On kinetic modeling of change in active sites upon hydrothermal aging of Cu-SSZ-13

Rohil Daya, Saurabh Joshi, Jinyong Luo, Rama Krishna Dadi, Neal Currier, Aleksey Yezerets

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09-18-2019

CLEERS 2019 Workshop

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Outline▪ Motivation and Background

▪ Modification of reaction rate forms to account for active site dynamics

▪ Developing constitutive relations for Hydrothermal Aging of Cu-SSZ-13

▪ Summary and Future Work

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Motivation – Tailpipe NOx and Durability Requirements

Future Application Challenges

2024+

Challenge

Real World 90%NOx

Add heat/CDA/ bypass

Sustained LT

Cold start

GHG Reduction

Engine efficiency

Electrification

Fuel type

…

Emission Warranty Extension

Forecast 2022 2024

HD 5Y 350K miles 5Y 435K miles ( maybe longer)

MD 5Y 150K miles 5Y 185K miles ( maybe longer)

Overall approach to meet these challenges

includes

a) Adaptable Controls

b) Improved understanding and modeling

of real world aging

c) Improved Component durability (higher

resistance to real world aging)Class 2022 2027 (forecast)

HD 5Y 350K miles 14Y 800K miles

MD 5Y 150K miles 14Y 400K miles

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Two chemically distinct monomeric Cu2+ active sites in Cu-SSZ-13; typically solvated and mobile

under reaction conditions; undergo redox during SCR (Cu1+ observed during standard SCR)

Mild-hydrothermal aging leads to

decrease in number density of ZCuOH sites (𝛺𝑍𝐶𝑢𝑂𝐻),and concomitant increase in number density of Z2Cu

sites (𝛺𝑍2𝐶𝑢) [1]. Severe HTA leads to dealumination,

increase in CuOx, and eventually structural collapse

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Active Sites in Cu-SSZ-13 and influence of aging

Hydrothermal Aging Sulfur Poisoning

ZCuOH

Z2Cu

[1] Luo, J., An, H., Kamasamudram, K., Currier, N., Yezerets, A., Watkins, T., & Allard, L.

(2015). Impact of Accelerated Hydrothermal Aging on Structure and Performance of Cu-SSZ-

13 SCR Catalysts. SAE International Journal of Engines, 8(3), 1181-1186.

Sulfur selectively poisons Copper sites.

ZCuOH stores more sulfur and is harder to

regenerate relative to Z2Cu [2]

[2] Shih, A. J., Khurana, I., Li, H., González, J., Kumar, A., Paolucci, C., ... & Yezerets, A.

(2019). Spectroscopic and kinetic responses of Cu-SSZ-13 to SO2 exposure and implications

for NOx selective catalytic reduction. Applied Catalysis A: General, 574, 122-131.

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Monolith reactors can be modeled as plug flow reactors. Equations below

are sufficient to describe SCR behavior under realistic operating conditions

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Classical Conservation Equations for Monolith

Reactors

𝒖𝝏𝒚𝒊𝝏𝒛

= −𝟒𝒌𝒎,𝒊𝒅𝒉𝒚𝒅

(𝒚𝒊 − 𝒚𝒊𝒘𝒄)𝝂𝒋,𝒊𝒓𝒋

𝑪𝟎= −𝑫𝒆𝒇𝒇

𝝏𝟐𝒚𝒊𝒘𝒄𝝏𝒙𝟐

𝜴𝒎𝝏𝜽𝒌𝝏𝒕

= 𝝂𝒋,𝒊𝒓𝒋

Convection Rate = Diffusion Rate Surface Coverage ConservationReaction Rate = Diffusion Rate

𝒌𝒎,𝒊 𝒚𝒊 − 𝒚𝒊𝒘𝒄 = −𝑫𝒆𝒇𝒇𝝏𝒚𝒊𝒘𝒄𝝏𝒙

𝒂𝒕 𝒙 = 𝟎subject to

[3] Copeland, C., Pesiridis, A., Martinez-Botas, R., Rajoo, S., Romagnoli, A., & Mamat, A.

(2014). Automotive Exhaust Waste Heat Recovery Technologies. In Automotive Exhaust

Emissions and Energy Recovery. Nova Science Publishers.

