Resolving N2O formation dynamics during lean/rich cycling of a commercial LNT

Transcription

1 Resolving N2O formation dynamics during lean/rich cycling of a commercial LNT Jae-Soon Choi, Josh A. Pihl, Mi-Young Kim, Todd J. Toops, William P. Partridge Oak Ridge National Laboratory Petr Kočí, David Mráček, Šárka Bártová, Miloš Marek University of Chemistry and Technology, Prague CLEERS Workshop April 28, 2015 Dearborn, MI ORNL is managed by UT-Battelle for the US Department of Energy

2 LNTs remove NO x via cyclic lean/rich operation Storage (lean) s Regeneration (rich) 1-10 s prim. N 2 CO lean phase (storage) prim. sec. N 2 sec. rich phase (reduction) 2 complex spatiotemporal development of regeneration reactions

3 Significant can form during LNT regeneration Cycle-averaged data Temporal profiles lean 5s rich lean Cycle-averaged yield can reach ~20% (lab reactor study, 60/5-s cycling, no reductant in lean) Sensitive to temperature and reductant type Two peaks At regeneration inception: primary peak At rich-to-lean transition: secondary peak Mechanisms not well understood strong greenhouse gas (need to minimize) 3

4 This study aimed to clarify formation mechanisms UCT Prague-ORNL collaboration ( ) Experiments Commercial catalysts BMW Lean GDI LNT Bench reactor experiments Fast lean/rich cycling Transient response method Specialized measurements High speed FTIR SpaciMS 15 NO DRIFTS Develop models & mitigation strategies Rich Lean lean rich lean S NO + S N2O + S N2 + S NH3 = 1.0 in entire range. Covered by reductive species Local reduction of PGM sites at the rich regeneration front N 2 NO Covered by oxidative species lean rich CO sec. slightly lean CO lean small sec. lean rich slightly lean lean CO no sec. Simulation & Analysis Conversion (%) Yield (%) NO x, (ppm) (ppm) Temperature ( C) Time (s) 4

5 Results formation in rich phase (regeneration) primary peak lean 5s rich lean 5

6 NO x reduction over incompletely reduced PGM sites leads to Spatial profile along catalyst reduction front moves H 2 CO stored NO x stored oxygen NO reduction over partially reduced PGM *: PGM sites 2NO O * * * * O O N N O * * * * inlet outlet formed locally at the reduction front Reductants first reach oxidized zone (early regen) yields highest near light-off temperature for each reductant type H 2, (intermediate), CO, HCs greater chance to have this surface composition 6

7 Implementation of variable selectivity in global kinetic model Local selectivity of the stored NO x reduction to NO,, N 2 or linked with coverage of oxidative or reductive species on locally available PGM sites Approximate selectivity functions are used in the global kinetics LNT model S NO + S N2O + S N2 + S NH3 = 1.0 in entire range. Local reduction of PGM sites at the regeneration front N 2 NO Simplified PGM state reactions Pt + ½ O 2 Pt-O Pt-O + CO Pt + CO 2 Pt-O + H 2 Pt + H 2 O Pt-O + 1/9 C 3 H 6 Pt + 1/3 H 2 O + 1/3 CO 2 Pt-O + 2/5 2/5 NO + 3/5 H 2 O Covered by reductive species Covered by oxidative species (Pt-O represents a lumped characteristic of the PGM state, not the real oxygen coverage) 7

8 Implementation of variable selectivity in global kinetic model (cont.) Full range of possible N-products considered Ba(NO 3 ) 2 + S NO *3 H 2 + S N2O *4 H 2 + S N2 *5 H 2 + S NH3 *8 H 2 BaO + S NO *2 NO + S N2O *1 + S N2 *1 N 2 + S NH3 *2 + H 2 O Ba(NO 3 ) 2 + S NO *3 CO + S N2O *4 CO + S N2 *5 CO + S NH3 *(8 CO + 3 H 2 O) BaO + S NO *2 NO + S N2O *1 + S N2 *1 N 2 + S NH3 *2 + CO 2 Ba(NO 3 ) 2 + S NO *3/9 C 3 H 6 + S N2O *4/9 C 3 H 6 + S N2 *5/9 C 3 H 6 BaO + S NO *2 NO + S N2O *1 + S N2 *1 N 2 + CO 2 + H 2 O Ba(NO 3 ) 2 + S NO *6/5 + S N2O *2 + S N2 *10/3 BaO + S NO *16/5 NO + S N2O *2 + S N2 *8/3 N 2 + CO 2 + H 2 O High N 2 selectivity of OSC reduction with Ce 2 O 4 + 2/3 Ce 2 O 3 + 1/3 N 2 + H 2 O Choi, Partridge, Pihl, Kim, Kočí. Daw., Catalysis Today 184 (2012) Kočí, Bártová, Mráček, Marek, Choi, Kim, Pihl, Partridge, Topics in Catalysis 56 (2013) Bártová, Mráček, Kočí,, Marek, Choi, Chemical Engineering Journal (2015) doi: /j.cej LNT pre-oxidized with O 2 Inert bypass 200 C 300 C 400 C Inert 8 ~0 ppm at all temps

