The main harmful components of automobile exhaust are hydrocarbons (CnHm), carbon monoxide (CO) and nitrogen oxides (nitrogen oxides). These three substances are toxic to human body, among which CnHm and nitrogen oxides will also form photochemical smog under suitable conditions such as sunlight, which is more harmful. There are two main schemes to eliminate these harmful components in automobile exhaust: one is to improve the combustion mode of the engine to reduce the emission of harmful gases; The other is to use catalytic converter to purify harmful gases in tail gas. First of all, in 1975, the United States installed catalytic converters on new cars, and then Japan, Western Europe and other countries also adopted catalytic converters to meet the requirements of their own automobile emission regulations. There are two types of automotive catalytic converters. One is oxidation catalytic reactor, which makes CnHm and CO in the tail gas react with the residual oxygen in the tail gas to generate harmless H2O and CO2, thus achieving the purpose of purification.
Due to the mandatory emission standards of pollutants such as nitrogen oxides and the requirement of reducing fuel consumption, on the one hand, the air-fuel ratio should be controlled at 14.6 as much as possible, and on the other hand, methods such as controlling ignition timing and exhaust gas recirculation should be adopted to reduce nitrogen oxides in tail gas. However, the disadvantage of these methods is that they often increase CnHm and CO in tail gas. In order to solve this problem, three-way catalyst (TWC) appeared. The characteristic of this catalyst is that it can simultaneously purify carbon monoxide (CO), hydrocarbons (CnHm) and nitrogen oxides (nitrogen oxides) in automobile exhaust with one catalyst. However, in order to exert its catalytic performance, the air-fuel ratio must always be controlled at around 14.6 0. 1. This catalytic purifier has high purification rate, but it needs oxygen sensor, multi-point fuel injection and electronic ignition. This catalytic purifier takes O2 and nitrogen oxides in the tail gas as oxidants and CO, CnHm (represented by CH2) and H2 as reductants, and the following reactions can occur near the theoretical air-fuel ratio:
2CO+O2=2CO2
2CO+2NO=N2+2CO2
CH2 +3nNO= nN2+nCO2+nH2O
2NO+2H2=N2+2H2O
At present, most of the three-way catalysts used are based on porous ceramics, and then the so-called coating is attached, and finally the active components are adsorbed by impregnation. The active components of the catalyst are mainly precious metals, such as platinum (Pt), palladium (Pd) and rhodium (Rh). Because precious metal resources are scarce and expensive, scientists all over the world are committed to the research of rare earth/palladium three-way catalyst which is economically and technically feasible. It is expected that the catalyst has a good application prospect.
Three-way catalytic purifier has the advantages of good purification rate and fuel economy, but the main problem is high cost. Because there is a lot of residual oxygen in the gas emitted by diesel engine, the control of oxygen sensor is not sensitive, so the three-way catalytic purifier is generally not used in diesel engine, but only suitable for gasoline engine.