Download - Methodology for Estimating Ea for Catalyst Deactivation · 2017. 6. 14. · Apply the protocol to SCR catalyst aging and evaluate applicability of the Arrhenius expression for representing

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Methodology for Estimating Ea for Catalyst Deactivation

Bukky Oladipo, Tom Pauly, Marco Lopez

May 2, 2012

2012 DOE Crosscut Workshop on Lean Emissions Reduction Simulation


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Rationale for Current Work

¶  Assessing catalyst system performance deterioration over in-use lifetime very costly and time-consuming

¶ Need to correctly reflect impact of thermal aging and chemical exposure can complicate bench aging acceleration to mimic field aging

¶  Typically, aging acceleration simulated through oven-, burner-, and engine bench-aging with select time @ temperature specifications

¶  For gasoline application, Arrhenius expression has been a successful tool used for determining equivalent aging acceleration over the years

¶  Industry interest is growing to develop appropriate protocol for accelerating diesel catalyst system aging to demonstrate end-of-life performance for various heavy-duty applications


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Objectives

¶  Employ the Arrhenius expression as a tool for developing accelerated aging protocol for diesel catalyst systems

¶  Apply the protocol to SCR catalyst aging and evaluate applicability of the Arrhenius expression for representing loss of overall NOx conversion with aging and/or the inherent functionalities of the SCR

¶  Establish Ea (energy of activation) for the deactivation of the catalyst and establish the variants related to functional deactivation due to thermal aging and chemical exposure

¶  Identify conditions under which the global Arrhenius method is not sufficient to determine the required aging acceleration


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Arrhenius Time and Temperature Dependency of Catalyst System Aging

Arrhenius equation relates rate of a reaction to temperature

Example: Ea = 96.5 kJ/mol for CO oxidation on Pt 111 face (gasoline) Source: SwRI – HD-DAAAC Consortium EPA Presentation, 17 March 2009

RTEa

Aek−

=

activation energy

gas constant

temperature (Kelvin)

pre-exponential factor

rate constant

natural log, e

kA

T

eEa

R


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Our Ultimate Objective is to Establish Performance DF for the Catalyst

AGING DURATION

NOx

CONV

ERSI

ON

NOx Conversion Deterioration With Oven-Aging at 675 °C; Evaluation Testing at 250 deg C SCR Inlet Temp

•  Aging model effectively establishing DF for simulating end-of-life (EUL) performance

• HD Truck: 435,000 miles (~8,000 hrs)

•  Locomotive line-haul: >64,000 hours EUL

• Can Arrhenius-type expression work to predict EUL activity?


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In the literature on catalyst simulation, inhibition terms are usually expressed in the Arrhenius form:

For NO oxidation over DOC as an example,

where the inhibition terms K1, K2, K3, & K4 have Arrhenius dependence just like the main reaction rate term k3

Source: Pandya, Mmbaga, Hayes, Hauptmann and Votsmeier, “Global Kinetic Model and Parameter Optimization for a Diesel Oxidation Catalyst,” National Research Council Canada, Pan2009


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For simulation purposes, can we then separate deactivation into distinct sources?

);( poisoningagingthermalfonDeactivati =

)()( poisoninggthermalfref

×=Δ

ηη

⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

Δ

aging

ompositeccomposite

ref RTE

A expηη

Therefore, can we write

leading to:

where poisoningthermalcomposite EEE +=


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SCR1 ∅10.5“x 6.0“

8.5 L ∅10.5“x 6.0“

8.5 L = 17.0 L

Performance evaluation with NO2

CDPF Pt/Pd

DOC Pt/Pd

Aged

∅10.5“x 6.0“ 300/5

8.5 L

∅10.5“x 12.0“ 200/12 AC

17.0 L

A simple example with hydrothermal aging: Fe-Zeolite SCR over short to long aging duration

Aged 16H @ 750C

Temperature

Time

¶ 400°C

¶ 350°C

¶ 300°C

¶ 250°C

¶ Temp. ¶ NO2-Content

¶ 56 %

¶ 59 %

¶ 46 %

¶ 27 %

¶ Space Velocity

¶ SCR Volume

¶ 33.0 k

¶ 25.5 k

¶ 21.5 k

¶ 19.5 k

¶ 17 L

Time Dependence of Thermal Aging: 550 C 200h 550 C 400h 550 C 800h Temperature Dependence of Thermal Aging: §  16 hr 550°C §  16 hr 700°C 16 hr 800°C

SCR2


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Approach

¶  Arrhenius expression to describe performance loss due to aging:

¶  For (global) SCR NOx conversion reaction, consider: •  Deactivation = Change in NOx conversion from Fresh to Aged State

= Function (time, temperature) •  Assuming:

§  Linearity with aging time

§  Exponential with temperature

•  Deactivation where Δη/taging is the rate of loss of NOx conversion efficiency and

Δη is normalized by appropriate reference value ηref, e.g. 100 for alpha =1.0, etc.

