Michael Wörner

Vacuum-ultraviolet photochemically initiated modification of polystyrene surfaces: morphological changes and mechanistic investigations.

Vacuum-ultraviolet (VUV) irradiation (lambda(exc): 172 +/- 12 nm) of polystyrene films in the presence of oxygen produced not only oxidatively functionalized surfaces, but generated also morphological changes. Whereas OH- and C=O-functionalized surfaces might be used for e.g. secondary functionalization, enhanced aggregation or printing, processes leading to morphological changes open new possibilities of microstructurization. Series of experiments made under different experimental conditions brought evidence of two different reaction pathways: introduction of OH- and C=O-groups at the polystyrene pathways is mainly due to the reaction of reactive oxygen species (hydroxyl radicals, atomic oxygen, ozone) produced in the gas phase between the VUV-radiation source and the substrate. However, oxidative fragmentation leading to morphological changes, oxidation products of low molecular weight and eventually to mineralization of the organic substrate is initiated by electronic excitation of the polymer leading to C-C-bond homolysis and to a complex oxidation manifold after trapping of the C-centred radicals by molecular oxygen. The pathways of oxidative functionalization or fragmentation could be differentiated by FTIR-ATR analysis of irradiated polystyrene surfaces before and after washing with acetonitrile and microscopic fluorescence analysis of the surfaces secondarily functionalized with the N,N,N-tridodecyl-triaza-triangulenium (TATA) cation. Ozonization of the polystyrene leads to oxidative functionalization of the polymer surface but cannot initiate the fragmentation of the polymer backbone. Oxidative fragmentation is initiated by electronic excitation of the polymer (contact-mode AFM analysis), and evidence of the generation of intermediate C-centred radicals is given e.g. by experiments in the absence of oxygen leading to cross-linking (solubility effects, optical microscopy, friction-mode AFM) and disproportionation (fluorescence).

Photoelectrochemical characterization of p- and n-doped single crystalline silicon carbide and photoinduced reductive dehalogenation of organic pollutants at p-doped silicon carbide

Abstract In this work, the photoelectrochemical properties of single crystalline p- and n-SiC, now available from Cree Research, Inc., will be presented. 4H- and 6H-SiC were investigated by electrochemical impedance spectroscopy and cyclic voltammetry. Flatband potentials of p-SiC determined by application of the Mott–Schottky-relation varied from 1.8 to 2.5 V versus NHE depending on the C or Si face and the doping concentration of the single crystalline samples. The flatband potential of n-SiC was found to be −1.3 V vs. NHE. A nearly Nernstian dependence on the pH was found for both the n-SiC and the p-SiC-types. The onset of the cathodic photocurrent at p-SiC occurred with a large overpotential in aqueous solution. P-SiC was used as photocathode for the reductive dehalogenation of halogenated acetic acids due to its chemical stability, its highly energetic conduction band and the overvoltage for the hydrogen evolution. It was especially suitable for the reduction of less reducible chlorinated acetic acids in the aqueous phase. The dehalogenation of brominated acetic acids resulted in the complete release of bromine in form of bromide. The bromide yields were about 98%. In case of chlorinated acetic acids, the dehalogenation stopped at the level of monochloroacetic acid, since this halide was very slowly reduced.

Electrochemical reduction of the iodinated contrast medium iomeprol: iodine mass balance and identification of transformation products

AbstractPotentiostatic-controlled electrochemical reduction of iomeprol was used to deiodinate iomeprol (IMP), a representative of the iodinated X-ray contrast media. The reduction process was followed by product analysis with liquid chromatography-electrospray ionization-tandem mass spectrometry and ion chromatography-inductively coupled plasma-mass spectrometry. The identification is mainly based on the interpretation of the mass fragmentation. The product analysis showed a rather selective deiodination process with the successive occurrence of IMP-I, IMP-2I, IMP-3I, and a transformation product (TP), respectively. The TP was formed from IMP-3I by a further cleavage of an amide bond and release of a (C = O)CHOH group from the side chain of IMP. The iodine mass balance on the basis of IMP and iodide showed a gap of about 26% at the beginning of the electrolysis process which could be completely closed by taking the intermediates IMP-I and IMP-2I into consideration. This means that the major intermediates and the TPs were considered and that the reduction process is a rather selective one to remove organically bound iodine from X-ray contrast media. An attractive application area would be the electrochemical deiodination of X-ray contrast media in urine of patients or hospital effluents. Mass fragmentation of iomeprol and its deiodination products

Dissolution and mineralization of ion exchange resins: differentiation between heterogeneous and homogeneous (photo-)Fenton processes

