition metals. The tendency of some elements to prefer higher oxidation states than usual was utilized to intentionally introduce reductive conditions. With this approach, reduction of selenium(IV) and tellurium(IV) oxides to their chalcogenides was achieved by using arsenic(III) oxide as reducing agent. In solution, monochalcogenide and dichalcogenide anions as well as the new (SeTe)2– anions were obtained. In addition, millimeter-sized crystals of the chalcogenides K2Se3 and K2Te3 and the previously unknown K2Se2Te were crystallized. This unexpected redox chemistry is far from what the standard potentials would suggest. The activity of water is considerably reduced by the ultra-alkaline conditions, which does not only decrease its vapor pressure and drives the reaction but obviously prevents the hydrolysis of the water sensitive chalcogenides. Overall, a preparatively simple, time-saving and secure approach compared to traditional methods like the synthesis in liquid ammonia was developed. Moreover, this method allows known and new potassium trichalcogenides to be obtained in larger amounts and in form of millimeter-sized single-crystals. A transfer of the approach to other systems should be promising. Reaction parameters described in literature were mostly confirmed and some details were added. For example, the selection of mineralizers was extended, reaction times and temperatures were specified, and a method for purifying the reaction products was added. With the exception of base concentration and concentration-dependent product formation, both of which have barely been studied so far. An example is the iron(III)-KOH hydroflux system, where four different products are accessible with increasing base-concentrations: α-Fe2O3, K2–xFe4O7–x(OH)x, K2Fe2O3(OH)2 and KFeO2. Overall, two trends are evident with increasing base concentration. First, the alkali metal content within the product rises or the alkali metal is incorporated in the structure in the first place. Second, the hydrogen content of the products constantly decreases. The latter is attributed to the increasing hygroscopicity of the reaction medium at higher hydroxide concentrations, which also reduces the activity of water in the hydroflux medium, so that water-sensitive compounds are stabilized.

Oxygenierungs- und Oxidationsreaktionen sind in unserem Alltag allgegenwärtig und von großer Bedeutung. Sie finden Anwendung von der Natur bis hin zur Industrie. Der Einsatz von O2 als Sauerstoffquelle bzw. Oxidationsmittel ist besonders erstrebenswert. Die Erforschung der Aktivierung von O2 an Übergangsmetallkomplexen und Untersuchung der Reaktivität der resultierenden Verbindung ist von großer Bedeutung für das Verständnis dieser Reaktionen und Mechanismen. In dieser Arbeit wurde zunächst der Komplex trans-[Ir(4-C5F4N)(CNtBu)(PiPr3)2] synthetisiert, welcher mit dem 4-C5F4N- und dem CNtBu-Liganden stabilisierende Komponenten und wertvolle analytische Sonden enthält. Ausgehend von dieser Iridium(I)-Verbindung konnte auf verschiedenen Wegen der Peroxidokomplex trans-[Ir(4-C5F4N)(O2)(CNtBu)(PiPr3)2] erhalten und umfangreich charakterisiert werden. In Gegenwart von [Fe(C5H5)2][PF6] konnten Hinweise auf einen redoxkatalysierten Mechanismus gewonnen werden. Im nächsten Abschnitt konnte gezeigt werden, dass sich der Peroxidokomplex durch Bronstedsäuren aktivieren lässt. So wurde z.B. unter der Verwendung von Säuren wie HCl, CF3COOH oder HF die Bildung von H2O2 erzielt. Dabei entstehen die entsprechenden Iridium(III)-Komplexe mit den koordinierten Säureanionen. In Gegenwart von HCOOH werden ein Carbonatokomplex und H2 als Hauptprodukte gebildet und es konnten Hinweise zum Mechanismus dieser komplexen Reaktion gewonnen werden. Des Weiteren sind Lewissäuren und Elektrophile in der Lage, die metallgebundene Disauerstoffeinheit des Peroxidokomplexes zu aktivieren. Dazu wurden unter Anderem Silane und Borane eingesetzt. Im Fall von ClSiMe3 und BClCy2 konnten während der Reaktion Intermediate detektiert und analysiert werden. Tragen die Lewissäuren Chloratome