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

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The three-way catalytic converter

4.3.7 Chemical reactions: summary

  1. Surface studies of the adsorption of CO and O2 on single crystals and model catalysts have led to the development of a possible mechanism for the oxidation of CO. Dissociatively adsorbed O atoms undergo a surface reaction with adsorbed CO, to form CO2.

  2. Under slightly fuel-rich conditions, where there is insufficient oxygen present for complete oxidation, CO can be removed by the water-gas shift reaction, using water produced in the combustion process in the engine. This is promoted by ceria.

  3. Hydrocarbons can be removed by oxidation or by reaction with water (a process known as steam reforming).

  4. Both CO and hydrocarbons can be removed by reaction with NO under stoichiometric or fuel-rich conditions. The NO-CO redox reaction is believed to proceed either by dissociation of NO(ad) followed by N atom combination, or by pairing of NO(ad) to give a dinitrosyl species, followed by dissociation. Whatever the detailed mechanism, Rh is particularly active for this reaction, and as such is currently an essential ingredient of the three-way catalyst.

  5. Ceria plays a number of important roles in the three-way catalyst: it is a structural promoter, stabilising both the noble metals and the support against particle growth and sintering; it is an oxygen-storage component, storing oxygen under fuel-lean conditions, and releasing it under fuel-rich conditions; it is a promoter for the water-gas shift and steam reforming reactions and it can enhance the low-temperature activity of the catalyst after certain types of pretreatment.

Study comment

The following SAQs invite you to collect together the different roles proposed for each component of the three-way catalyst. It would be a good plan to try them, and check your answers, before moving on.

SAQ 4

What are the major roles proposed for the different components of the Pt-Rh/CeO2–Al2O3 three-way catalyst?

Answer

The major roles proposed for the different components of the Pt-Rh/CeO2–Al2O3 three-way catalyst can be summarised as follows:

Platinum

  • CO oxidation

  • HC oxidation

  • steam reforming and water-gas shift reactions (with CeO2)

Rhodium

  • CO–NO redox reaction

  • steam reforming and water-gas shift reactions (with CeO2)

Ceria

  • oxgen storage

  • structural promoter maintaining both metal and support surface areas

  • chemical promoter for the water-gas shift and steam reforming reactions

  • enhancement in low-temperature activity of Pt after reducing pretreatment

SAQ 5

In Figures 13, 14 and 15, Pd in its fresh state is seen to be superior to Pt for the conversion of all three pollutants. Considering the properties we require of a catalyst, what possible reason can you suggest for the current widespread use of Pt in three-way catalysts, rather than Pd?

Answer

Pd is in fact cheaper than Pt (Box 1), so the reason for using Pt is not an economic one. The principal properties we require of a catalyst are activity, selectivity and stability. Because we have seen that the first two of these are at least as good for Pd as for Pt, this should lead us to consider the third, stability. In fact, as we shall discover in section 5.3, Pd has a high susceptibility to poisons, especially lead and sulfur. In addition, it sinters in reducing atmospheres, and can also form an alloy with Rh, reducing the activity of the latter.

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