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Bimetallic Silver Catalysts for the Reformate-Assisted Hydrocarbon Selective Catalytic Reduction...

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  • Slide 1
  • Bimetallic Silver Catalysts for the Reformate-Assisted Hydrocarbon Selective Catalytic Reduction (HC-SCR) of Nitrogen Oxides Richard Ezike Ph.D. Defense Department of Chemical Engineering The University of Michigan July 29, 2011
  • Slide 2
  • Smog http://upload.wikimedia.org/wikipedia/commons/9/96/SmogNY.jpg Consists of particulate matter and ground-level ozone Caused by reaction of NO x and hydrocarbons in the presence of sunlight Contributes to a number of health issues Emphysema, asthma, bronchitis, shortness of breath
  • Slide 3
  • Where is majority of NO x generated? EPA 2011 Increasing number of vehicles are lean-burn Diesel, gasoline- powered Lean-burn engines produce more NO x
  • Slide 4
  • NO x Emission Standards Increasingly stringent emission standards require significant technological advances 1,2 1.U.S. EPA Office of Mobile Sources 2.California Air Resources Board LEV Level III Vehicle Model Year NO x Standard (g/mile) 72%
  • Slide 5
  • NO x Technologies TechnologyBenefitsChallengesAuthors NO x decompositionMost direct method High reaction temperatures (~900 O C), thermodynamically difficult Iwamoto et al., App. Cat., 1991 NO x storage- reduction/Lean NO x traps Well established method, no additional reducing agent SO 2 deactivation, thermal degradation Matsumoto, Cat. Today, 2004 Selective Catalytic Reduction (SCR)with Urea Well established for heavy duty vehicles, no fuel penalty Urea freezes at - 10 o C, hydrocarbon poisoning at low temps Koebel et al., Cat. Today, 2000
  • Slide 6
  • Hydrocarbon Selective Catalytic Reduction (HC-SCR) of NO x HC + NO x + O 2 N 2 (desired) + CO 2 + H 2 O HC + NO x + O 2 N 2 O (undesired) + CO 2 + H 2 O Many supported metals shown to be active as catalysts Silver (Ag), Palladium (Pd), Platinum (Pt), Rhodium (Rh), Iron (Fe), Cobalt, (Co), Gold (Au) Zeolites, Metal Oxides Silver supported on alumina (Ag/Al 2 O 3 ) has been widely investigated Activity is negligible below 400 o C 1 Burch et al., Topics in Catalysis, 2004
  • Slide 7
  • Ag/Al 2 O 3 catalyst Temperature( o C) Conversion (%) Hydrocarbon w/o H 2 NO x w/o H 2 720 ppm NO 4340 ppm reductant (as C 1 ) 4.3% O 2 7.2% H 2 O Burch et al., Topics in Catalysis, 2004
  • Slide 8
  • Hydrogen Promotion of HC-SCR NO x light-off temperature decreased significantly with H 2 addition NO x light-off coincides with hydrocarbon light-off Burch et al., Topics in Catalysis, 2004 Temperature( o C) Conversion (%) 0.72% H 2 720 ppm NO 4340 ppm reductant (as C 1 ) 4.3% O 2 7.2% H 2 O NO x with H 2 Hydrocarbon with H 2
  • Slide 9
  • Platinum Group Metal Addition 1. Obuchi et al., App. Cat. B, Env, 1993 2. Burch et al., App. Cat. B, Env, 2002 3. He et al., App. Cat. B, Env, 2003 4. Sato et al., Cat. Comm., 2003 5. Kotsifa et al., Cat. Letters, 2002 Platinum Group Metals (such as Pd, Pt, and Rh) reduce NO x at lower temperatures compared to Ag 1-2 Effect is significant on NO x conversion between 200-600 o C 3-5 200-400 o C is the range of focus of my research Temperature( o C) NO x Conversion (%) NO x catalytic activity of C 3 H 6 -SCR 3
