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

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. Acknowledgements.

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

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  1. 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

  2. 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

  3. 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

  4. 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

  5. Background of SCR Catalysts • Metal oxide SCRcatalysts: 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

  6. 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

  7. 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

  8. Engine-Based Aging

  9. Schematic of Engine AftertreatmentFe-Zeolite SCR catalysts evaluated as system with DOC and DPF

  10. Specifications of Aftertreatment System DOC Exit gas SCR Exit Gas DOC Mixer SCR DPF Cat Out Cat In DPF Exit gas DOC Inlet gas SCR Inlet gas DPF T1

  11. Engine Bench Setup Located at NTRC Exhaust Sampling SCR Catalysts Exhaust Fuel Injection Drive Motor 500cc Hatz Engine Syringe Pump Throttle Controller

  12. 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 N2mixture such that α = NH3/NO = 0.6 to minimize NH3 slip

  13. 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

  14. SEM Micrographs of Fresh Fe-Zeolite SCR Catalyst • Channels shown are uniform • Washcoat thickness varies

  15. 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

  16. 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)

  17. 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)

  18. 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)

  19. NOx emissions during SCR catalyst aging on the engine bench at 750°C Cat In NO and NOx (no NH3) Cat Out NOx (with NH3) • Cycle to cycle variation of NOx emissions during aging • NOx conversion evaluated with α = 0.6 Cat Out NO (no NH3) Cat Out NO (with NH3)

  20. 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)

  21. NOx emissions during SCR catalyst aging on engine bench at 850°C Cat In NO and NOx (no NH3) Cat Out NOx (with NH3) •Cycle to cycle variation of NOx emissions during aging •NOx conversion evaluated with α = 0.6 Cat Out NO (no NH3) Cat Out NO (with NH3)

  22. Bench-Flow Reactor Performance Evaluation

  23. Bench-Flow Reactor Evaluation • 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

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

  25. 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

  26. Thermocouple locations in SCR catalyst 57mm 38mm Flow 5mm 5mm 75.5mm 19mm

  27. Composition of Simulated Exhaust Gases NOx performance evaluations areperformed at GHSV of 30,000 h-1

  28. Typical SCRcatalyst temperature at different locations during evaluation at 500ºC, GHSV = 30,000 h-1 • Steady state temperatures obtained during evaluation • ∆Tmax=19°C

  29. 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

  30. 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

  31. XRD patterns of fresh and aged Fe-Zeolite SCR Catalysts ∆ ∆ - CexZr1-xO2 ◊ - ZrO2 □ - Zeolite ○ - CeO2 ● - Cordierite † - Fe2O3 ♠ - Al2O3 ♦ - Si ∆ ∆ •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

  32. Expanded XRD patterns of fresh and engine-aged Fe-Zeolite SCR catalysts Zeolite --------Fresh -------- 800°C

  33. 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

  34. EPMA elemental maps of fresh Fe-Zeolite SCR catalyst Zn P • 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 S Ca Fe Si Zr Ce

  35. EPMA elemental maps of Fe-SCR catalyst aged for 31 cycles at 650°C (front) P Pt • 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 S Ca Fe Si Zr Ce

  36. EPMA elemental maps of Fe-Zeolite SCR catalyst engine-aged for 31 cycles at 650°C (rear) P Pt •Fe is well-dispersed in washcoat •Trace of Pt is detected •Agglomeration of ZrO2 and CeO2 • Less P deposited in rear section S Ca Fe Si Zr Ce

  37. EPMA elemental maps of Fe-Zeolite SCR catalyst engine-aged for 50 cycles at 750°C (front) P Pt • Cracking of washcoat is visible in front section due to higher temperature • Contamination from lube oil, i.e., P Ca S Fe Si Zr Ce

  38. EPMA elemental maps of Fe-Zeolite SCR catalyst engine- aged for 50 cycles at 750°C (rear) P Pt •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 Ca S Fe Si Zr Ce

  39. EPMA elemental maps of Fe-ZeoliteSCR catalyst engine-aged for 13 cycles at 850°C (front) P Zn • Severe crackings of washcoat Ca S Fe Si Zr Ce

  40. EPMA elemental maps of Fe-SCR engine-aged for 13 cycles at 850°C (rear) P Zn •Severe cracking of washcoat Ca S Fe Si Zr Ce

  41. 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 ofCeO2 and ZrO2 • Contamination from lube-oil

  42. Thank You Questions ????

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