Accelerated Aging of Fe-Zeolite SCR Catalysts: Engine and Bench-Flow Approaches Prof. Ke Nguyen...

Accelerated Aging of Fe-Zeolite SCR Catalysts: Engine and Bench-Flow Approaches

Prof. Ke Nguyen

Mechanical, Aerospace and Biomedical Engineering

University of Tennessee at Knoxville, USA

Presented

at

HCM City University of Technology

May 27, 2008

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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY

Acknowledgements

Research is supported by the U.S. Department of Energy (DOE), Office of FreedomCAR and Vehicle Technologies, Fuels Technology Program.

Fe-Zeolite SCR catalysts used were provided by Catalytic Solutions

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Outline

Engine-based aging and performance evaluation

Bench flow reactor-based performance evaluation of fresh and engine-aged Fe-Zeolite SCR catalysts

Results of surface characterization studies of fresh and engine-aged Fe-Zeolite SCR catalysts

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High-Temperature Thermal Aging of Fe-Zeolite SCR Catalysts

Key concern for Fe-Zeolite SCR catalysts durability High temperatures periodically required to

regenerate DPFs

Expected deactivation mechanisms Lube-oil poisoning Contaminations from DOC Changes in zeolite structure Washcoat breakdown, i.e., crackings

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Background of SCR Catalysts Metal oxide SCR catalysts: V2O5 supported on either TiO2 or Al2O3

- High NOx reduction ( ~90%) - Low temperature (350 - 450°C) - Lack of high-temperature durability - Emitting volatile vanadyl species - High catalyzing potential to oxidize SO2 to SO3

SO3 reacts with NH3 to form ammonium sulfate and bisulfate: catalyst poisoning

Zeolite SCR catalysts: most common Cu and Fe-Zeolite - High NOx reduction (~90%) over large temperature range(300-600°C) - High temperature heavy-duty diesel applications - Low potential to oxidize SO2 to SO3

- Cu-Zeolite produces more N2O than Fe-Zeolite

Focus only on Fe-Zeolite SCR catalysts

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SCR Reactions Between NH3 and NOx

Major reactions in the SCR process :

4NH3 + 4NO + O2 4N2 + 6H20 (1)

4NH3 + 2NO + 2NO2 4N2 + 6H20 (2)

4NH3 + 3NO2 3.5N2 + 6H20 (3)

Reaction 2 is the fastest reaction Reaction 1 is the intermediate reaction Reaction 3 is the slowest reaction

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SCR of NOx by NH3

Alpha (α) is defined as the ratio of NH3 to NOx

Exhaust gases consist mainly of NO (~90%), an α of 1 is required to reduce NOx (reaction 1)

In most systems some NO is oxidized to NO2 with a DOC to increase the ratio NO2:NO in exhaust gases

SCR generally proceeds according to reaction 2 NO2/NO must not be greater than 1, since

reaction 3 is slowest NH3 can be solely introduced to the exhaust gases

without prior oxidation of NO to NO2

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Engine-Based Aging

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Schematic of Engine Aftertreatment

Fe-Zeolite SCR catalysts evaluated as system with DOC and DPF

Drive motor

T1 P1 T2

T3

T4 T5 P2

T6 T7 T8

T9

517 cc diesel engine

Fuel pump

Temperature controller

Sample line

Sample line

NH3 line

DOC NH3 mixer SCR DPF Supplemental air

T1 Inlet Air

T2 Oil Sump

T3 Exhaust Manifold

T4 DOC Inlet gas

T5 DOC Exit gas

T6 SCR Inlet gas

T7 SCR Exit/DPF Inlet gas

T8 DPF Exit gas

T9 DPF Internal

P1 Oil Pressure

P2 Exhaust Pressure

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Specifications of Aftertreatment System

DOC Mixer SCR DPF

DOC Inlet gas

DOC Exit gas

SCR Inlet gas

SCR Exit Gas

DPF T1

DPF Exit gas

Cat In Cat Out

DOC SCR DPF Substrate material

cordierite cordierite cordierite

cpsi 400 400 200

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Engine Bench Setup Located at NTRC

SCR Catalysts

Syringe Pump

Drive Motor

500cc HatzEngine

ThrottleController

ExhaustSampling

Exhaust FuelInjection

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Engine-Based Aging and Evaluation Engine operates steady state at 1500 rpm

