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Understanding the Distributed Intra-Catalyst Impact of Sulfation on Water Gas Shift
in a Lean NOx Trap Catalyst
Bill Partridge1, Jae-Soon Choi1, Josh Pihl1, Todd Toops1, Jim Parks1, Nathan Ottinger2, Alex Yezerets2 , Neal Currier2
1: Oak Ridge National Laboratory2: Cummins Inc
DEER 2010 ConferenceEmissions Control TechnologiesMarriott Renaissance CenterSeptember 28, 2010Detroit, Michigan
U.S. DOE Program Management Team:Ken Howden, Gurpreet Singh, Steve Goguen
2
• Background & Motivation
• Experimental Approach
• Sulfation Results NSR: NOx Storage & Reduction OSC: Oxygen Storage Capacity WGS: Water Gas Shift
• Global Conceptual Model
• Conclusions
Outline
3
Background
H2
HC
NOO2
NO2NO2
O2NO CO2 Substrate
Washcoat
Pt
KNO2,3
K2CO3
Substrate
Washcoat
Pt
KNO2,3
K2CO3NO2HC,CO,H2
NO2HC,CO,H2 H2O
N2CO
Storage
RegenerationH2
HC
NOO2
NO2NO2
O2NO CO2 Substrate
Washcoat
Pt
KNO2,3
K2CO3
Substrate
Washcoat
Pt
KNO2,3
K2CO3NO2HC,CO,H2
NO2HC,CO,H2 H2O
N2CO
Storage
Regeneration
• LNT catalysts are sensitive to sulfur poisoning• Sulfates more stable than nitrates: SO2 + 1/2O2 + Pt + BaO ----> Pt + BaSO4
2NO + 3/2O2 + Pt + BaSO4 ----> Ba(NO3)2 + Pt + SO3
Sulfation changes reaction distributions
• High-temperature DeS can damage catalyst Irreversible thermal aging: e.g., Pt sintering Fuel penalty cost of process
Choi, et al., Appl. Catal. B 77 (2007) 145.
• LNTs (e.g., Pt/Ba/Al2O3) operate in cyclic mode LEAN-phase storage:2NO + 3/2O2 + Pt + BaO ----> Pt + Ba(NO3)2
RICH-phase reduction:5H2 + Pt + Ba(NO3)2 ----> N2 + 5H2O + Pt + BaO Internal spatial & temporal variations
●
4
Motivation• Need to minimize high-T DeS events
Basic control commands DeS too often and too long
• WGS enables advanced control Cummins OBD Patent (US Patent App. 20080168824) Active on-board assessment of catalyst state Only DeS when & for as long as required
Better efficiency (lower fuel penalty) Better durability (catalyst & engine last longer)
Intake Air
TurboExhaust LNT
Catalyst
Parks, Swartz, Huff, West. ORNL. DEER 2006, August 20-24, 2006, Detroit, MI
UEGO Sensors
LNT WGS α Ageing
UEGO Signal α WGS
UEGO has unique H2 cross sensitivity
0.94
0.95
0.96
0.97
0.98
0.99
1
1.01
1.02
1.03
0.97 0.98 0.99 1 1.01 1.02 1.03SCAT LAMBDA
LAM
BDA
SENS
OR
O2 (0.96)
CO (0.76)
NO (1.34)
C3H6 (0.51)
H2 (3.02)
(Gradient)
O2, CO, NO, C3H6
H2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
95000 100000 105000 110000 115000 120000 125000 130000
Time / ms
Sens
or V
olta
ge /
V
InletOutlet
LEAN
RICH
STOIC