Fenton and photo-Fenton processes lead under defined experimental conditions to the oxidative degradation and complete apparent dissolution of ion exchange resins (IER) based on copolymers of sulfonated styrene and divinylbenzene, as well as to the mineralization of the dissolved organic fragments. Using the optimal experimental design methodology (OED), the initial Fe(II)-concentration ([Fe(II)]0) was found to control the time needed to completely degrade the IER into soluble fragments, whereas the H2O2 concentration was of minor impact. The photo-Fenton process enhanced primarily the rate of mineralization compared to the dark reaction. The results of process modeling for Fenton and photo-Fenton processes, investigations on the evolution of sulfate (SO42−), CO2, formic and oxalic acids, as well as the comparison between results of photo-Fenton and VUV-photolysis experiments confirm: (i) the existence of two distinct Fenton processes taking place at the surface of the IER beads and in the aqueous bulk, (ii) the desulfonation as the reaction triggering the oxidative degradation and apparent dissolution of IER, (iii) the release of Fe(III) into the aqueous medium and its subsequent reduction, as well as the recycling of the Fe(II)-complexes at the surface of the IER, (iv) the low reactivity of HO˙ generated in solution toward the solid organic substrate, and (v) the important effect of Fe(III)-complexation by oxalic acid. Results also support the hypothesis that formic and oxalic acids are of different origin, and their probing might prove useful for other degradation processes as well.

Formation of hydrogen peroxide by VUV-photolysis of water and aqueous solutions with methanol

The hydrogen peroxide production upon vacuum ultraviolet (VUV) irradiation of water is reviewed, because published results from the last 10 years lead to conflicting mechanistic interpretations. This work confirms that in pure water, hydrogen peroxide is only produced in the presence of molecular oxygen. Mechanistic schemes explain these findings and confirm earlier statements that recombination of hydroxyl radicals is kinetically disfavoured. In agreement with other recent publications, this work confirms that enhanced hydrogen peroxide production takes place upon VUV irradiation of aqueous solutions of organic compounds. For these investigations, methanol was chosen as an organic model compound. During photolyses, hydrogen peroxide, dissolved molecular oxygen, pH-value of the reaction system, methanol and its products of oxidative degradation were analyzed, and kinetic studies were undertaken to explain the evolution of the concentrations of these components.

[FeII(bpy)3]2+/TiO2-codoped zeolites: synthesis, characterization, and first application in photocatalysis.

TiO2-codoped [FeII(bpy)3]2+/zeolite Y systems are revealed to be efficient photocatalysts for photochemically enhanced Fenton processes. Preparation, characterization, and tests using 2,4-xylidine as a model pollutant are described, and a first mechanistic hypothesis explaining the rather high photochemical reactivity of the catalyst system is given.

UV aerosol synthesis: a one-step route to silica, organic-silica and surfactant/silica nanostructured materials

Aerosol flame technology has been used for decades to fabricate on an industrial scale a range of inorganic (nano)particles, including carbon blacks, titania, and fumed silica. Nevertheless, the high local temperatures inherent to this technique prevent direct organic functionalization or loading by organic derivatives, which is essential in many applications to achieve specific properties and optimal dispersion within an organic matrix. We describe herein a novel eco-friendly UV process allowing single-step manufacture of high-value silica and organosilica particles at ambient temperature. Atomized alkoxysilane precursor droplets are produced within an annular photoreactor including 6 fluorescent UV lamps (maximum emission: 312 nm), and photocondensed continuously after a 1 min single pass. Droplet condensation is controlled by the release of a photoacid catalyst localized in the droplets, affording spherical polydisperse powder particles with a mean diameter ranging around 400–700 nm. In the presence of an amphiphilic block copolymer template, a silica/surfactant mesostructured film was deposited using this UV aerosol technology, resulting in a wormlike mesoporous structure after calcination.

Photoinitiated miniemulsion polymerization in microfluidic chips on automated liquid handling stations: Proof of concept

Photoinitiated polymerization is usually applied in the area of specialty chemicals and UV curing but can also be employed in the production of biofunctional polymers and nanocapsules. A promising approach for the preparation of those polymers is the photoinitiated polymerization of miniemulsions. Here, the mild reaction conditions are beneficial for the integration of sensitive biomaterials into the process. The optimal combination of different varied cause variables like irradiance, irradiation time, and height of the irradiated volume is crucial for an appropriate polymerization result. For the presented proof of concept study, a microfluidic tool was established to screen these cause variables in an automated high throughput manner on a liquid handling station (LHS). The experiments are planned and executed by means of a design of experiments (DoE) approach to investigate the effect of the variables on the residual monomer content (RMC) within the stated design space. As the RMC is considered as crucial for bioapplications, the optimal parameter combinations for the complete monomer conversion have been determined. The model‐based evaluation of the executed experiments resulted a channel height respectively optical path of 200–400 μm, an irradiance of 65% (equals 44.2 mW/cm²) and an irradiation time of 30 s for complete styrene conversion.