901 research outputs found
Layered cesium copper titanate for photocatalytic hydrogen production
No full textLayered cesium copper titanate as well as the unmodified cesium titanate Cs0.68Ti1.83□0.17O4 (□: vacancy) were synthesized by a solution-based approach. The insertion of small amounts of copper into the vacancies of Cs0.68Ti1.83□0.17O4 led to a significant red shift of the band gap energy from 3.4 eV to 2.9 eV. During photocatalytic H2 production experiments, a steady increase in the evolution rate was detected, which can be referred to the in-situ reduction of incorporated copper ions to metallic Cu. The reduced copper ions leach out of the lattice to the catalyst surface and act as co-catalyst for H2 formation, considerably exceeding the activity achieved with Cs0.68Ti1.83□0.17O4 modified with 0.075 wt.-% of Rh as co-catalyst. The use of diffuse reflectance spectroscopy enabled a direct measurement of the copper nanoparticle formation by following their rising plasmon resonance at operating conditions. Characterization by X-ray diffraction (XRD) revealed a significant change in the crystal structure upon photocatalysis
Exercise, Nutrition, and Bone Health
No full textOptimal bone metabolism is the result of hormonal, nutritional, and mechanical harmony, and a deficit in one area is usually impossible to overcome by improvements in others. Exercise during growth influences bone modeling locally at the regions being loaded, whereas calcium is thought to act systemically to influence bone remodeling. Despite acting through different mechanisms, a growing body of research suggests that exercise and calcium may not operate independently. Low dietary calcium intake or reduced bioavailability may minimize the adaptive response to exercise-induced bone loading. Conversely, adequate levels of calcium intake can maximize the positive effect of physical activity on bone health during the growth period of children and adolescents. Research also suggests that adequate levels of calcium intake can maximize bone density at the regions being loaded during exercise. Achieving optimal bone health and minimizing one’s risk of osteoporotic fracture later in life depend on a lifelong approach. This approach relies on the establishment of an optimum level of bone during the growth years, with a subsequent goal to maintain and slow the rate of age-related bone loss thereafter. Exercise, adequate nutrition, and optimal hormone levels are the components that influence the bone outcome. Making healthy nutritional choices, engaging in weight-bearing physical activity, and ensuring optimal hormone levels during growth provides a window of opportunity to build optimal bone mass, to reduce the risk of fracture later in life. Concurrent management of fracture risk with a physical activity prescription, adequate nutrition, and pharmacotherapy for osteoporosis when required offers the best approach to optimal bone health throughout adulthood
Anwendungsbereiche von Brennstoffzellen
No full textVorlesung im im Modul Ressourcenschonung VAK 5.07.491
Erneuerbare Energien von Prof. Dr. Michael Wark am 15.12.2022 in Oldenbur
Michael Fernandes : Room of Fears, Fixing Room and 4 Short Performances
No full textCatalogue for Michael Fernandes' participatory installation, Room of Fears
Elektrochemische Reduktion von quarternären Phosphonium-Derivaten – ein systematischer Optimierungsansatz bis zum geschlossenen Wittig-Zyklus