  • Slide 10
  • Objectives: Develop active and selective bimetallic catalysts for NO x reduction Define effects of H 2 and PGMs on the activity and selectivity of Ag/Al 2 O 3 Examine the effect of impregnation order on NO x reduction performance
  • Slide 11
  • Presentation Overview I.Introduction II.Catalyst Characterization and Screening III.Results: Effect of Loading IV.Results: Effect of Loading Order V.Summary and Future Work
  • Slide 12
  • Experimental Setup - Celero Ar NO/Ar C 3 H 6 / Ar CO/CO 2 H 2 /Ar O2O2 H2OH2O Water Trap Varian 4900 micro GC Chemiluminescent NO x Analyzer Vent Allows up to 8 catalysts to be tested in one experiment Fully automated Celero R1 R2 R3 R4 R5 R6 R7R8 T
  • Slide 13
  • Experimental Conditions ComponentConcentration NO600 ppm CO800 ppm CO 2 4% H2OH2O O2O2 10% H2H2 3200 ppm C3H6C3H6 1800 ppm Arbalance
  • Slide 14
  • Characterization Elemental Analysis Catalyst Actual Ag Metal Loading Target Noble Metal Loading Actual Noble Metal Loading Ag/Al 2 O 3 1.55 0.03% Ag-- Ag-1% Pd/Al 2 O 3 2.13 0.01% Ag0.019% Pd0.009 0.001% Pd Ag-1% Pt/Al 2 O 3 1.27 0.01% Ag0.035% Pt0.020 0.003% Pt Ag-1% Rh/Al 2 O 3 1.82 0.02% Ag0.02% Rh 0.008 0.001% Rh Ag-10% Pd/Al 2 O 3 2.10 0.02% Ag0.19% Pd0.17 0.01% Pd Ag-10% Pt/Al 2 O 3 1.83 0.02% Ag0.35% Pt0.24 0.01% Pt Ag-10% Rh/Al 2 O 3 1.47 0.03% Ag0.20% Rh0.09 0.01% Rh
  • Slide 15
  • NO x Conversion Al 2 O 3 -Supported Bimetallic Catalysts For low-loading, Pd and Pt had no effect: Rh suppressed activity For high-loading, Pd, Pt, Rh affect activity, reaching maximum at 300 o C
  • Slide 16
  • N 2 Selectivity Al 2 O 3 -Supported Bimetallic Catalysts For low-loading, selectivity increased with temperature For high-loading, selectivity decreased with temperature
  • Slide 17
  • Presentation Overview I.Introduction II.Catalyst Characterization and Screening III.Results: Effect of Loading IV.Results: Effect of Loading Order V.Summary and Future Work
  • Slide 18
  • Design of Experiment (DOE) Setup FactorsLevels HC/NO x ratio3:16:19:1 H 2 /CO ratio0:12:14:1 Second Metal Atomic Loading0%1%10% Second Metal TypePdPtRh Temperature ( o C)200300400 3 k full factorial design (k = 5) total of 243 independent observations Levels and responses normalized from 0 to 1
  • Slide 19
  • Normalization
  • Slide 20
  • DOE Procedure Develop HypothesisSelect Factors and Levels Run Experiment and Collect DataCalculate p values Plot EffectsDetermine Statistical Significance
  • Slide 21
  • Main Effects: NO x Conversion Mean NO x Conversion P value 0.00 P value 0.05 P value 0.00 All factors are significant Increasing loading caused decrease in conversion
  • Slide 22
  • Main Effects: N 2 Selectivity Mean N 2 Selectivity P value 0.06 P value 0.00 P value 0.02 P value 0.01 P value 0.00 H 2 /CO ratio, second metal loading, second metal type, temperature significant Presence of PGM causes N 2 O formation (increasing as more is added)
  • Slide 23
  • Significant Interactions Loading/Temperature Temperature ( o C) Mean NO x Conversion 200 300 400 Characteristics of detrimental effect of loading 10% PGM Loading > 300 o C P value 0.00 0% 1% 10%
