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To meet stringent emissions regulations, high conversion efficiency is required. This calls for advanced catalyst substrates with thinner walls and higher cell density. Higher cell density is needed because it brings higher mass transfer from the gas to the substrate wall. Basically, the increase in total surface area (TSA) causes higher mass transfer. However, not only the TSA, but the cell shape also has a great effect on mass transfer. There are two main kinds of substrates. One is the extruded ceramic substrate and the other is the metal foil type substrate. These have different cell shapes due to different manufacturing processes. For the extruded ceramic substrate, it is possible to fabricate various cell shapes such as triangle, hexagon, etc. as well as the square shape. The difference in the cell shape changes not only the mass transfer rate, but also causes pressure loss change. This is an important item to be considered in the substrate design.