(762f) Spectroscopic and Kinetic Responses of Cu-SSZ-13 to SO2 Exposure and Implications for NOx Selective Catalytic Reduction

Authors: 
Shih, A. J., Purdue University
Khurana, I., Purdue University
Li, H., University of Notre Dame
Gonzalez, J. M., Purdue University
Kumar, A., Cummins Inc.
Paolucci, C., University of Notre Dame
Lardinois, T., Charles D. Davidson School of Chemical Engineering, Purdue University
Jones, C. B., Purdue University
Albarracin Caballero, J. D., Purdue University
Kamasamudram, K., Cummins Inc.
Yezerets, A., Cummins Inc.
Delgass, W. N., Purdue University
Miller, J. T., Purdue University
Villa, A. L., Universidad de Antioquia
Schneider, W. F., University of Notre Dame
Gounder, R., Purdue University
Ribeiro, F. H., Purdue University
Reduction of NOx emissions from diesel engine exhaust is an environmental issue driven by increasingly stringent regulations. Cu-SSZ-13 catalysts are used commercially for this application, but they are susceptible to deactivation by SOx formed from the combustion of ppm levels of sulfur present in diesel fuel. Here, we report on the effects of SO2-poisoning on the two types of Cu active sites that are responsible for NOx selective catalytic reduction, Cu2+ exchanged at two framework Al sites, and [CuOH]1+ exchanged at one framework Al site [1,2].

Two Cu-SSZ-13 catalysts, one with only Cu2+ active sites (3.8 Cu wt%, 100% Cu2+) and another with predominantly [CuOH]1+ active sites (1.5 Cu wt%, 80% CuOH) were synthesized and characterized [3,4]. Our model Cu2+ and [CuOH]1+ catalysts were sulfated with dry SO2 at 200°C using a flow treatment that exposed samples to 5 times the number of SO2 (per Cu), resulting in molar S:Cu ratios of 0.3 and 0.7, respectively. Consistent with the experimental observation of the stronger preference for adsorption of SO2-derived species to [CuOH]1+, density functional theory (DFT) calculations indicate that the binding energy of SO2 on Cu2+ and [CuOH]1+ are -30 and -90 kJ mol-1, respectively. Sulfation decreased the SCR rate (300 ppm NO, 300 ppm NH3, 60% O2, 2% H2O, 8% CO2, balance N2, 200°C, per total Cu) by 26% for the Cu2+ catalyst and 64% for the [CuOH]1+ catalyst. Reaction rates are constant when normalized to the number of non-poisoned Cu sites (moles Cu – moles S) for all samples when rates are measured under in the reduction-limited regime (Eapp = 65 ± 5 kJ mol-1), which indicates that sulfur deactivates Cu sites at a 1:1 S:Cu molar ratio on both Cu2+ and [CuOH]1+ active sites. The coordination environment probed by UV-Visible indicate that S-Cu interactions are observed on [CuOH]1+ but not on Cu2+. Thus, even though sulfur poisons both Cu2+ and [CuOH]1+ sites at a 1:1 molar ratio, spectroscopic observations reveal different sulfur interactions with [CuOH]1+ and Cu2+.

Cu2+ sites are preferred over [CuOH]1+ sites as more SO2-resistant sites based on theoretical DFT calculations and experimental elemental analysis. Reaction kinetics indicate that each SO2 poison binds to only one Cu site, while other non-poisoned Cu sites continue to turn over the SCR cycle. These results predict that synthesizing catalysts with higher fractions of Cu2+ sites will lead to improved emission control catalysts for commercial applications.

References:

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[3] 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.
[4] Shih, A.J.; Khurana, I.; Li, H.; González, J.; Kumar, A.; Paolucci, C.; Lardinois, T.M.; Jones, C.B.; Albarracin-Caballero, J.D.; Kamasamudram, K.; Yezerets, A.; Delgass, W.N.; Miller, J.T.; Villa, A.L.; Schneider, W.F.; Gounder, R.; Ribeiro, F.H. Appl. Catal., A 2019 574, 122.