LNT In
LNT Out
α WGS
H2
COTHC
●
5
• Background & Motivation
• Experimental Approach
• Sulfation Results NSR: NOx Storage & Reduction OSC: Oxygen Storage Capacity WGS: Water Gas Shift
• Global Conceptual Model
• Conclusions
Outline
6
Approach: Controlled Bench Reactor Experiments with Spatially & Temporally Resolved Gas Analyses
Model LNT Catalyst
● Substrate: 300-cpsi cordierite
● Washcoat: Pt/Ba/Al2O3
● No Oxygen-Storage Capacity
(OSC) such as Ce
● Evaluated as a 3/4” x 3” core
Procedure● Baseline: 0 g/L S
● Performance evaluationNeutral; OSC; NSR
● 1st S dosing: 0.85 g/L S
● Performance evaluationNeutral; OSC; NSR
● 2nd S dosing: <1.7 g/L S
● Performance evaluationNeutral; OSC; NSR
● Post mortem analysisDRIFTSMicro Reactor
In Situ Intra-Channel Speciation
SpaciMS
0 76L (mm)Cat-In
SamplingCapillary
Cat-Out
Gas Flow
7
WGS converts CO to H2 via: CO + H2O → H2 + CO2
Lean-phase composition dictates CO reaction possibilities NSR: WGSR vs. OSC vs. LNT regeneration OSC: WGSR vs. OSC Neutral: WGSR only
Fast Cycling (60:5-s lean:rich cycling) Temperature: 325°C
Systematically Vary WGS Competition for CO Reductant
RICH (5s) LEAN (60s)
CO H2O NO O2 H2O
NSR 2% 5% 300ppm 10% 5%
OSC 2% 5% 0 10% 5%
Neutral 2% 5% 0 0 5%
8
• Background & Motivation
• Experimental Approach
• Sulfation Results NSR: NOx Storage & Reduction OSC: Oxygen Storage Capacity WGS: Water Gas Shift
• Global Conceptual Model
• Conclusions
Outline
9
Baseline (0 gS / Lcat): NSR in front ½
Back ½ unused
1st Sulfation (0.85 gS / Lcat): Front ¼ inactive
NSR in back ¾
2nd Sulfation (<1.7 gS / Lcat): Front ½ inactive
NSR in back 1/2
Sulfation Progressively Poisons NSR in Plug-Like Fashion
0
20
40
60
80
100
0.00 0.25 0.50 0.75 1.00
Relative Axial Catalyst Location (\)
% N
Ox
Trap
ped
Baseline0.85 gS / L< 1.7 gS / L
NSR CyclingNOx Storage
10
• Background & Motivation
• Experimental Approach
• Sulfation Results NSR: NOx Storage & Reduction OSC: Oxygen Storage Capacity WGS: Water Gas Shift
• Global Conceptual Model
• Conclusions
Outline
11
0.00
0.25
0.50
0.75
1.00
0.00 0.25 0.50 0.75 1.00Relative Axial Catalyst Location (\)
CO
2 fro
m O
SC (%
)
Baseline0.85 gS / L< 1.7 gS / L
OSC due to Pt redox No Ce or support OSC
~ active Pt area
OSC ~ uniform along catalyst i.e., uniform Pt distribution
Minor sulfation impact on OSC ~3-17% loss max minor OSC on Ba?
OSC active in sulfated zone Pt remains S free cf. poisoned NSR!
Can change input to NSR zone
Sulfation Has Little Impact on “OSC”
OSC Cycling
“OSC”: Pt Area
~17% Loss max
gS/Lcat 1st Q. 2nd Q. 3rd Q. 4th Q.
