Regeneration Mechanism of Lean NOx Trap (LNT) Catalysts

For removing NO_x from lean burn engine exhausts, lean NO_x trap (LNT) or NO_x storage and reduction (NSR) catalysts are considered as one of the technologies which has the most potential.

In the NSR reaction, NO_x is stored under lean conditions and subsequently reduced by hydrocarbons, hydrogen or carbon monoxide to nitrogen during a short rich period. However, there is a lack of understanding of the reaction mechanism, particularly when standard reaction conditions are used.

Study of Regeneration Mechanism of LNT Catalysts

This study highlights the effect of utilizing NO in both rich and lean periods throughout the NSR reaction across a 1.2 wt% Pt/15 wt%Ba/Al₂O₃ catalyst. The temporary kinetic switches, utilizing isotopically labeled ¹⁵NO during the regeneration period and ¹⁴NO during the course of the storage period allows for the analysis of ammonia and nitrogen that form when stored nitrates are reduced.

A Hiden Analytical HPR-20 mass spectrometer was used to monitor the evolution of gas phase species. For nitrogen formation, three different routes are suggested depending on various masses identified during the course of regeneration – ¹⁴N₂ (m/e=28), ¹⁴N¹⁵N (m/e=29) and ¹⁵N₂ (m/e=30) may take place.

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) or ¹⁵NO (black points). (a) 190ºC; (b) 340ºC.

During the formation of nitrogen through Route 1, a reaction occurs between ¹⁴NO_x and hydrogen to form ¹⁴NH₃. Ammonia further reacts with ¹⁴NO_x placed downstream to form ¹⁴N₂. In Route 2, it is assumed that the incoming ¹⁵NO reacts with hydrogen, forming ¹⁵NH₃ in the reactor zone where the trap has been already regenerated.

The isotopically labeled ammonia passes via the catalyst bed until it reaches the regeneration front, from where it takes part in the reduction of stored nitrates (¹⁴NO_x) to form ¹⁴N¹⁵N. ¹⁵N₂ formation through Route 3 is believed to occur through the reaction between incoming H₂ and ¹⁵NO.

References

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 N. Ireland UK.

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 QIC TMS Transient MS

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