Bimetallic Silver Catalysts for the Reformate-Assisted Hydrocarbon
Selective Catalytic Reduction (HC-SCR) of Nitrogen Oxides
Richard Ezike Ph.D. Defense
Department of Chemical EngineeringThe University of Michigan
July 29, 2011
Smog
http://upload.wikimedia.org/wikipedia/commons/9/96/SmogNY.jpg
• Consists of particulate matter and ground-level ozone
• Caused by reaction of NOx and hydrocarbons in the presence of sunlight
• Contributes to a number of health issues• Emphysema, asthma,
bronchitis, shortness of breath
Where is majority of NOx
generated?
EPA 2011
Mobile Sources 54%
Stationary Sources 36%
Other Sources 10%
• Increasing number of vehicles are lean-burn• Diesel,
gasoline-powered
• Lean-burn engines produce more NOx
NOx Emission Standards
• Increasingly stringent emission standards require significant technological advances1,2
1. U.S. EPA Office of Mobile Sources2. California Air Resources Board LEV Level III
1975 1977 1981 1994 1999 2004-2010
2014-2022
0
0.5
1
1.5
2
2.5
3
3.53.1
2
1
0.60.3
0.07 0.02
Vehicle Model Year
NO
x Sta
ndar
d (g
/mile
)
72%
NOx Technologies
Technology Benefits Challenges Authors
NOx decomposition Most direct method
High reaction temperatures
(~900OC), thermodynamically
difficult
Iwamoto et al., App. Cat.,
1991
NOx storage-reduction/Lean NOx
traps
Well established method, no
additional reducing agent
SO2 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 -10oC, hydrocarbon poisoning at low
temps
Koebel et al., Cat. Today,
2000
Hydrocarbon Selective Catalytic Reduction (HC-SCR) of NOx
HC + NOx + O2 → N2 (desired) + CO2 + H2OHC + NOx + O2 → N2O (undesired) + CO2 + H2O• 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/Al2O3) has been widely investigated• Activity is negligible below 400oC1
Burch et al., Topics in Catalysis, 2004
Ag/Al2O3 catalyst
0102030405060708090100
0 100 200 300 400 500 600
Temperature(oC)
Conv
ersi
on (%
)
Hydrocarbon w/o H2
NOx w/o H2
720 ppm NO4340 ppm reductant (as C1)4.3% O2
7.2% H2O
Burch et al., Topics in Catalysis, 2004
Hydrogen Promotion of HC-SCR
0102030405060708090100
0 100 200 300 400 500 600
• NOx light-off temperature decreased significantly with H2 addition• NOx light-off coincides with hydrocarbon light-off
Burch et al., Topics in Catalysis, 2004
Temperature(oC)
Conv
ersi
on (%
) 0.72% H2
720 ppm NO4340 ppm reductant (as C1)4.3% O2
7.2% H2O
NOx with H2
Hydrocarbon with H2
Platinum Group Metal Addition
1. Obuchi et al., App. Cat. B, Env, 19932. Burch et al., App. Cat. B, Env, 20023. He et al., App. Cat. B, Env, 20034. Sato et al., Cat. Comm., 20035. Kotsifa et al., Cat. Letters, 2002
• Platinum Group Metals (such as Pd, Pt, and Rh) reduce NOx at lower temperatures compared to Ag1-2
• Effect is significant on NOx conversion between 200-600oC3-5
• 200-400oC is the range of focus of my research
Temperature(oC)
NO
x Con
vers
ion
(%)
NOx catalytic activity of C3H6-SCR3
Objectives:
• Develop active and selective bimetallic catalysts for NOx reduction
• Define effects of H2 and PGMs on the activity and selectivity of Ag/Al2O3
• Examine the effect of impregnation order on NOx reduction performance
Presentation Overview
I. IntroductionII. Catalyst Characterization and ScreeningIII. Results: Effect of LoadingIV. Results: Effect of Loading OrderV. Summary and Future Work
Experimental Setup - Celero
Ar
NO/Ar
C3H6/Ar
CO/CO2
H2/Ar
O2
H2OWater Trap
Varian 4900 micro GC
Chemiluminescent NOx Analyzer
Vent
• Allows up to 8 catalysts to be tested in one experiment
• Fully automated
Celero
R1 R2
R3 R4
R5 R6
R7 R8
T
Experimental Conditions
Component Concentration
NO 600 ppm
CO 800 ppm
CO2 4%
H2O 4%
O2 10%
H23200 ppm
C3H61800 ppm
Ar balance
• Preparation Method: Incipient Wetness• Bimetallic catalysts made by sequential
impregnation (Ag loaded first)