9 Results formation at rich-tolean transition lean 5s rich lean secondary peak 9

10 NO x reduction can be significant at rich-tolean transitions Regeneration by CO at 300 C High conversion (~100 %) No breakthrough of NO x in lean L=0 (inlet) (outlet) L=1 NO x storage zone O 2 storage only prim. N 2 prim. sec. N 2 sec. 10

11 NO x reduction can be significant at rich-tolean transitions Regeneration by CO at 300 C High conversion (~100 %) No breakthrough of NO x in lean prim. N 2 CO L=0 (inlet) (outlet) L=1 prim. sec. N 2 NO x storage zone O 2 storage only prim. N 2 prim. sec. N 2 sec. 11

12 Secondary more significant with less complete regeneration Regeneration by C 3 H 6 at 250 C Slow regen, low conversion NO x breakthrough in lean phase Residual stored NOx at end of regen. L=0 (inlet) (outlet) L=1 NO x storage zone prim. N 2 sec. N 2 sec. prim. 12

13 Secondary more significant with less complete regeneration (cont.) N 2 product product CO 3s C 3 H 6 3s C 3 H 6 5s CO 5s CO 3s CO 5s C 3 H 6 3s C 3 H 6 5s H 2 H 2 Contribution of secondary peaks increases with decreasing regeneration length More residual NO x remaining on the surface Secondary peaks observed up to 400 C for short regeneration May be relevant to high-temperature activity during rapid cycling (Di-Air) Secondary most significant around light-off temperatures (as for primary ) 13

14 A variety of reductive species available on the surface at the end of regeneration Proposed general mechanism for secondary peaks formation: residual stored NO x + adsorbed reductants & intermediates High PGM-CO coverage simple adsorbed reductant can contribute to secondary N 2 and formation Accumulation of isocyanate (-NCO) species Other potential surface reductants:, HCs After switch back to lean: O 2 + NCO highly selective to N 2 not a source of the secondary peak DRIFTS surface scan during regeneration by CO, 250 C Interactions of NCO & PGM-CO with the NO x released from the storage sites secondary 14

15 Extension of global kinetic model to capture peaks at rich-to-lean transitions Selectivity at rich to lean transition is determined by: Amount of residual stored NO x and adsorbed reductants/intermediates Reductant ads/des steps added to the model H 2 + * H 2 * CO + * CO* C 3 H 6 + * C 3 H 6 * + * * Ѳ void = 1 (Ѳ H2* + Ѳ CO* + Ѳ C3H6* + Ѳ NH3* ) Rich S NO + S N2O + S N2 + S NH3 = 1.0 in entire range. Covered by reductive species Local oxidation of PGM sites after switch back to lean phase N 2 NO Lean Covered by oxidative species PGM oxidation rate Leskovjan, Mráček, Kočí, Proceedings of the 41st International Conference of Slovak Society of Chemical Engineering (2014) Bártová, Kočí, Mráček, Marek, Pihl, Choi, Toops, Partridge, Catalysis Today 231 (2014) Mráček, Kočí, Marek, Choi, Pihl, Partridge, Applied Catalysis B: Environmental (2015)

16 Gained mechanistic insights led to an enhanced LNT model Model prediction of dynamics BMW LNT (250 C, hr -1 ) All: 300 ppm NO, 5% H 2 O, inert balance Lean (60 s): 10% O 2 Rich (5 s): 0.378% C 3 H 6 BMW LNT (250 C, hr -1 ) All: 300 ppm NO, 5% H 2 O, inert balance Lean (60 s): 10% O 2 Rich (5 s): 3.4% CO, (ppm) NO x in NO x (ppm), (ppm) NO x in NO x (ppm) Time (s) Time (s) 16

