Download - Bimetallic Silver Catalysts for the Reformate-Assisted Hydrocarbon Selective Catalytic Reduction (HC-SCR) of Nitrogen Oxides Richard Ezike Ph.D. Defense.

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


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


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 (%

)



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


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)



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



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


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


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