Copeland et al. [3]

0 200 400 600 800 1000 1200

Cu

mu

lati

ve N

Ox

[g]

Time [s]

Hot FTP Cumulative NOx

SCR-In NOxSCR-Out ExpSCR-Out Model

0 200 400 600 800 1000 1200

NH

3[p

pm

]

Time [s]

Hot FTP NH3 Slip

SCR-Out Exp

SCR-Out Model

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▪ Aging induces changes in catalyst state. These changes can be modeled in a classical framework

by deriving:

a) Constitutive relations/rate expressions for quantitative changes of all chemically distinct active sites

b) Intrinsic, site specific turnover rates for conversion of gas species on each active site

▪ In this representation, the state and behavior of each active site is defined completely by a set of

variables and constants with respect to aging

▪ This entails a combination of reaction engineering and more fundamental approaches incorporating

DFT predictions and microkinetics

▪ In the next few slides, we will show a first effort at developing constitutive relations for the change in

active sites induced by hydrothermal aging of Cu-SSZ-13 Public

Incorporating Aging in Conservation Equations

𝜴𝒎𝝏𝜽𝒌𝝏𝒕

= 𝝂𝒋,𝒊𝒓𝒋 𝜴𝒎′ (𝒕𝒂𝒈𝒆, 𝑻𝒂𝒈𝒆, 𝑺𝑶𝒙… )

𝝏𝜽𝒌𝝏𝒕

= 𝝂𝒋,𝒊𝒓𝒋′(𝒕𝒂𝒈𝒆, 𝑻𝒂𝒈𝒆, 𝑺𝑶𝒙… )

Fixed Age Variable Age

Z2Cu

Variable : 𝛺𝑍2𝐶𝑢 (𝑛𝑢𝑚𝑏𝑒𝑟 𝑑𝑒𝑛𝑠𝑖𝑡𝑦)

Constants (if fixed turnover rate) : 𝐴𝑁𝐻3𝐴𝑑𝑠 𝑍2𝐶𝑢 ,

∆𝑆𝑁𝐻3𝐴𝑑𝑠 𝑍2𝐶𝑢 , ∆𝐻𝑁𝐻3𝐴𝑑𝑠 𝑍2𝐶𝑢 , 𝐴𝑆𝐶𝑅𝑍2𝐶𝑢 , 𝐸𝑎𝑆𝐶𝑅 𝑍2𝐶𝑢…

ZCuOH

Variable : 𝛺𝑍2𝐶𝑢 (𝑛𝑢𝑚𝑏𝑒𝑟 𝑑𝑒𝑛𝑠𝑖𝑡𝑦)

Constants (if fixed turnover rate) : 𝐴𝑁𝐻3𝐴𝑑𝑠 𝑍𝐶𝑢𝑂𝐻 ,

∆𝑆𝑁𝐻3𝐴𝑑𝑠 𝑍𝐶𝑢𝑂𝐻 , ∆𝐻𝑁𝐻3𝐴𝑑𝑠 𝑍𝐶𝑢𝑂𝐻 , 𝐴𝑆𝐶𝑅𝑍𝐶𝑢𝑂𝐻 , 𝐸𝑎𝑆𝐶𝑅 𝑍𝐶𝑢𝑂𝐻 …

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Hydrothermal Aging Model Approach

NH3 storage kinetics on individual sites

• Site isolation approach

• 550°C-4h H-SSZ-13 data used for Brønsted acid site kinetics

• 750°C-4h Cu-SSZ-13 data used for lumped Copper site kinetics.

• 600°C-2h low temperature (

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Active Sites for NH3 Adsorption on H-SSZ-13▪ It has been shown that on H-SSZ-13, NH4

+ cations are NH3 –solvated below 400°C, based on in-

situ vibrational spectroscopy and DFT simulations [4-5]

[4] Giordanino, F., Borfecchia, E., Lomachenko, K. A., Lazzarini, A., Agostini, G., Gallo, E., ... & Lamberti, C.

(2014). Interaction of NH3 with Cu-SSZ-13 catalyst: a complementary FTIR, XANES, and XES study. The

journal of physical chemistry letters, 5(9), 1552-1559.

[5] Li, S., Zheng, Y., Gao, F., Szanyi, J., & Schneider, W. F. (2017). Experimental and computational

interrogation of fast SCR mechanism and active sites on H-Form SSZ-13. ACS Catalysis, 7(8), 5087-5096.