• Calcined in air at 600oC for 3 hours
• Noble metal loadings based on total amount of atomic Ag on a 2% Ag/Al2O3 catalyst• Atomic count = 1.1*1020 Ag atoms/g catalyst
Characterization – Elemental Analysis
CatalystActual Ag Metal
Loading Target Noble Metal Loading
Actual Noble Metal Loading
Ag/Al2O3 1.55 ± 0.03% Ag - -
Ag-1% Pd/Al2O3 2.13 ± 0.01% Ag 0.019% Pd 0.009± 0.001% Pd
Ag-1% Pt/Al2O3 1.27 ± 0.01% Ag 0.035% Pt 0.020 ± 0.003% Pt
Ag-1% Rh/Al2O3 1.82 ± 0.02% Ag 0.02% Rh 0.008 ± 0.001% Rh
Ag-10% Pd/Al2O3 2.10 ± 0.02% Ag 0.19% Pd 0.17 ± 0.01% Pd
Ag-10% Pt/Al2O3 1.83 ± 0.02% Ag 0.35% Pt 0.24 ± 0.01% Pt
Ag-10% Rh/Al2O3 1.47 ± 0.03% Ag 0.20% Rh 0.09 ± 0.01% Rh
NOx Conversion – Al2O3-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
300oC
150 200 250 300 350 400 4500
20
40
60
80
100AgAg-1% PdAg-1% PtAg-1% RhAg-10% PdAg-10% PtAg-10% Rh
Temperature (oC)
NO
x Co
nver
sion
(%)
N2 Selectivity – Al2O3-Supported Bimetallic Catalysts
• For low-loading, selectivity increased with temperature• For high-loading, selectivity decreased with temperature
150 200 250 300 350 400 45085
90
95
100
AgAg-1% PdAg-1% PtAg-1% RhAg-10% PdAg-10% PtAg-10% Rh
Temperature (oC)
N2
Sele
ctivi
ty (%
)
Presentation Overview
I. IntroductionII. Catalyst Characterization and ScreeningIII. Results: Effect of LoadingIV. Results: Effect of Loading OrderV. Summary and Future Work
Design of Experiment (DOE) SetupFactors Levels
HC/NOx ratio 3:1 6:1 9:1
H2/CO ratio 0:1 2:1 4:1
Second Metal Atomic Loading 0% 1% 10%
Second Metal Type Pd Pt RhTemperature (oC) 200 300 400
• 3k full factorial design (k = 5)• total of 243 independent observations • Levels and responses normalized from 0 to 1
Normalization
•
DOE Procedure
Develop Hypothesis Select Factors and Levels
Run Experiment and Collect DataCalculate p values
Plot Effects Determine Statistical Significance
Main Effects: NOx Conversion
9:16:13:1
0.5
0.4
0.3
0.2
0.1
4:12:10:1 10%1%0%
RhPtPd
0.5
0.4
0.3
0.2
0.1
400300200
HC/NOx Ratio H2/CO Ratio Second Metal Loading
0.3271
Second Metal Type Temperature
0.3271
Mea
n N
Ox C
onve
rsio
n
P value ≈ 0.00 P value ≈ 0.00 P value ≈ 0.00
P value ≈ 0.05
P value ≈ 0.00
• All factors are significant
• Increasing loading caused decrease in conversion
9:16:13:1
0.9
0.8
0.7
0.6
4:12:10:1 10%1%0%
RhPtPd
0.9
0.8
0.7
0.6
400300200
HC/NOx Ratio H2/CO Ratio Second Metal Loading
0.8012
Second Metal Type Temperature
0.8012
Main Effects: N2 SelectivityM
ean
N2 S
elec
tivity
P value ≈ 0.06 P value ≈ 0.00
P value ≈ 0.02
P value ≈ 0.01
P value ≈ 0.00
• H2/CO ratio, second metal loading, second metal type, temperature significant
• Presence of PGM causes N2O formation (increasing as more is added)
Significant Interactions – Loading/Temperature
Temperature (oC)
Mea
n N
Ox
Conv
ersi
on
200 300 400
• Characteristics of detrimental effect of loading• 10% PGM Loading• > 300oC
P value ≈ 0.00
0%1%10%
Effect of Loading
Possible Reasons:1. Unselective combustion of the hydrocarbon and
increased formation of N2O by noble metals 2. Site blocking of Ag by noble metals
Observation: Increasing amount of second metal onto Ag/Al2O3 is detrimental on NOx conversion and N2 selectivity (primarily at high loadings at 300oC)
Loading Effect – Bimetallic Catalysts
• HC conversion increases to 100% at high loadings• NOx conversion subsequently decreases
0102030405060708090
100
NOx Conver-sionHC Con-version
Conv
ersi
on (%
)
T = 300oCGHSV = 60000 hr-1
H2/CO = 4HC/NOx = 9
Ag Ag-1% Pd
Ag-1% Pt
Ag-1% Rh
Ag-10% Pd
Ag-10% Pt
Ag-10% Rh
N2 Selectivity – Al2O3-supportedMonometallic 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
150 200 250 300 350 400 45060
70
80
90
100
Ag1% Pd1% Pt1% Rh10% Pd10% Pt10% Rh
Temperature (oC)
N2
Sele
ctivi
ty (%
)
Is Site Blocking an Issue?
