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Development and Validation of a Two-Site Kinetic Model for NH3-SCR over Cu-SSZ-13. Part 1. Detailed Global Kinetics Development Based on Mechanistic Considerations

  • Special Issue: 2017 CLEERS October 3 - 5, Ann Arbor, MI, USA
  • Published:
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Abstract

In this study, we present the first of two parts in the development and validation of a two-site detailed global kinetic model for NH3 Selective Catalytic Reduction (SCR) over a well-characterized Cu-SSZ-13 catalyst. Based on fundamental literature studies and experimental data for two distinct hydrothermal aging conditions, it was observed that at least two distinct sites were necessary to describe the storage, oxidation, and SCR behavior for this catalyst. The site definitions were allocated based on analysis of simulated net desorption rates during the NH3-TPD experiment. The S1 sites were associated with extra framework copper ions stabilized by -OH ligands (ZCuOH), likely located near the eight-membered ring CHA cages, along with Brønsted acid sites, while S2 sites were associated with copper ions attached directly to the repeating units (Z2Cu) near the six-membered rings, along with physisorbed NH3 sites and low temperature transient copper dimers. The selective NH3 oxidation and NO oxidation reactions were only modeled over S1, in line with the expected catalytic sites for these reactions. Standard, fast, and NO2 SCR reactions were modeled on both sites, with different activation energies. Finally, noticeable nitrate-based hysteresis effects were observed, in both N2O concentrations and fast SCR NO conversions. These were accounted for by explicitly modeling nitrate formation, titration by NO, and thermal decomposition to N2O. The developed SCR model was validated with additional reactor data at nominal inlet NH3-to-NOx ratios (ANRs) of 0.8 and 1.2. In general, the model showed good predictability in the temperature range of 150–550 °C for both hydrothermal ageing conditions and space velocities. Further full-scale engine dynamometer validation and development of a downstream NH3 slip catalyst (ASC) reaction-diffusion model will be reported in the second part of the paper.

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Notes

  1. It was observed that the quality of the N2 readings from the MS was relatively poor when CO2 was present in the feed gas. Ordinarily, N2 at a mass-to-charge (m/e) ratio of 28 is relatively easy to measure with the MS. However, 5% CO2 in the gas stream created a problem, as there was sizeable fragmentation from CO2 to CO+, which is picked up at m/e 28. At a typical electron filament energy of 70 ev, the CO+ from the CO2 completely overwhelmed the small signal from N2 at m/e 28. Since CO2 had absolutely no influence over any of the experiments (this was confirmed in further independent experiments).

Abbreviations

Ω k :

Active site density for coverage, k mol-site/m3reactor

a j :

Active site density of reaction j, mol-site/m3reactor

C pg :

Heat capacity of gas, J/(kg-K)

D e, i :

Binary diffusion coefficient of species i in the mixture, m2/s

f :

Friction factor

f wc :

Solid fraction of washcoat

f sb :

Solid fraction of substrate

h :

Heat transfer coefficient, W/(m2-K)

h x :

External heat transfer coefficient, W/(m2-K)

k m, i :

Species i mass transfer coefficient in the gas phase, kg/(m2-s)

k :

Reaction rate constant, 1/s

M i :

Molecular weight of species i, kg/mol

p :

Pressure, Pa

R i :

Species mass rate, kg/(m3-s)

r j :

Washcoat-averaged reaction rate for reaction j, mol/m3reactor-s

s ij :

Stoichiometric coefficient of species i in reaction j

S :

Surface area per reactor volume, 1/m

S x :

External surface area per reactor volume, 1/m

T g :

Temperature of bulk gas in reactor channels, K

T s :

Solid phase temperature, K

T x :

External temperature, K

v :

Interstitial velocity, m/s

V :

Reactor volume, m3

ΔH j :

Enthalpy of reaction (negative for exothermic reactions), J/mol

δ :

Average washcoat thickness, m

ε :

Void fraction of reactor

θ :

Surface coverage

λ sb :

Thermal conductivity of substrate, W/(m-K)

ρ g :

Density of bulk gas in reactor channels, kg/m3

ρ s :

Density of gas at catalyst surface, kg/m3

σ kj :

Stoichiometric coefficient of coverage k in reaction j

ψ s :

Effective heat capacity of reactor, J/(m3-K)

ω g, i :

Species i mass fraction in the gas phase

ω s, i :

Species i mass fraction in the washcoat

ω sf, i :

Species i mass fraction at the washcoat/channel interface

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Acknowledgments

The authors would like to acknowledge Cormetech Inc. for executing the test protocol, supplying the reactor data, and assisting with the reactor setup description. Furthermore, the Gamma Technologies Aftertreatment support team helped us with useful discussions and continuous assistance with modeling work.

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Authors and Affiliations

  1. Isuzu Technical Center of America, 46401 Commerce Center Drive, Plymouth, MI, 48170, USA

    Rohil Daya, Chintan Desai & Bruce Vernham

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  1. Rohil Daya
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  2. Chintan Desai
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  3. Bruce Vernham
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Correspondence to Rohil Daya.

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Appendix: Kinetic Modeling Results at GHSV 30,000/h

Appendix: Kinetic Modeling Results at GHSV 30,000/h

Fig. 22
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Steady-state NH3 oxidation experiment and simulation results including NO formation and N2O formation from 250 to 550 °C

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Fig. 23
figure 23

Steady-state NO oxidation results with outlet NO2/NOx ratios at inlet NO2/NOx = 0 and 0.33 from 250 to 550 °C

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Fig. 24
figure 24

Steady-state standard SCR NOx conversion, NH3 conversion, and N2O slip results from 150 to 550 °C

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Fig. 25
figure 25

Steady-state fast SCR NO conversion, NH3 conversion, and N2O slip results from 175 to 550 °C (experiment did not reach steady state at 150 °C)

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Fig. 26
figure 26

Steady-state NO2 SCR NO2 conversion NH3 conversion and N2O slip results from 225 to 550 °C

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Daya, R., Desai, C. & Vernham, B. Development and Validation of a Two-Site Kinetic Model for NH3-SCR over Cu-SSZ-13. Part 1. Detailed Global Kinetics Development Based on Mechanistic Considerations. Emiss. Control Sci. Technol. 4, 143–171 (2018). https://doi.org/10.1007/s40825-018-0095-5

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