  • Slide 24
  • Effect of Loading Possible Reasons: 1.Unselective combustion of the hydrocarbon and increased formation of N 2 O by noble metals 2.Site blocking of Ag by noble metals Observation: Increasing amount of second metal onto Ag/Al 2 O 3 is detrimental on NO x conversion and N 2 selectivity (primarily at high loadings at 300 o C)
  • Slide 25
  • Loading Effect Bimetallic Catalysts HC conversion increases to 100% at high loadings NO x conversion subsequently decreases AgAg-1% Pd Ag-1% Pt Ag-1% Rh Ag-10% Pd Ag-10% Pt Ag-10% Rh
  • Slide 26
  • N 2 Selectivity Al 2 O 3 -supported Monometallic Catalysts Ag exhibits high selectivity throughout temperature range Selectivity not significantly affected at 1% loading At 10% loading, significant decreases in selectivity occur over PGM
  • Slide 27
  • Is Site Blocking an Issue? Catalyst O 2 uptake ( mol/g) Ag/Al 2 O 3 2 1 Ag-10% Pd/Al 2 O 3 1.0 0.1 Ag-10% Pt/Al 2 O 3 1.2 0.3 Ag-10% Rh/Al 2 O 3 3 13 1 O 2 uptake virtually unchanged with addition of PGM Suggests site blocking is not an issue Oxidized in air at 600 o C for 1 hour Degassed in He for 1 hour Reduced in 10% H 2 /Ar at 250 o C for 2 hours Degassed in He at 260 o C for 1 hour Pulsed 1% O 2 /He at 170 o C
  • Slide 28
  • Slide 29
  • Effect of Loading Possible Reasons: 1.Unselective combustion of the hydrocarbon and increased formation of N 2 O by noble metals 2.Site blocking of Ag by noble metals Observation: Increasing amount of second metal onto Ag/Al 2 O 3 is detrimental on NO x conversion and N 2 selectivity (primarily at high loadings at 300 o C)
  • Slide 30
  • Presentation Overview I.Introduction II.Catalyst Characterization and Screening III.Results: Effect of Loading IV.Results: Effect of Loading Order V.Summary and Future Work
  • Slide 31
  • Loading Order Prior Research NO x reduction improved when adding Ag after addition of small amount of Rh metal with decane 1 HC-SCR with CH 4 activity improved when Co was added after Zn on a Co-Zn/HZSM-5 catalyst 2 1.Sato et al., Cat. Comm., 2003 2.Ren et al., App. Cat. B: Env., 2002
  • Slide 32
  • Significant Interactions Loading Order with Metal Type NO x Conversion Insignificant for Pt and Rh (within error) Addition of Ag after Pd results in 6% improvement in conversion Loading Order Mean NO x Conversion P value 0.02 Error = 1.8% 0.35 0.3
  • Slide 33
  • Significant Interactions Loading Order with Metal Type N 2 selectivity Insignificant for Pt and Rh (within error) Addition of Ag after Pd results in 12% improvement in selectivity Loading Order Mean N 2 Selectivity P value 0.04 Error = 3.2% 0.7 0.8
  • Slide 34
  • Effect of Loading Order Possible Reasons: 1.Greater surface concentration of Ag on the surface when added after Pd 2.Pd miscible with Ag Observation: Switching the order improves performance only for Pd bimetallic catalysts
  • Slide 35
  • TPR: Pd-based catalysts H 2 consumption peak from Ag significantly when Ag added second Suggests higher Ag surface concentration 1% Pd/Al 2 O 3 Ag/Al 2 O 3 Al 2 O 3 Ag-1% Pd/Al 2 O 3 1% Pd-Ag/Al 2 O 3 10% Pd/Al 2 O 3 Ag-10% Pd/Al 2 O 3 10% Pd-Ag/Al 2 O 3 Oxidized in air at 600 o C for 1 hour Degassed in Ar for 1 hour Cooled to RT in Ar Ramped from RT to 500 o C in 10% H 2 /Ar at 20 o C/min
