Supported catalysts are essential components of many industrial processes and applications, ranging from petrochemical and catalytic converters to fuel cells. They are generally required because of their high surface area, reduced amount of active agent, and high thermal stability. The performance of a catalytic process is often intimately related to the catalyst design and the distribution of metal in the support. There are four main categories of metal profiles, uniform, egg-yolk, egg-shell and egg-white. The choice of the optimal metal profile in the support is determined by the required activity and selectivity, as well as other characteristics of the chemical reaction (kinetics, mass transfer). 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.

For impregnation, both “diffusional impregnation” (pre-wetted supports) and “capillary impregnation” (dry supports prior to impregnation) are simulated in this work. We have also developed a drying model, which takes into account the convective flow of the liquid solvent due to the capillary force, the diffusion of the metal due to the gradient of concentration, metal adsorption onto the support, the convective and diffusive transport in the gas phase, and heat transport. The initial input of the drying model comes from the results from the impregnation simulations. Therefore, we can combine impregnation and drying together to analyze how they impact the distribution of metal in supported catalysts. The models used in this work can capture the essential physics of impregnation and dying and provide physical insight into the fundamentals of the impregnation and drying processes while still maintaining a level of generality. To validate our simulations, companion experiments are carried out.

In the present study, uniform or egg-shell profiles are obtained by impregnation of one component in the support, while egg-white or egg-yolk profiles are obtained by competitive impregnation of two components, where the coimpregnant metal has a higher affinity for the support than the desired metal. We investigate the importance of the physical properties of the solid support (permeability, pellet size), liquid solution (diffusion coefficient, initial metal precursor concentration, pH) and process conditions (humidity and temperature in the drying air), and study the effect of interactions that exist between the dissolved metal and the solid support (physical adsorption, crystallization). This parametric investigation is used to improve our understanding of what are the key parameters in impregnation and drying and how to adjust these parameters to produce a desired metal profile.

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