~800 ppm NOx, exhaust temperature ~350°C, GHSV = 60000 h-1

SCR catalysts are aged by raising exhaust gas temperature during active DPF regeneration Supplemental fuel is injected into the exhaust upstream of the DOC

Aging cycles Up to 50 aging cycles of 25 minutes each Atomized fuel is injected in five-minute increments until exhaust

temperatures of 600, 750 and 850°C are achieved then cut off fuel Desired exhaust temperatures are obtained at fuel cut-off

Evaluation cycles Evaluation is carried out after a given number of aging cycles at an

exhaust temperature of ~350°C Inject 5% of NH3 in N2 mixture such that α = NH3/NO = 0.6 to minimize

NH3 slip

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SCR Catalysts Used in Engine Bench Fe-Zeolite SCR catalysts of 400 cpsi and cordierite substrate mainly used for heavy-duty diesel applications are provided by Catalytic

Solutions

7.6 cm 15.2 cm

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SEM Micrographs of Fresh Fe-Zeolite SCR Catalyst

• Channels shown are uniform • Washcoat thickness varies

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Energy Dispersive Spectroscopy (EDS) of Fresh Fe-Zeolite SCR Catalyst

•EDS performed on washcoat corner of few atomic layer

•Al, Si, O, Fe, Ce, Zr and Au peaks detected

•Al, Si, O and Fe are components of zeolite

•Ce and Zr are components of oxygen storage materials

•Au peak is from gold coating applied to the sample before performing SEM

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Typical temperature profiles during SCR catalyst aging on engine bench at 650°C(α = 0.6)

•Repeatable temperature profile during aging

• Axial temperature variation in SCR catalyst

•SCR catalyst inlet experiences higher temperature than exit (~90°C)

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NOx emissions during SCR catalyst aging on engine bench at 650°C (α = 0.6)

•Cycle to cycle variation of NOx emissions during aging

•Half of NO is oxidized to NO2 by DOC and DPF

•During engine evaluation less than half of NOx out is NO

•NOx conversion of ~50% is obtained with α = 0.6

Cat In NO and NOx (no NH3)

Cat Out NOx (with NH3)

Cat Out NO (no NH3)

Cat Out NO (with NH3)

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Typical temperature profiles during SCR catalyst aging on engine bench at 750°C (α = 0.6)

• Temperature profile is repeatable

• Axial temperature variation in SCR catalyst

• Front section of SCR catalyst experiences higher temperature than rear section (~100°C)

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NOx emissions during SCR catalyst aging on the engine bench at 750°C


•NOx conversion evaluated with α = 0.6



Cat Out NO (no NH3)


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Typical temperature profiles during SCR catalyst aging on engine bench at 850ºC (α = 0.6)

•Repeatable temperature profile

•Axial temperature variation along SCR catalyst

•Smaller temperature difference between SCR inlet and exit (~ 40°C)

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NOx emissions during SCR catalyst aging on engine bench at 850°C


•NOx conversion evaluated with α = 0.6

Cat Out NO (no NH3)




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Bench-Flow Reactor Performance Evaluation

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Bench reactor evaluation of fresh and engine-aged Fe-Zeolite SCR catalysts Temperature varied between 200 and 600°C α varied between 0.5 and 1.2

Surface characterization studies SEM-EDS, EPMA, XRD and BET

Bench-Flow Reactor Evaluation

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Performance Evaluation of Fresh and Engine-Aged SCR Catalysts is Carried Out on Bench-Flow Reactor

SCR Catalyst Reactor

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Fe-Zeolite SCR Catalysts

- Fresh Fe-Zeolite SCR catalysts

- Engine-aged Fe-Zeolite catalysts • Two cored samples are obtained from engine-aged catalysts: front half and rear half • Evaluation is performed separately on both engine-age samples

22 mm 75 mm

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Thermocouple locations in SCR catalyst

5mm5mm

19mm

38mm

57mm

Flow

75.5mm

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Composition of Simulated Exhaust Gases

Gas Species Concentration

H2O (g) 5%

O2 14%

CO2 5%

NO 350ppm

NH3 175-350ppm

N2 balance

NOx performance evaluations are performed atGHSV of 30,000 h-1

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Typical SCR catalyst temperature at different locations during evaluation at 500ºC, GHSV = 30,000 h-1

