(247j) Insight into Selective Catalytic Reduction on Cu-SSZ-13 from Molecular Simulation

Insight into Selective Catalytic Reduction on Cu-SSZ-13 from Molecular Simulation

Christopher Paolucci¹, Atish A. Parekh², Ishant Khurana², John Di Iorio², Hui Li¹, Trunojoyo Anggara¹, Jeffrey T. Miller², Rajamani Gounder², W. Nicholas Delgass², Fabio H. Ribeiro², and William F. Schneider¹*

¹Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame IN 46556 (USA)

²School of Chemical Engineering, Purdue University, West Lafayette, IN 47907 (USA)

The selective catalytic reduction (SCR) of NO_x in excess oxygen is a key challenge in meeting increasingly stringent emission regulations.

4 NH₃ + 4 NO + O₂  → 4 N₂ + 6 H₂O  (SCR)

The metal-exchanged zeolite Cu-SSZ-13 in particular exhibits sufficient activity and hydrothermal stability for practical applications.  A primary obstacle for further improvement of SCR catalysts is a lack of molecular-level understanding of the active sites and mechanism.  Here, using Monte Carlo (MC) simulation, density functional theory (DFT) calculations, ab-initio molecular dynamics (AIMD), and ab-initio thermodynamic modeling we show that:

- SCR occurs on isolated metal sites in zeolites. The types and maximum number of these sites are estimated through a MC/DFT method[1]. This result is experimentally validated through a variety of techniques including temperature programmed desorption (TPD), X-ray absorption spectroscopy (XAS), UV-Visible spectroscopy, and kinetic experiments on zeolites of varying compositions.[2] 


- Develop a redox mechanism that describes the SCR reaction at a molecular level on these sites based off of AIMD simulations and DFT calculations. The mechanism and its intermediate species are validated through XAS experiments at Argonne national laboratory and acid site titration experiments at Purdue University.[3] 


- Model the kinetics of SCR and validate the model against operando XAS experiments at a variety of catalytic conditions. 


We show SCR occurs through a redox mechanism on all isolated Cu sites and identify the key reaction steps in the reduction and oxidation half cycles [3].

References

[1]  Bates SA, Verma AA, Paolucci C, Parekh AA, Anggara T, Yezerets A, Schneider WF, Miller JT, Delgass WN, Ribeiro FH. Identification of the active Cu site in standard selective catalytic reduction with ammonia on Cu-SSZ-13. Journal of Catalysis. 2014;312:87–97. 


[2]  Verma AA, Bates SA, Anggara T, Paolucci C, Parekh AA, Kamasamudram K, Yezerets A, Miller JT, Delgass WN, Schneider WF, Ribeiro FH. NO oxidation: A probe reaction on Cu-SSZ-13. Journal of Catalysis. 2014;312:179–190. 


[3]  Paolucci C, Verma AA, Bates SA, Kispersky VF, Miller JT, Gounder R, Delgass WN, Ribeiro FH, Schneider WF. Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu-SSZ-13. Angewandte Chemie International Edition. 2014;53(44):11828–11833.