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

Reduction of NO_x 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 SO_x 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 (E_app = 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:

[1] Jangjou, Y.; Do, Q.; Gu, Y.; Lim, L.-G.; Sun, H.; Wang, D.;Kumar, A.; Li, J.; Grabow, L.C.; Epling, W.S. ACS Catal. 2018, 8, 2.
[2] Luo, J.; Wang, D.; Kumar, A.; Li, J.; Kamasamudram, K.; Currier, N.; Yezerets, A. Catal. Today 2016, 267, 3.
[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.