Electrocatalytic materials and techniques for the anodic oxidation of various organic compounds

The research presented within this thesis is devoted to the development of electrocatalytic materials and methods employed to catalyze oxygen-transfer reactions in aqueous media. The use of rotated disk electrodes for the generation of Koutecky-Levich plots was evaluated for several mechanisms to identify the efficacy of estimating the number of electrons (n ) and the heterogeneous rate constant (kh) from such plots. Two mathematical models for oxygen-transfer reactions were derived for no adsorption and weak adsorption of a model reactant to the rotated disk electrode. Theoretical equations were compared to experimental data and show good agreement. Lead dioxide film anodes were doped with Fe(III) and Bi(V) and compared to the activity of undoped electrodes for the voltammetric response of toluene and m-xylene. Very slightly doped Fe(III)-PbO 2 films showed equal or greater heterogeneous rate constants than very highly doped Bi(V)-PbO2. A single-pass electrochemical cell was developed for the electrochemical incineration of dilute chromatographic eluent solutions. Manganese sulfate co-dissolved with the sample compounds was found to deposit onto the Pt substrate and catalyze the anodic oxidation of many compounds used for chromatographic separations. Ultraviolet light was used as a photocatalyst in the oxidation of selected organic compounds at the anodes Fe(III)-doped PbO2 and Fe(III)-doped MnO2. The ultraviolet light was found to catalyze the oxidation of both UV absorbing and non-UV absorbing molecules, indicating that the UV light activates the semi-conducting metal oxide film rather than the molecule present within the diffusion layer of the electrode. Finally, initial investigations using a Fe(III)/Mn(IV)-doped PbO2 film indicate that this film selectively oxidizes phenol and aniline to acetic acid in the presence of ultraviolet light

(S)norlaudanosoline synthase. the first enzyme in the benzylisoquinoline biosynthetic pathway

1. introduction Isoquinoline alkaloids form the largest group of alkaloids in the plant kingdom. Numerous publications deal with aspects of the biosynthesis of these compounds in vivo [ 11, while isoquinoline biosynthesis at the cell-free level had hardly been touched [Z]. The initial reaction in isoquinoline biosynthesis has long [3] been assumed to be a condensation of two aromatic units, both derived from tyrosine, namely dopamine and 3,4-d~ydroxyphenylacet~dehyde [4]. This scheme was later modified [5] where it was reported that condensation of dopamine with 3,4-dihydroxyphenylpyruvate would lead to an amino acid, norlaudanosoline-l-carboxylic acid, which in turn, by decarboxylation, would yield norlaudanosoline

Hochauflösende phasenbasierte Röntgenstreuung eine mögliche Methode und ihre Eignung zur Früherkennung von Krebszellen aufgrund gewebespezifischer Veränderungen

This thesis describes the potential of phase based X ray scattering in order to detect cancer cells in an early stage of development. A faster growing cell nucleus and enlargement of its density are first indications of cancer development. At that stage a cancer cell hardly differs from an healthy one, and can be satisfactorily distinguished just by light microscopy, if at all. However, in this case only a small fraction of a biopsy can be investigated due to the reduced field of view of a microscope. In the case of scattering methods the spatial resolution is worse but the detection of small changes of physical parameters size, density is much more sensitive. With the proposed phased based X ray scattering it is demonstrated that in principle cancer cells in their early stage of development can be detected in apparently healthy biopsies. Scattering curves from a biopsy cell system are calculated by means of Radon Transform of the cell system cells and their randomly placed nuclei, both randomly orientated with respect to the X ray beam yielding the particular phase shift due to the cells and their nuclei. Subsequent Fraunhofer diffraction takes into account cells healthy and cancer ones that can differ in size and density of their nuclei. Graphs and numbers for increased nucleus size and density are given to estimate the fraction of cancer cells in a biopsy. This work proves the feasibility of phase based X ray scattering, and that in principle cancer cells be detected in apparently healthy biopsies and that it can assist as additional tool in cancer detection for established methods in diagnostics of cancer suspicious samples