⎟⎠

⎞⎜⎝

⎛−=RTEAk aexp

⎟⎟⎠

⎞⎜⎜⎝

⎛=

Δ=

agingagingref RTEA

tRateonDeactivati exp

ηη


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Variation of NOx Conversion Efficiency with Aging Temperature (Aging Duration = 16 hrs)

α = 1.0

30 40 50 60 70 80 90

100

250 300 350 400 Temperature [°C]

NO

x C

onve

rsio

n [%

]

0 50 100 150 200 250 300 350

NH

3 Slip [ppm

]

16 hr @ 550 C 16 hr @ 800 C 16 hr @ 700 C NH3-Slip 16 hr @ 550 C NH3-Slip 16 hr @ 800 C NH3-Slip 16 hr @ 700 C


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Variation of NOx Conversion Efficiency with Aging Duration (Aging Temperature = 550 C)

30 40 50 60 70 80 90

100

250 300 350 400 Temperature [°C]

NO

x C

onve

rsio

n [%

]

0 50 100 150 200 250 300 350

NH

3 Slip [ppm

]

16 hr @ 550 C 200 hr @ 550 C 400 hr @ 550 C 800 hr @ 550 C NH3-Slip 16 hr @ 550 C NH3-Slip 200 hr @ 550 C NH3-Slip 400 hr @ 550 C NH3-Slip 800 hr@ 550 C

Two Major Observations: �  There is no significant loss of NOx conversion except @ 400 deg C temperature; �  Aging temperature influences deactivation more than aging duration


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Estimating Ea for Degreened & Aging Modes (NOx Eff)ref, % 100Time Aged Temp NOx Eff Eff Deac. Inv Temp Deac/time (D) ln(D)hr deg C % - 1/K 1/s16 550 94.38 5.62 0.00121 9.757E-07 -13.84

16 700 91.25 8.75 0.00103 1.519E-06 -13.4016 800 80.5 19.5 0.00093 3.385E-06 -12.6016 550 94.44 5.56 0.00121 9.653E-07 -13.85200 550 92.68 7.32 0.00121 1.017E-07 -16.10400 550 90.17 9.83 0.00121 6.826E-08 -16.50800 550 90 10 0.00121 3.472E-08 -17.18

y = -2392.9389188x - 10.9384323R2 = 0.9995698

y = -14121.1373361x + 0.5625532R2 = 0.9530623

-18.00

-17.00

-16.00

-15.00

-14.00

-13.00

-12.00

-11.00

-10.00

0.0009 0.0010 0.0011 0.0012 0.0013

1/T

LN(D)

Degrnd Mode Aging Mode

Linear (Degrnd Mode) Linear (Aging Mode)

y = -10496x - 2.7345R2 = 0.4878

-18.00

-17.00

-16.00

-15.00

-14.00

-13.00

-12.00

-11.00

-10.00

0.0009 0.0010 0.0011 0.0012 0.00131/T

LN(D)

All Aging Levels Linear (All Aging Levels)

Estimation of Activation Energy (Ea) for the Deactivation Mechanism (400 deg C data)

⎟⎠

⎞⎜⎝

⎛−=

Δ

RTEA

ta

agingref

NOx expηη

We consider:

hence,

( ) ⎟⎠

⎞⎜⎝

⎛−=⎟

⎟⎠

⎞⎜⎜⎝

⎛ Δ

TREA

ta

agingref

NOx 1LNLNηη

Normal aging mode Degreened mode

Degrn'd Aging-Ea/R -2,393 -14,121R 8.314 8.314 kJ/kmolEa 19,894.9 117,403.1 kJ/kmolln(A) -10.93843 0.562553A 1.78E-05 1.755148 1/s


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Fe-SCR example shows aging influence only at the high end (400 deg C) of aging temperature

¶  This is likely due to loss of NH3 storage capability; dealumination of Zeolite or other mechanisms

¶ Must look at component functionalities rather than global NOx conversion efficiency •  NO oxidation •  NH3 storage capacity •  Surface coverage dependent NOx conversion •  NH3 oxidation

¶  Attempt to correlate the component processes (functionalities) using the Arrhenius expression


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Another Example Using Cu-Zeolite SCR (Hydrothermal Oven Aging)

0

10

20

30

40

50

60

70

80

90

100

100 150 200 250 300 350 400 450 500 550 600 650

T in front of SCR [°C]

NO

con

vers

ion,

nor

med

by

alph

a [%

]

