1 Department of Chemical and Biochemical Engineering, Technical University of Denmark
The scope of this work is to investigate flame synthesis of oxides and oxide composites for catalytic applications. Vaporized acetylcetonate precursors are combusted in a flame leading to the formation of metal oxides with high specific surface areas. The employed flame setup is a premixed flat flame where fuel, air and the precursors are mixed prior to ignition. Metal acetylacetonates are used as the precursors due to a high thermal stability in air at temperatures up to 200-250̊C. The surface area, particle morphology and crystalline structure of the oxides are controlled by changing the flame temperature, the high temperature residence time and the precursor concentration. The Cu/ZnO/Al2O3 methanol catalyst is used as a model system for the preparation of catalytic materials. The flame synthesized catalyst exhibits a high and reproducible activity for methanol formation from synthesis gas (CO/CO2/H2) and an excellent thermal stability. Addition of alumina as a structural promoter is necessary in order to obtain a high activity for methanol formation. The binary systems, i.e., CuO/ZnO, ZnO/Al2O3 and CuO/Al2O3 are investigated as a prelude to the preparation of the ternary catalyst. These investigations prove that synthesis in a premixed flame is a very attractive method for the preparation of high surface area spinel structures with a high degree of crystallinity and a good resistance against sintering. ZnAl2O4, CuAl2O4 and MgAl2O4 spinel structures have been synthesized. The CuAl2O4 spinel exhibits a high activity for alcohol dehydrogenation due to a high reduced copper surface area. The copper surface areas of the reduced copper catalysts are measured employing N2O-titration. Treating the reduced copper catalysts with N2O results in a mild oxidation and only the surface layer of the copper crystallites is oxidized. A number of complications may arise using the N2O-titration. It may be difficult to obtain full oxidation of the copper surface without having some oxidation of the bulk. Secondly, some sintering of the nano-sized copper crystallites may occur due to the exothermic nature of the surface oxidation. A new improved approach for the N2O-titration, which provides the possibility of distinguishing surface oxidation and bulk oxidation, is developed. Furthermore, there is no indication of any sintering during the measurement of the copper surface areas. Experiments with synthesis of pure oxides have also been carried out. Pure alumina is synthesized with specific surface areas between 164 and 407 m2/g . The crystal structure varies from γ-Al2O3 to amorphous alumina. The alumina powder consists of agglomerated dense primary particles. Typical for flame synthesis, the individual agglomerates have a highly dendritic structure with a low density. The particle formation during alumina synthesis is modelled employing either a monodisperse model or a self-preserving model for coagulation in combination with a hybrid model describing the sintering kinetics. The hybrid model includes two mechanisms for the sintering, which allows the individual mechanisms to control the sintering in different temperature regimes. Simulation of the specific surface areas and collision diameters of the synthesized powder fits measured values nicely when the hybrid sintering model is applied. The temperature distribution in the flame used for the simulations is based on measurements, which are corrected for radiation losses. The method for the temperature correction has been checked by computational fluid dynamics (CFD) and the method for correction has proved to be reliable.