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Sonstrom P.,Institute of Applied and Physical Chemistry | Adam M.,Ceramic Materials and Components | Wang X.,Institute of Applied and Physical Chemistry | Wilhelm M.,Ceramic Materials and Components | And 2 more authors.
Journal of Physical Chemistry C | Year: 2010

Ceramers constitute a new type of porous hybrid ceramic which is obtained by pyrolysis of polysiloxane precursors with organic side groups at temperatures low enough that part of the polymer has not yet been decomposed (≈500 °C). By using different precursors, interesting possibilities arise to change the structural and mechanical properties of this monolithic material over a wide range. Moreover, the surface chemical properties (such as basicity or hydrophilicity) can be modified by varying the organic side groups. In order to add a catalytic function to such a ceramer, different preparation techniques were employed in this study with the objective of incorporating nanoparticles into the ceramers (addition of ionic precursors vs preformed colloidally synthesized nanoparticles during the synthesis). The resulting materials were structurally characterized and catalytically studied using the oxidation of CO as a test reaction. Our results reveal that especially colloidally prepared nanoparticles provide attractive options to manufacture tailored catalysts since the particle sizes, as controlled by the colloidal synthesis, can be well-preserved. It could be shown that aminopropyltriethoxysilane (APTE) leads to a homogeneous distribution of nanoparticles in the ceramer matrix and an enhanced CO oxidation activity after sample activation, an effect that can be attributed to a more effective binding of the Pt nanoparticles to the precursors of the material hindering sintering and agglomeration. The mass transport limitation due to pore diffusion was characterized for samples in the form of larger grains and finely ground powders by determining the macro kinetics (reaction orders and activation energies) and calculating the Thiele modulus. The comparison reveals mass transport limitation due to microporosity. We discuss strategies to avoid such restrictions and optimize mass transport of gaseous reactants paving the way to ceramer-based monolithic catalyst with high structural stability and an optimized distribution of monodisperse nanoparticles. © 2010 American Chemical Society. Source

Filser J.,University of Bremen | Filser J.,Center for Environmental Research and Sustainable Technology | Arndt D.,University of Bremen | Arndt D.,Center for Environmental Research and Sustainable Technology | And 42 more authors.
Nanoscale | Year: 2013

Iron oxide nanoparticles (IONP) are currently being studied as green magnet resonance imaging (MRI) contrast agents. They are also used in huge quantities for environmental remediation and water treatment purposes, although very little is known on the consequences of such applications for organisms and ecosystems. In order to address these questions, we synthesised polyvinylpyrrolidone-coated IONP, characterised the particle dispersion in various media and investigated the consequences of an IONP exposure using an array of biochemical and biological assays. Several theoretical approaches complemented the measurements. In aqueous dispersion IONP had an average hydrodynamic diameter of 25 nm and were stable over six days in most test media, which could also be predicted by stability modelling. The particles were tested in concentrations of up to 100 mg Fe per L. The activity of the enzymes glutathione reductase and acetylcholine esterase was not affected, nor were proliferation, morphology or vitality of mammalian OLN-93 cells although exposure of the cells to 100 mg Fe per L increased the cellular iron content substantially. Only at this concentration, acute toxicity tests with the freshwater flea Daphnia magna revealed slightly, yet insignificantly increased mortality. Two fundamentally different bacterial assays, anaerobic activated sludge bacteria inhibition and a modified sediment contact test with Arthrobacter globiformis, both rendered results contrary to the other assays: at the lowest test concentration (1 mg Fe per L), IONP caused a pronounced inhibition whereas higher concentrations were not effective or even stimulating. Preliminary and prospective risk assessment was exemplified by comparing the application of IONP with gadolinium-based nanoparticles as MRI contrast agents. Predicted environmental concentrations were modelled in two different scenarios, showing that IONP could reduce the environmental exposure of toxic Gd-based particles by more than 50%. Application of the Swiss "Precautionary Matrix for Synthetic Nanomaterials" rendered a low precautionary need for using our IONP as MRI agents and a higher one when using them for remediation or water treatment. Since IONP and (considerably more reactive) zerovalent iron nanoparticles are being used in huge quantities for environmental remediation purposes, it has to be ascertained that these particles pose no risk to either human health or to the environment. © 2013 The Royal Society of Chemistry. Source

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