contd...
NOx storage-reduction (NSR) catalyst
In the NSR catalysts NOx is trapped under oxygen-rich conditions in an alkaline earth material like BaO which is incorporated in the noble-metal containing washcoat of 3-way catalyst. The NOx trap concept is shown schematically in Fig. 5.24 .
| Figure 5.24 |
Schematic of a NSR Catalyst (NO x trap). |
NO is first converted to NO2 over Pt catalyst from the oxygen rich exhaust
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(5.7) |
The NO2 is temporarily stored on alkaline metal oxide BaO forming its nitrate,
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(5.8) |
The trap is saturated with NOx in about 60 seconds. A precisely controlled spike of rich mixture is then supplied to the engine so that HC and CO in significant amounts are present in the gases. The required rich mixture for SI engine may be obtained by the
synchronized control of fuel injection pulse width. The stored NOx is reduced by the HC and CO) during a short period of about a second.. The NOx reduction takes place on a noble metal catalyst.
 |
(5.9) |
Lean NOx traps have high conversion efficiency in a relatively narrow temperature range of 350 –450º C. However, different vehicle operating modes produce widely differing exhaust gas temperatures and the overall conversion efficiency of NSR catalysts is 30 to 35% only. Another problem is poisoning of the trap by fuel sulphur. Fuel sulphur of less than 5 ppm is necessary for operation of NSR catalysts.
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