Zugl.: Würzburg, University, Diss., 1999.

No description available.

No description available.

The small size regime and bulky hydrocarbon exterior of dendrimer encapsulated nanoparticles (DENs) often make characterization of these materials a unique challenge. Here, I report on three studies utilizing the techniques of extended X-ray absorption fine structure (EXAFS) and electrochemistry to probe the properties and behavior of these materials. First, the synthesis and characterization of PdCu bimetallic nanoparticles, and Pd and Cu monometallic nanoparticles, consisting of an average of ~64 atoms is described. The bimetallic nanoparticles were prepared by co-complexation of Pd²⁺ and Cu²⁺ salts to interior functional groups of a dendrimer template followed by chemical reduction to yield DENs. EXAFS spectroscopy indicates that these particles have an alloy structure. This is a rare example of a stable nanoparticle in this size range that consists of one reactive metal and one substantially more noble metal. Second, in-situ electrochemical EXAFS is used to evaluate the structure of Pt DENs during the oxygen reduction reaction (ORR). The DENs contained an average of just 225 atoms each. The results indicate that the Pt coordination number (CN) decreases when the electrode potential is moved to positive values. The results are interpreted in terms of an ordered core, disordered shell model. The structure of the DENs is not significantly impacted by the presence of dioxygen, but other electrogenerated species may have a significant impact on nanoparticle structure. Third, the electrochemical dissolution of Cu DENs is investigated using anodic stripping voltammetry (ASV). The effect of the scan rate and Cu loading on the electrode to the stripping wave is performed. The results indicate a large, positive shift of the stripping potential for the dendrimer-metal composites, but no size-dependent changes to peak position. / text

Chitosan-based copolymers (CH-GL1:6, CH-GL1:1, CH-GL1:0.5, and CH-GL1:0.25) were prepared at variable weight ratios of chitosan (CH) to glutaraldehyde (GL). Physiochemical properties of cross-linked copolymers were characterized using FTIR (Fourier Transform Infrared) spectroscopy, PXRD (Powder X-ray Diffraction), CHN analysis, and thermogravimetric analysis (TGA). The swelling behaviour of the polymers along with chitosan was investigated. The sorption properties of copolymers with arsenate oxoanions were investigated at various pH using 10 mM phosphate buffer systems and also in aqueous solution without buffer. The Sips sorption model describes the best fit parameters for adsorption. The relative monolayer sorption capacities Qm (mg/g) of the adsorbents are given in parentheses in the following order: CH-GL1:1(14.4) > CH-GL1:0.5(12.0) > CH-GL1:0.25(10.3) > CH-GL1:6(2.24). In general, the sorption capacities are listed in descending order as follow: un-buffered > buffered (pH 5.0) > buffered (pH 8.5). The removal efficiencies for 20 mg of polymers over a variable concentration range ( 1-200 mg/L) of arsenate in aqueous solution without buffer are as follow: CH-GL-1:1(20-95%), CH-GL1:0.5(14-97%), CH-GL1:0.25(10-98%), CH-GL1:6(2.0-56%), and CH (0.007-3.9%). The sorption properties of the adsorbents were also determined in bicarbonate buffer to evaluate the competitive effect of phosphate buffer on adsorption of arsenate oxoanions. X-ray absorption spectroscopy (XAS) of chitosan and CH-GL1:1 was performed after adsorption at different pH conditions using two buffer systems to evaluate the chemical environment around the arsenate species. In addition, sorptive properties of phenolic adsorbate (i.e. PNP) were estimated with CH-GL copolymers at various pH conditions. The estimated sorptive capacities (Qm; mmol/g) for PNP are in the range 0.07-0.21(mmol/g) while removal efficiencies for PNP are greater at lower pH conditions.

No description available.

