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

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The three-way catalytic converter

4.2.2 Catalyst performance

Figure 3 shows the difference in the emission levels for CO, VOC and NOx for a vehicle, with and without a three-way catalytic converter. It is evident that the catalytic converter reduces the emissions of all three classes of pollutants quite dramatically over a wide range of speeds. Before we discuss the data in any detail, a few words about how they were obtained are in order.

Figure 3
Figure 3 Emission levels for CO, VOC and NOx for petrol-engined vehicles as a function of speed, with (green) and without (black) a three-way catalytic converter.

Federal and European Test Procedures are used to test emissions from a complete ‘finished’ converter and engine together, to ensure that a new car model, for instance, will meet the current emissions legislation. Some sort of smaller-scale testing is obviously required in the laboratory. In the research and development of automotive catalysts, activity testing fulfils the function of screening and comparing novel and modified catalysts, and examining their performance under different conditions. The process of screening must provide a reliable means of identifying materials that will perform as active, selective and durable catalysts under automotive conditions. The approach usually taken is to measure conversion of the pollutants as a function of temperature, using a simulated exhaust-gas mixture flowing through a bed of powdered catalyst: the flow-rate has to be high enough to mimic the ‘through-put’ or space velocity of a catalytic converter (typically a contact time for the gases with the catalyst of 72 milliseconds is used). The test is then repeated using a different simulated exhaust-gas to represent a different engine mode. Ageing studies are performed by exposing the catalyst to different, and often extreme conditions, for varying lengths of time.

Figure 4 shows a typical graph of catalytic performance over the normal range of operating temperature, 100–600 °C. Until the incoming gases have heated the catalyst to around 250–300 °C, the activity of the catalyst is low. This temperature, at which the efficiency of the catalyst rapidly increases, is known as the light-off temperature. Until this temperature is reached, the catalyst is not working at full efficiency, and so CO, NOx and hydrocarbons will all be emitted from the exhaust pipe in significant amounts. This problem is known as cold start. Ideally the light-off temperature should be as low as possible.

Figure 4
Figure 4 Activity of a three-way catalyst for the simultaneous conversion of CO (black), NOx (solid green) and the hydrocarbon propene (C3H6) (dotted green)

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