Reduction and 3-Way Catalytic Converters

For reduction of NO_x, the CO, HC and H₂which are present in the exhaust during rich engine operation are made to react with NO_x over a catalyst when NO_x is converted to N₂ . Possible reactions under reducing conditions are given in Table 5.3.

1.	NO + CO + CO2
2.	NO + 5CO + 3H2 O 2NH3 + 5CO2
3.	2NO + CO N2O + CO2
4.	NO + H2 + H2O
5.	2NO + 5H2 2NH3 + 2H2O
6.	2NO + H2 N2 O + H2O

A number of catalysts like CuO, NiO, monel etc., were studied for NO_x reduction However, their NOx conversion efficiency being just 25 to 35 % only, they did not find acceptance. Rhodium (Rh) from the noble metal group has been found to be the most successful reduction catalyst so far and is being used in actual systems. .
The concentration of NO_x being relatively small, only small fractions of CO, HC present in exhaust is utilized in the reduction reactions. Most of HC and CO is required to oxidized simultaneously or in a separate reactor.. Initially, separate reduction and oxidation catalyst systems were considered but their operation was complex, the engine had to be operated rich. Also, some ammonia is generated during reduction reactions, which is oxidized again to NO in the downstream oxidation reactor. Due to these reasons, the dual-bed catalysts did not find much acceptance and the 3-Way catalyst was developed and used on cars around 1980.