Regeneration mechanism of Pt/BaO/Al2O3 lean NOx trap catalyst with H2

Regeneration of a Pt/BaO/Al2O3 NOx storage-reduction catalyst was studied using H-2 as a model reductant. We propose that the regeneration process involves a localized reaction front of the reductant that travels through the catalyst bed with complete regeneration of the trapping sites. When NH3 was used as a regenerating gas instead of H-2, the process was found to be equivalent to that with H-2, and NH3 was found to be as effective as H-2 in regenerating the trap catalyst. The process was limited only by the supply of the hydrogen atoms, irrespective of the source of hydrogen (H-2 or NH3) and was also not affected by temperature changes or by the presence Of CO2 and H2O. Even though Pt by itself is not very selective in reducing the NO, to N-2 in a NOx/H-2 feed stream, a high N-2 selectivity is maintained over the Ba-containing trap catalyst since the undesirable byproducts, mainly NH3, continue to react further until they are optimally converted to N-2. In fact, we propose that the regeneration of the trap catalyst using H-2 as the reductant involves the formation of NH3 as an intermediate from the stored NOx, and NH3 acts as a carrier of H-atoms. The NH3 ultimately gets further oxidized to N-2 by the stored NOx, the oxygen source on the catalyst, giving high selectivity towards N-2. (C) 2008 Elsevier B.V. All rights reserved.