The catalytic converter makes harmful substances (CO, HC and NOx) react chemically with harmless ones (H2O, CO2, N2) according to the exhaust gas passed. Generally platinum, palladium, iridium and rhodium, etc., are used as catalysts for automobiles.
(1) Types of catalyst
Oxidation catalystOxidize HC or CO and form nonpolluting H2O or CO2.
Deoxidization catalystRemove oxygen from NOx and form non-polluting N2.
Oxidation/Deoxidization catalystPerform above two functions at the same time. (Oxidation/Deoxidization catalyst for automobiles is called Three-Way Catalytic Converter (TWC)because three harmful substances, CO/HC/NOx, are transformed into non-polluting substances at the same time. Oxidation/Deoxidization catalyst is used for most automobiles nowadays.)
(2) Operating temperature catalyst
For the catalyst, the purification rate changes according to the temperature. As shown in the graph, the purification rate is close to 100% and purifies the exhaust gas effectively when the catalyst temperature rises over 40C.
Vehicles equipped with the catalytic converter need to use unleaded gasoline, because lead adheres to the surface of catalyst and the oxygen sensor (O2 sensor), and the appropriate effect cannot be attained if leaded gasoline is used.
(3) Three-Way Catalyst (TWC) system
The TWC is the system that oxidizes CO and HC in the exhaust gas and deoxidizes NOx at the same time, to purify them into CO2, H2O and N2. Recently the monolithic type is used as shown in the illustration. Alumina or catalytic substances coat the grated pattern of monolith which has many holes. The harmful substances are purified by passing through the holes. There are two types of monolith; ceramic type and metal type. The thinner the grate is, the higher the purification ability becomes. The TWC can be most effective around the theoretical air-fuel ratio. Therefore, the air-fuel ratio feedback system is needed to keep the air-fuel ratio around the theoretical air-fuel ratio. The air-fuel ratio feedback system detects oxygen in the exhaust gas, using the oxygen sensor attached in the exhaust manifold. Then the fuel injection volume is adjusted by the engine ECU to control the air-fuel ratio at all times so that the TWC performs properly.