(484f) Nature of SO2 Poisoned Cu-SSZ-13 Catalysts Under Ammonia Selective Catalytic Reduction (NH3-SCR) Conditions

Arthur J. Shih¹, Hui Li², Ashok Kumar³, Juan M. Gonzalez^1,4, Ishant Khurana¹, Christopher Paolucci², Jonatan D. Albarracin Caballero¹, Atish A. Parekh¹, Aída Luz Villa⁴, W. Nicholas Delgass¹, Rajamani Gounder¹, Aleksey Yezerets³, William F. Schneider², Jeffrey T. Miller¹, and Fabio H. Ribeiro¹

¹Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN

²Department of Chemical and Biomolecular Engineering, University of Notre Dame, IN

³Cummins Incorporated, Columbus, IN

⁴Chemical Engineering Department, Universidad de Antioquia, Medellín, Colombia

Reduction of NO_x emissions from diesel engine exhaust is an environmental issue driven by increasingly stringent emission regulations. Cu-SSZ-13 catalysts have been shown to be promising for this application due to their hydrothermal stability and selectivity in reducing NO_x to N₂ using NH₃ compared to other zeolite catalysts. SO_x formed from the combustion of ppm levels of sulfur in diesel fuel deactivates the Cu-SSZ-13 catalyst. However, its effect on Cu²⁺ and CuOH active sites and the SCR mechanism is unclear and under debate.[1-4]

Two Cu-SSZ-13 catalysts, one with only Cu²⁺ (3.8 Cu wt%, 100% Cu²⁺)and another with primarily [CuOH]¹⁺ (1.5 Cu wt%, 80% CuOH) active sites were synthesized, hereby denoted as Z₂Cu and ZCuOH, respectively.[5] Each catalyst was sulfated with dry SO₂ at 200°C and 400°C, resulting in sulfur contents of 0.7 wt% and 1.0 wt%, respectively. Sulfation at 400°C decreased the standard SCR rate by 90% for the ZCuOH catalyst but by only 60% for the Z₂Cu catalyst. In addition, the apparent activation energy for ZCuOH sites decreased from 70 kJ mol^-1 to 11 kJ mol^-1 with increasing extents of sulfur poisoning but remained constant at 70 kJ mol^-1 for the Z₂Cu catalyst. These kinetic results suggest two different deactivation mechanisms for the Z₂Cu and ZCuOH sites. The decrease in apparent activation energy for the ZCuOH catalyst is attributed to changes in the binding energy of NH₃ to the Cu active sites, corroborated by evidence from UV-Visible where sulfur-Cu ligands were observed on ZCuOH but not on Z₂Cu. The unchanged apparent activation energy and reactant orders for the Z₂Cu catalyst after sulfation are attributed to pore-filling by ammonium sulfate salts; this reduces the total number of accessible Cu, as supported by argon micropore measurements.

Z₂Cu sites are preferred over ZCuOH sites due to the lower impact of sulfur poisoning on the rate and apparent activation energy. These results predict that synthesizing catalysts with higher fractions of Z₂Cu sites will lead to improved emission control catalysts for commercial application.

References:

1. Jangjou, Y.; Wang D.;Kumar, A.; Li J.; Epling, W.S. ACS Catal. 2016, 6, 10.
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5. Paolucci, C.; Parekh, A.A.; Khurana, I.; Di Iorio, J.R.; Li, H.; Albarracin Caballero, J.D.; Shih, A.J.; Anggara, T.; Delgass, W.N.; Miller, J.T.; Ribeiro, F.H.; Gounder, R.; Schneider, W.F. J. Am. Chem. Soc. 2016, 138, 18.