4.4 The chemical reactions
Since its development, the three-way catalyst has been exposed to the full spectrum of techniques available for the characterisation of catalytic materials. The data provided can be correlated with the results of activity tests and kinetic measurements, which provide information on the performance of the catalyst. This reveals that although the catalyst functions as a composite material, it can be divided into distinct groups of catalytic centres that provide several different types of site, active for one or more of the many different reactions. The participation of a particular type of site at any given moment will depend on the conditions experienced by the catalyst; for example, whether the gases are a net reducing, stoichiometric, or oxidising mixture.
Measurements of intrinsic kinetics are usually carried out on simple gas mixtures to allow activation energies and reaction orders to be calculated for specific reactions. The data can often contribute to an understanding of the mechanisms by which the surface reactions occur. They are also used to create reaction models that will predict the performance of the catalyst under various anticipated conditions.
The overall reaction scheme is complicated, with many contributing processes. The strategy of the three-way catalyst is to simultaneously remove CO, HC and NOx, and our treatment will accordingly be divided into three subsections. The desired reactions can be expressed in simple terms as follows:
Removal of CO
Water-gas shift (WGS) reaction:
Removal of hydrocarbons
Removal of NO (plus CO or HC (not shown))
CO + NO redox reaction:
or with hydrogen:
Any number of these reactions may be occurring simultaneously as the A/F ratio goes through its cycle about the stoichiometric composition. The following subsections will look more closely at the removal of each of the pollutants under various conditions, and will also examine the role of the catalyst components.
The supported commercial catalyst is the one most difficult to study because of its complexity, with a large number of different components – Pt, Rh, Al2O3, CeO2, BaO, etc. – present in one catalyst. It is therefore often simpler to study model systems, such as Pt/Al2O3 or Rh/CeO2, and if certain surface-science techniques are to be used, the ‘catalyst’ under study has to be even simpler – a particular face of a metal single crystal. These studies, often performed under ultrahigh vacuum (UHV), are far removed from the real catalyst system and the conditions it experiences. Hence, it cannot be assumed automatically that the results will be directly relevant to what is actually happening in a converter fitted to an operational vehicle.