DFT Optimized Structures [5]

Strong-H Weak-H

▪ Furthermore, in-house experimental data for H-SSZ-13 shows indirect evidence of second order adsorption,

consistent with solvation of NH4+ ions by NH3 . Thus, NH3 adsorption on Brønsted acid sites modeled as a

Type-II BET isotherm

𝑟𝑏−𝑆𝐻 = 𝑘𝑏−𝑆𝐻𝜃𝑍.𝑁𝐻4𝛺𝐻

𝑁𝐻3 + 𝑍𝐻 ⇿ 𝑍.𝑁𝐻4𝑟𝑓−𝑆𝐻 = 𝑘𝑓−𝑆𝐻𝑦𝑁𝐻3𝜃𝑍𝐻𝛺𝐻

𝑁𝐻3 + 𝑍.𝑁𝐻4 ⇿ 𝑍.𝑁𝐻4. 𝑁𝐻3𝑟𝑓−𝑊𝐻 = 𝑘𝑓−𝑊𝐻𝑦𝑁𝐻3𝜃𝑍.𝑁𝐻4𝛺𝐻𝑟𝑏−𝑊𝐻 = 𝑘𝑏−𝑆𝐻𝜃𝑍.𝑁𝐻

4.𝑁𝐻

3𝛺𝐻

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Second Order NH3-Adsorption on H-SSZ-13▪ NH3 release during isothermal adsorption and TPD can be described quantitatively using the Type-II BET

adsorption model

▪ Results from typical first order Langmuir/Temkin adsorption model show that while such a model can capture

the desorption peak with similar accuracy, NH3 release isothermal NH3 adsorption cannot be described

accurately

550°C-4h H-SSZ-13 NH3 TPD 550°C-4h H-SSZ-13 Total Storage

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Additional Active Sites for NH3 Adsorption on Cu-SSZ-13In addition to the Strong-H and Weak-H sites, two additional sites considered for NH3 adsorption:

Copper Sites (Temkin isotherm model)

Physisorbed NH3 (to account for increased storage at high feeds and low temperatures. Langmuir

isotherm model)

𝑟𝑏−𝐶𝑢 = 𝑘𝑏−𝐶𝑢𝜃𝐶𝑢.𝑁𝐻3𝛺𝐶𝑢.𝑁𝐻3

𝑁𝐻3 + 𝐶𝑢 ⇿ 𝐶𝑢.𝑁𝐻3𝑟𝑓−𝐶𝑢 = 𝑘𝑓−𝐶𝑢𝑦𝑁𝐻3𝜃𝐶𝑢𝛺𝐶𝑢.𝑁𝐻3

𝑟𝑏−𝑃 = 𝑘𝑏−𝑃𝜃𝑃.𝑁𝐻3𝛺𝑃

𝑁𝐻3 + 𝑃 ⇿ 𝑃.𝑁𝐻3𝑟𝑓−𝑃 = 𝑘𝑓−𝑃𝑦𝑁𝐻3𝜃𝑃𝛺𝑃

ZCu, Z2Cu and ZCuOH sites lumped

into global Copper site, due to

inability of NH3-TPD to resolve

individual copper sites

Isolated Cu2+ positions from AIMD [6]

[6] Paolucci, C., Di Iorio, J. R., Ribeiro, F. H., Gounder, R., & Schneider, W. F. (2016). Catalysis science of NOx selective catalytic

reduction with ammonia over Cu-SSZ-13 and Cu-SAPO-34. In Advances in Catalysis (Vol. 59, pp. 1-107). Academic Press.

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Copper and Physisorbed Site Kinetics on Cu-SSZ-13

▪ NH3-TPD at 750°C-4h shows a primary release feature at 290°C, with a minor shoulder at 400°C

▪ Significant low temperature isothermal desorption attributed to a global physisorbed site. Isothermally

desorbed NH3 shows non-monotonic trend with increased temperature

Age : 600°C-2h

Experiment : 1h isothermal

adsorption followed by 40

minutes isothermal desorption

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Site-Specific Adsorption-Desorption Dynamics▪ Quantitative predictions of the influence of catalyst temperature on storage of individual sites

▪ Steady-state total storage on 750°C-4h aged Cu-SSZ-13 compares well with model predictions

Age : 600°C-2h

Experiment : 1h isothermal

adsorption followed by 40

minutes isothermal desorption

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NH3 Adsorption Equilibrium Thermodynamics

[7] Paolucci, C., Parekh, A. A., Khurana, I., Di Iorio, J. R., Li, H., Albarracin Caballero, J. D., ... & Ribeiro, F. H.

