The converters unlimited’s construction is as follows:
- The converters unlimited support or substrate. For automotive , the core is usually a ceramic monolith that has a honeycomb structure (commonly square, not hexagonal). (Prior to the mid 1980s, the converters unlimited material was deposited on a packed bed of alumina pellets in early GM applications.) Metallic foil monoliths made of Kanthal (FeCrAl)[16] are used in applications where particularly high heat resistance is required.[16] The substrate is structured to produce a large surface area. The cordierite ceramic substrate used in most catalytic converters was invented by Rodney Bagley, Irwin Lachman, and Ronald Lewis at Corning Glass, for which they were inducted into the National Inventors Hall of Fame in 2002.[4]
- The washcoat. A washcoat is a carrier for the converters unlimited materials and is used to disperse the materials over a large surface area. Aluminum oxide, titanium dioxide, silicon dioxide, or a mixture of silica and alumina can be used. The catalytic materials are suspended in the washcoat prior to applying to the core. Washcoat materials are selected to form a rough, irregular surface, which increases the surface area compared to the smooth surface of the bare substrate.[17]
- Ceria or ceria-zirconia. These oxides are mainly added as oxygen storage promoters.[18]
- The catalyst itself is most often a mix of precious metals, mostly from the platinum group. Platinum is the most active catalyst and is widely used, but is not suitable for all applications because of unwanted additional reactions and high cost. Palladium and rhodium are two other precious metals used. Rhodium is used as a reduction catalyst, palladium is used as an oxidation catalyst, and platinum is used both for reduction and oxidation. Cerium, iron, manganese, and nickel are also used, although each has limitations. Nickel is not legal for use in the European Union because of its reaction with carbon monoxide into toxic nickel tetracarbonyl Copper can be used everywhere except Japan.
Upon failure, a converters unlimited can be recycled into scrap. The precious metals inside the converter, including platinum, palladium, and rhodium, are extracted.