The three-way catalytic converter
The three-way catalytic converter

This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course

The three-way catalytic converter

4.2 The three-way catalytic converter

4.2.1 Composition

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.

Figure 1
Figure Schematic diagram of the three-way catalytic converter. The catalytic converter, in a metal canister, is placed in the exhaust system of the vehicle. As the exhaust gases pass through it, they flow through the channels in the ceramic monolith, where they encounter the particles of alumina impregnated with the metal catalysts.
Figure 2
Figure 2 Electron micrograph of a cross section of a ceramic monolith coated with an alumina washcoat

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.

S342_1

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to university level study, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus