12 Three way catalytic converter

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Three way catalytic converter The most efficient way to clean the exhaust gases from pollutants is to force them through the catalytic converter (or three-way catalyst). The operation made by a three-way catalyst are: - Reduction of NOx - Oxidation of CO and HC To maximize the catalyst efficiency, a large surface area in contact with gas flows is necessary, because first of all reactants must be absorbed into the active surface sites, then reactions occur and then the products have to be desorbed. Thus the exhaust gases are guided by a metal casing through the passageways of a ceramic monolith with a honeycomb structure. This solution offers the advantages of a high contact surface, low fluid dynamic losses as well as good mechanical and thermal strength. The active catalysts are impregnated into a wash-coat of highly porous alumina. Noble metals are the most suitable as catalytic materials: Platinum (Pt) and palladium (Pa) are used to create highly active sites for oxidation of CO and HC. Rodium (Rh) acts in reduction of NOx Cerium oxides are very useful: they store oxygen in the lean cycles (formation of Ce2O3), making it available when the engine is running with a rich air/fuel ratio to oxidise CO, H2 and HC (formation of CeO2). The catalytic converter is very efficient (>80%) in a narrow window close to the stoichiometric. Catalyst conversion efficiency depends also on: - Gas residence time inside the converter - Substrate temperature The optimal range is (300 –800 °C). T < 300 °C: low conversion efficiency T > 800 °C: active site sintering with agglomeration of catalyst and decrease of the active area - Light-off temperature: ensures at least a 50% conversion efficiency. Needs to be reached as soon as possible and influence the positioning of the catalyst in the…