4h/800°C hydrothermal 4h/850°C hydrothermal 4h/875°C hydrothermal4h/900°C hydrothermal 4h/950°C hydrothermal

Short hydrothermal aging reaching up to 950 deg C

Estimating Ea for Aging Modes (NOx Eff)ref, % 100

Time Aged Temp NOx Eff Eff Deac. Inv Temp Deac/time (D) ln(D)hr deg C % - 1/K 1/hr

4 800 98.0 2.0 0.00093 1.389E-06 -13.494 850 92.0 8.0 0.00089 5.556E-06 -12.104 875 83.0 17.0 0.00087 1.181E-05 -11.354 900 57.5 42.5 0.00085 2.951E-05 -10.434 950 6.5 93.5 0.00082 6.493E-05 -9.64

Performance Deterioration at 200 deg C

⎟⎠

⎞⎜⎝

⎛−=

Δ

RTEA

ta

agingref

NOx expηη


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Potential to Identify Transition Points for Change in Aging (Deactivation) Mechanism

Inverse Temp (1/K)

⎟⎟ ⎠⎞⎜⎜ ⎝⎛ Δ

tLN

refNOX

ηη

Ea=188,135 kJ/kmol

Ea=290,031 kJ/kmol

HIGH THERMAL DEACTIVATION

LOW TO MEDIUM THERMAL

DEACTIVATION

Increasing Aging Temperature

High Temperature Aging Deactivation

Potentially a result of:

•  Zeolite collapse

•  Phase change

•  Cu sintering

•  Cristobalite

Deactivation under Normal/Medium Temperature

Potentially a result of (some generic chemistry facts):

•  Loss of catalytic sites

•  Cu migration

•  Dealumination

•  Chemical poisoning (P; S;…)

•  ….

Aging Impact on Low Temperature Deactivation; GHSV = 30,000 (1/hr)


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Identifying Aging Limit for SCR Deactivation Source: “CLEERS SCR Teleconference,” Stephen J Schmieg, GM R&D


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Need to look at inherent “component” processes rather than global NOx conversion only

¶ Recall that the Fe-SCR example shows aging influence only at relatively high operating temperature (400 deg C)

¶  It suggests potential benefit of looking at the detailed processes

¶ Component functionalities to consider: •  NO oxidation •  NH3 storage capacity •  Surface coverage dependent NOx conversion •  NH3 oxidation

¶ Can these inherent processes (functionalities) be correlated with the Arrhenius even when overall NOx conversion shows no impact?


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Component Functionalities of SCR NOx Conversion Source: “CLEERS SCR Teleconference,” Stephen J Schmieg, GM R&D


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0

50

100

150

200

250

300

Time

PPM

/ de

g C

0

1

2

3

4

5

6

7

8

9

Test

Ste

p

Temp NO NH3 Step Temp NO NH3 Step Temp NO NH3 Step

Influence of 650 °C Hydrothermal Aging Change in NH3 storage at 175 °C reveal impact of aging

Aging hours as indicated on the curves

NH3 storage capacity is almost entirely gone after 1000 hrs of hydrothermal aging at 650 deg C

NO Efficiency almost unchanged

250 hrs 1000 hrs

16 hrs


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Influence of 650 °C Hydrothermal Aging No change in NH3 storage at 400 °C with the aging

Aging hours as indicated on the curves

0

50

100

150

200

250

300

350

400

450

Time

ppm

or

o C

0

1

2

3

4

5

6

7

8

Temp NO NH3 Temp NO NH3 Temp NO NH3 Step

16 hrs 250 hrs 1000 hrs

NO Efficiency equal


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We decide to employ a DoE approach involving aging temperature and duration

5

10

1

8

3

2, 4, 9

7

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7 Relevant aspects to consider: •  Response of SCR component functionalities

to aging level (time @ temp)

•  Contribution of exposure to chemical poisoning elements (P, S, Zn, Ca, etc.)


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If Successful, Potential Application of the Arrhenius Correlation for Aging Representation

§  Estimate likely performance deactivation for aging duration and temperature of a given application

§  May also lend itself to correlation of different aging platforms (e.g. burner versus engine aging; etc)

§  Determining new aging time (or temperature) corresponding to a baseline or field aging

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

121

2 11expTTR

Ekk a

k1 = deactivation rate at temperature T1

k2 = deactivation rate at temperature T2

With Ea known, calculate k2 corresponding to given aging temperature T2


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Next Steps

¶ Complete performance testing and analysis of the aging DoE

¶  Establish impact of chemical exposure and attempt to correlate combined thermal and chemical aging effects

¶ Determine suitability of the procedure for extrapolating required accelerated aging for extended-duration applications e.g. locomotive

¶  Apply same principle to DOC and CDPF aging