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

3

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

4

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

5

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

7

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

9

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

10

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

13

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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