Regeneration Mechanism of Lean NOx Trap (LNT) Catalysts

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

In the NSR reaction, NOx 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.

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.

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 14NOx and hydrogen to form 14NH3. Ammonia further reacts with 14NOx 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 (14NOx) 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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This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.

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