No full textTriphenylphosphine is one of the crucial reagents in Wittig olefination reactions. Over the past decade, many scientists have focused their studies on the reduction of triphenylphosphine oxide (TPPO) towards triphenylphosphine (TPP). As part of the reaction TPP is sacrificed to yield the desired olefine (E/Z) as well as a triphenylphosphine oxide (TPPO) containing a strong oxygen-phosphorus (P=O) double bond. TPPO is considered thermodynamically stable but economically undesired. It is a waste product of the Wittig reaction which has been the most famous olefination reaction for decades due to its high E/Z selectivity. One of the most prominent products of the WOR is the industrial synthesis of Vitamin A (BASF, Rhône-Poulenc, DSM) whose annual production has tripled over the past decade (2020: 7500 t/a). Conventional chemical recycling methods to reduce P(V) to P(III) utilise sacrificial reduction agents (silanes, boranes, allanes). In recent years many studies focused on less toxic and sacrificial agent free alternative pathways. This study focuses on the optimization of an indirect electrochemical reduction of TPPO towards TPP in an undivided and divided cell setup in presence of scandium triflate at silver and tin working electrodes. The electro-reduction becomes possible after activation of TPPO with methyl triflate forming a quarternary phosphonium salt (TPPOMe). Under optimized conditions, the P=O bond of TPPO is weakened, which allows for reduction of TPP-OMe with current efficiencies of up to 50% and a chemical selectivity of 99% (towards TPP). Scandium triflate is not required if tin electrodes are used, hence a proposal was made for further studies in the field of electrocatalysis for the electrochemical reduction of phosphine oxide compounds (volcano curves). The optimization of the indirect electrochemical reduction of TPPO to TPP in an undivided and divided cell, as presented here, resulted in a reactor design (flow cell) and a process concept for a closed Wittig cycle on an industrial scale.Triphenylphosphin ist eines der wichtigsten Reagenzien in Wittig-Olefenierungsreaktionen (WOR). In den letzten zehn Jahren haben viele Wissenschaftler und Wissenschaftlerinnen sich mit der Reduktion von Triphenylphosphinoxid (TPPO) zu Triphenylphosphin (TPP) beschäftigt. Als Teil der Olefinsynthese wird TPP geopfert, um das gewünschte Alken (E/Z) zu bilden. Dabei wird jedoch in stöchiometrischen Mengen TPPO gebildet, welches aufgrund der starken P=O-Doppelbindung als thermodynamisch stabil, aber wirtschaftlich unattraktiv gilt. Die Wittig-Reaktion ist aufgrund der hohen E/ZSelektivität seit Jahrzehnten eine der am meisten verwendeten Olefinsynthesereaktionen im industriellen Maßstab. Eines der bekanntesten Produkte der WOR ist die industrielle Synthese von Vitamin A (BASF, Rhône-Poulenc, DSM), dessen jährliche Produktion sich in den letzten zehn Jahren verdreifacht hat (2020: 7500 t/a). Konventionelle chemische Recyclingmethoden zur Reduktion von P(V) zu P(III) verwenden meist überstöchiometrische Mengen an Opferreduktionsmittel (Silane, Borane, Allane). In den letzten Jahren haben sich jedoch vermehrt Studien auf weniger toxische und opfermittelfreie alternative Wege konzentriert. Der Themenschwerpunkt dieser Dissertationsarbeit liegt in der Optimierung einer indirekten elektrochemischen Reduktion von TPPO zu TPP in einer ungeteilten und geteilten Zelle in Gegenwart von Scandiumtriflat an Silber- und Zinnarbeitsektroden. Die elektrochemische Reduktion wird nach Aktivierung von TPPO mit Methyltriflat unter Bildung eines quaternären Phosphoniumsalzes (TPP-OMe) erst ermöglicht. Unter optimierten Bedingungen wird die P=O-Doppelbindung von TPPO soweit geschwächt, dass bei einer elektrochemischen Reduktion des Phosphoniumsalzes eine Stromausbeute von bis zu 50% und eine chemische Selektivität von 99% (gegenüber TPP) erreicht werden konnte. Die Anwesenheit von Scandiumtriflat ist bei Verwendung von Zinnelektroden nicht notwendig, weshalb ein Vorschlag für weitere Studien im Bereich der Elektrokatalyse für die elektrochemische Reduktion von Phosphinoxidverbindungen gemacht wurde (Vulkan Kurven). Die hier vorgestellte Optimierung der indirekten elektrochemischen Reduktion in einer ungeteilten und geteilten Zelle von TPPO zu TPP, resultierte in einem Reaktordesign (Durchflusszelle) sowie Verfahrenskonzept für einen geschlossenen Wittig Zyklus im technisch größeren Maßstab.DFG, 390540038, EXC 2008: Unifying Systems in Catalysis "UniSysCat
Hack the System!
