Abstract
In this thesis rattle-type structured hollow sphere catalysts were developed to obtain materials with specific properties and functions. The main goal was the fabrication of high temperature stable systems, following the approach of encapsulation of Au nanoparticles (NP) in ZrO2 hollow spheres, which had previously been developed in our laboratories. This innovative concept was the basis for an extension to other encapsulated metal NP system.
With the aim to synthesize a thermally stable catalyst, zirconia hollow spheres with ca. 70 nm in diameter were filled with single platinum nanoparticles of 3-7 nm.
The synthesis route is based on the production of a monodisperse Pt colloid. Several platinum particles were then coated with a shell of silica. The Pt@SiO2 core-shell composite was then covered with a layer of zirconia. After calcination the silica core was leached out with NaOH, resulting in platinum nanoparticles incorporated in a mesoporous zirconium oxide shell. Besides the investigations of the influence of different synthesis parameters, the material was tested in the selective reduction of NOx with propene. However, the controlled and reproducible synthesis did not result in a similar high temperature stable system as the Au, @ZrO2 material. Instead, sintering of the Pt core particles was observed, even at temperatures below 800 °C.
Another goal of this work was the modification of the gold based catalyst system. It was shown that the size of Au@SiO2 can be tuned to produce template materials with different dimensions for the subsequent coating with zirconia. This resulted in a tuning of the catalyst loading in a range of 2-14 wt.% of Au. A higher gold loading improved the catalytic performance of the material in the CO oxidation. The Au@SiO2 composite with 50 nm suffered from an instable shell, resulting in coalescence of the gold nanoparticles. The fabricated material with a diameter of 70 nm was nearly unaffected by the thermal treatment at 800°C: it showed a similar performance in the CO oxidation like the untreated particles. Additionally, the gold catalyst was varied in its chemical composition. A hollow titania shell system was developed by the coating of a Au@SiO2 composite with a diameter of 200 nm with TiO2. Since many studies showed that titania supported gold catalysts have a higher X activity in the conversion of CO to CO2, the titania hollow sphere catalyst should also show a better performance. In fact, for the Au, @TiO2 catalyst a higher catalytic activity (T50% = ca. 120°C), compared to the known Au, @ZrO2 100 nm system (T50% = ca. 170°C), was achieved. Unfortunately, the titania shell showed a loss of integrity after the thermal treatment at 800°C.
For a further increase of the activity, a pathway to an Au, @ZrO2 (Au < 15 nm) was developed by the coating of several small Au nanoparticles with silica. Subsequent zirconia covering was followed by calcination and the leaching of the silica with NaOH. This led to a single Au nanoparticle incorporated in a porous zirconia shell. The experiments were accompanied by the observation of the influence of single reaction parameters such as temperature and the concentration of reactants. It was found that the temperature, as well as the amount of precursors had a significant impact on the quality of the colloidal Au@SiO2. High requirements on the exactness of the synthesis route made this procedure rather difficult. Additionally, the yield of the final Au, @ZrO2 was very low so that a full characterization of the material was not possible. The smaller particles showed an increased activity in the CO oxidation.
The core-shell particles were used to find new synthesis options in the fabrication of rattle-type structured materials. An ex post size control of encapsulated gold nanoparticles was performed to shrink the dimension of the catalytically active gold core. This conceptual approach showed a high potential for a further development of this material, since an enhancement of the activity was observed. Hollow silica spheres were produced by a complete leaching of the gold of an Au@SiO2 composite. The siliceous “nanoreactors” were used as template for the synthesis from metallic particles (e.g. Pt, Ru) inside of the cavity. This approach expanded the window of possibilities to produce rattle-type materials. The chemically modified materials can be coated with a shell of a metal oxide and after calcination and leaching of SiO2 one will obtain a single metal nanoparticle in a mesoporous shell of a metal oxide.