0 NSR unused
0.85 Inactive degraded
<1.7
Qualitative Pictorial Representation of NSR Activity
●
12
• Background & Motivation
• Experimental Approach
• Sulfation Results NSR: NOx Storage & Reduction OSC: Oxygen Storage Capacity WGS: Water Gas Shift
• Global Conceptual Model
• Conclusions
Outline
13
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.00 0.25 0.50 0.75 1.00
Relative Axial Catalyst Location (\)
H2 C
once
ntra
tion
(%)
Baseline
0.85 gS / L
< 1.7 gS / L
Baseline (0 gS / Lcat): WGS throughout
1st Sulfation (0.85 gS / Lcat): Front ½ : “Max” degradation
~90-95% loss from Baseline
WGS in back ½
2nd Sulfation (<1.7 gS / Lcat): Front 3/4 : “Max” degradation
WGS in back ¼
WGS S-front leads NSR S-front by ~ ¼ catalyst
WGS inactive upstream of NSR
WGS more sensitive to S than NSR
WGS Very Sensitive to Sulfur Degradation
Neutral Cycling
WGS
~95% Loss
0
20
40
60
80
100
0.00 0.25 0.50 0.75 1.00
Relative Axial Catalyst Location (\)
% N
Ox
Trap
ped
Baseline0.85 gS / L< 1.7 gS / L
NSR CyclingNOx Storage
●
14
• Background & Motivation
• Experimental Approach
• Sulfation Results NSR: NOx Storage & Reduction OSC: Oxygen Storage Capacity WGS: Water Gas Shift
• Global Conceptual Model
• Conclusions
Outline
15
Global Model of Distributed S Impact on NSR, WGS & OSC
• Fully active in S-free zone
• Initial Sulfation:• WGS very sensitive to S - probably due
to changes in Pt-support interface structure
• NSR insensitive due to spillover, surface diffusion and NO2,gas accessing S-free Ba in field
• Progressive Sulfation:• Further incremental WGS degradation
to max • Progressive NSR degradation as field
Ba is sulfated• Ultimate NSR poisoning• Minor OSC degradation
Sulfated Ba Pt S-Free Ba
Poisoned Degraded Active
Progressively Degraded Active
Active
Maximum Degradation
Progressively Degraded
Sulfated Ba Pt S-Free Ba
Poisoned Degraded Active
Progressively Degraded Active
Active
Maximum Degradation
Sulfated Ba Pt S-Free BaSulfated Ba Pt S-Free Ba
Poisoned Degraded Active
Progressively Degraded Active
Active
Maximum Degradation
Progressively Degraded
16
• Background & Motivation
• Experimental Approach
• Sulfation Results NSR: NOx Storage & Reduction OSC: Oxygen Storage Capacity WGS: Water Gas Shift
• Global Conceptual Model
• Conclusions
Outline
17
Conclusions
• WGS occurs on Ba LNT catalysts (not just Ce-containing catalysts)
• Each LNT function has a different response to sulfation− WGS: very sensitive to initial S− NSR: Progressively degraded and poisoned− OSC: Minor degradation
• The S distribution is different w.r.t. each LNT function
• Conceptual model of distributed S impact on different LNT functions
• So what:– Improved understanding of LNT sulfation– Enable better models and catalyst system design (device size/capacity)– Enable improved OBD & control (cf. Cummins Control Patent)– Better emissions control, efficiency & durability
18
Acknowledgments
Sponsor:U.S. DOE Office of Vehicle Technologies,Ken Howden, Gurpreet Singh, Steve Goguen
Thank YouBill Partridge865-946-1234
19
How doesSulfur
Degrade WGS?
20
Oxygen Mitigates Sulfur Degradation of WGS
OSC enhances WGS in Sulfated states
~5-10% gain vs. Neutral
Little recovery vs. ~95% loss w/ Sulfation
O2 readily displaces S from Pt
Sgas adsorbs on Pt during rich Based on DRIFTS measurements
Fridell et al., Topics in Catal. V16/17, 133, 2001
Pt is S free in OSC fast-cycling conditions
O2 improves WGS by desorbing S from Pt
S adsorption on Pt has minor impact on WGS degradation
Other non-Pt-S route accounts for primary WGS S-degradation
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.00 0.25 0.50 0.75 1.00Relative Axial Catalyst Location (\)
H2 C
once
ntra
tion
(%)
0.85 gS / L, OSC
0.85 gS / L, Neutral
<1.7 gS / L, OSC
<1.7 gS / L, Neutral
WGS
OSC & Neutral Cycling
●
21
Does S Degrade WGS by Changing Pt Electronic Structure?
S doesn’t change Pt-CO affinity cf. flat CO peak position
Pt electronic density ~ constant
Pt sites are available cf. flat CO peak area
consistent w/ other observations showing that Pt is available
0
2
4
6
8
10
1.000.750.600.35Relative S
CO
Pea
k A
rea
(a.u
.)