• Steady state temperatures obtained during evaluation

• ∆Tmax=19°C

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NOx conversion of fresh Fe-Zeolite SCR catalysts with 350 ppm NO at GHSV of 30,000 h-1

•Maximum NOx conversion of 94% occurs at 400ºC at α = 1.2

•NOx conversion increases with α

•Lower NOx conversion at T>500°C due to oxidation of NH3

•Low NOx conversion at 200°C due to kinetics

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NOx Conversion of Engine-Aged Fe-Zeolite SCR Catalysts (α=1)

•Front section of both engine-aged catalysts degraded severely

•Reduction in NOx performance in front-half is possibly due to lube-oil poisoning, contaminations from DOC and cracking of washcoat

•Catalyst aged at 650°C is being evaluated

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XRD patterns of fresh and aged Fe-Zeolite SCR Catalysts

•Significantly less zeolite detected in aged samples

•Fe2O3 and Al2O3 peaks are discernable in aged samples

•Zeolite structure begins to breakdown around 800°C for many high-silica zeolites

□ ●

□

∆

●

○∆

∆∆

◊

∆

† †

††

† †

†♠♠

†♦

†♠

64 hrs on BFR (28ppm SO2)

13 cycles at 850°C (Front)

13 cycles at 850°C (Rear)

Fresh Fe-SCR

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Expanded XRD patterns of fresh and engine-aged Fe-Zeolite SCR catalysts

Zeolite

-------- Fresh

-------- 800°C

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BET surface area measurements of fresh and engine-aged Fe-Zeolite SCR catalysts

•Surface area includes zeolite and cordierite substrate

•Front sections of engine-aged SCR catalysts experience severe loss of surface area

•Surface area is higher if cordierite substrate is excluded

•Surface area reduction correlates with aging temperature

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EPMA elemental maps of fresh Fe-Zeolite SCR catalyst

• Fe is well-dispersed in washcoat

• ZrO2 and CeO2 are the oxygen storage materials

• Si is one of constituent of zeolite

• Trace of P is detected

Zn P

S Ca Fe

Si Zr Ce

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EPMA elemental maps of Fe-SCR catalyst aged for 31 cycles at 650°C (front)

• Fe is still well dispersed in washcoat

• Agglomeration of ZrO2

and CeO2

• More P is detected in front due to lube-oil contamination

• Trace of Pt is detected

Pt P

S Ca Fe

Si Zr Ce

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EPMA elemental maps of Fe-Zeolite SCR catalyst engine-aged for 31 cycles at 650°C (rear)

•Fe is well-dispersed in washcoat

•Trace of Pt is detected

•Agglomeration of ZrO2 and CeO2

• Less P deposited in rear section

Pt P

S Ca Fe

Si Zr Ce

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EPMA elemental maps of Fe-Zeolite SCR catalyst engine-aged for 50 cycles at 750°C (front)

• Cracking of washcoat is visible in front section due to higher temperature

• Contamination from lube oil, i.e., P

Pt P

S Ca Fe

Si Zr Ce

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EPMA elemental maps of Fe-Zeolite SCR catalyst engine- aged for 50 cycles at 750°C (rear)

•Temperature in rear-half is much less than 750C

•No cracking of washcoat is observed in rear-half, which explains higher NOx conversion

•Only trace of Pt is detected

Pt P

S Ca Fe

Si Zr Ce

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EPMA elemental maps of Fe-ZeoliteSCR catalyst engine-aged for 13 cycles at 850°C (front)

• Severe crackings of washcoat

Zn P

S Ca Fe

Si Zr Ce

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EPMA elemental maps of Fe-SCR engine-aged for 13 cycles at 850°C (rear)

•Severe cracking of washcoat

Zn P

S Ca Fe

Si Zr Ce

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Conclusions Loss of NOx performance of engine-

aged Fe-Zeolite SCR catalysts is mainly due to:

- Loss of surface area (BET)

- Breakdown of zeolite structure at temperatures above 800°C (XRD)

- Severe cracking of washcoat at temperatures above 750°C (EPMA)

- Agglomeration of CeO2 and ZrO2

- Contamination from lube-oil

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Thank You

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