Thermal Modelling of Aerogel-Based Washcoats for Three-Way Catalytic Conversion: Effects of Washcoat Composition and Thickness on Light-Off Time

Automotive three-way catalytic converters (TWCs) typically consist of an encased cordierite honeycomb structure, coated with an alumina washcoat impregnated with platinum group metals (PGMS). While effective under normal operating conditions, upon a cold start there is typically a significant warm-up time prior to the TWC achieving "light-off" and becoming effective. This cold-start transient is a significant (even dominant) contributor to total pollution emissions.  Aerogels are nanoporous materials that have a large surface area, low density, and low thermal conductivity. The use of aerogel in place of the denser and more thermally conductive alumina washcoat might reduce the time needed to achieve light-off and decrease overall pollutant emissions. This idea was investigated using a one-dimensional transient model to simulate heat transfer in a compound wall composed of a cordierite base with one of three different coatings: silica aerogel washcoat, a catalytically active copper-alumina (CuAl) aerogel washcoat, and a traditional alumina washcoat. Simulations were performed using a transient finite difference model in MATLAB and confirmed using Abaqus FEA Software. The exhaust gas was assumed to flow over the surface at a constant temperature of 600 K with a heat transfer coefficient of 35 W/m²K; values typical for a catalytic converter. Square honeycomb structures were analyzed with a 75-µm-thick or a 150-µm-thick cordierite wall, to simulate 400 and 300 cells per square inch (CPSI) cordierite honeycombs respectively. A nominal washcoat thickness of 20 µm was modelled, and the surface temperature of the washcoat in direct contact with the exhaust was analyzed over time. A number of scenarios were examined including: (a) the effect of washcoat properties; (b) the effect of the percent (0-100%) of the washcoat thickness in the total wall composition; (c) the effect of washcoat thickness (10-100 µm) for fixed cordierite thickness; and (d) the effect of a transient exhaust temperature. The results show a large initial increase in surface temperature for the aerogel-based washcoats (compared to that of the alumina) which then levels off as heat penetrates into the cordierite layer. A 20-µm-layer aerogel-based washcoat reaches light-off temperature 2-3 seconds faster than the alumina washcoat representing an 8-17% decrease in light-off time. For a 100-µm-layer, the aerogel washcoats reach light off 13-14 seconds faster representing a 30% decrease in light-off time. However, light-off time increases with washcoat layer thickness so using a thinner coating is better. If the overall wall thickness is kept the same, but thickness of the washcoat increases, the time to light-off for the aerogel washcoats is reduced by 28 sec (300 CPSI honeycomb) and by 16 sec (400 CPSI) while the alumina washcoat light-off time only decreases by 0.7 s. When using a more realistic model with an exhaust temperature that increases with time, similar trends are observed. Although there are a number of challenges associated with using an aerogel-based washcoat, these results indicate that their use could reduce TWC light-off time. Finding a way to maintain the initial surface temperature rise would allow even shorter light-off times to be achieved.