wird die entsprechende Dichloridoiridium(III)-Verbindung gebildet. Durch Einsatz von BPh3 konnte eine veränderte Reaktivität erreicht werden und der Ausgangskomplex trans-[Ir(4-C5F4N)(CNtBu)(PiPr3)2] zurückerhalten werden. / Oxygenation- and oxidation reactions are ubiquitous and of great importance to our daily life. They find application from nature to industry. The use of O2 as an oxygen source or oxidation reagent, respectively, is particularly desirable. The research on the activation of O2 at transition metal complexes and the investigations of the reactivity of the resulting compounds is of great significance for the understanding of these reactions and mechanisms. Herein, this work describes the synthesis of the complex trans-[Ir(4-C5F4N)(CNtBu)(PiPr3)2], which incorporates stabilizing and valuable analytical elements provided by the 4-C5F4N and CNtBu ligands. Starting from this iridium(I) compound, the peroxido complex trans-[Ir(4-C5F4N)(O2)(CNtBu)(PiPr3)2] could be synthesized using different methods. In the presence of [Fe(C5H5)2][PF6], indications for a redox catalyzed mechanism could be provided. The next chapter shows that Bronsted acids are capable of activating the peroxido complex. With the help of acids like HCl, CF3COOH or HF, for example, the formation of H2O2 was achieved. Thereby, the corresponding iridium(III) complexes with the coordinating anions are formed. In the presence of HCOOH, a carbonato complex and H2 are formed as main products and ideas for the mechanism of this complex reaction were indicated. Furthermore, Lewis acids and electrophiles have the ability to activate the metal bound dioxygen moiety of the peroxido complex. Silanes and boranes were used for this purpose amongst others. In case of ClSiMe3 and BClCy2, intermediates of the reactions could be detected and analysed. If the Lewis acids carry chloride atoms, the corresponding dichlorido iridium(III) compounds were formed. BPh3 enabled a different reactivity and allowed the isolation of the starting material trans-[Ir(4C5F4N)(CNtBu)(PiPr3)2].

Lithium-ion batteries have transformed energy storage and technological applications. They stand poised to convert transportation from combustion to electric engines. The discharge/charge rate is a key parameter that determines battery power output and recharge time; typically, operation is on the timescale of hours but reducing this would improve existing applications and open up new possibilities. Conventionally, the rate at which a battery can operate has been improved by synthetic strategies to decrease the solid-state diffusion length of lithium ions by decreasing particle sizes down to the nanoscale. In this work, a different approach is taken toward next-generation high-power and fast charging lithium-ion battery electrode materials. The phenomenon of high-rate charge storage without nanostructuring is discovered in niobium oxide and the mechanism is explained in the context of the structure–property relationships of Nb2O5. Three polymorphs, T-Nb2O5, B-Nb2O5, and H-Nb2O5, take bronze-like, rutile-like, and crystallographic shear structures, respectively. The bronze and crystallographic shear compounds, with unique electrochemical properties, can be described as ordered, anion-deficient nonstoichiometric defect structures derived from ReO3. The lessons learned in niobia serve as a platform to identify other compounds with related structural motifs that apparently facilitate high-rate lithium insertion and extraction. This leads to the synthesis, characterisation, and electrochemical evaluation of the even more complicated composition–structure–property relationships in ternary TiO2–Nb2O5 and Nb2O5–WO3 phases. Advanced structural characterisation including multinuclear solid-state nuclear magnetic resonance spectroscopy, density functional theory, X-ray absorption spectroscopy, operando high-rate X-ray diffraction, and neutron diffraction is conducted throughout to understand the evolution of local and long-range atomic structure and changes in electronic states.