17 and control strategies Lambda control impact on secondary peak selectivity BMW LNT (225 C, hr -1 ) All: 300 ppm NO, 5% H 2 O, inert balance Lean (60 s): 10% O 2 Rich (5 s): 3.4% CO Slightly lean (6 or 25 s): 0.5% CO, 0.28% O 2 17

18 Summary LNT regeneration dynamics & mechanisms controlling N-product selectivity clarified by reactor study and modeling Primary peak in rich phase Caused by reaction btwn. N* + NO* formed over partially reduced PGM High at early regen. times, at low temperature, with less effective reductants Selectivity determined by NO x release rate + PGM reduction rate Secondary peak at rich-to-lean transition Caused by reaction btwn. residual NO x and reductive species on the surface Selectivity determined by amount of residual stored NO x and adsorbed reductants/intermediates + PGM oxidation rate Insights led to enhanced kinetic model & mitigation strategies Avoid regeneration at low temperatures Minimize hydrocarbon content at low-intermediate temperatures (via engine control, reforming catalyst utilization upstream) Lambda control after rich regeneration 18

19 Acknowledgements Funding U.S. Department of Energy, Vehicle Technologies Office Program managers: Gurpreet Singh, Ken Howden, Leo Breton Czech Ministry of Education Kontakt II project: LH Jae-Soon Choi Petr Kočí 19

20 Additional Slides 20

21 trend consistent w/ PGM redox state NO x conversion (%) cycle-averaged NO x conversion 100 H CO C 3 H Temperature ( C) yield (%) cycle-averaged yield 20 H 2 15 C 3 H 6 10 CO Temperature ( C) maximized near light-off temperature for a given reductant H 2 < CO < C 3 H 6 (consistent with reduction efficiency) One exception: H 2 produces high above light-off T (<150 C) Likely due to large -intermediate formation (major product) is not effective at low temperatures 21

22 Oxidation of stored not likely a major contributor to 2 nd peak Gas-phase NO in lean not critical (results not shown) Regeneration with H 2 at T=150 C lean rich lean lean rich lean NH 3 no secondary peak NH 3 secondary peak N 2 O N 2 O Note: complete NO x conversion (no breakthrough) in both cases Without CO 2, higher, but negligible secondary CO 2 involved in surface intermediate formation? 22

23 Lean-phase reduction of residual surface NO x leads to secondary Regeneration with H 2 at T=150 C without CO 2 with CO 2 3-s rich 25-s rich CO 2 not critical in secondary formation, but seems to promote by delaying regeneration (see NO x breakthrough profiles) 23

24 Possible surface reductants for secondary formation during regeneration with H 2 ( C):, C lean Regeneration with at T=200 C with CO 2 NO x inert rich inert NO x DRIFTS with CO 2, T=200 C Absorbance Regeneration with H 2 (DRIFTS) at T=200 C with CO Al / Ba -NCO at rich to purge trigger 60 s after rich to purge trigger 20 s after purge to lean trigger PGM-CO Wavenumbers (cm -1 ) stable in inert reacting in lean Switch from (rich) to inert 2 nd Destabilization of strongly held by PGM and reaction with NO x -NCO not observed PGM-CO formed readily Inhibition of reactions over PGM 24

25 Possible surface reductants responsible for secondary formation during regeneration with CO ( C): CO NO x lean rich Regeneration with CO at T=250 C lean 25 Absorbance gasphase CO 2100 PGM-CO at rich to purge trigger 60 s after rich to purge trigger at 5 s purge to lean trigger 1900 Wavenumbers (cm -1 ) Stable PGM-CO formed in rich & gradually disappeared in lean -NCO formed but not very stable is effective at this temperature (not strongly bound to PGM) CO adsorbed on PGM sites likely reductant

26 Possible surface reductants responsible for secondary formation during regeneration with C 3 H 6 ( C): C x H y Regeneration with C 3 H 6 at T=300 C lean rich lean No significant -NCO or PGM-CO on the surface (DRIFTS results not shown) NO x Sharp CO 2 peak observed at rich/lean transition indicates presence of surface C species (results not shown) H 2, CO, all effective at 300 C (not strongly bound to PGM) PGM-H x C y likely reductant 26