CatalystO2 uptake
(μmol/g)
Ag/Al2O3 2 ± 1
Ag-10% Pd/Al2O3 1.0 ± 0.1
Ag-10% Pt/Al2O3 1.2 ± 0.3
Ag-10% Rh/Al2O3 3 ±1
• O2 uptake virtually unchanged with addition of PGM
• Suggests site blocking is not an issue
• Oxidized in air at 600oC for 1 hour• Degassed in He for 1 hour• Reduced in 10% H2/Ar at 250oC for 2 hours• Degassed in He at 260oC for 1 hour• Pulsed 1% O2/He at 170oC
Effect of Loading
Possible Reasons:1. Unselective combustion of the hydrocarbon and
increased formation of N2O by noble metals 2. Site blocking of Ag by noble metals
Observation: Increasing amount of second metal onto Ag/Al2O3 is detrimental on NOx conversion and N2 selectivity (primarily at high loadings at 300oC)
Presentation Overview
I. IntroductionII. Catalyst Characterization and ScreeningIII. Results: Effect of LoadingIV. Results: Effect of Loading OrderV. Summary and Future Work
Loading Order – Prior Research
• NOx reduction improved when adding Ag after addition of small amount of Rh metal with decane1
• HC-SCR with CH4 activity improved when Co was added after Zn on a Co-Zn/HZSM-5 catalyst2
1.Sato et al., Cat. Comm., 20032.Ren et al., App. Cat. B: Env., 2002
Significant Interactions – Loading Order with Metal Type – NOx Conversion
• Insignificant for Pt and Rh (within error)
• Addition of Ag after Pd results in ≈ 6% improvement in conversion
Ag-secondAg-first
37363534333231302928
PdPtRh
TypeMetalSecond
Loading Order
Mea
n N
Ox C
onve
rsio
n
P value ≈ 0.02
Error = 1.8%
0.35
0.3
Significant Interactions – Loading Order with Metal Type – N2 selectivity
• Insignificant for Pt and Rh (within error)
• Addition of Ag after Pd results in ≈ 12% improvement in selectivity
Loading Order
Mea
n N
2 Sel
ectiv
ity
Ag-secondAg-first
88868482807876747270
PdPtRh
TypeMetalSecond
P value ≈ 0.04
Error = 3.2%
0.7
0.8
Effect of Loading Order
Possible Reasons:1. Greater surface concentration of Ag on the surface
when added after Pd2. Pd miscible with Ag
Observation: Switching the order improves performance only for Pd bimetallic catalysts
TPR: Pd-based catalysts
• H2 consumption peak from Ag significantly when Ag added second
• Suggests higher Ag surface concentration
1% Pd/Al2O3
Ag/Al2O3
Al2O3
Ag-1% Pd/Al2O3
1% Pd-Ag/Al2O3
10% Pd/Al2O3
Ag-10% Pd/Al2O3
10% Pd-Ag/Al2O3
• Oxidized in air at 600oC for 1 hour
• Degassed in Ar for 1 hour• Cooled to RT in Ar• Ramped from RT to 500oC
in 10% H2/Ar at 20oC/min
Ag-Pd – miscibility or surface interactions?
