Zusammenfassung
In this thesis, different types of nanostructured composites were synthesized by novel synthetic pathways to address the specific requirements for liquid phase catalysis, biomedical applications, and use at high temperature. To achieve the target morphologies with iron based nanoparticles as core material, novel preparation pathways were developed to allow defined and reproducible coatings with polymer, silica and carbon in gram-scale quantities. Furthermore, the chosen formation mechanisms were carefully studied in order to create more complex and asymmetrically shaped building blocks on the nanometer scale.
Magnetically recoverable catalysts, consisting of 10 nm sized Fe3O4 particles in SO3H-functionalized polystyrene spheres of about 100 nm were prepared. By changing the cross-linking degree of the polymer spheres, two highly active, superparamagnetic catalysts with different physical and mechanical properties could be obtained. The materials were characterized with respect to their long term stability in the condensation reaction of benzaldehyde and ethylene glycol. The performed test reactions revealed the strong impact of the cross-linking degree on the oxidation protection of the magnetic core and on the swelling properties of the catalysts.
Furthermore, colloidal mushroom structures with sizes from 80 to 180 nm were obtained by asymmetrically induced growth of silica on polystyrene nanospheres. In this work, it was proven that well positioned iron oxide nanoparticles on negatively charged polymers can be used to obtain those structures by means of the well know Stöber-process. This was demonstrated for magnetite as well as for hematite species. Further, it could be shown that the incorporated iron oxides could be easily removed by acid leaching to obtain defined cavities inside the mushroom shaped composites. The prepared materials might be attractive building blocks for new type of carriers for drug delivery, catalysis, multifunctional sensoring etc.
In this work, also magnetic nanoparticles, individually coated in graphitic shells, were reproducibly obtained via pyrolysis of novel precursor colloids. The self prepared precursor materials consisted of iron oxide nanoparticles in cross-linked polystyrene spheres, which are separately coated with 340 nm thick silica shells. Under reductive atmosphere at 800 °C, the incorporated iron oxide species could be reduced to iron, while the polymer was catalytically graphitized at the formed metal surfaces. After removal of the SiO2 shells, the desired products consisting of 30 nm sized Fe/Fe3C cores in dense graphite shells could be obtained. The water dispersable colloids show a remarkable, defined morphology, strong magnetic properties and high oxidation resistance, even in concentrated acids. To prove their long term stability as solid acid catalyst, the graphite coated particles were functionalized with SO3H-groups and tested, like the polymer based catalysts, in the condensation reaction of benzaldehyde and ethylene glycol.
In addition, high temperature stable iron based catalysts for ammonia decomposition reaction have been developed. Again, the advantageous features of two components were exploited by means of core-shell structures in the nanometer range. In this case, the catalytically active iron oxide cores were coated by 20 nm thick porous silica shell, which prevent particle growth processes at elevated temperatures but keep the active phase accessible for educt and product molecules. The highly active catalysts showed conversions of 73 kg NH3 kgcat-1 h-1 at a reaction temperature of 750 °C without any detectable diffusion limitation and degradation. Based on the high stability further studies, like in-situ XRD, could be performed under simulated reaction conditions. The experiments reveal the presence of α-Fe as the prevailing phase above 600 °C.