Regensburg, Univ., Diss., 2008

Les connaissances des propriétés des fluides hydrothermaux, c'est-à-dire des solutions aqueuses à haute température (T) et haute pression (P), sont essentielles dans les domaines des sciences de la terre, chimie prébiotique, industrie nucléaire ou encore des sciences environnementales. Près du point critique (Pc, Tc) et dans les conditions pseudocritiques (i.e. à la densité critique au dessus de la Tc-Pc), les propriétés des fluides changent de manière radicale. Ces modifications ont lieu à différentes échelles: à l’échelle macroscopique (changements de densité et de compressibilité), à l’échelle mésoscopique (processus d’agglomération entre les différentes clusters) et enfin à l’échelle moléculaire (organisation local entre les atomes et les molécules, par exemple les liaisons hydrogène, H).Cette étude est centrée sur les effets de la solvatation aqueuse de différents électrolytes en fonction de la température, pression et concentration. L'étude expérimentale a été conduite par les mesures d’Absorption X, technique de choix pour mesurer d'une part l'évolution de la densité du fluide et d'autre part sonder l'environnement atomique local autour des solutés. Cette étude a été faite sur l'eau pure et sur différentes solutions salines à différentes pressions (jusqu'à ~1.3Pc) et à température variable (jusqu'à ~2Tc), pour passer du domaine liquide au domaine supercritique, dans une cellule permettant de découpler totalement l’effet de la pression et de la température.Nous avons obtenu des preuves expérimentales du déplacement du point critique et de l'isochore et de leur dépendance en fonction de la concentration en sel pour NaCl (0,3, 0,5 et 1,0 moles NaCl par kilogramme d'eau). Nous avons également observé une anomalie de densité dans la région supercritique (SC) et l'apparition d'une séparation des phases liquide-vapeur pour certaines solutions aqueuses. L'augmentation de la densité relative dans cette zone critique est plus prononcée pour Cs>Rb>K>Na>Li pour les bromures et les chlorures. Le changement structural dans cette région a été suivi par spectroscopie XANES haute résolution au seuil K du brome pour différents bromures d'alcalin. L'interprétation des spectres XANES indiquent clairement un changement drastique dans la structure locale du fluide au moment de l'apparition de cette anomalie pouvant être interprétée par l'apparition des paires ioniques.Ces nouvelles observations sont a relier au changement structurale du solvant et principalement à l'évolution de son pouvoir de solvatation lié à l'évolution des liaisons hydrogène à hautes T-P. / Knowledge of the properties of hydrothermal fluids, i.e. aqueous solutions at high temperature (T) and high pressure (P), are essential in the fields of earth sciences, prebiotic chemistry, nuclear industry and environmental sciences. Near the critical point (Pc, Tc) and under the pseudocritic conditions (i.e. at the critical density at T-P above Tc-Pc), the fluid properties change radically. These changes take place at different scales: the macroscopic scale (density and compressibility changes), the mesoscopic scale (agglomeration process between different clusters) and finally, the molecular scale (local organization of atoms and molecules, for example hydrogen bonds, H).This study focuses on the solvation effects in different electrolytes as a function of temperature, pressure and concentration. The experimental study was carried out by X-ray absorption measurements, the technique suitable for probing both fluid density evolution and local atomic environment around the solutes. This study was carried out on pure water and salt solutions at different pressures (up to ~ 1.3Pc) and temperatures (up to ~ 2Tc), to pass from the liquid to the supercritical domain, using a cell permitting to completely decouple the effects of pressure and temperature.We have obtained experimental proofs of the displacement of the critical point and the isochore and their dependence on the salt concentration in the case of NaCl (0.3, 0.5 and 1.0 moles NaCl per kilogram of water). We have also observed a density anomaly in the supercritical region (SC) and the appearance of liquid-vapor phase separation for some aqueous solutions. The relative density increase in the critical zone is more pronounced for Cs>Rb>K>Na>Li for bromides and chlorides. The structural change in this region was followed by high-resolution XANES spectroscopy at the K-threshold of bromine for various alkali bromide. The interpretation of the XANES spectra clearly indicates a drastic changes in the fluid structure related to this anomaly, which can be interpreted by the appearance of ionic pairs.These new observations are in link with water structure and solvation properties evaluation and consequently with hydrogen bonding changes under high T-P.

No description available.

No description available.