  • Slide 36
  • Ag-Pd miscibility or surface interactions? Ag and Pd are not miscible until 900 o C 1 Surface energies suggest Pd migrates to surface more readily and could interact with Ag compared to Pt and Rh 2 Metal Surface Energies (J/m 2 ) Ag1.2 Pd1.9 Pt2.3 Rh2.5 1.I. Karakaya, Journal of Phase Equlibria, 1986 2. Vitos et al., Surface Science, 1998 1600 900 Temperature ( o C)
  • Slide 37
  • Ag metal dispersion - Ag-first vs. Ag-second loaded Catalyst O 2 uptake ( mol/g) Ag-10% Pd/Al 2 O 3 1.0 0.1 10% Pd-Ag/Al 2 O 3 6 1 Ag-10% Pt/Al 2 O 3 1.2 0.3 10% Pt-Ag/Al 2 O 3 2 1 Ag-10% Rh/Al 2 O 3 3 1 10% Rh-Ag/Al 2 O 3 3 1 O 2 uptake significantly increases when Ag added after Pd Negligible change for Pt bimetallics: no change for Rh catalysts
  • Slide 38
  • Effect of Loading Order Possible Reasons: 1.Greater surface concentration of Ag on the surface when added after Pd 2.Pd miscible with Ag Observation: Switching the order improves performance only for Pd bimetallic catalysts
  • Slide 39
  • Presentation Overview I.Introduction II.Catalyst Characterization and Screening III.Results: Effect of Loading IV.Results: Effect of Loading Order V.Summary and Future Work
  • Slide 40
  • Summary and Conclusions Increasing loading of second metal was detrimental on the NO x conversion and N 2 selectivity Due to increased unselective combustion of C 3 H 6 and formation of N 2 O with increased noble metal concentration Loading order mattered only for Pd bimetallic catalysts Increased surface concentration of Ag when Ag added after Pd The combination of both H 2 and noble metal addition did not result in a better performing catalyst compared to Ag/Al 2 O 3 by itself
  • Slide 41
  • Suggestions for Future Work Reduce noble metal amounts even smaller EXAFS (Extended X-Ray Absorption Fine Structure) to identify electronic effects of loading order on Ag- Pd bimetallic catalysts Further investigate differences in metal type
  • Slide 42
  • Acknowledgements Professor Levi Thompson My Committee Professor Galen Fisher Professor Erdogan Gulari Professor Phillip Savage Professor Arvind Atreya Quantum Sciences Inc. Past and Present Members of Thompson Group Friends (especially in SMES-G, SCOR, and AGEP) My Family God
  • Slide 43
  • Bimetallic Silver Catalysts for the Reformate-Assisted Hydrocarbon Selective Catalytic Reduction (HC-SCR) of Nitrogen Oxides Richard Ezike Ph.D. Defense Department of Chemical Engineering The University of Michigan July 29, 2011
  • Slide 44
  • Engine Exhaust Characteristics (before treatment) Exhaust Component Diesel EngineGasoline Engine Nitrogen Oxides200-1000 ppm100-4000 ppm Total Hydrocarbons 10-330 ppm400-5000 ppm CO150-1200 ppm0.1-6% O2O2 5-15%0.2-2% H2OH2O1-7%10-12% CO 2 3-13%10-18% Sulfur Oxides10-100 ppm15-60 ppm Particulates50-400 mg/m 3 n/a TemperatureRT-700 o CRT-1100 o C From Supported Metals in Catalysis, Anderson and Garcia, 2005. Three way catalytic converter (TWC) can reduce Nox emissions from gas engines up to 90% TWC cannot do this in oxidizing environment