(2016). Catalysis in a cage: condition-dependent speciation and dynamics of exchanged Cu cations in SSZ-13

zeolites. Journal of the American Chemical Society, 138(18), 6028-6048.

▪ Estimated adsorption enthalpies and entropic penalties are aligned with DFT/AIMD predicted values in

literature

▪ Entropic penalties in line with the notion of mobile NH3 complexes on zeolite surface

[5] Li, S., Zheng, Y., Gao, F., Szanyi, J., & Schneider, W. F. (2017). Experimental and computational interrogation of fast SCR

mechanism and active sites on H-Form SSZ-13. ACS Catalysis, 7(8), 5087-5096.

Model ParametersRecent DFT/AIMD Predictions [5, 7-8]

[8] Li, H., Paolucci, C., & Schneider, W. F. (2018). Zeolite adsorption free energies from ab initio

potentials of mean force. Journal of chemical theory and computation, 14(2), 929-938.

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Modeling the change in NH3-TPD with hydrothermal

aging▪ NH3-TPD experiments indicate a monotonic decrease in high temperature peak and increase in low

temperature peak with increased aging [9]

▪ This behavior was modeled with:

a) Fixed turnover rates (identified previously)

b) Increased NH3 storage ability on Copper sites with aging

c) Decreased number density of Brønsted acid sites with aging

[9] Luo, J., Gao, F., Kamasamudram, K., Currier, N., Peden, C. H., & Yezerets, A.

(2017). New insights into Cu/SSZ-13 SCR catalyst acidity. Part I: Nature of acidic

sites probed by NH 3 titration. Journal of Catalysis, 348, 291-299.

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Tracking Storage Capacity on Copper Sites and

Number Density of Brønsted Acid Sites▪ Increase in NH3 storage on copper sites is linearly related to the loss of Brønsted acid sites

▪ The loss of Brønsted acid sites with aging is used to develop a constitutive relationship for the

evolution of these sites, yielding the hydrothermal aging equation

NH3 Storage Ability on

Copper Sites

Number Density of

Brønsted acid Sites

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Hydrothermal Aging Equation

𝟏

𝜴𝑯𝒏𝒐𝒓𝒎= 𝟐. 𝟓𝟑 ∗ 𝟏𝟎𝟖𝒆𝒙𝒑 −

𝟏𝟔𝟖𝟕𝟑𝟑

𝑹𝑻𝒂𝒈𝒆𝒕𝒂𝒈𝒆 + 𝟏

Ratio-based calculator reported previously [10] Identical deactivation energies from two

different derivation methods

▪ A linear relationship is found between the inverse Brønsted acid site density and aging time,

implying a second order Arrhenius rate. The rate constant and activation energy are derived

Arrhenius Plot

[10] Luo, J., Kamasamudram, K., Currier, N., & Yezerets, A. (2018). NH3-TPD methodology for quantifying

hydrothermal aging of Cu/SSZ-13 SCR catalysts. Chemical Engineering Science, 190, 60-67.

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Model Limitations – Copper Site Deconvolution

▪ NH3 adsorption at 200°C leads to replacement of approximately two framework oxygen atoms on

Z2Cu sites and approximately one framework oxygen atom on ZCuOH sites [9]

