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In their most common design, metallic catalyst substrates are made of thin metal foils, flat and corrugated, formed into a honeycomb structure which is placed inside a metal shell, as shown in Figure 1. The advantages of metal substrates are their high geometric surface area and low pressure drop associated with the thin walls. Advanced substrates where the foils are brazed/welded together also show good mechanical durability and resistance to thermal shock. The major disadvantage of high quality metallic substrates is their high cost.
Figure 1. Metallic Catalyst Substrate (Emitec)
The major area of application of metallic substrates are close-coupled pre-converters in gasoline cars. In this high-temperature application, metallic substrates allow to eliminate the ceramic mounting mats, thus resulting in a robust catalytic converter system. In some cases, mostly in certain more expensive car models, metallic substrates have also been used in the main catalytic converter. Metallic substrates are uncommon in OEM diesel applications—light or heavy-duty alike—but have been utilized for many diesel aftermarket/retrofit converters.
Most metallic converters are cellular structures made of thin foils in various configurations, with channels formed through corrugation of the foil. Examples of other concepts can be given, such as metal foams  or assemblies of flat, perforated foils . These alternative designs have not yet found commercial use.