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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. The foils in metallic substrates can be brazed/welded together to provide 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 first major area of application of metallic substrates was as close-coupled pre-converters in gasoline cars. In this high-temperature application, metallic substrates allow elimination of the ceramic mounting mats, thus resulting in a robust catalytic converter system. In some cases, mostly in some luxury car models, metallic substrates have also been used in the main catalytic converter.
There have been relatively few applications of metallic substrates in OEM diesel engines—examples include the 2007-2012 Dodge Ram with NAC and 2010 and later Navistar Maxxforce engines that used no NOx aftertreatment. Metallic substrates have been more commonly utilized for diesel aftermarket/retrofit converters. Another common application of metallic substrates is catalytic converters for stationary engines, where the metallic foil construction allows for much larger diameters than ceramic honeycombs.
Most metallic converters have cellular structures made of thin foils in various configurations, with channels formed through the corrugation of the foil. A number of alternative concepts have been proposed—such as metal foams  or assemblies of flat, perforated foils —but these have not gained wide acceptance.
Substrates made of metal foil can offer a significant degree of flexibility in shaping the channels. For instance, conical shaped substrates can be assembled to improve exhaust gas flow distribution through the converter . A number of specialized designs have been developed to address various application needs—including turbulent flow designs, particle oxidation catalysts and electrically heated catalysts.