Simulation of Solvent-Water Diffusion Processes during the Synthesis of Ordered Zirconia and Ceria-Zirconia-Yttria Mesostructures

Zirconia materials are commonly used in transportation (catalytic converters) and energy (fuel cells) applications. Variants, such as ceria-zirconia-yttria (CZY) oxides, are widely used as supports for three-way catalysts for automotive exhaust emission control. For this application, the creation of CZY materials with an ordered arrangement of mesopores should lead to enhanced catalyst performance by improving gas transport and reducing the sintering of supported noble metals. Ordered mesoporous zirconia and CZY catalysts were synthesized via evaporation induced self-assembly (EISA) using surfactants as the structure directing agent (SDA). Ordered mesoporous structures were only synthesized when the solvent evaporation rate during synthesis was sufficiently slow to allow for the self-arrangement of SDA micelles and the controlled influx of the hydrolyzing agent (water). Previously reported syntheses of ordered zirconia materials failed to examine how the diffusion of solvent and reagents in the precursor sol-gel impacts the final morphology of the mesostructures.

A study is presented on the relationship between the appearance of ordered structures and the diffusion rates of solvent and water during the synthesis process. Multi-physics finite element models (using COMSOL) were employed to simulate the EISA process, where evaporation and absorption processes were incorporated into a system with moving boundaries. This model describes the effects of evaporation rate, concentration of the surfactant during the process, vessel geometry, and other parameters relevant to the synthesis. More importantly, the model provides us an in-depth understanding of how diffusion processes impact the EISA process, which can help in process scale-up for industrial purposes.