There are three precious metals, platinum, rhodium, and palladium inside the TWC Universal Catalytic Converters, which can promote the harmful gases HC, CO, and nitrogen oxides produced by the engine to react with each other, and finally produce harmless carbon dioxide, nitrogen, and water.inquiry
The three-way universal catalytic converters use precious metals as catalysts and uses CO and HC in the exhaust gas as reducing agents to reduce NOx to N2 and O2, and CO and HC are oxidized to CO2 and H2O in the reduction reaction.
Structure: shield, shock absorption layer, heat insulation layer, catalyst substrate, and catalyst coating.
Catalyst Substrate and Coating
Cordierite (2MgO·2Al2O3·5SiO2), due to its low thermal expansion coefficient, outstanding thermal shock resistance, easy availability of raw materials, and low cost, is widely used as a vehicle exhaust catalyst substrate. 95% of the currently used automotive catalyst substrate are honeycomb cordierite ceramic substrates.
The coating is also called the “second catalyst substrate”, which is a porous material, insoluble in water, soluble in strong acid or alkali solutions, and has a strong surface adsorption capacity. Its function is to provide a large surface area to attach catalytic components such as precious metals. Compared with the specific surface area of the cordierite substrate of 1m2/g, the specific surface area per gram of the catalyst coating is as high as hundreds of square meters.
There is a thin layer of platinum, palladium, or rhodium on the surface of the small holes of the honeycomb ceramic block in the converter.
Why choose Pt (Platinum), Pd (Palladium), and Rh (Ming)?
1. Compared with base metals, precious metals have better thermal stability, durability, and lower reaction tendency with carrier materials.
2. Among the platinum group metals, the reason for not choosing other metals is: Ru (ruthenium), Ir (iridium), and Os (osmium) will form volatile oxides.
3. Platinum and palladium are catalysts for oxidation reactions, and palladium is a catalyst for reduction reactions
The interaction of precious metals and catalyst substrate:
1. The carrier can improve the dispersion of precious metals
2. A good carrier can inhibit the sintering of precious metals at high temperatures
3. Noble metals can improve the redox and oxygen storage properties of the carrier
4. The interaction of noble metal and carrier improves the catalytic activity of exhaust gas