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SCR Catalysts
Selective reduction catalysts applied in SI engines work on the principle of direct reduction of NOx by hydrocarbons injected into exhaust stream. In the diesel engines the SCR system uses ammonia produced from urea is used as the reducing agent or ‘reductant'. The urea SCR systems would be discussed in detail in Module 6. In the lean-mixture environment, NOx may be reduced by HC as per the following reaction:
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(5.10) |
The exhaust gas stream should have right type of HC in right concentrations to complete the above reaction and reduce nitrogen oxides. Propane is effective at around 500º C and ethylene at 160-200º C. Zeolites like Cu/ZSM-5 have been studied as SCR catalysts. However, these catalysts are sensitive to water vapour and sulphur dioxide, and hence so far, have had only a limited success.
CATALYST DEACTIVATION
The automotive catalysts in the USA are required to meet the emission standards for 192,000/240,000 km. of life, The fresh catalysts while meet the standards but during vehicle operation their conversion efficiency deteriorates due to ageing and poisoning effects by the contaminants that may come from fuel or engine lubricating oil that burns in the cylinder. The catalyst is subjected to high temperatures exceeding 900 ºC, thermal shocks and mechanical vibrations. Contaminants originating from fuel that cause serious catalyst poisoning, are sulphur and lead (now the gasoline is free of lead), and from lubricating oil are zinc and phosphorous compounds. |
