Catalysts

The need to control, but above all limit atmospheric pollution has given a considerable boost to research into increasingly more innovative systems able to cut the emission of pollutants from diesel and petrol engines. In this respect, the by now well know catalyst is an instrument of crucial importance inasmuch as it is able to cut the emissions of carbon oxide (CO) and hydrocarbons (HC). The catalyst favours the oxidisation of the hydrocarbons and carbon oxide, transforming them into water and carbon dioxide. The catalyst does not take an active part in the reaction which it triggers and can thus be used for long periods of time and many operating hours before being replaced. The catalyst generally consists of a steel casing containing the metal honeycomb support representing the heart of the catalysts itself. Noble metals (generally platinum, rhodium and palladium) are deposited in ideal quantities on the metal support; the quantity and the type of metal are decided according to the specific application, the type of engine, the operating procedures and the performance levels to be obtained. Normally, for diesel engines, the support is only impregnated with platinum and/or palladium because in the case of diesel engines enough oxygen is available to allow the catalytic reaction. To petrol-fuelled engines, three-way catalysts are normally fitted (metal support impregnated with platinum, palladium and rhodium); into these engines, oxygen has to be introduced (by means of a Lambda probe) in order to favour the catalytic reaction. Catalysts can however be poisoned with lead and so only lead-free petrol must be used. The best catalyst performances are only achieved when the temperature of exhaust gases is high (180 – 200°C); consequently, the catalyst does not start operating immediately when the engine is started, but only after a certain time required to reach the necessary temperature