1. The core, or substrate. The core is often a ceramic honeycomb in modern catalytic converters, but stainless steel foil honeycombs are also used. The honeycomb surface increases the amount of surface area available to support the catalyst, and therefore is often called a “catalyst support”. The ceramic substrate 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.

2. The washcoat. A washcoat is used to make converters more efficient, often as a mixture of silica and alumina. The washcoat, when added to the core, forms a rough, irregular surface, which has a far-greater surface area than the flat-core surfaces do, which then gives the converter core a larger surface area, and therefore more places for active precious-metal sites. The catalyst is added to the washcoat (in suspension) before being applied to the core.

3. The catalyst itself is most often a precious metal. Platinum is the most-active catalyst and is widely used. It is not suitable for all applications, however, because of unwanted additional reactions and/or cost. Palladium and rhodium are two other precious metals used. Platinum and rhodium are used as a reduction catalyst, while platinum and palladium are used as an oxidation catalyst. Cerium, iron, manganese and nickel are also used, although each has its own limitations. Nickel is not legal for use in the European Union (because of its reaction with carbon monoxide). Copper can be used everywhere except North America,[clarification needed] where its use is illegal because of the formation of dioxin.