Three-way catalysis means the catalysis that converts the CO, HC, NOX and other harmful gases discharged from automobile exhaust gas into harmless carbon dioxide, water and nitrogen through oxidation and reduction reaction. It is mainly use three-way catalysts. The carrier part of three-way catalysts is a piece of porous ceramic material, which is installed in the tailor-made exhaust pipe. Name it as carrier because it does not participate in catalytic reactions, but is covered with a layer of platinum, rhodium, palladium and other precious metals and rare earth coating. It is the most important external purifying device installed in automobile exhaust system.