• Ag and Pd are not miscible until 900oC1
• Surface energies suggest Pd migrates to surface more readily and could interact with Ag compared to Pt and Rh2
Metal Surface Energies (J/m2)
Ag 1.2
Pd 1.9
Pt 2.3
Rh 2.5
1.I. Karakaya, Journal of Phase Equlibria, 19862. Vitos et al., Surface Science, 1998
1600
900
Tem
pera
ture
(o C)
Ag metal dispersion - Ag-first vs.Ag-second loaded
CatalystO2 uptake
(μmol/g)
Ag-10% Pd/Al2O3
1.0 ± 0.1
10% Pd-Ag/Al2O3
6 ± 1
Ag-10% Pt/Al2O3 1.2 ± 0.3
10% Pt-Ag/Al2O3 2 ± 1
Ag-10% Rh/Al2O3 3 ± 1
10% Rh-Ag/Al2O3 3 ± 1
• O2 uptake significantly increases when Ag added after Pd
• Negligible change for Pt bimetallics: no change for Rh catalysts
Effect of Loading Order
Possible Reasons:1. Greater surface concentration of Ag on the surface
when added after Pd2. Pd miscible with Ag
Observation: Switching the order improves performance only for Pd bimetallic catalysts
Presentation Overview
I. IntroductionII. Catalyst Characterization and ScreeningIII. Results: Effect of LoadingIV. Results: Effect of Loading OrderV. Summary and Future Work
Summary and Conclusions
• Increasing loading of second metal was detrimental on the NOx conversion and N2 selectivity• Due to increased unselective combustion of C3H6 and
formation of N2O 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 H2 and noble metal addition did not result in a better performing catalyst compared to Ag/Al2O3 by itself
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
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
Bimetallic Silver Catalysts for the Reformate-Assisted Hydrocarbon
Selective Catalytic Reduction (HC-SCR) of Nitrogen Oxides
Richard Ezike Ph.D. Defense
Department of Chemical EngineeringThe University of Michigan
July 29, 2011
Engine Exhaust Characteristics (before treatment)
Exhaust Component
Diesel Engine Gasoline Engine
Nitrogen Oxides
200-1000 ppm 100-4000 ppm
Total Hydrocarbons
10-330 ppm 400-5000 ppm
CO 150-1200 ppm 0.1-6%
O2 5-15% 0.2-2%
H2O 1-7% 10-12%
CO2 3-13% 10-18%
Sulfur Oxides 10-100 ppm 15-60 ppm
Particulates 50-400 mg/m3 n/a
Temperature RT-700oC RT-1100oCFrom 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
Noble Metal HC-SCR
• Good low temperature performance (200-400oC)
• Characterized by volcano plot behavior
• Tend to produce significant amounts of N2O
1. Burch and Millington, Cat. Today, 1996
Activity of 1% loaded noble metal catalysts for NOx reduction byC3H6
1
Proposed Mechanism – BaseMetal Oxides
• NO or hydrocarbon will react with O2 to form adsorbed NOx or acetate species
• Decomposition could create isocyanate, cyanate, ammonia intermediates
• Reduce to N2
Burch et al., Topics in Catalysis, 2004
Proposed Mechanism – BaseMetal Oxides – H2 Enhancement
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
F Distribution Table
Factor degrees of freedom (number of levels – 1)
Resi
dual
deg
rees
of f
reed
om (n
umbe
r of t
reat
men
ts -
num
ber o
f lev
els)
• Use F distribution table to get specific Fobs values
• Compare with calculated F values from ANOVA
C3H6 Conversion – MonometallicCatalysts
9:16:13:1
0.9
0.8
0.7
0.64:12:10:1 0.10%0.01%0.00%
RhPtPd
0.9
0.8
0.7
0.6400300200
HC/ NOx Ratio
Ave
rag
e N
itro
ge
n S
ele
ctiv
ity
H2/ CO Ratio Second Metal Loading
Second Metal Type Temperature
Main Effects: N2 SelectivityH2/CO ratio, second metal loading, temperature significant
presence of H2 and increasing temperature enhance NOx conversion over Ag/Al2O3 and therefore N2 formation
Presence of noble metal causes N2O formation (increasing as more is added)
P value ≈0.21 P value ≈ 0.00P value ≈ 0.00
P value ≈ 0.07 P value ≈ 0.00
DRIFTS – Bimetallics at 400oChydroxyls Trace CO2(g) formatesC=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 = 400oCH2/CO = 4HC/NOx = 9
TPR: Pt-based catalysts
0.01 Pt/Al2O3
Ag/Al2O3
Al2O3
Ag-0.01 Pt/Al2O3
0.01 Pt-Ag/Al2O3
0.1 Pt/Al2O3
Ag-0.1 Pt/Al2O3
0.1 Pt-Ag/Al2O3
• Reduction of PtO2 at 180oC
180oC
• H2 consumption peak from Ag significantly larger on Ag-second loaded catalysts
• Observe shift in Ag reduction peak on high-loading Ag-Pt catalysts (from 230oC-200oC)
TPR: Rh-based catalysts
• Reduction of Rh2O3 at 120oC
0.01 Rh/Al2O3
Ag/Al2O3
Al2O3
Ag-0.01 Rh/Al2O3
0.01 Rh-Ag/Al2O3
0.1 Rh/Al2O3
Ag-0.1 Rh/Al2O3
0.1 Rh-Ag/Al2O3
120oC
• Observe shift in Ag reduction peak on high-loading Ag-Rh catalysts (from 230oC-140oC)
Surface Coverage
• Surface Coverage ~ 15% for Ag, .1% for 0.01 loaded catalysts and 1% for 0.1 loaded catalysts
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.
Competitiveness Factor
150 200 250 300 350 400 4500
10
20
30
40
50
60
70
80AgAg-1% PdAg-1% PtAg-1% RhAg-10% PdAg-10% PtAg-10% Rh
Temperature (oC)
Com
petiti
vene
ss F
acto
r