269160 Impact of Drying Approaches On the Metal Distribution of Preparation of Supported Catalysts
Supported catalysts are used in many industrial processes and applications, ranging from petrochemical and catalytic converters to fuel cells. These catalysts have many advantages, such as a high surface area, a low amount of the often expensive active component (Pd, Pt, etc.) and high mechanical and thermal stability. Clearly, the catalyst design has a pronounced effect on the performance of a catalytic process. With respect to the distribution of the active component in the support materials, four main categories of metal profiles can be distinguished, i.e., uniform, egg-yolk, egg-shell and egg-white profiles. The choice of the desired metal profile is determined by the required activity and selectivity, and tailored for specific reactions and/or processes. Although the development and preparation of supported catalysts have been investigated for many years, many aspects of the various catalyst manufacturing steps are still not fully understood, and in industry the design of catalysts is predominated by trial and error experiments,
Supported catalysts are usually prepared by impregnation, where a porous support is contacted with a liquid solution that contains the desired metal as a dissolved salt. This step is usually followed by the evaporation of the liquid solvent and that is drying. After drying reduction and calcination are carried out. It is generally believed that the metal profile is controlled by the conditions that are applied during impregnation where the metal contacts the solid support for the first time. However, experimental work has shown that drying may also significantly impact the metal distribution within the support. Therefore, to achieve a desired metal profile we need to understand both impregnation and drying steps.
In this work we have carried out experiments and simulations for the drying process of Ni/Alumina, Cu/Alumina, and Mo/Alumina systems. The metal profiles are investigated in cylinder, ring and trilobe carriers. Both microwave drying and regular oven drying are used in this work. A microwave oven with a turntable leads to the ionic species in solution being subject to an oscillating electric field. This will increase diffusivity of the metal ionic species in solution (inside the support) through ion conductivity. In addition during microwave drying, vapor is generated internally as a result of volumetric heating, leading to the development of a pressure gradient that helps transport moisture and speed the drying process. Microwave drying can provide rapid and uniform drying within the porous support, leading to uniform evaporation of the liquid solvent, which will most likely result in a uniform distribution of the active metal within the support. Therefore, microwave drying is distinctly different from regular oven drying. For all cases examined in this work, we find that microwave drying gives us a much more uniform metal distribution than regular oven drying for both a uniform initial distribution or an egg-shell distribution after the impregnation step. The impact of the energy intensity on the microwave drying process is significant. The drying time required greatly increases with a decrease in the input energy. We have also developed theoretical models to simulate microwave drying and regular oven drying processes and compared the simulations with experiments.
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