Deactivation Correlations of Pd/Rh Three-way Catalysts Designed for Euro IV Emission Limits: Effect of Ageing Atmosphere, Temperature and Time
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Chapter 2. Three-way catalysis

Table of Contents
2.1. General
2.2. Structure of a three-way catalyst
2.3. Phenomena in the three-way catalyst operation

2.1. General

Automotive exhaust gases formed in the gasoline engines contain many environmentally harmful compounds. As a result of incomplete combustion, exhaust gases can include carbon monoxide (CO) and hydrocarbons (HC). A typical composition of exhaust gases is presented in Table 1. Catalytic purification has proven to be an efficient way to reduce emissions from exhaust gases. Due to the increased demand for low-emission vehicles, most automobiles are currently supplied with a three-way catalytic converter (TWC) for the simultaneous removal of major pollutants CO, NO_X and HC from a gasoline engine’s exhaust gases (Heck & Farrauto 2001). The current and forthcoming European emission limits for gasoline engines are presented in Table 2.

Table 1. Typical concentrations of the exhaust gas constituents of gasoline-fuelled engines. Air-to-fuel ratio contributes significantly to these concentrations (Taylor 1993).

HC	750 ppm*	CO₂	13.5 vol-%
NO_X	1050 ppm	O₂	0.51 vol-%
CO	0.68 vol-%	H₂O	12.5 vol-%
H₂	0.23 vol-%	N₂	72.5 vol-%
* Based on C₃

Table 2. European emission limits (g/km) for gasoline-fuelled passenger cars and light commercial vehicles (Directive 98/69/EC).

Stage:	In new types:	CO (g/km)	HC (g/km)	NO_X (g/km)
EURO III	1.10.2000	2.3	0.20	0.15
EURO IV	1.10.2005	1.0	0.10	0.08

The currently-used exhaust gas purification system consists of a catalytic converter and an electronically controlled air/fuel management system as shown in Fig. 1. The oxygen sensor measures the net oxygen content that is proportional to stoichiometry in the exhaust gas. The air inlet and fuel injection are controlled to provide a stoichiometric ratio between oxygen (air) and fuel. The objective is to keep the air-to-fuel ratio (A/F-ratio) within the so-called lambda window (as presented in Fig. 2). In this narrow window, the high conversions (> 80-90%) of CO, HC and NO_X are achieved simultaneously. If the A/F-ratio is below 14.6, the exhaust gas contains more reducing reactants (CO, HC) than oxidizing reactants (O₂, NO_X) and the engine operates under rich conditions. If the A/F-ratio exceeds 14.6, the engine operates under lean conditions. The reduction reactions of NO_X are favoured under rich conditions, whereas the lean conditions favour the catalytic oxidation reactions of CO and hydrocarbons (Lox & Engler 1997). The refinement of the engine management system affects both the performance and the durability of the emission control system. Due to its performance in promoting the main reactions to reach completion and at the same time minimizing the extent of the secondary reactions, the closed-loop-controlled three-way catalyst has become the most widely applied technique for catalytic emission control. (Shelef & McCabe 2000)

Figure 1. Purification system of exhaust gases of gasoline engines. Purification system includes an electrically controlled air-to-fuel management system (cf. Holmgren 1998).

Figure 2. The conversion efficiency (%) of a three-way catalyst as a function of A/F-ratio. The lambda window, an A/F-ratio of 14.6 corresponds to stoichiometric operation, λ =1 (cf. Holmgren 1998).

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Scope and outline of the work		Structure of a three-way catalyst