Application of spaciMS to the study of ammonia formation in lean NOx trap catalysts

Abstract SpaciMS was employed to understand the factors influencing the selectivity of NO x reduction in two fully formulated LNT catalysts, both degreened and thermally aged. Both catalysts contained Pt, Rh, BaO and Al 2 O 3 , while one of them also contained La-stabilized CeO 2 . The amount of reductant required to fully regenerate each catalyst was first determined experimentally based on the OSC of the catalyst and the NO x storage capacity (NSC). In this way a correction was made for the change in catalyst OSC and NSC after aging, thereby eliminating these as factors which could affect catalyst selectivity to NH 3 . For both catalysts, aging resulted in an elongation of the NO x storage–reduction (NSR) zone due to a decrease in the concentration of NO x storage sites per unit catalyst length. In addition to decreased lean phase NO x storage efficiency, stretching of the NSR zone affected catalyst regeneration. Three main effects were identified, the first being an increase of the NO x “puff” that appeared during the onset of the rich front as it traversed the catalyst. Spatially, NO x release tracked the NSR zone, with the result that the NO x concentration peaked closer to the rear of the aged catalysts. Hence the probability that NO x could re-adsorb downstream of the reduction front and subsequently undergo reduction by NH 3 (formed in the reduction front) was diminished, resulting in higher rich phase NO x slip. Second, the stretching of the NSR zone resulted in increased selectivity to NH 3 due to the fact that less catalyst (corresponding to the OSC-only zone downstream of the NSR zone) was available to consume NH 3 by either the NH 3 -NO x SCR reaction or the NH 3 -O 2 reaction. Third, the loss of OSC and NO x storage sites, along with the decreased rate of NO x diffusion to Pt/Rh sites (as a result of Pt/Rh–Ba phase segregation), led to an increase in the rate of propagation of the reductant front after aging. This in turn resulted in increased H 2 :NO x ratios at the Pt/Rh sites and consequently increased selectivity to NH 3 .

[1]  Do Heui Kim,et al.  Promotional Effects of H2O Treatment on NOx Storage Over Fresh and Thermally Aged Pt–BaO/Al2O3 Lean NOx Trap Catalysts , 2008 .

[2]  George G. Muntean,et al.  Relationship of pt particle size to the NOx storage performance of thermally aged Pt/BaO/Al2O3 lean NOx trap catalysts , 2006 .

[3]  Michael P. Harold,et al.  NOx storage and reduction with H2 on Pt/BaO/Al2O3 monolith : Spatio-temporal resolution of product distribution , 2008 .

[4]  Jae-Soon Choi,et al.  Sulfur impact on NOx storage, oxygen storage, and ammonia breakthrough during cyclic lean/rich operation of a commercial lean NOx trap , 2007 .

[5]  T. Egami,et al.  Lattice Defects and Oxygen Storage Capacity of Nanocrystalline Ceria and Ceria-Zirconia , 2000 .

[6]  Michael P. Harold,et al.  Pt dispersion effects during NOx storage and reduction on Pt/BaO/Al2O3 catalysts , 2009 .

[7]  Jae-Soon Choi,et al.  NOx storage–reduction characteristics of Ba-based lean NOx trap catalysts subjected to simulated road aging , 2010 .

[8]  Todd J. Toops,et al.  Intra-fuel cell stack measurements of transient concentration distributions , 2006 .

[9]  P. Forzatti,et al.  The reduction of NOx stored on LNT and combined LNT–SCR systems , 2010 .

[10]  R. Zukerman,et al.  Modeling and simulation of a smart catalytic converter combining NOx storage, ammonia production and SCR , 2009 .

[11]  Todd J. Toops,et al.  Effect of engine-based thermal aging on surface morphology and performance of Lean NOx Traps , 2007 .

[12]  Jae-Soon Choi,et al.  Spatiotemporal distribution of NOx storage and impact on NH3 and N2O selectivities during lean/rich cycling of a Ba-based lean NOx trap catalyst , 2012 .

[13]  Pio Forzatti,et al.  Mechanistic aspects of the reduction of stored NOx over Pt–Ba/Al2O3 lean NOx trap systems , 2008 .

[14]  Bernd Krutzsch,et al.  Modelling of a combined NOx storage and NH3-SCR catalytic system for Diesel exhaust gas aftertreatment , 2010 .

[15]  M. Fernández-García,et al.  Influence of thermal sintering on the activity for CO–O2 and CO–O2–NO stoichiometric reactions over Pd/(Ce, Zr)Ox/Al2O3 catalysts , 2002 .

[16]  Michael P. Harold,et al.  Modeling the effect of Pt dispersion and temperature during anaerobic regeneration of a lean NOx trap catalyst , 2010 .

[17]  M. Crocker,et al.  The effect of regeneration conditions on the selectivity of NOx reduction in a fully formulated lean NOx trap catalyst , 2011 .

[18]  Jae-Soon Choi,et al.  Sulfur and temperature effects on the spatial distribution of reactions inside a lean NOx trap and resulting changes in global performance , 2008 .

[19]  Pio Forzatti,et al.  Role of ammonia in the reduction by hydrogen of NOx stored over Pt–Ba/Al2O3 lean NOx trap catalysts , 2008 .

[20]  Michael P. Harold,et al.  Selective catalytic reduction of NO by H2 in O2 on Pt/BaO/Al2O3 monolith NOx storage catalysts☆ , 2008 .

[21]  Jae-Soon Choi,et al.  NH3 formation and utilization in regeneration of Pt/Ba/Al2O3 NOx storage-reduction catalyst with H2 , 2009 .

[22]  Jae-Soon Choi,et al.  Influence of Ceria on the NOx Storage/Reduction Behavior of Lean NOx Trap Catalysts , 2008 .

[23]  U. Graham,et al.  NOx Reduction on Fully Formulated Lean NOx Trap Catalysts Subjected to Simulated Road Aging: Insights from Steady-State Experiments , 2011 .

[24]  Jae-Soon Choi,et al.  Axial length effects on lean NOx trap performance , 2009 .

[25]  J. Grunwaldt,et al.  The fate of platinum in Pt/Ba/CeO2 and Pt/Ba/Al2O3 catalysts during thermal aging , 2007 .

[26]  William S. Epling,et al.  Spatially resolving LNT desulfation: re-adsorption induced by oxygen storage materials , 2011 .

[27]  Louise Olsson,et al.  Reduction of NOx over a combined NSR and SCR system , 2010 .

[28]  Jae-Soon Choi,et al.  Effect of Aging on the NOx Storage and Regeneration Characteristics of Fully Formulated Lean NOx Trap Catalysts , 2011 .

[29]  F. Ribeiro,et al.  Regeneration mechanism of Pt/BaO/Al2O3 lean NOx trap catalyst with H2 , 2008 .

[30]  Jae-Soon Choi,et al.  Spatially resolved in situ measurements of transient species breakthrough during cyclic, low-temperature regeneration of a monolithic Pt/K/Al2O3 NOx storage-reduction catalyst , 2005 .

[31]  Neal W. Currier,et al.  Ammonia is a hydrogen carrier in the regeneration of Pt/BaO/Al2O3 NOx traps with H2 , 2007 .

[32]  W. Epling,et al.  Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/Reduction Catalysts , 2004 .

[33]  Jae-Soon Choi,et al.  Intra-channel evolution of carbon monoxide and its implication on the regeneration of a monolithic Pt/K/Al2O3 NOx storage-reduction catalyst , 2006 .

[34]  Bernd Krutzsch,et al.  Evaluation of NOx Storage Catalysts for Lean Burn Gasoline Fueled Passenger Cars , 1997 .