NOx Storage and Reduction (NSR) or Lean NOx Trap (LNT) catalysts are considered to be one of the most promising technologies for NOx removal from lean burn engine exhausts. In the NSR reaction NOx 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 14NOx to form mainly 14NH3. Then, ammonia further reacts with 14NOx 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 (14NOx) 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 NOx 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