Influence of the Washcoat Structure in the Performance of Automotive Three Way Catalysts 2013-24-0166
Transport limitations inside the porous washcoat layer have an important influence in the light-off and overall conversions even in the case of relatively thin layers used in automotive three way catalysts (TWC). The porous structure of the washcoat layer is controlled at two levels: i) at the level of mesoporous structure, which can be determined by the use of specific synthesis techniques (e.g., sol-gel or pore-templating method), and ii) at the level of macroporous structure, which is influenced by the particle size distribution of mesoporous in a slurry that has undergone a specific thermal treatment. This paper investigates the influence of the washcoat structure in the performance of automotive TWC. Furthermore, the article presents a method that allows to quantify the magnitude of the reaction resistance (chemical kinetics), internal mass transfer resistance (washcoat diffusion), and external mass transfer resistance on the TWC conversions. From experimental data gathered at different operating temperatures (from light-off to 1100 K) and applying the developed methodology for resistances quantification, it was found that the internal mass transfer limitations play a major role in the TWC conversions. The main conclusions from this study are: i) the effective diffusivity strongly depends on the washcoat layer structure, which in turn depends on its preparation process; ii) the resistance quantification analysis reveals that the lower effective diffusivity interval is more adequate for the TWC used in the present investigation, which also indicates that the TWC washcoat has a low percentage of macropores; and iii) to decrease the relevance of the internal mass transfer limitation requires an increase in the effective diffusivity and/or a thinner washcoat layer.