Regeneration mechanism of a Lean NOx Trap (LNT) catalyst in the presence of NO investigated using isotope labelling techniques

The presence of NO during the regeneration period of a Pt–Ba/Al2O3 Lean NOx Trap (LNT) catalyst modifies significantly the evolution of products formed from the reduction of stored nitrates, particularly nitrogen and ammonia. The use of isotope labelling techniques, feeding 14NO during the storage period and 15NO during regeneration allows us to propose three different routes 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). It is proposed that the formation of nitrogen via Route 1 involves the reaction between hydrogen and 14NOx released from the storage component to form 14NH3 mainly. 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 form 15NH3 in the reactor zone where the trap has been already regenerated. This isotopically labelled 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. The modification of the hydrogen concentration fed during regeneration affects the relative importance of H2 or 15NH3 as reductants and thus the production of 14N2 via Route 1 and 14N15N via Route 2.