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How are honeycomb ceramic monoliths used in automotive catalytic converters?

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advanced structural ceramics Figure 1: Resistance to cracking in transformation-toughened zirconia. In a ceramic composed of tetragonal zirconia dispersed in a zirconia matrix, the stress field advancing ahead of a propagating crack transforms the small tetragonal particles to larger monoclinic particles. The larger particles exert a crack-closing force in the process zone behind the crack tip, effectively resisting propagation of the crack.

In automotive catalytic converters, honeycomb monoliths serve as essential support structures for catalysts that reduce harmful exhaust emissions, such as nitrogen oxides, carbon monoxide, and unburned hydrocarbons.1

These monoliths are typically manufactured from cordierite, a magnesium aluminosilicate chosen for its low thermal expansion, which helps the structure withstand the high temperatures of an exhaust system. The honeycomb design features 1,000 to 2,000 longitudinal pores, approximately one millimetre in size, separated by thin walls. This geometry is achieved through extrusion, a process refined by using polymer resins to maintain the structure's strength before firing.12

To function as a catalyst, the monolith is coated with a wash of alumina, which provides a high surface area to support platinum-group metals or base-metal catalysts. Compared to pellet-based supports, honeycomb monoliths are preferred because they cause a smaller pressure drop in the exhaust system, improving overall engine efficiency. However, because these ceramic structures are inherently friable, they require careful containment to prevent damage from vibration and external stress.1