OXIDATION AND 3-WAY CATALYTIC CONVERTERS

Since 1975, in the production gasoline vehicles two main types of catalytic converters have been used:

• Oxidation catalytic converters
• 3-Way catalytic converters.

The catalytic converters for the first time were used to reduce only HC and CO emissions from the US gasoline passenger cars in 1975. As these converters reduced HC and CO by oxidation, they were called as ‘oxidation' catalytic converters. NO_x emission standards were met by use of EGR at that time. The engines were operated on rich mixtures and with application of EGR engine out NO_x emissions were reduced. Secondary air was injected in the exhaust system upstream of the converter to provide sufficient oxygen for oxidation of CO and HC on the catalyst. Later, when the NO_x standards were made stringent from 1981, reduction catalysts were also developed. An exhaust gas oxygen sensor developed in the early 1980s facilitated engine operation at near stoichiometric mixtures that made it possible to simultaneously oxidize CO and HC to CO₂ and H₂O and reduce NO_x to N₂ . As all the three pollutants were converted simultaneously in the same reactor these were termed as 3-way catalytic converters. T he three-way catalytic converter is presently a standard fitment on most gasoline passenger cars.
During mid-1990s, gasoline direct injection (GDI) engines with charge stratification were introduced in the market by two Japanese car manufacturers. During part load city operation, these GDI engines work as stratified-charge engines with overall very lean air/fuel mixtures. For NO_x control in these engines, lean de-NO_xcatalytic converters have been developed. The de-NO_x x catalysts and other advanced catalyst systems such as for cold start HC control are discussed later.