Ceramic substrates have been at the heart of mobile emissions control since the technology’s beginning in the early 1970s when Corning developed synthetic cordierite and the die for ceramic honeycomb extrusion. The honeycomb substrates are filled with thousands of tiny, parallel channels open on each end, allowing the vehicle’s exhaust to flow through. These channels provide a large interior surface area to support catalytic activity. When the substrate is about the size of a soda can, its interior surface (including the high surface area washcoat) has a surface area about the size of an American football field.

These substrates can withstand high temperatures >1100°C (2000°F) and are extremely thermal shock resistant (so that they survive rapid heat-ups on frigid mornings). These substrates are very adaptable and can accept a wide variety of catalyst formulations to eliminate pollution from gasoline, diesel, natural gas, hydrogen and other fuels.

The completed substrate is coated with a combination of the appropriate catalytic materials that turn it into a mini chemistry lab and allows large volumes of exhaust to be efficiently treated. At high temperatures of an operational engine, exhaust gases like nitrogen oxides and carbon monoxide meet the catalyst and are converted into harmless nitrogen and water and less harmful carbon dioxide.

The first commercial ceramic substrates were low cell density (about 200 cells/in²) with thicker walls (about 12 mil or 0.012” or 0.3 mm) with a substrate volume about four times that of engine displacement (i.e., cylinder volume of the engine). As material and processing technology progressed, higher cell densities, thinner walls, and higher porosities became possible.

In today’s average U.S. gasoline or hybrid sedan, there are two or three substrates at work to meet strict U.S. gaseous emissions standards. Right off the engine, close-coupled substrate(s) with high cell density provide a lot of geometric surface area to allow the catalyst to do the initial gaseous conversions. In the underfloor position there is generally a lower cell density substrate to help clean up final emissions.