Oxidation State Optimization for Maximum Efficiency of NOx Adsorber Catalysts
Junhui Li, Neal W. Currier, Aleksey Yezerets, Cummins Inc.
Haiying Chen, Howard Hess, Shadab MullaJohnson Matthey
LEV II-ULEV Certified Systemwith Cummins 6.7L Engine and A/T System
Exhaust Flow
LNT
DP
F
System Function
Close-Coupled Oxidation Catalyst (CCC)
Lean NOx Trap Catalyst (LNT)
Diesel Particulate Filter (DPF)
Lean HC, CO, NO Oxidation NOx trapping Soot trapping
De-NOx Residual O2 removalIn -situ POX, WGS NOx > N2 reduction
Regeneration slip species removal
De-SOx Lean/rich cyclingHC oxidation - Heat
Sulfur release Regeneration slip species removal
De-Soot HC oxidation - Heat HC oxidation - Heat PM oxidation
HC oxidation - Heat
NOx release
Complex multi-component, multi-functional catalyst Major advances in the fundamental understanding and application
of the technology
[1] Epling, Yezerets, Currier et al. “Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/ Reduction Catalysts”. Catalysis Reviews; V46(2004), p.163-245
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Lean
Rich
NO > NO2 NO2 Storage Enrichment & ReductantEvolution
Lean
Rich Release Conversion
Lean
Rich
NO > NO2 NO2 Storage Enrichment & ReductantEvolution
Lean
Rich Release Conversion
Lean
Rich
NO > NO2 NO2 Storage Enrichment & ReductantEvolution
Lean
Rich Release Conversion
Fundamental challenges[1]: Multi-component, multi-functional
catalyst: • At least 3 components, with
different functions• Both red-ox and acid-base
catalyst chemistry 5 sequentially-coupled process Memory effects
Short-term memory Amount of NOx on the catalyst Spatial profile of reactions
Mid-term memory Amount and form of sulfur
Slow, reversible morphological changes Ba redistribution Ba aluminate formation and break-up Oxidation state of Rh
Long-term memory Thermal deactivation (Pt sintering, loss of Pt/Bainterface)
NOx Adsorber Technology
Lean (Rh oxidation):4Rh + 3O2 2Rh2O3
Rich (Rh reduction):Rh2O3+3H22Rh+3H2ORh2O3+3CO2Rh+3CO2
3Rh2O3+C3H66Rh+3H2O+3CO2
OutlineWhat is the function of Rh in the catalyst (why Rh is important) How important is Rh oxidation state Rh oxidation
– temperature and duration
Rh reduction– Role of reductant speciation, temp, richness
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Why Rh is Important?
Reduced metal has better reactivity than oxidized metal. Rh is more active than Pt for NO reduction by CO. The reactivity of Rh is much more sensitive to its oxidation state
than Pt
Rh is the key component for NO reduction in the presence of CO
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0
20
40
60
80
100
150 200 250 300 350 400
NO
xco
nver
sion
(%)
Temp (C)
Rh
NO
N O
N2 CO2
How Important of Rh Oxidation States Rh is the key component for NO reduction in LNT catalysts NO dissociation is the rate-limiting step of NO reduction
Rh2O3O
OO
OOO
O
Oxidized Rh has substantially fewer sites for NO adsorption. The adsorbed NO is more difficult to dissociate on the oxidized Rh. Fewer adjacent sites for NO dissociation Thermodynamically less favorable
Reduced Rh Oxidized Rh
Granger et al, J. Catal. 175 (1998) 194-2036
Effect of Temperature on “Memory” of Rh Function
0
50
100
150
200
250
300
150 350 550 750 950 1150time
sign
al
rich, 350 C, NOx conv. 89.3%lean, 350 C, NOx conv. 90.0%
NOx trace Air/700C/16hr DT-62 (at 280C)
0
50
100
150
200
150 350 550 750 950 1150time
signa
l
rich, 280 C, NOx conv. 75.1%
lean, 280 C, NOx conv. 62.8%
The reduced Rh is more active of NO reduction than oxidized Rh. The memory effect is highly dependant on temperature
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rich, 280 C, NOx conv. 75.1%lean, 280 C, NOx conv. 62.8%
sign
al
0
200
400
600
800
1000
1200
1400
0 100 200 300 400 500Time (s)
NO
x co
ncen
trat
ion
(ppm
) 1.25%CO1.25%H20.8%C3H6
0
200
400
600
800
1000
1200
1400
0 100 200 300 400 500Time (s)
NO
x C
once
ntra
tion
(ppm
)
1.25%CO1.25%H20.8%C3H6
Effect of Reductant Type on “Memory” of Rh Function
300ºC 350ºC
H2 is very effective for both NOx release and NOx conversion to N2. CO is effective for NOx release but not as effective for NOx conversion
to N2, compared with H2
C3H6 is not effective for either NOx release or NOx conversion.