𝜴𝑪𝒖.𝑵𝑯𝟑 = 𝟐𝜴𝒁𝟐𝑪𝒖′ + 𝟏𝜴𝒁𝑪𝒖𝑶𝑯

′

𝜴𝒁𝑪𝒖𝑶𝑯′

𝜴𝒁𝟐𝑪𝒖′ + 𝜴𝒁𝑪𝒖𝑶𝑯

′ = 𝟎. 𝟖𝟕 𝒂𝒕 𝟓𝟓𝟎°𝑪 − 𝟒𝒉𝒅𝜴𝒁𝟐𝑪𝒖

′

𝒅𝒕𝒂𝒈𝒆= −

𝒅𝜴𝒁𝑪𝒖𝑶𝑯′

𝒅𝒕𝒂𝒈𝒆

−𝒅𝜴𝑯

𝒅𝒕𝒂𝒈𝒆= −

𝒅𝜴𝒁𝑪𝒖𝑶𝑯′

𝒅𝒕𝒂𝒈𝒆+ 𝟎. 𝟑𝟏

𝒅𝜴𝒁𝟐𝑪𝒖′

𝒅𝒕𝒂𝒈𝒆

Known from NH3-TPD model

▪ Model does not explicitly predict the evolution of individual Copper sites with hydrothermal aging.

However, combining model results with other characterization and experimental data can provide

some insight

Reducible Cu2+ densities

▪ Additional characterization data on same catalyst :

H2-TPR on degreened catalyst [9]

▪ Correlating Brønsted acid site loss rate with

individual copper site change rate :

NO + NH3 titration

experiments

[9] Luo, J., Gao, F., Kamasamudram, K., Currier, N., Peden, C. H., & Yezerets, A.

(2017). New insights into Cu/SSZ-13 SCR catalyst acidity. Part I: Nature of acidic

sites probed by NH 3 titration. Journal of Catalysis, 348, 291-299.

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Summary and Future Work▪ Modeling of SCR catalyst aging is necessary to meet the next phase of tailpipe regulations and

durability requirements

▪ If successfully developed and integrated with performance predictions, these models have

significant applications in improved understanding and design of SCR catalysts, along with better

catalyst health monitoring and control

▪ These models can be enabled by intrinsic micro-kinetics for species conversion on each active site

and experimental quantification of active site evolution with aging. For instance, the loss of

Brønsted acid sites with hydrothermal aging can be expressed by :

▪ Constitutive relations must be discovered for the response of all chemically distinct active sites to

different SCR degradation mechanisms

▪ Once the catalyst state is known, the performance can be estimated using site-specific turnover

rates identified through experimental and theoretical understanding

𝟏

𝜴𝑯𝒏𝒐𝒓𝒎= 𝟐. 𝟓𝟑 ∗ 𝟏𝟎𝟖𝒆𝒙𝒑 −

𝟏𝟔𝟖𝟕𝟑𝟑

𝑹𝑻𝒂𝒈𝒆𝒕𝒂𝒈𝒆 + 𝟏

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Backup Slides

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Transient and Steady-State Coverages on H-SSZ-13▪ The surface coverages are plotted as a function of time (transient TPD) and temperature (steady-state).

Steady-state coverages compare qualitatively well with computational results in [5]

[5] Li, S., Zheng, Y., Gao, F., Szanyi, J., & Schneider, W. F. (2017). Experimental and computational interrogation of fast SCR

mechanism and active sites on H-Form SSZ-13. ACS Catalysis, 7(8), 5087-5096.

Surface Coverages in [5]

550°C-4h H-SSZ-13 NH3 TPD550°C-4h H-SSZ-13 Total Storage

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Experimental Data : Jinyong Luo

Transient and Steady-State Coverages on Cu-SSZ-13▪ The surface coverages are plotted as a function of time (transient TPD) and temperature (steady-state)

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Validation of modeling approach with steady-state

Storage measurements▪ Evolution of total NH3 storage with hydrothermal aging can be understood using the model

▪ With hydrothermal aging, the significance of Brønsted acid sites as a reservoirs of NH3 decreases

Surprising experimental result at 450°C being

investigated further

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Analytical Expressions for Total and Site Specific Storage▪ The equilibrium constants on each site can be utilized to calculate site specific surface coverage using

Langmuir’s adsorption isotherm

𝐿𝑎𝑛𝑔𝑚𝑢𝑖𝑟/𝑇𝑒𝑚𝑘𝑖𝑛 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐶𝑜𝑣𝑒𝑟𝑎𝑔𝑒 −→ 𝜽𝑬𝒒𝒊(𝒚𝑵𝑯𝟑 , 𝑻) =𝑲𝑬𝒒𝒊𝒚𝑵𝑯𝟑

