Materials Used in Catalytic Converters
The catalytic converter is made up of several materials. The catalyst core or substrate varies according to the vehicle. For example, in automotive catalytic converters, the core is usually a ceramic monolith with a honeycomb structure.
When manufactured in large quantities, ceramic cores can be inexpensive. Metallic foil monoliths are made of iron-chromium-aluminum combination, and used in some applications. Metallic cores are less expensive when manufactured for use in small production runs, such as in sportscars in which low back pressure and reliability under constant high load is essential. Both these materials are designed to provide a high surface area to support the catalyst washcoat. Cordierite ceramic substrates can also be used in most catalytic converters.
The catalyst washcoat is a carrier for the catalytic materials, which is used to disperse the materials over a high surface area. Titanium dioxide, aluminium oxide, silicon dioxide, or a combination of silica and alumina can be used. The catalytic materials are suspended in the washcoat before application to the core. Washcoat materials have rough, irregular surface to increase surface area, which helps to maximize the catalytically active surface available to react with the engine exhaust.
The catalyst used in the converter is mostly a precious metal such as platinum, palladium and rhodium. Platinum is used as a reduction catalyst and as an oxidation catalyst. Although platinum is a very active catalyst and widely used, it is very expensive and not suitable for all applications. Rhodium is used as a reduction catalyst, while palladium is used as an oxidation catalyst.
In some cases, cerium, iron, manganese and nickel are also used. However, some companies forbid the use of some of these.