Regeneration mechanism of Lean NOx Trap (LNT) catalyst

NO_x Storage and Reduction (NSR) or Lean NO_x Trap (LNT) catalysts are considered to be one of the most promising technologies for NO_x removal from lean burn engine exhausts. In the NSR reaction NO_x is stored under lean conditions and then reduced by H₂ or CO or hydrocarbons to N₂ during a short rich period. However, the reaction mechanism is not well-understood especially when using typical reaction conditions.

The current study highlights the effect of using NO in both lean and rich periods during the NSR reaction over a 1.2 wt% Pt/15 wt%Ba/Al₂O₃ catalyst. The transient kinetic switches, using ¹⁴NO during the storage period and isotopically labelled ¹⁵NO during the regeneration, allows us to analyse for nitrogen and ammonia formed from the reduction of stored nitrates. The evolution of gas phase species was monitored by a Hiden Analytical HPR 20 mass spectrometer. Three different routes are proposed for nitrogen formation based on the different masses detected during regeneration, i.e. ¹⁴N₂ (m/e=28), ¹⁴N¹⁵N (m/e=29) and ¹⁵N₂ (m/e=30) may take place. The formation of nitrogen via Route 1 involves the reaction between hydrogen and ¹⁴NO_x to form mainly ¹⁴NH₃. Then, ammonia further reacts with ¹⁴NO_x located downstream to form ¹⁴N₂. In Route 2 it is postulated that the incoming ¹⁵NO reacts with hydrogen to form¹⁵NH₃ in the reactor zone where the trap has been already regenerated. This isotopically labeled ammonia travels through the catalyst bed until it reaches the regeneration front where it participates in the reduction of stored nitrates (¹⁴NO_x) to form ¹⁴N¹⁵N. The formation of ¹⁵N₂ via Route 3 is believed to occur by the reaction between incoming ¹⁵NO and H₂.

Figure 1: Evolution of ¹⁵N₂ or ¹⁴NO (m/e=30), ¹⁴N₂ (m/e=28) and ¹⁵N¹⁴N (m/e=29) during LNT regeneration, in the presence of ¹⁴NO (red points) or¹⁵NO (black points). (a) 190ºC; (b) 340ºC.

Project Summary By: Sarayute Chansai, Ph.D., Research Fellow, CenTACat, School of Chemistry and Chemical Engineering, Queen’s University Belfast, DKB, Stranmilis Road, Belfast BT9 5AG

Paper Reference: Beñat Pereda-Ayo et al. (2012) “Regeneration mechanism of a Lean NO_x Trap (LNT) catalyst in the presence of NO investigated using isotope labelling techniques” Journal of Catalysis 285 (1), 177-186

Hiden Product: HPR-20 TMS Transient MS

Read Full Article:  Customer Research Review : Gas reaction studies & catalysis research [issue : 1120/10]