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
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
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 2 + + 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
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 150-550 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
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
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 1. 300 ppm (or 300 ppm + 0.5% H 2 ) pulse input 2. Initial LNT surface: reduced, oxidized or nitrated 3. Steady flow 1. 500 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
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
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
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
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) 500 400 300 200 100 0 500 CO NO N 2 O N 2 0 100 200 300 400 500 catalyst temperature ( C) 400 300 200 100 CO concentration (ppm) NO NO 2 N 2 O N 2 CO Pihl et al., SAE Technical Paper 2006-01-3441 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
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
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
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) 80 60 60 40 40 20 N 2 O N N 2 O 2 O N 2 O 20 0 0 100 200 300 400 500 600100 200 300 400 500 600 100 200 300 400 500 600100 200 300 400 500 600 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
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
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 865-946-1368 choijs@ornl.gov