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Main points: Catalyst Poisoning, Storage-Reduction Catalyst, Catalyst Deactivation, Selective Catalytic Reduction, Alkaline Metal Oxide, Noble Metal Catalyst, High Conversion Efficiency, Lean-Mixture Environment, Thermal Deactivation
Typology: Study notes
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Lean de-NO (^) x Catalysts
NO (^) x storage-reduction (NSR) catalyst
SCR Catalysts
CATALYST DEACTIVATION
Catalyst Poisoning
The gasoline direct injection (GDI) engine operating in stratified charge mode is a a lean-burn spark- ignited engine that gives 20 – 30% higher fuel efficiency compared to the conventional stoichiometric engine The diesel engines also operate with 40% or more excess air. The 3-way catalytic converter cannot provide NO (^) x reduction^ in^ the^ lean^ burn^ SI^ and^ the^ diesel^ engines. Lean de-NO^ x catalyst
technology has been developed to meet the needs of these engines. Two main types of lean de-NO (^) x
catalyst technology are;
NOx storage-reduction (NSR) catalyst or NO (^) x trap Selective catalytic reduction (SCR)
Selective reduction catalysts applied in SI engines work on the principle of direct reduction of NO (^) x 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, NO (^) x may be reduced by HC as per
the following reaction:
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.
The automotive catalysts in the USA are required to meet the emission standards for 192,000/240, 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.
which seals the passage of exhaust gas to the catalyst sites. Silicon coming from contamination of fuel clogs the protective sheath of the sensor restricting the diffusion of gases to the surface of the sensor element. It affects the response of oxygen sensor which adversely affects conversion efficiency of the closed loop controlled three-way catalysts.