No full textA review of McKenzie Wark\u27s A Hacker Manifesto (Harvard University Press, Cambridge, 2005)
Rational Design of Proton Conducting Coordination Polymers
No full textIn der hier vorliegenden Arbeit wurden Koordinationspolymere (CPs) hinsichtlich ihrer imidazol- beziehungsweise wasservermittelten Protonenleitung systematisch untersucht.Within the scope of this thesis, the proton conductivity of coordination polymers (CPs) was systematically investigated towards the imidazole-based conductivity as well as the influence of additional acidic protons in a water-driven proton conductivity
Synthesis of photosensitizers and electrodeposition of sensitized nanostructured zinc oxide electrodes
No full textThe major topic of this thesis was the preparation of a photosensitization cell, like the Grätzel-cell. Instead of titaniumdioxid also ZnO can be used for this kind of cells. The great advantage of ZnO is that it can be electrodeposited from aqueous zinc salt solutions at 70 °C. The sensitizer can be added directly to the deposition solution and will be incorporated to the growing ZnO One-step method.One task in this Thesis was the parallel incorporation of different dyes at the same time, so that addition of the absorption regions of different dyes in one film could be achieved. The following panchromatic effect should lead to a better efficiency of the electrode.On the other hand the morphology of the growing ZnO films should be influenced by the addition of detergents to the deposition mixture. Detergents are well known as structure directing agents, for example in the synthesis of porous molecular sieves. Therefore their use in electrodeposition of porous ZnO electrodes seems to be promising, although this is a total new approach in the electrodeposition of ZnO.Sodium lauryl sulphate (SDS) turned out to be a good candidate for the pre-structuring of the electrode material. After desorption of SDS, dyes should be re-adsorbed to the semi conducting material in a sufficient high concentration. Different methods for desorption of the detergent have been investigated and afterwards different dyes and their mixtures have been re-adsorbed. The final goal was to achieve the highest possible load of monomeric dye molecules to a highly porous ZnO surface and therefore an efficient absorption of light in the visible ranch and a good contact with an electrolyte in a photovoltaic cell
Protonentransport in Additiven der Polymerelektrolytemembran zur Verwendung in Brennstoffzellen
No full textThe enhancement of proton transport in polymer electrolyte membranes is an important issue for the development of fuel cell technology. The objective is a material providing proton transport at a temperature range of 350 K to 450 K independent from a purely water based mechanism. To enhance the PEM properties of standard polymer materials, a class of additives is studied by means of atomistic simulations consisting of functionalised mesoporous silicon dioxide particles. The functional molecules are imidazole or sulphonic acid, covalently bound to the surface via a carbon chain with a surface density of about 1.0 nm−2 groups. At first, the proton transport mechanism is explored in a system of functional molecules in vacuum. The molecules are constrained by the terminal carbon groups according to the geometric arrangement in the porous silicon dioxide. The proton transport mechanism is characterised by structural properties obtained from classical molecular dynamics simulations and consists of the aggregation of two or more functional groups, a barrier free proton transport between these groups followed by the separation of the groups and formation of new aggregates due to fluctuations in the hydrogen bond network and movement of the carbon chain. For the different proton conducting groups, i.e. methyl imidazole, methyl sulphonic acid and water, the barrier free proton transport and the formation of protonated bimolecular complexes were addressed by potential energy calculations of the density functional based tight binding method (DFTB). For sulphonic acid even at a temperature of 450 K, relatively stable aggregates are formed, while most imidazole groups are isolated and the hydrogen bond fluctuations are high. However, high density of groups and elevated temperatures enhance the proton transport in both systems. Besides the anchorage and the density of the groups, the influence of the chemical environment on the proton transport was studied. Therefore, the uptake and distribution of water molecules was estimated from classical molecular dynamic simulations and the local chemical environment was determined for different functional groups. The sulphonic acid functionalised silicon dioxide pores are more hydrophilic than the unfunctionalised and the imidazole functionalised systems. At lower hydration, the distribution of water is inhomogeneous and the surface of the pore is covered by a water layer for all systems. In addition to the interaction with water, an interaction of functional groups with the surface is observed which is shielded under hydration. Due to these interactions, the number of isolated groups and their stability is increased under the influence of the environment that reduces the proton transport mechanism which has been described before. Apart from the proton transport mechanism known from the vacuum system, two additional mechanisms occur under the chemical environment. These mechanism directly involve water molecules. One possibility is the complete deprotonation of the functional group, followed by water based proton transport as expected for acidic system, e.g. sulphonic acid. Another possibility is a water based proton transport over short distances from one proton conducting group to another. The three competing mechanisms are studied by free energy calculations and their occurance is evaluated according to the local environment conditions. The proton transport mechanisms involving water are more favourable in sulphonic acid functionalised particles, while the dominating mechanism is comparable to the mechanism in vacuum for imidazole system
Einfluss von Cellulose-graft-Copolymeren auf die Faser/Matrixanbindung in naturfaserverstärkten Duromeren
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