4.2 The three-way catalytic converter
The current three-way catalyst, shown schematically in Figure 1, is generally a multicomponent material, containing the precious metals rhodium, platinum and (to a lesser extent) palladium, ceria (CeO2), γ-alumina (Al2O3), and other metal oxides. It typically consists of a ceramic monolith of cordierite (2Mg.2Al2O3. 5SiO2) with strong porous walls enclosing an array of parallel channels. A typical monolith has 64 channel openings per cm2 (400 per in2), This design allows a high rate of flow of exhaust gases Cordierite is used because it can withstand the high temperatures in the exhaust, and the high rate of thermal expansion encountered when the engine first starts – typically, the exhaust gas temperature can reach several hundred degrees in less than a minute. Metallic monoliths are also used, particularly for small converters, but these are more expensive.
To achieve a large surface area for catalysis, the internal surfaces of the monolith are covered with a thin coating (30–50 μm) of a highly porous material, known as the washcoat (Figure 2). The total surface area is now equivalent to that of about two or three football pitches. The washcoat generally consists of alumina (70–85%) with a large surface area, with oxides, such as BaO, added as structural promoters (stabilisers to maintain surface area) and others, for example CeO2, as chemical promoters. This system becomes the support for the precious metal components (Pt, Pd and Rh). These metals constitute only a small fraction (1–2%) of the total mass of the washcoat, but they are present in a highly dispersed form. They are generally applied by deposition from solution, although they may instead be introduced during formation of the washcoat itself. Exact catalyst formulations are, as one might expect, closely guarded secrets. Some compositions use all three metals; others use Rh together with only one of the other two, typically Pt, as in the present generation of Pt-Rh converters used in the UK, in which Pt constitutes 80–90% of the total precious metal mass.