Maximum-likelihood absorption tomography

Maximum-likelihood methods are applied to the problem of absorption tomography. The reconstruction is done with the help of an iterative algorithm. We show how the statistics of the illuminating beam can be incorporated into the reconstruction. The proposed reconstruction method can be considered as a useful alternative in the extreme cases where the standard ill-posed direct-inversion methods fail.Comment: 7 pages, 5 figure

Study of pyridine-mediated electrochemical reduction of CO2 to methanol at high CO2 pressure

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The recently proposed highly efficient route of pyridine-catalyzed CO 2 reduction to methanol was explored on platinum electrodes at high CO 2 pressure. At 55 bar (5.5 MPa) of CO 2 , the bulk electrolysis in both potentiostatic and galvanostatic regimes resulted in methanol production with Faradaic yields of up to 10 % for the first 5–10 C cm −2 of charge passed. For longer electrolysis, the methanol concentration failed to increase proportionally and was limited to sub-ppm levels irrespective of biasing conditions and pyridine concentration. This limitation cannot be removed by electrode reactivation and/or pre-electrolysis and appears to be an inherent feature of the reduction process. In agreement with bulk electrolysis findings, the CV analysis supported by simulation indicated that hydrogen evolution is still the dominant electrode reaction in pyridine-containing electrolyte solution, even with an excess CO 2 concentration in the solution. No prominent contribution from either a direct or coupled CO 2 reduction was found. The results obtained suggest that the reduction of CO 2 to methanol is a transient process that is largely decoupled from the electrode charge transfer

Thermodynamics of Meissner effect and flux pinning behavior in the bulk of single crystal La2 xSrxCuO4 x 0.09

We have studied the evolution of magnetic flux pinning behavior in the Meissner phase and the mixed state for the high- T c single-crystal La 2 − x Sr x CuO 4 ( x = 0.09 ) superconductor using the polarized neutron imaging method with varying magnetic field and temperature. In the Meissner state expulsion of magnetic field (switched on during the measurements) is visualized, and the signatures of a mixed state with increasing temperature are observed. However, for flux pinning behavior in the range 5 K ≤ T ≤ 15 K and H ext = 63.5 mT (switched off during the measurements), the evolution of the fringe pattern indicates magnetic flux pinning inside the bulk of the sample. At 25 K ≤ T ≤ 32 K , a continuous decrease in inhomogeneously distributed pinned magnetic flux is observed, with the sample reaching a normal conducting state at T c ( ≈ 32 K). The flux pinning behavior is also explored as a function of H ext at T = 5 K . As expected, with increasing H ext an increase in fringe density is observed, indicating an increase in magnetic flux pinning in the bulk of the sample. A comparison between calculated and experimentally visualized pinned magnetic fluxes shows good agreement. This implies quantification of pinned magnetic flux inside the sample, which is not possible with any other technique for bulk samples

Electrosynthesis of Hydrogen Peroxide by Phase Transfer Catalysis

The portable electrochemical generation of hydrogen peroxide from air and water would enable greater utilization of this versatile green oxidant in applications ranging from environmental remediation to portable sanitation. Currently, electrochemical H2O2 synthesis is hampered by the lack of low-cost, non-toxic catalysts that selectively reduce O2 to H2O2 and the lack of low-energy methods for separating the produced H2O2 from the electrolyte media. Herein, we show that a disulfonated anthraquinone can simultaneously catalyze the selective conversion of O2 to H2O2 and shuttle between immiscible aqueous and organic phases via ion exchange. We exploit both of these properties in a flow system to assemble an all Earth-abundant prototype device for the continuous generation and separation of H2O2 into an electrolyte-free water stream. The combination of molecular redox mediation and phase transfer catalysis demonstrated here has broad implications for the electrochemical synthesis and isolation of value-added chemicals and fuels