Correlation between LNT NH 3 and N 2 O selectivities under fast cycling conditions

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1 Correlation between LNT and N 2 O selectivities under fast cycling conditions Jae-Soon Choi, Josh Pihl, Miyoung Kim, Bill Partridge, Stuart Daw Oak Ridge National Laboratory Petr Kočí Institute of Chemical Technology, Prague 2011 CLEERS Workshop April 21, 2011 Dearborn, MI

2 LNT can emit large amount of & N 2 O Example: bench reactor experiments at ORNL CLEERS reference LNT; 60/5-s lean/rich cycling & N 2 O yields as high as 70 & 20%, respectively Highly dependent on reductant type & temperature Processes leading to these trends need to be understood 2 Managed by UT-Battelle To control emissions & mitigate environmental and health impacts To develop predictive models

3 Transient chemistry details necessary to accurately describe performance trends Example: bench reactor experiments at ORNL CLEERS reference LNT; 60/5-s lean/rich cycling; H 2 reductant Intermediate reductant roles of recently demonstrated NO x + H 2 N H 2 O + NO x N 2 + H 2 O + CeO 2 (OSC) N 2 + H 2 O + Ce 2 O 3 Limited information on N 2 O chemistry & relationship with 3 Managed by UT-Battelle

4 Objective: clarify N 2 O chemistry by probing transformation Performance trends Sweep reductant type H 2, CO, H 2 /CO, C 3 H 6, C 3 H 8, H 2 /HC, Sweep temperature C Improved understanding of N 2 O & chemistry CLEERS Ref Transient surface chemistry adsorption, decomposition, oxidation Gas & surface species DRIFTS Automated bench reactor Commercial lean GDI LNT Modeling Incorporate new findings Extend reaction kinetics model (N 2 O) Bench reactor 4 Managed by UT-Battelle Subject of the following talk by Partridge/Kočí Model

5 Catalyst studied: CLEERS reference LNT Lean GDI LNT (Umicore), 625 cpsi Domain Ba-rich Ce/Zr-rich Al-rich Mg/Al-rich Composition Ba (high), Ce/Zr, Pt, Pd Ba (low), Ce/Zr, Pt, Pd Al, Rh, Pd Mg/Al, Pt, Ce Large oxygen storage capacity (OSC: Ce/Zr mixed oxides) 5 Managed by UT-Battelle

6 Three types of experimental conditions 1. Lean/rich cycling 1. Lean (60 s): 300 ppm NO, 10% O 2 2. Rich (5 s): 3.4% reductant 2. Transient response ppm (or 300 ppm + 0.5% H 2 ) pulse input 2. Initial LNT surface: reduced, oxidized or nitrated 3. Steady flow ppm NO + 0.5% H 2 Time All experiments presented here conducted with Base gas: 5% H 2 O, 5% CO 2, N 2 balance SV: 30K h -1 6 Managed by UT-Battelle

7 decomposition (2 = N 2 + 3H 2 ) is significant under transient conditions Transient response experiment: pulse input LNT pre-reduced with H 2 followed by inert (N 2 ) purge Bypass 200 C 300 C 400 C release of adsorbed Note: no additional desorption during subsequent TPD Very high initial rates: zero slip at all temperatures Lower steady-state rates Steep leading edges of breakthrough profiles indicate adsorption/storage Not explained by storage: minor role (small desorption) 7 Managed by UT-Battelle H-adsorption or spillover

8 Decomposition is inhibited by hydrogen Transient response experiment : 300 C, pulse input w/ vs. w/o 0.5% H 2 LNT pre-reduced with H 2 followed by inert purge (or +H 2 ) +H 2 Bypass only Co-feeding H 2 suppresses both initial & steady-state decomposition Hydrogen inhibits decomposition over precious metal When H-adsorption or storage capacity is saturated 8 Managed by UT-Battelle

9 reduction of surface oxygen does not lead to N 2 O formation Transient response experiment : pulse input LNT pre-oxidized with O 2 followed by inert purge bypass 200 C 300 C 400 C N 2 O, 200, 300, 400 C reduction of stored oxygen (CeO 2 ): very efficient (plug-like front) Extent of surface reduction highly dependent on temperature reduction of CeO 2 not a major contributor to N 2 O 9 Managed by UT-Battelle

10 decomposition & OSC reduction explain lower apparent yields at higher temperature Lean/rich cycling 60 s: 300ppm NO, 10% O 2 5 s: 3.4% H 2 Steady rich flow 500ppm NO, 0.5% H 2 concentration of N-containing species (ppm) CO NO N 2 O N catalyst temperature ( C) CO concentration (ppm) NO NO 2 N 2 O N 2 CO Pihl et al., SAE Technical Paper Steady rich flow experiments show higher generation at higher T Another potential factor to consider: Lower H 2 /NO ratio (unfavorable for formation) due to faster NO x release 10 Managed by UT-Battelle

11 N 2 O formation mainly due to reaction with stored NO x Transient response experiment: 300 C, pulse input LNT pre-nitrated with 300 ppm NO x + 10% O 2 followed by inert purge bypass 200 C 200 C 400 C 300 C 300 C 400 C is efficient in reducing stored NO x also Major contributor to N 2 O 11 Managed by UT-Battelle

12 Gas-phase O 2 reaction with at rich/lean transition can lead to additional N 2 O Transient response experiment: 200 C, pulse input with 0.5% H 2 LNT pre-oxidized with 0.5% O 2 followed by inert purge +H 2 10% O 2 Lean/rich cycling 60 s: 300ppm NO, 10% O 2 5 s: 3.4% H 2 Rich Lean + O 2 reaction possible due to Axial backmixing (rich/lean transition) Slow release of adsorbed (subsequent lean) 12 Managed by UT-Battelle

13 N 2 O peaks near light-off T where reaction with stored NO x is maximized 100 H 2 NO x conv CO C 3 H 6 C 3 H 8 NO x conv NO x conv NO x conv 80 NO x conversion ( C) N 2 O N N 2 O 2 O N 2 O Temperature ( C) Temperature ( C) Temperature ( C) Temperature ( C), N 2 O yield (%) Light-off temperature is highly dependent on reductant type: H 2 < CO < C 3 H 6 < C 3 H 8 At light-off temperatures, Near max conv. reached using whole LNT Reactions forming (reductant + stored NO x ) & N 2 O ( + stored NO x ) maximized Above light-off temperatures, Max conv. using partial length of LNT Reactions consuming without generating N 2 O increase ( + CeO 2 ; decomposition) 13 Managed by UT-Battelle NO x storage NO x storage oxygen storage

14 Conclusions can be involved in various surface reactions Decomposition & H-spillover (rich) Adsorption (rich) & slow release (rich, subsequent lean) Reduction of stored NO x (rich), stored O (rich), gas-phase O 2 (rich/lean; lean) N 2 O is formed as a result of conversion reaction with stored NO x (rich): major contributor reaction with stored oxygen (rich): negligible contribution oxidation with gas-phase O 2 (rich/lean, lean): minor contribution & N 2 O highest near light-off T for a given reductant type Formation of (reductant + stored NO x ) & N 2 O ( + stored NO x ) maximized +CeO 2 & decomposition minimized Findings can enhance model, N 2 O capabilities See following talk by Partridge/Kočí 14 Managed by UT-Battelle

15 Acknowledgments Research sponsored by DOE, Vehicle Technologies Program Program Managers: Ken Howden, Gurpreet Singh Catalyst from Umicore Owen Bailey 15 Managed by UT-Battelle Jae-Soon Choi