Oxidized Rh
Oxidized Rh
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Experimental Procedure
650
140
400
NO LO after Rh oxidation
NO LO after Rh reduction
60 mins
30 mins
20 mins
30 mins
Gas species Rich Lean
NOx 200 ppm 200 ppm
C3H6 2000 ppm -
H2 1.25% -
CO 4.00% -
O2 0.00 % 5%
H2O 5 % 5 %
CO2 5 % 5 %
20 mins 1 min
NO LO after Rhoxidation at different
temperatures
LeanRich50s Lean/ 10s RichHe
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0
20
40
60
80
100
160 180 200 220 240
Temp (ºC)
NO
x co
nver
sion
(%)
reduced350C400C450C500C550C600C650C700C750C
170
180
190
200
210
220
230
240
200 400 600 800 1000Oxidation Temp (C)
Ligh
t off
Tem
p T 5
0
Reduced Rh
Effect of Temperature on Rh Oxidation
T50 = 184°C for reduced Rh vs. T50=223°C for oxidized RhOne-minute lean exposure at high temperature can cause Rh
oxidation
•Reduced Rh: pretreat sample at 400C, 20 L/R cycles with NO and cool down
in rich condition to 140C w/o NO
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Effect of Temp on Rh Oxidation
T50 = 184°C for reduced Rh vs. T50 = 230°C for oxidized RhOne-minute lean exposure at high temperature can cause Rh
oxidation
•Reduced Rh: pretreat sample at 400C, 20 L/R cycles with NO and cool down in rich condition to 140C w/o NO
170
180
190
200
210
220
230
240
200 400 600 800 1000
Oxidation Temp (C)
Ligh
t off
Tem
p T 5
0
Reduced Rh
Pretreated in Rich, cool down in Rich, 1min Lean w/
NO and cool down in He,
OxidizedRh at 650C
for 1h
Oxidized Rhat 650C for
10h
0
20
40
60
80
100
160 180 200 220 240
Temp (ºC)
NO
x co
nver
sion
(%)
reduced350C400C450C500C550C600C650C700C750C
T ↑
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Experimental Procedure
LeanRich50s Lean/ 10s RichHe
Gas species Rich Lean
NOx 200 ppm 200 ppm
C3H6 2000 ppm -
H2 1.25% -
CO 4.00% -
O2 0.00 % 5%
H2O 5 % 5 %
CO2 5 % 5 %
650
140
400
NO LO after Rh oxidation
60 mins
30 mins
60 mins
5 mins
30 mins
NO LO after Rh oxidation and reduced for 5 L/R cycles at
200, 250, 300, 350 and 400C
20 mins
30 mins
NO LO after Rh Red
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170
180
190
200
210
220
230
200 250 300 350 400 450
Red Temp (C)
Ligh
t off
tem
p (C
)
Effect of Temp on Rh Reduction During L/R Cycling
The Rh reach its most reduced state at 300ºC. The exposure to temperature higher than 300ºC will partially oxidize
the reduced Rh.
Note: NO present during the lean oxidation at 650C, cool down to target temp and 5 L/R cycles at the target temperature.
ImprovedReduction Increased
Oxidation0
20
40
60
80
100
150 170 190 210 230 250Temperature (C)
NO
x co
nver
sion
(%)
200C250C300C350C400C
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Effect of Rh Redox States on C3H6 Oxidation
Rh oxidation state affects both NO reduction and HC oxidation on NOx adsorber catalyst.
Cap.
150
160
170
180
190
200
210
220
230
240
200 300 400 500 600 700 800 900
Oxidation Temp (ºC)
NO
Red
uctio
n L
ight
-off
Tem
p T 5
0(º
C)
120
130
140
150
160
170
180
190
200
C3H
6O
xida
tion
Ligh
t-Off
Tem
p T 5
0(º
C)
Reduced Rh
Oxidized Rh
NO Reduction
C3H6 oxidation
Summary
Continued improvements in the understanding of the underlying chemistry of LNT operation and lifecycle offer opportunities for further system efficiency improvements– Operation– Catalyst design
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Catalyst Technology Team
Cummins– Neal Currier – Aleksey Yezerets
Many other colleagues at Cummins and JM
Johnson MattheyHaiying ChenHoward HessShadab Mulla
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