Nature of SO2-poisoning on ZCuOH and Z2Cu sites in Cu-SSZ-13 during the NH3 Selective Catalytic Reduction (NH3-SCR) of NOx

Arthur  Shih, Purdue University

Arthur J. Shih1, Juan M. Gonzalez1,4, Hui Li2, Ishant Khurana1, Ashok Kumar3, Christopher Paolucci2, W. Nicholas Delgass1, Jeffrey T. Miller1, Krishna Kamasamudram3, Aleksey Yezerets3, Aída Luz Villa4, William F. Schneider2, Rajamani Gounder1, 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 NOx 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 NOx to N2 using NH3 compared to other zeolite catalysts. SOx formed from the combustion of ppm levels of sulfur in diesel fuel deactivates the Cu-SSZ-13 catalyst. Its effect on Cu2+ and CuOH active sites and the SCR mechanism remains a matter of ongoing research. [1-5]

Two Cu-SSZ-13 catalysts, one with only Cu2+ active sites (3.8 Cu wt%, 100% Cu2+) and another with primarily [CuOH]1+ active sites (1.5 Cu wt%, 80% CuOH) were synthesized, hereby denoted as Z2Cu and ZCuOH, respectively.[6,7] 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 (per Cu before sulfation; 200°C) by 90% for the ZCuOH catalyst but only by 60% for the Z2Cu catalyst. Reaction rates and activation energies collected under the same reduction limited regime indicate that sulfur on the catalyst deactivates Cu sites at a 1:1 molar ratio, regardless of Cu speciation. The coordination environment probed by UV-Visible and EXAFS indicate that sulfur-Cu ligands were observed on ZCuOH but not on Z2Cu. Thus, though sulfur poisons Cu at a 1:1 molar ratio, Z2Cu and ZCuOH sites are poisoned via two different sulfur-related deactivation mechanisms.

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.

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