Abstract
Within the field of chemistry both nanotechnology as well as green chemistry are considered to be of considerable scientific as well as societal relevance. Nanotechnology describes the study of materials with at least one dimension not exceeding the 100 nm size range. These nanosized materials possess mechanical, optical, electrical and/or
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magnetic properties that deviate from those observed for the bulk material. Green chemistry involves the development of environmentally benign chemical processes using renewable resources to access reactants, reagents, solvents and thus sustainable products. The research described in this thesis combines both nanotechnology and green chemistry for the development of functional base metal nanomaterials for which a range of applications are foreseen. A novel method is developed to obtain base metal nanoparticles of copper, nickel, cobalt or iron supported onto (graphitic) carbon bodies. Spherical microcrystalline cellulose spheres are used as the precursor for these carbon bodies. The method entails the loading of hydrophilic microcrystalline cellulose spheres by wet impregnation with aqueous solutions of base metal salts followed by drying. During pyrolysis of the dried loaded microcrystalline cellulose spheres under stagnant and inert conditions in the temperature range of 400-800 °C several processes occur. Whereas the cellulose is converted into an amorphous carbonaceous material, the dispersed metal salts are converted into their metal oxides. Finally, the highly dispersed supported nanosized metal oxide particles are quantitatively reduced yielding dispersed base metal nanoparticles without the need of an external H2(g) source, i.e. explosive H2(g) is not required, instead cellulose pyrolysis products provide the required reducing atmosphere. It is shown that base metal nanoparticles capable of forming metal carbides (nickel, cobalt and iron) convert the amorphous carbonaceous support into graphitic carbon nanostructures. This is of considerable interest for the development of efficient (ferromagnetic) absorbents. Under appropriate conditions in which the size of the base metal nanoparticles (nickel or iron) is carefully controlled, it is found that the metal nanoparticles act as efficient (heterogeneous) growth catalysts for either carbon nanofiber or multi-walled carbon nanotube growth in the presence of an external carbon source. This allows for a large scale synthesis of these materials. In the case of the iron nanoparticles, conditions could be found in which upon heat treatment in a stagnant and inert atmosphere both the formation of iron nanoparticles as well as a concomitant growth of multi-wall carbon nanotubes occurred consecutively in the absence of both an external H2(g) as well as an external carbon source. These experiments suggest viable pathways for the still debatable formation of (multi-wall) carbon nanotubes in nature. These results indicate that these base metal nanoparticles will also be active as heterogeneous catalyst in other processes. Finally preliminary experiments have been executed showing that the materials developed can also be applied in electrochemistry (novel electrodes) and magnetic resonance imaging.
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