(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. Gonzalez1,4, 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 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. 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. 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.
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