2050
2060
2070
2080
2090
2100
CO
Pea
k Po
sitio
n (c
m-1
)
AreaPosition
CO DRIFTS @ 325C
Maybe initial S is detrimental to Pt-support\Ba interface Not yet verified for Pt\Ba\Al2O3 catalyst
But,…Pt\Ce extensively studied for WGS & Reverse WGS
&…common theme is importance of metal-support interface & activation on Pt
●
22
S May Concentrate or Minute S is Detrimental to Interface Structure Necessary for WGSR
Nonlinear S impact on WGS Small initial S dose has major impact on WGS (Fresh vs. 4th Q) 4th Q has significant NOx capacity
S has significant but limited impact i.e., non-zero asymptote0%
5%
10%
15%
20%
Fresh 4th Q, S
3rd Q, S
2nd Q, S
1st Q, SCO2
(% of feed)Catalyst Section & S State
WGSR Activity, 300 C
Increasing S
Sakamoto et al., J of Catal. 238, 361, 2006
Surface N Surface S
Different N & S deposition N goes through Pt N concentrated around Pt
S goes down everywhere
Lean will oxidize S on Pt Likely deposited close to Pt
May concentrated S around Pt
Goguet Model of WGS Carbon Poisoning
(Goguet et al., J. Phys. Chem. B 108, 20240, 2004)
Advancing Carbon Front
Pt“Ring of Active Area”
C-Island Radius
Curve shape uncertain
Non
-Ze
ro
Lim
it
0
WG
S Ac
tivity
C forms at Pt
C island grows around Pt
Proximal C impacts WGS
No impact of distal C
Looks like S impact
We don’t yet know……Further fundamental research needed●
23
0
20
40
60
80
100
0.00 0.25 0.50 0.75 1.00
Relative Axial Catalyst Location (\)
% N
Ox
Trap
ped
Baseline0.85 gS / L< 1.7 gS / L
Baseline (0 gS / Lcat): NSR in front ½
Back ½ unused
1st Sulfation (0.85 gS / Lcat): Front ¼ inactive
NSR in back ¾
Broadened NSR zone
(not perfectly “plug like”)
2nd Sulfation (>1.7 gS / Lcat): Front ½ inactive
NSR in back 1/2
Sulfation Progressively Poisons NSR in Plug-Like Fashion
gS/Lcat 1st Q. 2nd Q. 3rd Q. 4th Q.
0 NSR unused
0.85 Inactive degraded Degraded???
<1.7
NSR Cycling; 0, 0.85 & <1.7 g-S / LcatNOx Storage
Qualitative Pictorial Representation of NSR Activity
24
0.00
0.25
0.50
0.75
1.00
0.00 0.25 0.50 0.75 1.00Relative Axial Catalyst Location (\)
CO
2 fro
m O
SC (%
)
Baseline0.85 gS / L< 1.7 gS / L
OSC due to Pt redox No Ce or support OSC
~ active Pt area
OSC ~ uniform along catalyst i.e., uniform Pt distribution
Minor sulfation impact on OSC ~3-17% loss max minor OSC on Ba?
OSC active in sulfated zone Pt remains S free cf. poisoned NSR!
Sulfation Has Little Impact on “OSC”
OSC Cycling
“OSC”: Pt Area
~17% Loss max
25
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.00 0.25 0.50 0.75 1.00
Relative Axial Catalyst Location (\)
H2 C
once
ntra
tion
(%)
Baseline
0.85 gS / L
< 1.7 gS / L
Baseline (0 gS / Lcat): WGS throughout
1st Sulfation (0.85 gS / Lcat): Front ½ : “Max” degradation
~90-95% loss from Baseline
WGS in back ½
2nd Sulfation (<1.7 gS / Lcat): Front 3/4 : “Max” degradation
WGS in back ¼
WGS S-front leads NSR S-front By ~ ¼ catalyst
WGS & NSR S degradation differs
WGS more sensitive to S than NSR
WGS Very Sensitive to Sulfur Degradation
Neutral Cycling
WGS
~95% Loss
0
20
40
60
80
100
0.00 0.25 0.50 0.75 1.00
Relative Axial Catalyst Location (\)
% N
Ox
Trap
ped
Baseline0.85 gS / L< 1.7 gS / L
NSR CyclingNOx Storage
●