  • Slide 45
  • Noble Metal HC-SCR Good low temperature performance (200-400 o C) Characterized by volcano plot behavior Tend to produce significant amounts of N 2 O 1.Burch and Millington, Cat. Today, 1996 Activity of 1% loaded noble metal catalysts for NO x reduction by C 3 H 6 1
  • Slide 46
  • Proposed Mechanism Base Metal Oxides NO or hydrocarbon will react with O 2 to form adsorbed NO x or acetate species Decomposition could create isocyanate, cyanate, ammonia intermediates Reduce to N 2 Burch et al., Topics in Catalysis, 2004
  • Slide 47
  • Proposed Mechanism Base Metal Oxides H 2 Enhancement
  • Slide 48
  • Mechanism over Noble Metals Dissociation reduction (Burch et al., App. Cat. B. Env. 1994) Z is an adsorption site Reaction occurs on reduced noble metal
  • Slide 49
  • F Distribution Table Factor degrees of freedom (number of levels 1) Residual degrees of freedom (number of treatments - number of levels) Use F distribution table to get specific F obs values Compare with calculated F values from ANOVA
  • Slide 50
  • C 3 H 6 Conversion Monometallic Catalysts
  • Slide 51
  • Main Effects: N 2 Selectivity H 2 /CO ratio, second metal loading, temperature significant presence of H 2 and increasing temperature enhance NO x conversion over Ag/Al 2 O 3 and therefore N 2 formation Presence of noble metal causes N 2 O formation (increasing as more is added) P value 0.21 P value 0.00 P value 0.07 P value 0.00
  • Slide 52
  • DRIFTS Bimetallics at 400 o C hydroxyls Trace CO 2 (g) formates C=C? Presence of formates, hydroxyls and v(C=C) bond [He et al, APB: Env., 2003; Wichterlova et al., J. Cat., 2005] Surface species and intensities similar regardless of second metal type T = 400 o C H 2 /CO = 4 HC/NO x = 9
  • Slide 53
  • TPR: Pt-based catalysts 0.01 Pt/Al 2 O 3 Ag/Al 2 O 3 Al 2 O 3 Ag-0.01 Pt/Al 2 O 3 0.01 Pt-Ag/Al 2 O 3 0.1 Pt/Al 2 O 3 Ag-0.1 Pt/Al 2 O 3 0.1 Pt-Ag/Al 2 O 3 Reduction of PtO 2 at 180 o C 180 o C H 2 consumption peak from Ag significantly larger on Ag- second loaded catalysts Observe shift in Ag reduction peak on high- loading Ag-Pt catalysts (from 230 o C-200 o C)
  • Slide 54
  • TPR: Rh-based catalysts Reduction of Rh 2 O 3 at 120 o C 0.01 Rh/Al 2 O 3 Ag/Al 2 O 3 Al 2 O 3 Ag-0.01 Rh/Al 2 O 3 0.01 Rh-Ag/Al 2 O 3 0.1 Rh/Al 2 O 3 Ag-0.1 Rh/Al 2 O 3 0.1 Rh-Ag/Al 2 O 3 120 o C Observe shift in Ag reduction peak on high- loading Ag-Rh catalysts (from 230 o C-140 o C)
  • Slide 55
  • Surface Coverage Surface Coverage ~ 15% for Ag,.1% for 0.01 loaded catalysts and 1% for 0.1 loaded catalysts
  • Slide 56
  • Theoretical Ag-Pd Orientation Jaatinen et al., Vacuum, 2004 Top view of the geometry of (1 1 1) orientation for 3 X 3 surface system: The white circles are the surface atoms (if not labeled, those atoms are Ag.
  • Slide 57
  • Competitiveness Factor
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Bimetallic Silver Catalysts for the Reformate-Assisted Hydrocarbon Selective Catalytic Reduction (HC-SCR) of Nitrogen Oxides Richard Ezike Ph.D. Defense Department of Chemical Engineering The University of Michigan July 29, 2011
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