𝟏+𝑲𝑬𝒒𝒊𝒚𝑵𝑯𝟑𝑖 = 𝐶𝑢 𝑠𝑖𝑡𝑒𝑠, 𝑃ℎ𝑦𝑠 𝑠𝑖𝑡𝑒𝑠

▪ Once the surface coverage on each of the sites is known, the total ammonia storage can be calculated as

follows:

𝝎𝑵𝑯𝟑 = 𝑴𝑾𝑵𝑯𝟑𝑾𝑳

𝝆𝒘𝜴𝑪𝒖.𝑵𝑯𝟑𝜽𝑬𝒒𝑪𝒖 + 𝜴𝑯 𝜽𝑬𝒒𝒁𝑵𝑯𝟒

+ 𝟐𝜽𝑬𝒒𝒁𝑵𝑯𝟒𝑵𝑯𝟑+ 𝜴𝒑𝜽𝑬𝒒𝑷

Where 𝝎𝑵𝑯𝟑 is the NH3 storage in g/Lcat, 𝑴𝑾𝑵𝑯𝟑 is the molecular weight of NH3 in kg/mol, 𝑾𝑳 is washcoat loading in

g/Lcat, 𝝆𝒘 is washcoat density in kg/m3 (666 for DW3136 family), 𝜴𝒊 mol/m

3washcoat and 𝜽𝑬𝒒𝒊 is the equilibrium surface

coverage of active site 𝑖

▪ In order to estimate NH3 storage as a function of aging time and temperature, 𝜴𝒊 as a function of aging time and temperature must be calculated, and this is the objective of the aging model

𝐵𝐸𝑇 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐶𝑜𝑣𝑒𝑟𝑎𝑔𝑒 −→ 𝜽𝑬𝒒𝒁𝑵𝑯𝟒(𝒚𝑵𝑯𝟑 , 𝑻) =𝑲𝑬𝒒𝑺𝑯𝒚𝑵𝑯𝟑

𝟏+𝑲𝑬𝒒𝑺𝑯𝒚𝑵𝑯𝟑+𝑲𝑬𝒒𝑺𝑯𝑲𝑬𝒒𝑾𝑯𝒚𝑵𝑯𝟑𝟐 ; 𝜽𝑬𝒒𝒁𝑵𝑯𝟒𝑵𝑯𝟑(𝒚𝑵𝑯𝟑 , 𝑻) =

𝑲𝑬𝒒𝑺𝑯𝑲𝑬𝒒𝑾𝑯𝒚𝑵𝑯𝟑𝟐

𝟏+𝑲𝑬𝒒𝑺𝑯𝒚𝑵𝑯𝟑+𝑲𝑬𝒒𝑺𝑯𝑲𝑬𝒒𝑾𝑯𝒚𝑵𝑯𝟑𝟐

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Model Limitations – Severe Hydrothermal Aging

▪ Beyond a threshold aging, there is a change in

NH3 storage energetics on Copper sites,

characterized by a shift in the low temperature

release peak from 290°C to 265°C

▪ Systematic analysis of NH3 TPD profile shows

that are at least two different phases of

hydrothermal aging:

Phase I – Drop in ratio of high to low temperature

peaks, but negligible loss of Cu2+ (transformation of

ZCuOH to Z2Cu)

Phase II – Drop in Cu2+ sites. Brønsted acid sites

negligible. Change in energetics of low temperature

sites. Loss of NH3 storage

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Cu2+ site Distribution as a function of Hydrothermal

Age

Key Model Prediction : In the

degreened state, 30-35% of the

Copper does not store NH3, and

must therefore not exist as

isolated Cu2+

𝑍2𝐶𝑢 𝑤𝑡.% =𝛺𝑍

2𝐶𝑢 ∗ 𝑀𝑊𝐶𝑢 ∗ 𝑉𝑟𝐷𝐺 ∗ 610.237

𝑍𝐶𝑢𝑂𝐻 𝑤𝑡.% =𝛺𝑍𝐶𝑢𝑂𝐻 ∗ 𝑀𝑊𝐶𝑢 ∗ 𝑉𝑟

𝐷𝐺 ∗ 610.237

Where 𝑀𝑊𝐶𝑢 is the molecular weight of Cu in

g/mol, 𝑉𝑟 is ratio of washcoatvolume to reactor volume

and 𝐷𝐺 is dry gain in g/in3